CHAPTER 9 - Patterns of Inheritance State Standards Standard 3a:

Standard 3b

Purebreds and Mutts — A Difference of Heredity • Genetics is the science of heredity • These black Labrador puppies are purebred—their parents and grandparents were black Labs with very similar genetic makeups – Purebreds often suffer from serious genetic defects • The parents of these puppies were a mixture of different breeds - Their behavior and appearance is more varied as a result of their diverse genetic inheritance The science of genetics has ancient roots • The science of heredity dates back to ancient attempts at selective breeding • Until the 20th century, however, many biologists erroneously believed that – characteristics acquired during lifetime could be passed on – characteristics of both parents blended irreversibly in their offspring Figure 9.2 and 9.3

Experimental genetics began in an abbey garden • Modern genetics began with Gregor Mendel’s quantitative experiments with pea plants Figure 9.4 • Mendel crossed pea plants that differed in certain characteristics and traced the traits from generation to generation - This illustration shows his technique for cross fertilization Figure 9.5 • Mendel studied seven pea characteristics

•He hypothesized that

– alleles (although he did not use that term), the units that determine heredity Figure 9.6 Mendel’s principle of segregation describes the inheritance of a single characteristic • From his experimental data, Mendel deduced that an organism has two genes for each inherited characteristic

Figure 9.6 b • A sperm or egg carries only one gene of each pair

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This process describes Mendel’s law of segregation Alleles can be dominant or recessive

Principle of Segregation • Homologous pairs of genes segregate (separate) during gamete formation (meiosis).

Figure 9.7 Homologous chromosomes bear the two alleles for each characteristic • Alternative forms of a gene (alleles) reside at the same locus on homologous chromosomes Genetics Vocabulary Gene – a segment of DNA that contains the instructions that code for a particular trait Locus – specific location of a gene on a chromosome Allele – alternate versions of a gene at a single locus Homozygous (purebred/truebred) – when the alleles of a gene are the same on the homologous chromosomes Heterozygous (hybrid) – when the alleles of a gene are different on the homologous chromosomes Dominant – the allele that is expressed when the alleles are heterozygous. Represented by an upper case letter Recessive – the allele that is not expressed when the alleles are heterozygous. Represented by a lower case letter. To be expressed the cell must have 2 copies of the recessive allele Phenotype – the physical appearance of a trait in an organism Genotype – the genetic make up of an organism with respect to a trait. The genotype of a trait can be homozygous dominant, heterozygous or homozygous recessive Figure 9.11

Mendel’s principles reflect the rules of probability • Inheritance follows the rules of probability Predicting the Outcome of a Monohybrid Cross Predict the results of the following cross (using R to denote tongue-rolling ability): P generation: RR x RR 1. What genotype(s) will be found in the F1 generation? 2. What phenotype(s) will be found in the F1 generation? 3. Explain why you made these predictions.

Predict the results of the following cross: P generation: RR x rr 1. What genotype(s) will be found in the F1 generation? 2. What phenotype(s) will be found in the F1 generation? 3. Explain why you made these predictions. Predict the results of the following cross: P generation = Rr x Rr

1. Draw the Punnett square.

2. What are the possible genotypes in the F1 generation?

3. What is the genotypic ratio of this cross?

4. What are the possible phenotypes in the F1 generation?

5. What is the phenotypic ratio for this cross?

Practice Monohybrid Problems 1. The gene for tall (T) is dominant over dwarf (t) in the garden pea. A pea plant that comes from a line of plants that are all tall (true-breeding) is crossed with a dwarf pea plant. a. What is the phenotype of the F1 generation? b. What is (are) the genotypes?

2. In guinea pigs, rough coat (R) is dominant over smooth coat (r). Predict the genotypes and phenotypes of the offspring and give the genotypic and phenotypic ratios is a homozygous dominant guinea pig is crossed with a heterozygous guinea pig. 3. In humans, the ability to taste phenylthiourea (PTU) is dominant. “Tasters” (TT) or (Tt) perceive an extremely bitter taste of PTU, while “non tasters” (tt) experience no sensation, or taste. a. What are the genotypes of Mr. and Mrs. Meadowmuffin, who can taste PTU, and who have 3 children, one of whom is a non taster? b. What offspring phenotypes would be expected from the following crosses and in what ratios? 1. heterozygous x heterozygous 2. homozygous taster x heterozygous 3. heterozygous x non taster 4. In silkworms a single gene determines the color of the cocoon. The yellow cocoon allele is dominant to the white allele. What are the genotypic and phenotypic ratios of a cross between a homozygous dominant male and a heterozygous female? 5. In mice a single gene determines the color of the eyes. The black eyes allele is dominant to the red allele. What are the genotypic and phenotypic ratios of a cross between a heterozygous male and a heterozygous female? Figure 9.10 Geneticists use the testcross to determine unknown genotypes • The offspring of a testcross often reveal the genotype of an individual when it is unknown • A testcross is

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Usually performed when the phenotype of the unknown individual is dominant.

Understanding Test Cross Brown coat color (B) in rabbits is dominant and white coat color is recessive. 1. Suppose you have a group of rabbits – some brown and some white. a. For which phenotype(s) do you know the genotype(s)?

b. For which phenotype(s) are you unsure of the genotype(s)?

Using B and b to symbolize the brown and white alleles 2. What are the possible genotypes of a white rabbit in your group?

3. What are the possible genotypes of a brown rabbit?

4. Suppose you wanted to find out the genotype of a brown rabbit. What color rabbit would you mate it with?

A brown buck (male) is mated with a white doe (female). In their litter of 11 young, 6 are white and 5 are brown. 5. Using a Punnett square to check your answer, what is the genotype of the buck?

6. Use a Punnett square to determine the ratio of brown and white offspring that would have been produced by the above mating if the brown buck had been homozygous.

The principle of independent assortment is revealed by tracking two characteristics at once • By looking at two characteristics at once, Mendel found that the genes of a pair segregate independently of other gene pairs during gamete formation

Dihybrid cross An experimental mating of individuals in which the inheritance of 2 traits is tracked. When the inheritance of 2 traits is tracked in an individual, the dominant/recessive traits do not always appear together. The individual may be dominant in one of the traits and recessive in the other. • Genes for different characteristics are not connected and each pair of genes for a characteristic separate independently during meiosis.

Solving Dihybrid Problems 1. List the genotypes of each parent. 2. Make all possible combinations of the gametes 3. Construct a 16 square Punnett square. 4. List the possible genotypes of the offspring and determine the genotypic ratio. 5. List the possible phenotypes of the offspring and determine the phenotypic ratio. Example: In humans freckles (F) is dominant and no freckles (f) is recessive. Normal arches (A) are dominant and flat feet (a) is recessive. A man who has freckles and flat feet (FFaa) marries a woman without freckles and normal arches (ffAA). What are the possible genotypes and phenotypes of their children?

Practice Dihybrid Problems 1. What are the possible gametes formed from the independent assortment of genes for AABb?

2. In garden peas axial flower position (A) is dominant and terminal flower position (a) is recessive. Tall vine (T) is dominant and short vine (t) is recessive. A plant homozygous for tall vine and axial flowers was crossed with a plant having short vines and terminal flowers. What are the possible phenotypes and genotypes of the offspring? 3. The offspring from problem 2 were crossed with one another. What are the phenotypes of the offspring? 4. In horses black hair (B) is dominant and chestnut is recessive (b). The trotting gait is dominant (T) and pacing (t) is recessive. A homozygous black pacer is crossed with a homozygous chestnut trotter. What are the possible genotypes and phenotypes of their offspring? 5. In watermelons, the alleles for green color (G) and short shape (S) are dominant over the alleles for striped color (g) and long shape (s). A plant with long, striped fruits is crossed with a plant that is heterozygous for both characteristics. What genotypes and phenotypes will be found among the offspring and in what ratios?

Figure 9.13 Connection: Genetic traits in humans can be tracked through family pedigrees • The inheritance of many human traits follows Mendel’s principles and the rules of probability • Family pedigrees are used to determine patterns of inheritance and individual genotypes Figure 9.14 Connection: Many inherited disorders in humans are controlled by a single gene • Most such disorders are caused by autosomal recessive alleles – Examples:

Inherited Single Gene Disorders Recessive disorders • Most single gene disorders

• Most born to normal parents who are carriers – –

Carrier –

• Carriers have a 1 in 4 chance of having a child with a recessive disorder Figure 9.15 • A few are caused by dominant alleles -

Examples:

Connection: Fetal testing can spot many inherited disorders early in pregnancy • Karyotyping and biochemical tests of fetal cells and molecules can help people make reproductive decisions – Fetal cells can be obtained through amniocentesis • Chorionic villus sampling is another procedure that obtains fetal cells for karyotyping • Examination of the fetus with ultrasound is another helpful technique Mendel’s principles are valid for all sexually reproducing species – However, often the genotype does not dictate the phenotype in the simple way his principles describe Figure 9.16 Incomplete dominance results in intermediate phenotypes • When an offspring’s phenotype—such as flower color— is in between the phenotypes of its parents, it exhibits

Figure 9.17

• Incomplete dominance in human hypercholesterolemia • In a cross between a homozygous dominant parent and a homozygous recessive parent the phenotype of the offspring is in between the phenotypes of the parents. • Example: When red snapdragons are crossed with white snapdragons all the offspring have pink flowers Incomplete Dominance Problems 1. Roan is a name for cattle that show a heterozygous incomplete dominant genotype. A roan calf’s parents were a white cow and a red bull. a. What is the roan’s genotype? b. Can two roans mate and produce all roans? Explain.

2. Coat color in one breed of mice is controlled by incomplete dominant alleles so that yellow and white are homozygous, while cream is heterozygous. What phenotypes would the cross of two cream individuals produce? Show a Punnett square. Figure 9.19 Codomiance The alleles for A and B blood types are codominant, and

Codominance Problems 1. Color pattern in a species of duck is determined by a single pair of genes with three alleles. Alleles H and I are codominant and allele j is recessive to both. How many phenotypes are possible in a flock ducks that contains all the possible combinations of these three alleles? 2. Mary has blood type A and she marries John, whose blood type is B. They have three children: Joan, James, and Pete. Joan has blood type O, James has blood type A, and Pete has blood type B. Explain with Punnett squares how this is possible. 3. In a legal case, a man is accused of fathering a child of blood type AB. He is blood type O and the child’s mother is blood type AB. Can this man be the father? Explain your answer with Punnett squares.

Many genes have more than two alleles in the population Multiple allele traits

• Example: the three alleles (IA, IB, i) for ABO blood type in humans Figure 9.20 A single gene may affect many phenotypic characteristics Pleoitropy

• Example: the allele for sickle-cell disease Figure 9.21 A single characteristic may be influenced by many genes Polygenic traits

• This situation creates a continuum of phenotypes – When the range of traits is graphed a bell shaped curve is seen • Example: skin color, eye color Match the description with its pattern of inheritance 1. There are 3 different alleles for a blood group, IA, IB, and i, but an individual has only two at a time.

2. The sickle cell allele, s, is responsible for a variety of phenotypic effects, from pain and fever to damage to the heart, lungs, joints, brain or kidneys.

3. If a red shorthorn cow is mated with a white bull, all their offspring are roan, a phenotype that has a mixture of red and white hairs.

4. Independent genes at 4 different loci are responsible for determining a person’s HLA tissue type, important in organ transplants and certain diseases.

5. When graphed, the number of individuals of various heights forms a bell shaped curve.

6. Chickens homozygous for the black allele are black, and chickens homozygous for the white allele are white. Heterozygous chickens are gray.

Figure 9.23 The chromosomal basis of Mendel’s principles • Genes are located on chromosomes – Their behavior during meiosis accounts for inheritance patterns

Genes on the same chromosome tend to be inherited together Linked genes

• These genes usually do not follow Mendel’s principle of independent assortment Figure 9.28 Many animals including humans have a pair of sex chromosomes • A human male has one X chromosome and one Y chromosome • A human female has two X chromosomes

• Whether a sperm cell has an X or Y chromosome determines the sex of the offspring • Other systems of sex determination exist in other animals and plants Sex Chromosomes and Sex-Linked Genes • The genetic basis of sex determination isn’t fully understood: – Gene SRY on the Y chromosome plays a crucial role – SRY triggers

– Absence of SRY results in ovary development Figure 9.29 Sex-linked genes exhibit a unique pattern of inheritance • All genes on the sex chromosomes are said to be • In many organisms, the X chromosome carries many genes unrelated to sex – Fruit fly eye color is a sex-linked characteristic – Their inheritance pattern reflects the fact that males have one X chromosome and females have two Figure 9.30 Figure 9.31 Connection: Sex-linked disorders affect mostly males • Most sex-linked human disorders are due to recessive alleles – Examples: hemophilia, red-green color blindness

– A male receives a single X-linked allele from his mother, and will have the disorder, while a female has to receive the allele from both parents to be affected

Figure 9.32

• A high incidence of hemophilia has plagued the royal families of Europe Sex-Linked Disorders • Other sex-linked disorders are – Duchenne muscular dystrophy – weakening and loss of muscle tissue – Fragile X syndrome – abnormal X chromosome, most common cause of mental retardation in boys Solving Sex-Linked Problems • Example One: Eye color is a sex-linked trait in fruit flies and is carried on the X chromosome. Red eye color (R) is dominant over white eye color (r). What is the sex and eye color of the offsrping of a homozygous red eyed female and a white eyed male? • Example Two: What is the sex and eye color of the offspring of a heterozygous red eyed female fruit fly and a red eyed male fruit fly?

Sex-Linked Problems 1. A human female “carrier” who is heterozygous for the recessive, sex-linked trait causing redgreen color blindness, marries a color blind male. What proportion of their male offspring will have red-green color blindness? 2. In a marriage of two nonhemophiliac parents, a son with hemophilia is born. What are the probabilities of these parents giving birth to sons being hemophiliac, and to daughters being hemophiliac? Will any daughters be carriers? Use (H) for the normal “non-hemophiliac” allele and (h) for the hemophilia allele. 3. In humans colorblindness (b) is an example of a sex-linked recessive trait. In this problem, a male with colorblindness marries a female who is not colorblind but carries the (b) allele. Using a Punnett square, determine the genotypic and phenotypic probabilities for their potential offspring. 4. A certain form of muscular dystrophy is inherited as a sex-linked, recessive gene. Jack has muscular dystrophy. (Neither of his parents has this disease.) Jane, Jack’s wife, does not have muscular dystrophy, but her father does. What fraction of their daughters would you expect to have muscular dystrophy? What fraction of their sons would you expect to have muscular dystrophy?

5. (a) Why are men never heterozygous for an X-linked trait? (b) Why must men always inherit an Xlinked trait from their mothers? (c)Can a color-blind father pass this allele on to his sons? Explain. (d) Can a normal male ever have a daughter that is color-blind? Explain