## Biology Tutorial

#### Intro

Mendel formulated a theory of inheritance based on experiments with garden peas, proposing that parents pass on to their offspring discrete genes that retain their identity through generations. The fundamental principles of inheritance are called the law of segregation and the law of independent assortment.

#### Sample Problem

In a certain species of bird, yellow beaks are dominant to orange beaks, and blue feathers are dominant to black feathers.

Two heterozygous birds are crossed. What fraction of the offspring would be expected to have yellow beaks and blue feathers?

3/16

9/16

1/16

3/16

1/4

#### Solution

This question requires us to do a dihybrid cross. We can represent the gene for beak color with the symbol “A” for dominant yellow and “a” for recessive orange. For feather color, we can use “B” for blue feathers and “b” for black.

The problem states that both birds are heterozygous for each trait. This implies that our cross is between two birds with the genotype AaBb.

Look at the gametes that can be produced by these parents: AB, Ab, aB, and ab. These are used to make a punnet square.

Offspring: 1AABB, 3Aabb, 8AaBb, 3aaBb, 1aabb

There are 16 total offspring. 12 of them carry the dominant A allele. This gives them the yellow beak phenotype.

Yellow beaks: 1AABB, 3Aabb, 8AaBb

Of these 12, 9 carry the dominant B allele for blue feathers.

Yellow beaks and blue feathers: 1AABB, 8AaBb

This gives us a total of 9 out of the 16 offspring that will express both the yellow beak and blue feather phenotypes.