Genetics: The Punnett Square

In the early 1900’s, a geneticist by the name of Reginald Crundall Punnett introduced the Punnett square, a handy tool used to predict results in genetic crosses.

A Punnett square shows possible ways in which alleles can combine.

In a Punnett square, all possible gametes of one parent are lined up vertically, and all possible gametes of the other parent are lined up horizontally.  The resulting genotypes of the offspring are placed within the squares.  By simple inspection of the alleles of the offspring within the squares, one can easily determine the ratio of the genotypes and the phenotypes.

Sample Problems Using

Problem 1

In garden peas, round seeds are dominant to wrinkled seeds.  A plant that is homozygous for round seeds is crossed with a plant that is heterozygous.  What percentage of plants will have round seeds?  What is the chance of the offspring being heterozygous?

P1                    RR      x          Rr

GR:      2RR: 2Rr or 1RR: 1Rr or 50%RR: 50%Rr

PR:       4/4 or 100% round seeds

Answer:           

All plants (100%) will have round seeds.  An offspring has a 50% chance being heterozygous.

Problem 2

In humans, brown eyes are dominant over blue eyes.  If a couple who are both heterozygous for brown eyes reproduce with one another, will the child have brown eyes or blue eyes?

Key:    B = brown eyes

            b = blue eyes

P1:       Bb       x          BbGR:      ¼ BB:  2/4 Bb:   ¼ bb

PR:       ¾ or 75% brown eyes:   ¼ or 25% blue eyes

Answer:

The child could have brown eyes or blue eyes.  The probability that the child could be brown-eyed is 75% while the probability that it could be blue-eyed is only 25%.

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Category: Biology