The Basis of Heredity “Chapter 18”
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Curriculum Outcomes
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Key Terms
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Questions Questions 1-3 page 596
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Genes and Heredity Can you identify members of a family by physical traits?
Heredity – is the transmission of biological traits from parent to offspring. Genetics – Study of inheritance of biological traits.
Biological traits are determined by genes, which are specific segments of DNA. Humans are able to use this information to their advantage. Cows and Dogs produced Crop plants 6
Activity
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Mendelelian Genetics
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Gregor Mendel (1822-1884)
Responsible for the Laws governing Inheritance of Traits 9
Gregor Johann Mendel Austrian monk Studied the inheritance of traits in pea plants Developed the laws of inheritance Mendel's work was not recognized until the turn of the 20th century 10
Gregor Johann Mendel Between 1856 and 1863, Mendel cultivated and tested some 28,000 pea plants He found that the plants' offspring retained traits of the parents Called the “Father of Genetics" 11
Site of Gregor Mendel’s experimental garden in the Czech Republic
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Particulate Inheritance Mendel stated that physical traits are inherited as “particles” Mendel did not know that the “particles” were actually Chromosomes & DNA
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Gregor Mendel – Pioneer of Genetics
Mendel tracked and recorded the transmission of seven visible traits through several generations of the garden pea. To Keep track he called the first parents P and than Filial General F1 and so on.
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Why did he work with a garden pea? Garden peas have a number of Characteristics
How it reproduces- reproduces through self pollination. 14
Seven Characteristics Studied By Mendel
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The Principle of Dominance
When Mendel used pollen from a pea plant with round seeds to fertilize a pea plant with wrinkled seeds, he found that all the offspring (progeny) in the F1 generation had round seeds.
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Progeny – new individual that result from reproduction; offspring. Did this mean that pollen determines shape?
So, he did the opposite and again all the progeny had round seeds. Round-seed shape was always the dominant trait. Mendel called the other wrinkled shaped seeds the recessive trait. 16
Questions Questions 1-5 page 600
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Genetic Terminology Trait - any characteristic that can be passed from parent to offspring Heredity - passing of traits from parent to offspring Genetics - study of heredity
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Types of Genetic Crosses Monohybrid cross - cross involving a single trait e.g. flower color Dihybrid cross - cross involving two traits e.g. flower color & plant height 19
Punnett Square Used to help solve genetics problems
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Designer “Genes” Alleles - two forms of a gene (dominant & recessive) Dominant - stronger of two genes expressed in the hybrid; represented by a capital letter (R) Recessive - gene that shows up less often in a cross; represented by a lowercase letter (r) 22
More Terminology Genotype - gene combination for a trait (e.g. RR, Rr, rr) Phenotype - the physical feature resulting from a genotype (e.g. red, white) Segregation – the separation of alleles during meiosis
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Genotype & Phenotype in Flowers Genotype of alleles: R = red flower r = yellow flower
All genes occur in pairs, so 2 alleles affect a characteristic Possible combinations are: Genotypes
RR
Rr
rr
Phenotypes
RED
RED
YELLOW 24
Genotypes Homozygous genotype - gene combination involving 2 dominant or 2 recessive genes (e.g. RR or rr); also called pure Heterozygous genotype - gene combination of one dominant & one recessive allele (e.g. Rr); also called hybrid 25
Result of Peas being Crossed
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Probability and Inheritance of Single Traits
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Phenotypic ratio – the ratio of offspring with a dominant trait to the alternative recessive trait
Punnett Square – a chart used to determine the predicted outcome of a genetic cross. Genotypic ratio – the ratio of offspring with each possible allele combination from a particular cross.
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P = # of ways that a given outcome can occur Total # of possible outcomes 27
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Review Questions Page 475 Questions 1-4 (old text) Questions 1-3 page 604
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Genes and Environment Determine Characteristics
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Mendel’s Pea Plant Experiments
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Why peas, Pisum sativum? Can be grown in a small area Produce lots of offspring Produce pure plants when allowed to self-pollinate several generations Can be artificially cross-pollinated 34
Reproduction in Flowering Plants Pollen contains sperm Produced by the stamen Ovary contains eggs Found inside the flower Pollen carries sperm to the eggs for fertilization Self-fertilization can occur in the same flower Cross-fertilization can occur between flowers 35
Mendel’s Experimental Methods Mendel hand-pollinated flowers using a paintbrush He could snip the stamens to prevent self-pollination He traced traits through the several generations 36
How Mendel Began Mendel produced pure strains by allowing the plants to selfpollinate for several generations 37
Eight Pea Plant Traits Seed shape --- Round (R) or Wrinkled (r) Seed Color ---- Yellow (Y) or Green (y) Pod Shape --- Smooth (S) or wrinkled (s) Pod Color --- Green (G) or Yellow (g) Seed Coat Color ---Gray (G) or White (g) Flower position---Axial (A) or Terminal (a) Plant Height --- Tall (T) or Short (t) Flower color --- Purple (P) or white (p) 38
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Mendel’s Experimental Results
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Did the observed ratio match the theoretical ratio? The theoretical or expected ratio of plants producing round or wrinkled seeds is 3 round :1 wrinkled Mendel’s observed ratio was 2.96:1 The discrepancy is due to statistical error The larger the sample the more nearly the results approximate to the theoretical ratio 42
Generation “Gap” Parental P1 Generation = the parental generation in a breeding experiment. F1 generation = the first-generation offspring in a breeding experiment. (1st filial generation) From breeding individuals from the P1 generation F2 generation = the second-generation offspring in a breeding experiment. (2nd filial generation) From breeding individuals from the F1 generation 43
Following the Generations
Cross 2 Pure Plants TT x tt
Results in all Hybrids Tt
Cross 2 Hybrids get 3 Tall & 1 Short TT, Tt, tt 44
Monohybrid Crosses
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P1 Monohybrid Cross
Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Round seeds x Wrinkled seeds RR x rr Genotype: Rr
r
r
R
Rr
Rr
Genotypic Ratio: All alike
R
Rr
Rr
Phenotypic Ratio: All alike
Phenotype: Phenotype Round
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P1 Monohybrid Cross Review Homozygous dominant x Homozygous recessive Offspring all Heterozygous (hybrids) Offspring called F1 generation Genotypic & Phenotypic ratio is ALL ALIKE 47
F1 Monohybrid Cross
Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Round seeds x Round seeds Rr x Rr
R
r
R
RR
Rr
r
Rr
rr
Genotype: RR, Rr, rr Phenotype: Phenotype Round & wrinkled G.Ratio: 1:2:1 P.Ratio: 3:1 48
F1 Monohybrid Cross Review Heterozygous x heterozygous Offspring: 25% Homozygous dominant RR 50% Heterozygous Rr 25% Homozygous Recessive rr Offspring called F2 generation Genotypic ratio is 1:2:1 Phenotypic Ratio is 3:1 49
What Do the Peas Look Like?
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…And Now the Test Cross Mendel then crossed a pure & a hybrid from his F2 generation This is known as an F2 or test cross There are two possible testcrosses: Homozygous dominant x Hybrid Homozygous recessive x Hybrid 51
F2 Monohybrid Cross
st (1 )
Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Round seeds x Round seeds RR x Rr Genotype: RR, Rr
R
r
R
RR
Rr
Genotypic Ratio: 1:1
R
RR
Rr
Phenotypic Ratio: All alike
Phenotype: Phenotype Round
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F2 Monohybrid Cross (2nd)
Trait: Seed Shape Alleles: R – Round r – Wrinkled Cross: Wrinkled seeds x Round seeds rr x Rr
R r r
Rr Rr
r rr rr
Genotype: Rr, rr Phenotype: Phenotype Round & Wrinkled G. Ratio: 1:1 P.Ratio: 1:1 53
F2 Monohybrid Cross Review Homozygous x heterozygous(hybrid) Offspring: 50% Homozygous RR or rr 50% Heterozygous Rr Phenotypic Ratio is 1:1 Called Test Cross because the offspring have SAME genotype as parents 54
Practice Your Crosses Work the P1, F1, and both F2 Crosses for each of the other Seven Pea Plant Traits
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Mendel’s Laws
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Results of Monohybrid Crosses Inheritable factors or genes are responsible for all heritable characteristics Phenotype is based on Genotype Each trait is based on two genes, one from the mother and the other from the father True-breeding individuals are homozygous ( both alleles) are the same 57
Law of Dominance In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. All the offspring will be heterozygous and express only the dominant trait. RR x rr yields all Rr (round seeds) 58
Law of Dominance
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Law of Segregation During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring. 60
Applying the Law of Segregation
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Law of Independent Assortment Alleles for different traits are distributed to sex cells (& offspring) independently of one another. This law can be illustrated using dihybrid crosses.
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Dihybrid Cross A breeding experiment that tracks the inheritance of two traits. Mendel’s “Law of Independent Assortment” a. Each pair of independently b. Formula: 2n
alleles segregates during gamete formation (n = # of heterozygotes)
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Question: How many gametes will be produced for the following allele arrangements? Remember: 2n (n = # of heterozygotes) 1. RrYy 2. AaBbCCDd 3. MmNnOoPPQQRrssTtQq 64
Answer: 1. RrYy: 2n = 22 = 4 gametes RY Ry rY ry 2. AaBbCCDd: 2n ABCD ABCd aBCD aBCd
= 23 = AbCD abCD
8 gametes AbCd abCD
3. MmNnOoPPQQRrssTtQq: 2n = 26 = 64 gametes 65
Dihybrid Cross Traits: Seed shape & Seed color Alleles: R round r wrinkled Y yellow y green
RrYy RY Ry rY ry
x
RrYy RY Ry rY ry
All possible gamete combinations 66
Dihybrid Cross RY
Ry
rY
ry
RY Ry rY ry
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Dihybrid Cross RY RY RRYY Ry RRYy rY RrYY ry
RrYy
Ry
rY
ry
RRYy
RrYY
RrYy
RRyy RrYy Rryy
RrYy rrYY rrYy
Rryy rrYy rryy
Round/Yellow: Round/green:
9 3
wrinkled/Yellow: 3 wrinkled/green:
1
9:3:3:1 phenotypic ratio 68
Dihybrid Cross Round/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 1 9:3:3:1
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Test Crosses
Wool producers often prefer white wool, since black wool is brittle and difficult to dye. Black sheep can be avoided by breeding only homozygous white rams. However the allele for white wool (W) is dominant over the allele for black wool (w), so white rams can be heterozygous. How can a woo producer be sure that a white ram is homozygous?
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Test Cross – the cross of an individual of unknown genotype to an individual that is fully recessive. 70
Test Cross
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Test Cross A mating between an individual of unknown genotype and a homozygous recessive individual. Example: bbC__ x bbcc BB Bb bb
= = =
brown eyes brown eyes blue eyes
CC = curly hair Cc = curly hair cc = straight hair
bC
b___
bc
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Test Cross Possible results:
bc
bC
b___ C
bbCc
bbCc
or
bc
bC
b___ c
bbCc
bbcc
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Summary of Mendel’s laws LAW DOMINANCE
SEGREGATION
INDEPENDENT ASSORTMENT
PARENT CROSS
OFFSPRING
TT x tt tall x short
100% Tt tall
Tt x Tt tall x tall
75% tall 25% short
RrGg x RrGg round & green x round & green
9/16 round seeds & green pods 3/16 round seeds & yellow pods 3/16 wrinkled seeds & green pods 1/16 wrinkled seeds & yellow pods 74
Incomplete Dominance and Codominance
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Incomplete Dominance
Incomplete dominance – the expression of both forms of an allele in heterozygous individual in cells of an organism, producing an intermediate phenotype.
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Incomplete Dominance
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Incomplete Dominance F1 hybrids have an appearance somewhat in between the phenotypes of the two parental varieties. Example: snapdragons (flower) red (RR) x white (rr) r r RR = red flower rr = white flower
R R 78
Incomplete Dominance r
r
R Rr
Rr
R Rr
Rr
produces the F1 generation All Rr = pink (heterozygous pink) 79
Incomplete Dominance
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Codominance
Codominance – the expression of both forms of an allele in heterozygous individual in different cells of the same organism
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Codominance
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Codominance Two alleles are expressed (multiple alleles) in heterozygous individuals. Example: blood type 1. 2. 3. 4.
type type type type
A B AB O
= = = =
IAIA or IAi IBIB or IBi IAIB ii 83
Codominance Problem Example: homozygous male Type B (IBIB) x heterozygous female Type A (IAi) IA
i
IB
IAIB
IB i
IB
IAIB
IB i
1/2 = IAIB 1/2 = IBi
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Another Codominance Problem • Example: male Type O (ii) x female type AB (IAIB) IA
IB
i
IAi
IB i
i
IAi
IB i
1/2 = IAi 1/2 = IBi
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Codominance Question:
If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents?
boy - type O (ii) AB (IAIB)
X
girl - type
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Codominance Answer: IA IB i
i
IAIB ii
Parents: genotypes = IAi and IBi phenotypes = A and B
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Sex-linked Traits Traits (genes) located on the sex chromosomes Sex chromosomes are X and Y XX genotype for females XY genotype for males Many sex-linked traits carried on X chromosome 88
Sex-linked Traits Example: Eye color in fruit flies Sex Chromosomes fruit fly eye color
XX chromosome - female
Xy chromosome - male 89
Sex-linked Trait Problem Example: Eye color in fruit flies (red-eyed male) x (white-eyed female)
XRY
x
XrXr
Remember: the Y chromosome in males does not carry traits. Xr Xr RR = red eyed Rr = red eyed R X rr = white eyed XY = male Y XX = female
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Sex-linked Trait Solution: Xr XR
XR
Xr
Y
Xr Y
Xr XR
Xr
Xr Y
50% red eyed female 50% white eyed male
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Female Carriers
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Genetic Practice Problems
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Breed the P1 generation tall (TT) x dwarf (tt) pea plants t
t
T T 94
Solution: tall (TT) vs. dwarf (tt) pea plants t
t
T
Tt
Tt
produces the F1 generation
T
Tt
Tt
All Tt = tall (heterozygous tall) 95
Breed the F1 generation tall (Tt) vs. tall (Tt) pea plants T
t
T t 96
Solution: tall (Tt) x tall (Tt) pea plants T t T
TT
Tt
t
Tt
tt
produces the F2 generation
1/4 (25%) = TT 1/2 (50%) = Tt 1/4 (25%) = tt 1:2:1 genotype 3:1 phenotype 97
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Pedigree Charts
Pedigree chart – a chart used to record the transmission of a particular trait or traits over several generations. (like a family tree)
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Pedigree Chart
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Questions Questions 1-3 page 607
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Other Patterns of Inheritance
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Pleiotrophic Genes – affect many different characteristics. Ex: Sickle cell anemia. A blood disorder. Normal hemoglobin has the allele HbA. Sickle cell occurs in individuals who have two copies of the mutated allele HbS. This mutation causes abnormally shaped hemoglobin that interlock with one another. People with sickle cell, are fatigued, weak and have an enlarged spleen. Often show signs of heart lung, and kidney failure.
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Multiple Alleles
Multiple Alleles – when trait are determined by more that two alleles. Most commonly seen trait is call the wild type.
Mutant – any allele of a gene other than the wild type allele. Ex: fruit fly can have any one of four eye colours. Red is the wild type, eyes may also be apricot, honey and white. They have two alleles for eye colour.
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Review Questions Page 478 Questions 1-8 (old text)
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Environment and Phenotype
Himalayan rabbits have black fur when raised in low temperatures and white in high temperatures
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Questions Questions 1-3 page 612
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Dihybrid Crosses and Polygenic Traits
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Dihybrid cross – a genetic cross involving two genes, each of which has more than one allele.
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Dihybrid
Crosses
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Punnett Square
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Probability and Dihybrid Crosses P = # of ways that a given outcome can occur total # of possible outcomes
Questions page 615
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Selective breeding
Selective breeding – the crossing of desired traits from plants or animals to produce offspring with both characteristics
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Selective Breeding
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Inbreeding – the process whereby breeding stock is drawn from a limited number of individuals possessing desirable phenotypes.
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Polygenetic traits – inherited characteristics that are determined by more than one gene Epistatic gene – a gene that masks the expression of another gene or genes.
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Review
Questions
Page 483 Questions 9-11 (old text) Questions 1-4 page 619
Review Questions page 623- 625
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Review
Questions
Page 489 Questions 12-14
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Social Issue Social Issue Page 491
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Lab
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