Biology Help

In-class introduction to basic Punnett square set-up and problem solving, Part 1

Problem-solving tips:

· A Punnett square allows you to predict the possible genetic outcome of children based on the genetic make-up of the parents.

· First, read the problem and figure out whether the trait of interest or genetic disorder is found on the dominant allele or the recessive allele because that will have an impact on how you interpret the results of the Punnett square.

· Select a letter to represent the trait or disorder and define the dominant and recessive alleles. For example: For eye color, B (dominant) = brown eyes and b (recessive) = blue eyes. For achondroplasia (dwarfism), A (dominant) = achondroplasia and a (recessive) = normal allele.

· If it is a sex-linked question, remember to include the sexual genotypes of the parents (XX for mom and XY for dad).

· Write down all possible genotypes & phenotypes and use this information to help you set up the Punnett square.

1. Practice question on a human trait. In reality, eye color is controlled by multiple genes and is a complex trait. For simplicity, we’ll assume that brown eyes are dominant to blue eyes. Answer the questions below.

a) Select a letter for this trait and define the dominant and recessive alleles.

B (dominant) =

b (recessive) =

b) Write down all possible genotypes and phenotypes for individuals in the population

	Possible genotypes (the 2 alleles an individual has)	Possible phenotypes (the physical appearance of a trait)
Homozygous dominant individuals
Homozygous recessive individuals
Heterozygous individuals

c) Set up the Punnett square and solve this problem. Kristy is heterozygous and Mark has blue eyes. What percentage of their offspring will have blue eyes?

		Kristy's genotype
Mark's genotype

a) Select a letter for this genetic condition and define the dominant and recessive alleles.

F (dominant) =

f (recessive) =

b) Write down all possible genotypes and phenotypes for individuals in the population

	Possible genotypes (the 2 alleles an individual has)	Possible phenotypes (the physical appearance of a trait)
Homozygous dominant individuals
Homozygous recessive individuals
Heterozygous individuals

c) Set up the Punnett square and solve this problem. Kristy and Mark are carriers for cystic fibrosis. The term carrier is only used when a condition is on the recessive allele. Carriers are heterozygous individuals who are normal and show no symptoms of the disorder, but they have the ability to pass on the mutated recessive allele to their offspring. What percentage of their children will be normal? What percentage of their children will be carriers?

		Kristy's genotype
Mark's genotype

2. Practice question on a genetic condition. Cystic fibrosis (CF) is an autosomal, recessive condition that results in mucus buildup in the lungs and digestive system organs. As a result, CF individuals have difficulty with breathing and bowel movement is obstructed.