GRE Subject Test: Biology : Understanding Pedigrees and Punnett Squares

Example Questions

Example Question #1 : Genetics, Dna, And Molecular Biology

A scientist is working with a breed of dog and has noticed that two traits, ear length and color, behave in normal dominant-recessive hierarchies. Long ears (A) are dominant to short ears (a) and black coloration (B) is dominant to yellow coloration (b). If he breeds a long eared, black dog (AaBb) with a short eared yellow dog (aabb), what would be the resulting phenotypic ratios of the offspring?

15 long ears, black : 1 short ears, yellow

1 long ears, black : 1 long ears, yellow : 1 short ears, black : 1 short ears, yellow

9 long ears, black : 3 long ears, yellow : 4 short ears, black

9 long ears, black : 3 long ears, yellow : 3 short ears, black : 1 short ears, yellow

1 long ears, black : 1 long ears, yellow : 1 short ears, black : 1 short ears, yellow

Explanation:

This question can be solved by making a punnett square. The genotypes are given: AaBb x aabb.

The potential gametes the AaBb dog can produce are AB, Ab, aB, and ab. The aabb dog can only produce one gamete: ab.

Putting these gametes in our punnett square we can see that we end up with the following potential offspring: AaBb, Aabb, aaBb, and aabb.

Each of these possible offspring carries a different phenotype. AaBb will carry both dominant traits and be black with long ears. Aabb will be yellow with long earsaaBb will be black with short ears. Finally, aabb will be yellow with short ears. Each of these gametes is produced in the same ratio, making these phenotypes exist in a 1:1:1:1 probability.

Example Question #1 : Genetics, Dna, And Molecular Biology

Which of the following choices represents information contained in a punnett square?

I. Potential genotype ratios of offspring

II. Possible gametes produced by parent generation

III. Allele frequencies of the population

I, II, and III

I and II

II and III

I only

I and II

Explanation:

Punnett squares give information about the potential genotype ratios of offspring possible from the cross of two members of the parental generation. The letters represent alleles of various genes, but do not give any information about the allele frequencies. To get information about the allele frequencies, more information about the size and make-up of the population would be needed. The actual cross is between potential gametes produced by the parental generation. Each square shows the potential offspring from these potential gametes.

Example Question #1 : Understanding Pedigrees And Punnett Squares

Peas in pea plants can be either yellow or green, with yellow being the dominant color. The peas can also be smooth or wrinkled, with smooth being the dominant shape. Suppose that a pea plant that is heterozygous for both traits is self crossed.

What proportion of the next generation will have smooth, green peas?