Section 9-2 Solving Punnett Squares

1. Section 9-2 Solving Punnett Squares

2. The Purpose of Punnett Squares A Punnett square is a tool used to predict the probabilitythat traits found in parents will occur in the offspring. It was developed by Reginald Punnett who was a zoologist in the late 1800’s who began to study heredityafter college. He is responsible for helping to credit Mendel’s work and cofounded the Journal of Genetics which is still highly regarded today.

3. The Purpose of Punnett Squares Punnett squares allow for various combinations of each allele for a gene from both the mother and father to show all of the possibilitiesfor the children. They can be used to screen for some genetic diseases as well as provide information to pet breeders and florists.

4. Setting up a Punnett Square The male alleles go on the top and the female alleles go along the side. To fill in the square, one allele from each parent is placed in each box to represent the combinations for the offspring. Always write a capital letter before a lower case letter in the boxes. Because there are 2 parents with 2 alleles each, there are 4 possibilities for each trait. When one gene specifies one trait it is called a monohybrid cross.

5. Setting up a Punnett Square A word problem provides information about the parent’s alleles. If a parent is dominant for a trait there will have to be information in the problem that tells you if they have 2 capital letters (homozygous) or 1 capital and one lowercase letter (heterozygous). The words pure and true-breeding specify a homozygous combination. Heterozygous parents are indicated by stating that the parent has the dominant trait but one of the grandparents does not.

6. Complete Dominance Pattern For this pattern there are only 2 phenotypes but 3 genotypes. An example is the gene for cystic fibrosis. Having a normal CFTR protein is dominant so is represented by F while having a defective protein is recessive and is represented by f. A normal person can have the genotype FF or Ff. A person with cystic fibrosis always has the genotype ff.

7. Complete Dominance Pattern If man who is normal but whose mother has cystic fibrosis marries a woman who is pure for the normal condition, what are the chances that they will have a child with the disease? All of the children will show the normal phenotype but 50% of the children with have a heterozygous genotype. They could pass the recessive allele on to their children someday. The phenotype ratio is written as 4 : 0 (normal : cystic fibrosis) The genotype ratio is written as 2 : 2 : 0 (FF Ffff)

8. Incomplete Dominance Pattern Because of the blending that occurs when both alleles occur together this pattern has 3 genotypes and 3 phenotypes. An example is feather color in hens and roosters. Having black feathers is equally dominant to having a mottled pattern called splash. The combination is a bluish feather color.

9. Incomplete Dominance Pattern If two blue hens are mated, what possible offspring could they have? The picture below shows a more complicated way to write the alleles. We will use the same capital and lower case letter to show the pattern (one form will be assigned to each). In these problems the genotype ratio always matches the phenotype ratio. All possible combinations are shown to the right.

10. Sex-linked Pattern We will only consider genes on the X chromosome for this pattern so I state this in the problem or tell you that the trait is sex-linked. This pattern will only involve traits that show complete dominance. We will use an X with a superscipt of a capital or lower case letter. There is never a letter on the Y chromosome because it does not carry the same traits as the X chromosome.

11. Sex-linked Pattern The results will always include 2 daughters and 2 sons. The genotypes will need to be specified by gender since they will sometimes differ based on the parent’s alleles. The pedigree below shows differences in offspring based on whether the mom or dad has a recessive allele on the X chromosome.

12. Sex-linked Pattern An Example in humans is the blood clotting disorder hemophilia. H is normal and h is hemophilia. In the top Punnett square 0% of the daughters will have hemophilia but 50% of the sons will. In the bottom square 0% of daughters or sons will have hemophilia even though the father has the disease.