GRE Subject Test: Biology : Understanding Pedigrees and Punnett Squares

Study concepts, example questions & explanations for GRE Subject Test: Biology

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Example Questions

Example Question #1 : Understanding Pedigrees And Punnett Squares

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?

Possible Answers:

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

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

15 long ears, black : 1 short ears, yellow

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

Correct answer:

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


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 #2 : Understanding Pedigrees And Punnett Squares

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

Possible Answers:

II and III

I only

I and II

I, II, and III

Correct answer:

I and II


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 #3 : 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?

Possible Answers:

Correct answer:


The shortcut for this problem involves the standard phenotypic ratios for a dihybrid cross. Nine offspring will show both dominant traits. Three will show one dominant trait and the other recessive trait. Three will show the inverse phenotypes, with the opposite dominant trait and recessive trait combination. One offspring will show both recessive traits. Based on these ratios, we can see that three of the sixteen offspring will show the dominant smooth trait and the recessive green phenotype.

We can also solve by using a dihybrid punnett square. The cross described will be AaBb x AaBb, in which the A alleles signify color and the B alleles signify shape.

Consider the color of the peas. In order to have green peas, two recessive alleles must combine in the next generation. According to a punnett square where both sides are heterozygous for the trait, there is only a one in four chance of this taking place. Since smooth is the dominant shape for the peas, a punnett square where each side is heterozygous shows a three in four chance that pea plants will have this shape. By multiplying these two probabilities, we determine that three out of sixteen pea plants will have smooth, green peas.

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