Practice Problems for Genetics: Worksheet 1 1- In humans, there is a gene that influences how curly the hair is (actually several genes appear to influence this trait, but we’ll only consider one). Individuals with wavy hair are heterozygous and those with straight or curly hair are each homozygous for one of the two allelic forms. Also, the presence of freckles is a dominant trait and the absence of freckles is recessive (this is a case of complete dominance). Calculate the probability that a man with straight hair and freckles (his mother didn’t have freckles) and his wife (who has wavy hair and no freckles) could have a daughter with either straight hair and freckles or wavy hair and freckles.

2- A 3rd grader decided to breed guppies for her school science fair project. She bought a male guppy that was black with short fins and a female guppy that was white with long fins. To study the inheritance of these features and she recorded the outcome of the first mating. This produced 117 offspring that were black with long fins (Group A) and 121 that were white with long fins (Group B). To follow up on this experiment she selected several offspring from Group A and interbred these. She found that 16 of the 251 were white with short fins, 52 were white with long fins, 48 were black with short fins, and 135 were black with long fins. In another test, she selectively crossed fish from Group B. When these fish were interbred, they produced 178 progeny that were white with long fins and 63 that were white with short fins.

When addressing the following questions, be sure to use the letters G/g to describe fin lengths and the letters R/r to indicate the different colors. A) What is the genotype of the fish within group A?

B) What is the genotype of the fish within group B? C) Which trait is dominant, white or black?

D) What is the phenotypic ratio in offspring of Group A? E) Use a Chi Square test to determine whether deviations from this phenotypic ratio (for Group A) are significant or are just due to random variation. (We’ll do Chi Square tests in class later this week.). F) If group B members were backcrossed to the father, what would the phenotype(s) of the young be?

3- You’re developing a new variety of roses and are interested in producing a strain that has double flowers and is resistant to the plant disease blackspot. These traits are both autosomal dominant traits. Also, the genes controlling each trait assort independently and are transcribed from separate gene loci. Over the years, you’ve collected a yellow rose line that is pure breeding for single flowers that are not resistant to blackspot. At a national rose conference, you come across a plant of the same yellow rose that has double flowers and is resistant to blackspot. You purchase this plant and breed it to one from your original yellow rose line and find that ½ of the offspring have single flowers and are resistant to blackspot, while ½ have double flowers and are resistant to blackspot. Be sure to show all work.

A) What was the genotype of the plant that you bought at the conference? Assume that the blackspot trait is controlled by alleles B/b and the doubleflowering trait is controlled by alleles D/d. B) What was the genotype of your original plant stocks (the single flower roses that are not resistant to blackspot)? C) What type of cross is this? Be specific. D) If you breed the offspring that have double flowers and are resistant to blackspot to the parent that had single flowers and was not resistant to blackspot, what phenotypes will the new offspring have? E) If you breed the offspring that have double flowers and are resistant to blackspot to the parent that you bought at the conference, what percentage of the offspring will have single flowers but resistance to blackspot?

4- You cross a canary that is Aa Bb cc Dd ee Ff to one that is aa bb Cc dd Ee ff. In canaries, traits A/a, C/c, D/d, and F/f exhibit complete dominance and traits B/b and E/e exhibit codominance. Consider the F1 progeny from this cross, and assume independent assortment. A- How many phenotypic classes do you expect in the F1 generation?

B- What is the probability of the genotype aa Bb Cc dd Ee ff? C- What proportion would be expected to be homozygous recessive for all genes? 5- In tomatoes, two alleles of one gene determine the character difference of purple (P) versus green (G) stems and two alleles of a separate, independent gene determine the character difference of “cut” versus “potato” (Po) leaves. The results for five matings of tomato plants and the resulting phenotypes are shown below.

Parent Number of progeny

Mating Phenotype P, C P, Po G, C G, Po 1 P, C x G, C 321 101 310 107 2 P, C x P, Po 219 207 64 71 3 P, C x G, C 722 231 0 0 4 P, C x G, Po 404 0 387 0 5 P, Po x G, C 70 91 86 77 A) Determine which alleles are dominant. B) What are the most probable genotypes for the parents in each cross? Write your answer below.

For the stem color use alleles G/g…G is dominant to g. For the leaf type use the alleles R/r…R is dominant to r.

Mating 1 Mating 2 Mating 3 Mating 4 Mating 5

6- In humans, albinism is a heritable trait that results in loss of the pigment melanin in the hair, skin, and eyes. This is a case of complete dominance and affected offspring are homozygous recessive. What is the probability of two heterozygous individuals producing a family that has two children with normal pigmentation and one albino child? 7- (Advanced question…easiest method to do this will be explained in class on Monday) Some rabbit varieties have solid fur color intermixed with silver hairs. This trait is known as silvering. The allele for silvering, l, is recessive to the non-silvering allele, L. A) How often would you expect to observe 5 rabbits with silvering in their hair and 2 brown rabbits without silvering in a single litter from a mating pair of heterozygous rabbits?

B) How often would you expect a mating between a heterozygous parent without silvering and a homozygous parent with silvering to produce offspring born in the following order: 1st and 4th offspring have silvering, but the 2nd and 3rd lack silvering? 8- (Advanced question…check maternal inheritance in book…info will be covered in class next week)- In the Southeastern flour beetle, inheritance of hairs on the 3rd thoracic segment (we’ll call these hairy beetles) is maternally influenced (this is a recessive maternal genetic trait). A hairy female flour beetle with the genotype Bb is bred to a male flour beetle with the genotype bb. A) Show the genotype or genotypes of the offspring produced.

B) Give the proportions of each genotype produced.

C) Will the offspring of this cross be hairy, hairless, or a mix of both?

D) What was the genotype of the mother of the female used in this cross?

9- (Advanced question…check lethal traits in book…info will be covered in class next week)- Heterozygous Cp cp chickens express a condition called creeper, in which the leg and wing bones are shorter than normal (cp cp). The dominant Cp allele is lethal when homozygous. Two alleles of an independently segregating gene determine white (W_) versus yellow (ww) skin color. From matings between chickens heterozygous for both of these genes…

A) What genotypic classes will the chicks that hatch have? B) What are the relative frequencies of these genotypes? C) What phenotypic classes will be among the viable progeny?

D) What are the expected relative frequencies of these phenotypes? 10- (Advanced question…Epistasis….Will be covered in class later) - In shetland goats, there are two genes that control coat color. One gene controls the color of pigmentation in the hairs. The dominant form of this allele (H+_) causes brown hair coloration and homozygous recessive individuals are chesnut. A second gene site influences graying very early in life. The R+ allele is dominant and causes premature graying. The RR allele is recessive and does not cause graying. The R+ allele is epistatic to the gene that influences color of pigmentation in the hairs.

A- What form of epistasis is this? B- If individual 1 (H+hR+RR) is crossed to individual 2 (hhR+RR), what proportion of

the offspring will be gray?

C- What proportion will be chesnut?

D- What color is individual 1?

E- What color is individual 2?