What is a Punnett Square?

A Punnett square is used to show the probability of an offspring with a given genotype resulting from a certain cross.

How to Solve Punnett Square Problems Involving Autosomal Genes

Autosomal genes are genes located on one the 22 pairs of chromosomes other than the sex chromosome (chromosome 23). We will look at how to make predictions associated with sex-linked genes in the second part of this article. In the mean time, remember these two principles –

  1. The presence of one or two dominant alleles will mean that the offspring will express the dominant phenotype
  2. The presence of two recessive alleles will mean that the offspring will express the recessive gene

PROBLEM 1

Here is a likely problem that you would get to solve that involves autosomal chromosomes.

In humans, the allele for freckles is dominant (F) and the allele for no freckles is recessive (f). If your mom has freckles and is heterozygous for this trait and your dad does not have freckles, what is the probability that their siblings will have freckles?

Solution Steps

  1. Name the alleles using an appropriate letter
  2. Use the capital of that letter to identify the dominant allele and the common letter to identify the recessive allele
  3. Write “”mom” and “dad” at the top and left side of the Punnett square (it does not matter which one goes on top)
  4. Add letters to the top and sides, indicating the alleles of the mom and dad
  5. Now, combine the letters in the four smaller squares to determine the probability that the offspring will inherit the trait

There is a 50% chance that their siblings will have freckles.

PROBLEM 2

Marfan syndrome is an autosomal dominant disorder. What is the probability that a child will have Marfan Syndrome if her mom has this disease and is heterozygous, and her dad is normal?

Solution

There is a 50% chance that their siblings will have Marfan syndrome.

PROBLEM 3

Albinism is a skin abnormality caused by recessive autosomal genes. What is the probability that a child will be albino if his dad is an albino and his mon is homozygous and does not carry the recessive gene?

Solution

There is no chance that any of the offspring will be albino.

How to Solve Punnett Square Problems Involving Sex-Linked Genes

As mentioned earlier, sex-linked genes are the genes associated with the sex chromosome, i.e., chromosome 23. Punnett square problems with sex-linked genes follow the general principles outlines earlier with just a few differences. Consider the following problem –

PROBLEM 1

Hemophilia is an X-linked recessive trait. What is the probability that a male offspring will have Hemophilia if his mom is a carrier of the trait?

In this problem, the mother is a carrier. That means, the diseases is not expressed although she has the gene. The defective gene is recessive and the normal gene is dominant. Therefore, the dominant gene masks the defect.

Solution Steps

  1. Draw the Punnett square with the words mom and dad to the top and right of the square (it does not matter which one is on top)
  2. Label the chromosomes and note which one has the disease trait. Remember that females have XX chromosomes and males have XY
  3. Now, combine the letters in the four smaller squares to determine the probability of the offspring inheriting the trait

Solution

One of the male offspring will have the disease and the other will not. Therefore, there is a 50% chance that one of the male offspring will have the disease. Note that although the gene is recessive, it will behave like a dominant gene in males. This is because males only have a single copy of the X chromosome. Therefore is no dominant corresponding normal allele to mask the defect.

PROBLEM 2

What is the probability that a male offspring will have Hemophilia if his dad has the disease?

Solution

The female offspring will be carriers of the disease and none of the males will have it. Therefore, there is no chance that any of the offspring will have the disease.

Get more practice with Punnett Squares here.

Courtney Simons
Administrator
Courtney Simons is a food science professor. He holds a BS degree in food science and a Ph.D. in cereal science from North Dakota State University.
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