Unit 3

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BIOLOGY

Unit 3 Reproduction & Inheritance

Andy Todd

Unit 3 Biology Notes: Unit 3: Reproduction & Inheritance

Contents:

A: Reproduction

B: Inheritance

Andy Todd

Unit 3

A: Reproduction Students will be assessed on their ability to: 3.1 describe the differences between sexual and asexual reproduction 3.2 understand that fertilisation involves the fusion of a male and female gamete to produce a

Asexual Reproduction Definiton: Reproduction in which there is only one parent and the offspring are clones (have the same genes, unless mutations occur) Mitosis:  Process by which cells asexually reproduce  Produces offspring called ‘daughter cells’ who are genetically identical to the parent (unless a mutation occurs)  Used to replace dead or damaged diploid cells in plants and animals  Also used for normal growth of diploid cells  Diploid cells – body cells with a full set of chromosomes (46 in humans)  Does NOT occur in sex cells Process of Mitosis: 1. Chromosomes become coiled and visible 2. Each chromosome makes an exact copy of itself 3. Chromosomes and their copy line up down the equator of the cell 4. Chromosomes are pulled apart and move to opposite ends of the cell 5. Cell splits into two new daughter cells with the exact same chromosomes as the parents Advantages of Asexual Reproduction:  Only one parent required (no mates needed)  Quick way of producing a large number of cells  Daughter cells are genetically identical to parent cells, so parent cells are easy to replace  Simple and reliable process Disadvantages of Asexual Reproduction:  No genetic variation  cells have a very hard time adapting to change

Andy Todd

Unit 3

Sexual Reproduction Definition: Reproduction in which an individual is formed by the combination of 2 sex cells from two parents and offspring are not genetically identical   

New organisms are created by the fusion of gametes (sex cells) from two parents – called fertilisation Fertilisation creates a normal diploid cell – called a zygote – which then grows by mitosis Offspring’s genetic characteristics are created from a mixture of the 2 sets of chromosomes, so it will inherit features from both parents.

Meiosis:  Process by which gametes (sex cells) are made for sexual reproduction  Gametes are haploid – they contain half the chromosomes of a normal body cell (23 in humans)  In humans: o Male gametes = Sperm cells o Female gametes = Egg cells  Gametes are formed in testes and ovaries from the meiosis of reproductive cells  In plants: o Male gametes = Pollen cells o Female gametes = Ova or egg cells  Gametes are formed in Anther and ovaries from the meiosis of reproductive cells Process of Meiosis: 1. Chromosomes of the reproductive cell fatten and become visible 2. Each chromosome makes an exact cop of itself 3. Chromosomes line up down the equator in pairs 4. The chromosome pairs swap genes (crossing over), split up and move to opposite ends of the cell 5. The cell spits into two new cells 6. The chromosomes are pulled apart in each new cell and move to opposite ends of the cells 7. The two new cells spit again to make four new haploid cells, genetically different from the parent cell

Andy Todd

Unit 3 Advantages of Sexual Reproduction:  Genetic variation occurs in many stages: o Line up of pairs of chromosomes is random o Crossing over (swapping of genes) among pairs of chromosomes is random o All gametes are genetically different from their parents and an organisms can produce gametes with billions of different combinations of chromosomes o The fusion of male and female gametes is random (it could be any gamete from the male and female)  Species can adapt to changes in their environment Disadvantages of Sexual Reproduction:  Two parents required (organisms need to find a mate)  More complex process  there is a higher chance of a mutation occurring

Andy Todd

Unit 3

Fertilisation Definiton: The fusion of male and female gametes to produce a zygote  Occurs when a nucleus from a pollen grain fuses with the nucleus in the egg sac  Steps of fertilisation: 1. Pollen grain lands on top of the stigma 2. Pollen tube is formed by the tube nucleus, one of the two nuclei in the grain cell 3. Tube grows up to the micropyle in the egg sac, a tiny hole in the egg sac 4. Tube nucleus joins with another nucleus in the egg sac. The new nucleus will grow to form the endosperm, a food supply for the growing zygote 5. Second nucleus passes through the pollen tube to the egg cell and joins with the egg cell nucleus. This produces the zygote. 6. The plant starts to turn the carpel into a fruit and the ovule(s) become seed(s).  Seeds protect the zygote until it is ready to germinate, after the seed has been distributed

Andy Todd

Unit 3

Flowering plants Students will be assessed on their ability to: 3.3 describe the structures of an insect-pollinated and a wind-pollinated flower and explain how each is adapted for pollination 3.4 understand that the growth of the pollen tube followed by fertilisation leads to seed and fruit formation 3.5 recall the conditions needed for seed germination 3.6 understand how germinating seeds utilise food reserves until the seedling can carry out photosynthesis 3.7 understand that plants can reproduce asexually by natural methods (illustrated by runners) and by artificial methods (illustrated by cuttings)

Asexual Reproduction in Plants  

Also called vegetative reproduction Plants produced are genetically identical

Runners:  Occurs naturally in some plants (such as strawberries)  Runner grows out from the base of the parent plant  Grows along the ground until the runner reaches a suitable patch of soil  The runner develops roots in the soil  When the root system has grown enough, the stem connecting the offspring to the parent withers and falls off Cuttings:  Does not occur naturally – man-made method  Cutting is a piece of a plant that has been cut off from it  The cutting is placed in the ground and a root hormone compound is added  The cutting then grows into a plant that is genetically identical to the plant from which it came  Cuttings are useful for farmers who want to produce plants with the best genetics for fruit/vegetable/grain production. They can just take a cutting of the best plant and grow the same plant again.

Andy Todd

Unit 3

Sexual Reproduction in Plants  

Occurs in flowering plants (the most successful group of plants) Flowering plants reproducing sexually have these adaptations. They can: o Produce both male and female gametes o Transfer gametes between flowers o Fuse the gametes to produce a zygote o Produce seeds (the embryo with food stores) o Distribute seeds

Floral Structure 

All flowering plants have about the same arrangement of structures for sexual reproduction

Andy Todd

Unit 3 Male Parts:  Entire male part = Stamen  Consists of o Anther (at the top) of a stem-structure called the filament  All flowers have more than one stamen  Pollen grains (containing the male gametes) are developed in the anthers  Cells in the pollen sacs of the anthers divide by meiosis to create cells for pollen grains  When all pollen grains are formed, anther splits open to release them Female Parts:  Entire female part = Carpel  Carpel produce the female gametes (ova or egg cells)  Consists of: o Stigma at the top – collects pollen grains o Style – connects stigma to the ovary o Ovary/ovum – contains ovule o Ovule – contains the one ova produced by meiosis surrounded by an egg sac for protection  Carpel protects the ova from the external environment  Flowers contain one or more carpel

Pollination, Fertilisation and Germination: Pollination Definition: Process of transferring pollen from stamen to carpel

Cross-Pollination:  The transfer of pollen from one plant to another  Two types of cross-pollination: o Wind Pollination o Insect Pollination  Plants that pollinate using the wind and plants that pollinate using insects have different structures which enable the plant to pollinate effectively using the method

Andy Todd

Unit 3 Wind Pollination

Plants pollinated by wind will have tall stamens with anthers exposed to wind (usually above the petals). They produce light pollen grains which are easily carried by the wind. They also have feathery Stigmas to enable them to catch the light pollen grains from the wind. Not usually brightly coloured nor do they produce nectar as they do not rely on insects. E.g. Grasses, cereals, trees etc

Andy Todd

Unit 3 Insect Pollination Plants pollinated by insects will have tall stamens with anthers above the petals. They produce sticky pollen grains which are easily carried by the insect. They also have sticky Stigmas to enable them to catch the light pollen grains from the insect. Usually brightly coloured, produce fragrant smells and nectar to attract the insects. Insects goal is to retrieve the sweet nectar which they use for energy. While retrieving the nectar the pollen sticks to them and is transferred from plant to plant. E.g. Most flowering plants, honey suckle etc

Self-Pollination:     

Occurs when the pollen of a plant is transferred to the stigma of the same plant Most plant try to prevent self-pollination Some have flowers which are just one sex or the other Some have flowers with male and female parts that mature at different times Pollen from other plants are more likely to fertilise an egg in the ovule anyway because pollen grains from the same plant tend to grow slower than foreign grains

Andy Todd

Unit 3 Fertilisation & Pollen tubes Once the pollen tube lands on the stigma it starts to grow a pollen tube from the pollen grain. This pollen tube will follow sugar solution down the inside of the style into the ovary, into the ovule and allows the transfer of the pollen nucleus to the nucleus of the ovum As the pollen tube grows the nucleus of the pollen grain travels down it. Fertilisation occurs when the pollen nucleus fuses with the nucleus of the ovum

Ovary becomes Fruit

The Ovary swells and becomes the fruit The Fertilised ovum becomes the seed. Many pollen can fertilise many ovules and so many seeds can be formed. Andy Todd

Unit 3

Seed Structure:  Endospermic seeds contain large stores of endosperm to supply the plant with nutrients as it germinates  In non-endospermic seeds, the endosperm is used to make large storage organs called cotyledons  The cotyledons are the first leaves of the plant, even though they don’t resemble leaves  Plants that have two cotyledons are called dicotyledonous plants  Plants that have one are called monocotyledonous plants  Radicle = young root  Plumule = first shoot of the embryo

Andy Todd

Unit 3

Germination: Definition: Process by which a seed develops into a plant

Steps: 1. Seed absorbs a large amount of water (to help enzymes work and to inflate cells) 2. Enzymes break down starch from the food reserves (usually endosperm) into smaller sugars. Sugar is used for respiration to produce energy for the plant’s growth. The plant doesn’t have leaves so it can’t get its energy from photosynthesis yet. 3. Water potential drops as sugar is broken down so seed absorbs even more water 4. Respiration rate rises 5. Seed grows into embryo plant with a radicle and plumule, which eventually develops leaves. At this point the plant starts photosynthesising and the plant is no longer germinating. 

Light isn’t needed for germinating seeds to grown because they use food reserves instead of photosynthesis

Andy Todd

Unit 3

Questions Reproduction in flowering plants 1 Which is the most accurate statement? The principal role of a flower in the life cycle of a plant is: (a) attracting insects (b) producing seeds (c) producing pollen (d) producing nectar 3 What is (a) the male gamete, and (b) the female gamete in a flowering plant? 4 Complete the following paragraph selecting words from the list below. Pollination is the transfer of ……. from the …….. to the ……. in a flower. In cross-pollination, the ……. from a flower on one …….is transferred to the …….of another ……. of the same species. anthers, ovule, stigma, plant, flower, pollen, ovary, petal, style, receptacle, stamens 5 Complete the following sentences: In a flowering plant fertilisation occurs when the ……..of the ……. fuses with the …….of the ……… . After fertilisation , the ……. becomes the ……. and the ……. becomes the ……. . 6 Which of the following statements is correct? In flowering plants: (a) pollination can take place without fertilisation (b) fertilisation can take place without pollination (c) pollination and fertilisation are the same (d) pollination and fertilisation must occur at the same time 7 Some species of plant are strongly adapted to pollination by certain insects. Which of the following characteristics would you regard as adaptations to pollination by bees: (a) white or coloured petals (f) small green petals (b) light, smooth pollen grains (g) production of nectar (c) spiky or sticky pollen grains (h) production of pollen (d) anthers and stigma inside the flower (i) production of scent? (e) anthers and stigma protruding from the flower

Andy Todd

Unit 3 pollen grain

8 A bee visits several flowers in succession on a single willow herb plant. In doing so, the bee transfers pollen from the younger flowers, near the top of the inflorescence (group of flowers) to the older flowers near the base of the inflorescence. Is this an example of self-pollination or cross-pollination?

stigma

ovule

micropyle

9 Complete the drawing to show what has to happen before fertilisation can occur ovary

10 The drawings show seeds or fruits of different plants. (a) From the appearance of the structures, make a guess at how each one is dispersed giving reasons for your answers. (b) What are the advantages to a plant of an effective method of seed dispersal?

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Andy Todd

F

Unit 3

11 The root of the pea seedling is marked with equally spaced lines as shown here. Draw what you would expect to see in two days' time if the root (a) grew only from the tip (c) grew only at the top (b) grew uniformly along its length (d) did not grow.

12 (a) What conditions do most seeds need in order to begin germination? (b) What other condition do the seedlings need to continue growth to mature plants? 16 You are asked to set up an experiment to investigate the effect of temperature on the rate of germination. You place ten soaked peas in each of three flower pots containing moist sand. One pot is placed in a refrigerator at 4°C, one is placed in a cupboard at room temperature (about18°C) and the third is placed in an incubator at 25°C. You leave them for a week, checking each day that the sand is kept moist. (a) How would you judge the results? (b) Why was the pot, at room temperature, kept in a cupboard rather than on the laboratory bench? 17 Starch is one of the most common storage product in seeds. What happens to the starch before it can be used by the germinating seed? 18 The diagram represent an experiment to test the hypothesis that seeds need oxygen in order to germinate. (a) What is the liquid in A and what does it do? (b) What is the liquid in B and what does it do? (c) Which of the two flasks represents the control and what is its purpose? (d) What results would you expect (i) if oxygen is necessary for germination (ii) if oxygen is not necessary for germination? Andy Todd

A

B wet cotton wool cress seeds no oxygen

Unit 3

19 What differences would you expect to see between pea seedlings grown for 10 days in total darkness and pea seedlings grown in the light for the same period of time?

Andy Todd

Unit 3

Questions Asexual reproduction and cloning in plants 1 In natural vegetative propagation, which of the following structures are most likely to give rise to new individuals: (a) stems, (b) roots, (c) buds, (d) leaves, (e) flowers? 2 The drawing shows a plant which reproduces vegetatively. (a) What will need to happen before shoots A - C become independent plants? (b) How might a gardener assist this process? (c) What name is given to the horizontal stem in this kind of propagation? (d) Name a commercially grown fruit whose plants are propagated in this way

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3 Before stem cuttings are planted, the cut end of the stem is often dipped in a hormone powder. What is the point of this? 4 The following are thought to be some of the advantages of either vegetative reproduction or sexual reproduction: produces greater variety in the offspring, good at colonising new areas, reduces competition from other species, maintains desirable qualities in the offspring, good at colonising favourable areas Make a table with these qualities under the headings of 'Sexual reproduction' and 'Vegetative reproduction'. 5 If a gardener wanted to propagate a useful variety of apple tree in a way which maintained all its desirable qualities, which of the following techniques would be used: (a) planting stem cuttings in potting compost (b) grafting stem cuttings onto a rootstock (c) grafting buds on to a root stock (d) growing the seeds produced from the useful variety (e) cross-pollinating the variety with another good variety and growing the seeds resulting from the cross?

Andy Todd

Unit 3 6 What name is given to the population of genetically identical offspring which result from a process of asexual (vegetative) reproduction? 7 Which structures of a flowering plant give rise to (a) potatoes, (b) the fleshy scales of an onion? 8 In the process of tissue culture in plants, what is needed to induce the formation of a complete plant, in addition to a growth medium with nutrients?

Andy Todd

Unit 3

Humans Students will be assessed on their ability to: 3.8 recall the structure and function of the male and female reproductive systems 3.9 understand the roles of oestrogen and progesterone in the menstrual cycle 3.10 describe the role of the placenta in the nutrition of the developing embryo 3.11 understand how the developing embryo is protected by amniotic fluid 3.12 recall the roles of oestrogen and testosterone in the development of secondary sexual characteristics.

Sexual Reproduction in Humans Male Reproductive System:  Male reproductive organ = Testes  Testes produce sperm (male gametes) via meiosis and secrete testosterone (male sex hormone)  Also have to be kept at a lower temperature than the rest of the body and are supported by the scrotum  Testosterone causes secondary sexual characteristics in developing boys, such as: o Growth spurts o Hair growth on face and body o Development and growth of penis, testes and scrotum o Voice breaking o Body becomes broader and more muscular o Sexual ‘drive’ development  Prostate gland and seminal vesicles produce liquid in which sperm swim  Semen = mixture of sperm and fluids  Sperm ducts connect testes and penis  Penis is used for both excretion and ejaculation of semen  Urethra in the penis is the passage through which urine and sperm pass through  Ring of muscle closes on erection to prevent the mixture of sperm and urine

Andy Todd

Unit 3

Female Reproductive System:  Female reproductive organ = Ovaries  Ovaries produce and release egg cells and secretes the female sex hormones progesterone and oestrogen  They are located on either side the uterus and are connected to it by the two oviducts (Fallopian tubes)  Progesterone and oestrogen cause secondary sexual characteristics, such as: o Growth spurts o Breast development o Development of vagina, oviducts and uterus o Menstrual cycle starts o Hips Widen o Pubic hair and under-arm growth o Sexual ‘drive’ development  Hormones also control changes during menstrual cycle o Oestrogen: encourages the repair of uterus lining after bleeding o Progesterone: maintains lining o Together: control ovulation (egg release)

Andy Todd

Unit 3



Menstrual Cycle Every 28 days, an egg is released form one of the ovaries for possible fertilisation

Steps: 1. While the follicle (in which the egg is produced) matures, oestrogen from the ovaries causes the uterus lining to thicken into a spongy layer of blood vessels. This is in preparation for possible fertilisation. 2. The egg is release from the follicle (called ovulation). 3. As the egg passes through the oviducts, progesterone is released from the corpus luteum (the remaining part of the follicle) to maintain the uterus wall and to stop other eggs from being released. 4. If the egg reaches the uterus and it has not been fertilised, the lining breaks down and menstruation (bleeding) begins. 5. After lining has been broken down, oestrogen causes the lining to thicken again and the cycle starts again. 6. If the egg is fertilised, then progesterone will continue to be released from the corpus luteum throughout the entire pregnancy to maintain the uterus lining while the zygote grows and prevent further ovulation. 

These steps occur at different stages: o The bleeding starts at day 1 and lasts for 4 days o The lining of the uterus builds up from day 4-14 o Ovulation occurs at day 14 o Lining is maintained for 14 days until day 28. If no fertilisation occurs after day 28, cycle starts again at day 1.

Andy Todd

Unit 3

Sexual Intercourse: 

When the fusion of male and female gametes occurs

Steps of intercourse: 1. Semen is ejaculated into the vagina 2. Sperm swims up towards the oviducts, following a concentration gradient of chemicals to guide them to the egg 3. Many sperm do not make it to the egg but some will 4. If fertilisation is to occur, the egg must be travelling down the Fallopian tube at the same time the sperm are travelling up it 5. The first sperm to reach the egg fertilises it. No other sperm can then fertilise it.

Andy Todd

Unit 3

Development of the fetus:  After fertilisation, the zygote travels towards the uterus for three days  By the time the zygote reaches the uterus, it will have grown from a single diploid cell to a group of 64 cells via the process of mitosis  The embryo then implants itself in the uterus wall, where it will grow for 38 weeks  Rate of growth is at its highest during this time  This period of growth is called gestation

Andy Todd

Unit 3

Andy Todd

Unit 3

The Amniotic Sac The amniotic sac provides protection to the developing fetus. It contains a fluid called amniotic fluid which provides cushioning and acts as a shock absorber.

The Placenta: Definition: An organ that allows the embryo to obtain vital materials from the mother’s blood and expel waste products into the bloodstream   

Develops from fetal tissues Contains a large number of blood vessels Main purpose is to supply the fetus with nutrients and oxygen and dispose of waste materials  Also prevents maternal blood and fetal flood from mixing  If the blood mixed, the fetus would be damaged by the higher blood pressure and level of hormones in the mother’s blood. Also, since the fetus has different antigens than the mother, if the embryo’s antigens were released into the mother’s blood, the mother’s immune system would threat begin attacking the embryo.  Umbilical cord connects the fetus to the placenta  After birth, the cord degenerates  Placenta also contains amniotic fluid that protects the fetus from damage The bag of fluid bursting is the first sign of labour

Sex Hormones Adolescence:  

 

When puberty starts, the pituitary gland begins to make hormones that make the sex organs active. Sex organs produce sex hormones that create secondary sexual characteristics. E.g. women: development of breasts, start of menstrual cycle men: voice breaks, facial hair, ability to become more muscular Also in men: testes start making testosterone women: ovaries start making oestrogen. Phase of adolescence = duration of puberty

Andy Todd

Unit 3

In humans the development of secondary sexual characteristics is stimulated at puberty by the hormone oestrogen in women and testosterone in men. Testosterone is produced in the testes of males, while oestrogen and progesterone are produced in the ovaries of females. Women develop breasts and wider hips; men develop a deeper voice, body and facial hair, and stronger body odour; and both develop hair in underarm and genital regions. However, at puberty the reproductive organs also mature as a consequence

Andy Todd

Unit 3

Cell division and chromosomes QUESTIONS 1 A cell in the basal layer of the skin contains 46 chromosomes and divides by mitosis to produce new skin cells. After ten successive divisions, how many chromosomes will the basal cell have? 2 The drawings below depict stages in the mitotic division of a cell

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(a) Write the letters in the order in which these stages occur. (b) How many pairs of chromosomes are there in the cell? (c) What is the diploid number of chromosomes in these cells? 3 Choose the most appropriate word to complete the sentence. When chromosomes replicate, they produce ……... tissues, nuclei, chromatids, somatic cells 4 In which three of the following cells is mitosis unlikely to occur? a sperm cell, an epithelial cell of a villus, a hair cell, a cell in the red bone marrow, a red blood cell, a lymphocyte, a cell in the basal layer of the skin 5 An animal has 36 chromosomes in each of its body cells. How many of these chromosomes came from its male parent? 6 Which pairs of chromosomes in the cell shown here are homologous? 7 Fill in the missing words. The ….. A ….. of a cell contains a fixed number of chromosomes. Before mitosis, each chromosome ….. B ….. to produce two ….. C …...

A

B C D

E F

8 The following drawings-show the sequence of events early in cell division. (a) Is the division meiotic or mitotic? (b) How do you know?

Andy Todd

Unit 3

(i)

(ii)

(iii)

9 Give two examples in each case of organs or tissues in which you would expect (a) meiosis, (b) mitosis to be taking place. 11 From the list below, choose the most suitable words to complete the sentence. Mutations are changes which occur in a ….. A …..or a ….. B …... If a mutation occurs in a cell which is going to form a ….. C ….., the mutation may affect the whole ….. D ….. which develops. Down's syndrome results from a ….. E ….. mutation In the ….. F….. Sickle cell anaemia results from a ….. G ….. mutation which affects ….. H ….. of the blood system. cells, gene, gamete, chromosome, nucleus, ovum, organism 12 Exposure to ….. A ….., ….. B ….. or ….. C ….. may increase the rate of mutation. Suggest words or phrases for A - C 13 What kinds of mutation in disease-causing bacteria might make them more dangerous?

Andy Todd

Unit 3

Human Reproduction

QUESTIONS

1 Fertilisation occurs when the …..(A)….. of the sperm cell fuses with the…..(B) ….. of the….. (C) ….. . 2 State the differences between the male gametes and the female gametes with regard to (a) their size; (b) their structure, (c) their relative numbers. 3 Before fertilisation can occur, the sperms have to travel from the testes to meet an ovum in the female organs. Using the list below, name the organs, in the correct order, through which the sperms will have to pass. uterus, sperm duct, oviduct, urethra, cervix, vagina 4 (a) Explain what is meant by ovulation. (b) How often does it occur in humans? 5 Explain why the chance of fertilisation in humans is restricted to only a few days each month. 6 The diagram below represents the events leading up to fertilisation (v), implantation (vi) and development. In each case name the structures involved and, at the numbers, state briefly what is happening or what has happened previously. (v) F (vi) E G

(iv)

C

(ii) D B A

(i)

(iii)

7 Blood from the fetus circulates through the placenta. (a) What substances pass (i) from the maternal to the fetal blood, (ii) from the fetal to the maternal blood? 8 What is the function of the umbilical cord?

Andy Todd

Unit 3 10 Describe the events which lead to the formation of (a) identical twins, (b) fraternal twins. 11 At an ante-natal clinic what can (a) blood tests, (b) urine tests reveal? 12 Place the following events in the correct order for natural childbirth. amniotic fluid expelled, placenta expelled from uterus, baby's feet emerge from vagina, abdominal contractions begin, baby' head emerges from vagina, amnion breaks, cervix dilates, contractions of the uterus begin . 13 (a) What are the advantages of human milk over cows' milk for feeding babies? (b) Apart from the composition of the milk, what are the other advantages of breast- feeding? 14 Name (a) the male sex hormone and (b) the female sex hormone which help bring about the changes at puberty .

Andy Todd

Unit 3

B: Inheritance Students will be assessed on their ability to: 3.13 recall that the nucleus of a cell contains chromosomes on which genes are located 3.14 understand that a gene is a section of a molecule of DNA 3.15 describe a DNA molecule as two strands coiled to form a double helix, the strands being linked by a series of paired bases: adenine (A) with thymine (T), and cytosine (C) with guanine (G) 3.16 understand that genes exist in alternative forms called alleles which give rise to differences in inherited characteristics 3.17 recall the meaning of the terms: dominant, recessive, homozygous, heterozygous, phenotype, genotype and codominance 3.18 describe patterns of monohybrid inheritance using a genetic diagram 3.19 understand how to interpret family pedigrees 3.20 predict probabilities of outcomes from monohybrid crosses 3.21 recall that the sex of a person is controlled by one pair of chromosomes, XX in a female and XY in a male 3.22 describe the determination of the sex of offspring at fertilisation, using a genetic diagram 3.23 understand that division of a diploid cell by mitosis produces two cells which contain identical sets of chromosomes 3.24 understand that mitosis occurs during growth, repair, cloning and asexual reproduction 3.25 understand that division of a cell by meiosis produces four cells, each with half the number of chromosomes, and that this results in the formation of genetically different haploid gametes 3.26 understand that random fertilisation produces genetic variation of offspring 3.27 recall that in human cells the diploid number of chromosomes is 46 and the haploid number is 23 3.28 understand that variation within a species can be genetic, environmental, or a combination of both 3.29 recall that mutation is a rare, random change in genetic material that can be inherited 3.30 describe the process of evolution by means of natural selection 3.31 understand that many mutations are harmful but some are neutral and a few are beneficial 3.32 understand how resistance to antibiotics can increase in bacterial populations 3.33 understand that the incidence of mutations can be increased by exposure to ionising radiation (for example gamma rays, X-rays and ultraviolet rays) and some chemical mutagens (for example chemicals in tobacco).

Andy Todd

Unit 3

Inheritance Variation: Types of Variation:  

Discontinuous (discrete) variation – a characteristic that can have one of a certain number of specific alternatives Continuous variation – a characteristic that can have any value in a range

Causes of Variation: 

Environmental causes – factors in a person’s environment that affect certain characteristics (ex. diet, lifestyle, etc.) Genetic causes – genes control certain characteristics (ex. eye colour, hair colour, gender) Many characteristics are influenced by both

Andy Todd

Unit 3 Cell – Nucleus – Chromosome – Gene – DNA

Deoxyribose Nucleic Acid – DNA

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Unit 3

Genes: Def: Chemical instructions that direct the cells of the body how to grow and work (part of DNA)     

       

Contain all the information needed to grow a fertilised egg into an adult All nuclei contain genetic material called chromosomes Chromosomes are threads made of wound up and bundled DNA (deoxyribonucleic acid) DNA is an extremely long series of bonded chemicals that is the basis of inheritance Chemicals that compose DNA are called bases and are called: o Thymine o Adenine o Guanine o Cytosine Thymine and adenine always match together while guanine and cytosine always match each other. The order in which these pairs of bases appear called the genetic code The genetic code spells out the instructions for all processes in cells Each group of three bases codes for a particular amino acid and are called codons Each length of DNA that spells out a particular instruction is a gene Each chromosome contains thousands of genes Genes tell cells how to make different proteins such as enzymes that are vital for life processes within the cells Not all genes are used in each cell

Mutation: Def: A rare and random change in genetic material  

     

Can occur when a cell is reproducing and something goes wrong They are completely random but can the chances of them happening can be increased by certain factors such as: o Exposure to radiation, UV rays, and X-rays o Mutagens (chemicals that increase chance of mutations) Usually harmful If a mutation occurs in reproductive cells, the offspring may be abnormal or die If a mutation occurs in body cells, mutant cells could grow and multiply throughout the body (cancer) Sometimes mutations are good and give an organism a survival advantage Mutation can then spread throughout species by natural selection Examples of mutations: cystic fibrosis (caused by a single mutated gene) Down’s syndrome (extra chromosome)

Andy Todd

Unit 3 Causes of Mutations 1 Ionising radiation (E.g X-Rays, Gamma-Rays & Ultra-Violet) Biological effects of radiation on living cells may result in a variety of outcomes, including: 1. Cells experience DNA damage and are able to detect and repair the damage. 2. Cells experience DNA damage and are unable to repair the damage. These cells may go through the process of programmed cell death, or apoptosis, thus eliminating the potential genetic damage from the larger tissue. 3. Cells experience a nonlethal DNA mutation that is passed on to subsequent cell divisions. This mutation may contribute to the formation of a cancer. 2 Chemical Mutagens (Chemicla in Tobacco)

Alleles The DNA inside your cells was inherited from your parents. Both your mother and father gave you 23 Chromosomes and a human has 46 chromosomes organized into 23 pairs. You have therefore two versions of the same gene. These versions are called Alleles of a gene. They can be the same (homozygous) or different (heterozygous) Dominant and Recessive Alleles Definition of Allele: different version of the same gene     

Alleles are parts of genes that determine inherited features Usually occur in pairs of two A person receives one allele from each parent Which allele a person receives is based on the alleles of the parent but is still random Two types of alleles: o Recessive – two alleles are required for allele to be expressed o Dominant – only one allele is required for genetic expression as the it can override the recessive gene

Andy Todd

Unit 3 Definitions: -

Homozygous – characteristic is caused by two identical alleles (both dominant or both recessive) Heterozygous – characteristic is caused by two different alleles (one dominant and one recessive) Genotype – an individual’s genetic combination Phenotype – an individual’s physical features influenced by the genotype

Genetic diagrams:  Monohybrid crosses demonstrates influence of the genotype of the parents on the genotype of the offspring regarding one characteristic and show the probabilities of different possible outcomes of a cross  Actual results may due to randomness  Could also use a Punnett square  Uses a capital letter and a lower case letter to represent the dominant allele and a recessive allele, respectively  The first generation = F1 (second generation = F2, etc.)  Genotypes and phenotypes can be expressed as ratios

Andy Todd

Unit 3 Codominance  

Occurs when both alleles are expressed (neither is dominant or recessive) Occurs in blood types with the A allele and the B allele (they form the AB genotype together)

Inherited Diseases  

  

Diseases can be inherited Examples: o Cystic Fibrosis – lungs become clogged with mucus o Haemophilia – blood doesn’t clot o Diabetes – insulin is not produced in the pancreas o Sickle-cell Anaemia – Red blood cells are misshapen and don’t carry oxygen properly o Huntington’s disease – disorder of nervous system Usually caused by faulty genes Most of the alleles for these disease are recessive (two alleles are needed to show the disorder) Many people are carriers. They have one normal allele and one faulty one (they don’t express the allele, but carry it).

Sex Determination:   

There are two chromosomes that determine sex: o X chromosome o Y chromosome Girls have two X chromosomes, while boys have one of each Egg cells always contain one X chromosome, but sperm has an equal chance of containing either  baby has an equal chance of being a boy or a girl

Andy Todd

Unit 3

Sex- Linked Inheritance:   

Some diseases and disorders (haemophilia) are more common in men than women This is because the recessive allele is found on part of the X chromosome and the Y chromosome is not big enough to carry the normal allele  While men only need one copy of the allele to have the disorder, women need two copies and are therefore less likely to inherit the disorder

Mitosis:  Process by which cells asexually reproduce  Produces offspring called ‘daughter cells’ who are genetically identical to the parent (unless a mutation occurs)  Used to replace dead or damaged diploid cells in plants and animals  Also used for normal growth of diploid cells  Diploid cells – body cells with a full set of chromosomes (46 in humans)  Does NOT occur in sex cells

Process of Mitosis: 6. Chromosomes become coiled and visible 7. Each chromosome makes an exact copy of itself

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Unit 3

8. Chromosomes and their copy line up down the equator of the cell

9. Chromosomes are pulled apart and move to opposite ends of the cell

10. Cell splits into two new daughter cells with the exact same chromosomes as the parents

Meiosis:  Process by which gametes (sex cells) are made for sexual reproduction  Gametes are haploid – they contain half the chromosomes of a normal body cell (23 in humans)  In humans: o Male gametes = Sperm cells o Female gametes = Egg cells  Gametes are formed in testes and ovaries from the meiosis of reproductive cells  In plants: o Male gametes = Pollen cells o Female gametes = Ova or egg cells Andy Todd

Unit 3 

Gametes are formed in Anther and ovaries from the meiosis of reproductive cells

Process of Meiosis: 8. Chromosomes of the reproductive cell fatten and become visible 9. Each chromosome makes an exact cop of itself 10. Chromosomes line up down the equator in pairs 11. The chromosome pairs swap genes (crossing over), split up and move to opposite ends of the cell 12. The cell spits into two new cells 13. The chromosomes are pulled apart in each new cell and move to opposite ends of the cells 14. The two new cells spit again to make four new haploid cells, genetically different from the parent cell

Natural Selection (Evolution) 1. 2. 3. 4. 5.

Variation Competition Survival Reproduction Future Generations

Antibiotic resistance

Andy Todd

Unit 3

How to prevent Antibiotic Resistance 1 Only take antibiotics when absolutely necessary 2 Always finish the prescribed course of antibiotics

Andy Todd

Unit 3

Questions - Variation and selection 1 What are the two main causes of heritable variation? 2 Classify the following variations as either (i) caused entirely by genetic effects or (ii) caused by a combination of genetic and environmental effects. obesity, eye colour, tallness, ability to sing, maleness, masculinity, blood group, natural hair colour; sickle-cell anaemia, agility 3 Alleles are genes which occupy corresponding positions on….. A ….. chromosomes. They control the same ….. B ….. but not necessarily in the same way.

7 The histogram shows the range and frequency of occurrence of particular blood pressures (systolic) in a group of women in the 30-39 age group. (a) On this evidence, could you say that blood pressure is a discontinuous variable? (b) Justify your answer.

% people

4 (a) What new combinations of characteristics might arise in the offspring when a tall plant with white flowers is crossed with a dwarf plant (of the same species) with red flowers? (b) What selective advantage might either of the new varieties have?

blood pressure/ mmHg

8 Give two examples in each case of (a) continuous, (b) discontinuous variation in human populations. 9 Which one of the following statements is the least accurate? (a) Discontinuous variation results entirely from genetic differences. (b) Continuous variation can result from genetic differences. (c) Discontinuous variation cannot be altered by environmental effects. (d) Continuous variation results from environmental effects. 10 Explain why identical twins will have the same genotype. 13 (a) Give three examples of types of competition between members of an animal species in the same population. (b) In each case suggest a variation that might help an individual to compete more effectively.

Andy Todd

Unit 3

14 For a beneficial variation to have a selective advantage in the course of evolution, it must be ….. A ….. by the offspring. 15 Evolution is thought to come about as a result of ….. A ….. which produce new ….. B ….. These gradually replace the original population as a result of …..C ….. 16 Which of the following statements is most acceptable from an evolutionary point of view? (a) Apes and humans have evolved from a common ancestor. (b) Humans have evolved from apes. (c) Certain apes have gradually evolved into humans. (d) Apes and humans are related. 17 What characteristics might a breeder select for in (i) a cereal crop, (ii) a farm animal?

Andy Todd

Unit 3

Heredity NOTE: Alleles are alternative forms of a gene which occupies a particular position in

a chromosome. Alleles affect the same characteristic (e.g. blood group) but not necessarily in the same way. lA, IB and i are alleles of a gene which controls the ABO blood groups. 1 A plant with red flowers is crossed with a white-flowered plant of the same species. All the seeds, when grown, produce plants with red flowers. Assuming that the flower colour is controlled by a single pair of alleles, which allele is dominant and which is recessive? 2 If a dominant allele for tall plants is represented by the letter D, what letter should represent the corresponding recessive allele? 3 In cats, the allele (S) for short fur is dominant to the allele (s) for long fur. (a) What is the genotype of a true-breeding, long-furred cat? (b) What is the phenotype of a cat with the genotype Ss? (c) In an Ss genotype, which allele is expressed in the phenotype? (d) Which of the fo1lowing genotypes is (i) heterozygous (ii) homozygous dominant? SS, Ss, ss 4 In rabbits, assume that the dominant allele (B) produces black fur. The allele (b) for white fur is recessive to B. (a) What colour fur will each of the following rabbits have? Rabbit 1 genotype BB

Rabbit 2 Bb

Rabbit 3 bB

Rabbit 4 bb

(b) Which of them will breed true? (c) Which rabbits are homozygous for coat colour? (d) If rabbits 1 and 4 were mated together and had 12 babies, how many of these would you expect to be black? (e) If rabbits 2 and 3 are interbred and produce several litters, totalling 48 babies, how many white babies would be predicted by the laws of genetics? (f) If rabbits 3 and 4 are mated together on several occasions and have 50 babies altogether, how many of their babies would you 'expect' to be black? NOTE: In this context, 'expect' implies the perfect Mendelian ratio. In practice you

would not expect to achieve this ratio with as few as 50 offspring.

Andy Todd

Unit 3 5 The alleles controlling the ABO blood groups are given the letters IA (group A), IB (group B) and i (group O). On the drawings below, write in the alleles on the chromosomes for each of the blood groups. (The first one has been done for you) group A

IA

group B

group AB

group O

IA

or

or

6 In shorthorn cattle, the coat colours red or white are controlled by a single pair of alleles. A calf which receives the allele for red coat from its mother and the allele for white coat from its father is called a 'roan'. It has an equal number of red and white hairs in its coat. (a) Is this an example of codominance or of incomplete dominance? (b) Give a reason for your answer. (c) Give one example in each case of (i) codominance, (ii) incomplete dominance, in humans. 7 Give three examples of human disorders which are caused by the action of a single pair of alleles. In each case say whether the harmful allele is dominant or recessive to the non-harmful allele. 8 In humans, maleness or femaleness is determined by a pair of sex chromosomes called X and Y. (a) What is the genotype for males? (b) What is the genotype for females? 9 (a) In humans, is it the sperm or the ovum which determines the sex of the offspring? (b) Give a reason for your answer. 10 In fruit flies, the allele (n) for ebony (black) body is recessive to the allele (N) for normal (grey) body. (a) Complete the Punnett square, for a cross between normal (grey-bodied) flies which are heterozygous for this allele (i.e. Nn genotypes). (b) State the expected proportion of normal and ebony-bodied flies in a large sample of Andy Todd

gametes gametes

Unit 3 the offspring. (c) State the proportion of the normal phenotypes which would be true breeding. 11 When a particular gene is said to be ‘sex-linked’, on which chromosome is that gene usually present? 12 The genetic disorder phenylketonuria (PKU) is caused by a recessive allele (n). The family tree below shows the incidence of the disease over three generations.

grandparents

parents wife

Peter

Alan

Jane

husband

children KEY

normal male

male with PKU

normal female

female with PKU

(a) What can you deduce about the genotypes of the grandparents? (b) Explain your reasoning. (c) What is the genotype of Jane's husband? (d) Explain your reasoning. (e) What are the chances that Peter is the carrier of the PKU allele that resulted in his having an affected son? (f) If Jane had been normal, what are the possible genotypes of the grandparents? (g) Is it possible that the allele for PKU is sex-linked? 13 One form of colour-blindness is a sex-linked inherited condition controlled by a recessive allele. Use the symbols X and Y for the sex chromosomes and N and n for the alleles for normal or defective colour vision to show the genotypes of (a) a normal male (d) a colour-blind female (b) a colour-blind male (e) a normal (carrier) female. (c) a normal (non-carrier) female

Andy Todd

Unit 3 14 Use the genotypes you have written for your answer to question 13 to show the chances of (a) a son being colour blind, (b) a daughter being a carrier, resulting from a marriage between a normal man and a carrier woman.

Andy Todd