Complexities in the patterns of inheritance

Genetics 301

Complexities in the patterns of inheritance (chapter 2)

I. Incomplete dominance: a heterozygote's phenotype is between that of the 2 homozygote extremes

A. Sickle cell anemia

This disease is considered to be inherited as an autosomal recessive disorder. At the molecular level and even at the phenotypic level, however, we should maybe call sickle cell anemia a disease showing incomplete dominance. Why?

B. Colias philodice (sulfur butterfly) enzyme phosphoglucoisomerase (PGI)

C. Flower color in four o'clock plants (see pp. 23)

II. Co-dominance (e.g., ABO blood groups--see chapter 2): a heterozygote expresses (at the phenotype level) both alleles simultaneously

III. Multiple alleles: three or more alleles for a gene (very common!)

A. Colias spp. phosphoglucoisomerase enzyme (PGI)

B. ABO blood groups

C. Coat color in rabbits: there are 4 alleles of a single gene involved in coat color of rabbits:

C = full coat color; dominant to all other alleles

c^ch = chinchilla coat, a partial defect in pigmentation; dominant to ch and c

c^h = himalayan coat, color in only certain parts of body; dominant to c

c = albino, no color; recessive to all other alleles

A rabbit with chinchilla fur is mated to a himalayan. Some of their F1 offspring have himalyan fur, some have chinchilla fur and some are albino. Name the genotypes of the parents and the genotypic ratios of the F1 offspring.

If these parent rabbits (see above question) have 5 offspring, what is the probability that 3 will be albino and 2 chinchilla?

IV. Penetrance and Expressivity (material from chapter 5)

A. Penetrance: a dominant allele doesn't always penetrate into the phenotype

B. Expressivity: given that a genotype is expressed, the level of expression is sometimes variable.

Piebald spotting (S^P) in beagles shows variable expressivity. What does this mean?

V. Pleiotropy: When a single gene influences many traits

VI. Polygenic inheritance: when many genes contribute to 1 phenotypic trait

VII. Epistasis: non-allelic genes interacting in some way to influence the expression of the phenotype

A. In mice (and other mammals), coat color is influenced by many genes. In mice, the A gene determines the distribution of pigment in the hair. The dominant A allele codes for agouti distribution of pigment (solid hair color with yellow band in it). The recessive a allele results in non-agouti patterns (solid hair color). In mice, the B gene determines the color of hair pigments. The dominant B allele results in black coloration, and the recessive b allele results in brown coloration. What are the phenotypes of the following individuals: AaBb x AaBb? If these individuals are bred, what are the expected phenotypic ratios of the offspring?

B. In mice, the C gene permits/inhibits the expression of color in hair. The dominant C allele permits color, and the recessive c allele inhibits the expression of color. In mice with the genotype cc, hair is colorless (despite the make-up of genes A and B). This is an example of recessive epistasis. Suppose the following individuals are bred: AaBBCc x AaBBCc. What colors are they? What proportion of their offspring are expected to be black? Agouti? White (albino)?

C. In sweet peas, there are 2 different lines of true-breeding plants that produce white-flowering plants (line 1, line 2). When line 1 white-flowering plants are crossed with line 2 plants, the F₁ offspring are all purple-flowering plants. When F₁ peas are self-fertilized, the phenotypic ratio of the F₂ generation is: 9 purple-flowering : 7 white-flowering. Explain these results.

D. If the dominant gene K is necessary for hearing and the dominant gene M results in deafness no matter what other genes are present, what is the expected ratio of a dihybrid cross? If a dihybrid individual is test-crossed, what is the expected phenotypic ratio?

E. In shepherd's purse, a common weed, plants true-breeding for round-seeded pods crossed with plants true-breeding for narrow-seeded pods produced 115 plants--all with round-seeded pods. When F₁ offspring are self-fertilized, the F₂ offspring included: 155 plants with round-seeded pods, and 12 plants with narrow-seeded pods. Explain these results.

F. A form of blindness in humans--retinitis pigmentosa--is caused by either a dominant gene R or a recessive gene a. Thus, only rrA- individuals are normal sighted. Two sighted people have a blind son (child with retinitis pigmentosa). What are the genotypes of the parents and son? If this son marries a blind woman with the genotype RrAa, what is the probability they will have a child with retinitis pigmentosa?

G. Question 41 in Tamarin's text. You are working with an exotic organism, Phobia laboris, and are interested in obtaining mutants that work hard. Normal (wildtype) phobes are lazy. Perseverance pays off, and you successfully isolate a true-breeding line of hard-workers. You begin a detailed genetic analysis of this trait. To date, you have obtained the following data: hardworker x non-worker results in F₁ offspring that are all non-workers. When F₁ non-worker offspring are mated with workers, 3/4 of their offspring are workers and 1/4 are non-workers (about). Explain these results. If F1 non-workers were mated amongst themselves (F₁ x F₁cross), what phenotypic ratio of workers to non-workers do you expect to see in the F₂ generation?