Genetics Answers

In humans, albinism is inherited as a recessive trait. Two normally pigmented

parents had an albino child. What is the probability (or percentage) of this couple

having another child with albinism?

A rooster with striped feathers and a pink comb is crossed with a hen which has striped

feathers and a pea comb. After many hatching instances, the F1 generation is made up of the

same number of male and female chicks and which present the following phenotypic

proportions:

For the males: 1/4 black striped; 1/4 pink striped; 1/4 pea; 1/4 simply striped.

For the females: 1/8 pink striped; 1/8 walnut striped; 1/8 pea striped; 1/8 simply striped; 1/8

plain walnut; 1/8 plain pink; 1/8 pea and plain; 1/8 simply plain.

What was the genotype of the parents?

Mendel found that full pods are dominant over constricted pods while round seeds are

dominant over wrinkled seeds. One of this crosses was between full, round plants and

restricted, wrinkled plants. From this cross, he obtained an F1 generation that was all full and

round. In the F2 generation, Mendel obtained his classical 9:3:3:1 ratio. Using this

information, determine the expected F1 and F2 generation result of a cross between

homozygote restricted, round plants and full, wrinkled plants.

In another cross involving parent plants of unknown genotype and phenotype, the following

offspring were obtained:

F1 3/8 full, round: 3/8 full, wrinkled; 1/8 constricted, round; 1/8 constricted, wrinkled.

Determine the genotype and the phenotype of the parents

Test the hypothesis: You cross the dwarf and tall plants and then self-cross the offspring. For best results, this is repeated with hundreds or even thousands of pea plants. What special precautions should be taken in the crosses and in growing the plants? 

Analyze your data: You observe the following plant phenotypes in the F2 generation: 2706 tall/inflated, 930 tall/constricted, 888 dwarf/inflated, and 300 dwarf/constricted. Reduce these findings to a ratio and determine if they are consistent with Mendelian laws. 

Form a conclusion: Were the results close to the expected 9:3:3:1 phenotypic ratio? Do the results support the prediction? What might be observed if far fewer plants were used, given that alleles segregate randomly into gametes? Try to imagine growing that many pea plants, and consider the potential for experimental error. For instance, what would happen if it was extremely windy one day?

1)     A farmer breeds a chicken with all white plumage with a chicken with all black plumage. All of the F1 offspring have bluish-gray plumage. When two of the bluish gray birds are crossed, black, white and bluish-gray offspring are produced. What is the mechanism of inheritance and the genotypes of the parents, F1’s and F2’s?

In the fly Drosophila, the allele for dumpy wings (d) is recessive to the normal longwing allele,     (D) and the allele for white eye (w) is recessive to the normal red-eye allele (W). In a cross of DDWW with Ddww, what proportion of the offspring are expected to be “normal” (long wings and red eyes)? What proportion are expected to have dumpy wings and white eyes?

1)    Phenylketonuria (PKU) is a human autosomal genetic disorder in which the affected individual cannot metabolize the amino acid phenylalanine. The disease is characterized by severe mental retardation if left untreated. The disease is caused by homozygosity for a recessive, mutant allele. If two parents are heterozygous for the allele, what is the probability that their child will have PKU?

1)    Lesch-Nyhan syndrome is a metabolic defect caused by the lack of the enzyme hypoxanthine-guanine phosphoribosyl transferase (HGPRT). This disease is the result of an X-linked, recessive mutation and results in mental retardation, selfmutilation and early death. A normal man marries a woman who had a brother who died of LNS. Your are the genetic counselor who will advise them on the likelihood of their having a LNS baby.

a)     What are the possible genotypes for the woman? What is the man’s genotype?

b)    For each of the woman’s possible genotypes, what are the possible genotypes of her offspring and the implications for survival for each of them?

In humans, hemophilia (bleeder’s disease) is a recessive, X-linked trait. Use a Punnett square to show the possible genotypes of offspring of a man who is a hemophiliac, and a normal, but heterozygous woman, where X^H contains the normal clotting gene and X^h contains the gene for hemophilia.

1)    In guinea pigs, black fur is dominant over white fur. How could an animal breeder test whether a black guinea pig is homozygous or heterozygous?