H2 Biology Core 2 - Dihybrid Cross with Independent Assortment

Step 1 - Set up the genetic problem

In this video we will work through a dihybrid cross involving two unlinked genes.

Step 1 - Set up the genetic problem

Imagine a plant where seed shape and seed colour are controlled by two separate genes on different chromosomes.

Step 1 - Set up the genetic problem

Round seed shape, capital R, is dominant over wrinkled, lowercase r.

Step 1 - Set up the genetic problem

Yellow seed colour, capital Y, is dominant over green, lowercase y.

Step 1 - Set up the genetic problem

We cross two plants that are both heterozygous for both traits, so each parent is R r Y y.

Step 2 - Determine the gametes using independent assortment

Because the two genes are on different chromosomes, they assort independently during meiosis one.

Step 2 - Determine the gametes using independent assortment

Each parent can produce four types of gametes.

Step 2 - Determine the gametes using independent assortment

These are R Y, R y, r Y, and r y.

Step 2 - Determine the gametes using independent assortment

Each gamete type is equally likely, with a probability of one quarter.

Step 3 - Construct the Punnett square

We now set up a four by four Punnett square with the gametes of each parent along the edges.

Step 3 - Construct the Punnett square

Fill in each cell by combining one gamete from each parent.

Step 3 - Construct the Punnett square

For example, R Y from the mother and R Y from the father gives R R Y Y, which is round and yellow.

Step 3 - Construct the Punnett square

Complete all sixteen cells systematically.

Step 4 - Count the phenotypic ratio

Now group the sixteen outcomes by phenotype.

Step 4 - Count the phenotypic ratio

Round yellow appears nine times - these are all genotypes with at least one R and at least one Y.

Step 4 - Count the phenotypic ratio

Round green appears three times - at least one R but homozygous recessive y y.

Step 4 - Count the phenotypic ratio

Wrinkled yellow also appears three times - homozygous r r but at least one Y.

Step 4 - Count the phenotypic ratio

Wrinkled green appears just once - the double homozygous recessive r r y y.

Step 5 - When to use the chi-squared test

In an exam, you may be given observed data from an actual cross and asked whether it fits the expected nine to three to three to one ratio.

Step 5 - When to use the chi-squared test

The chi squared test compares observed and expected frequencies.

Step 5 - When to use the chi-squared test

Calculate chi squared by summing the quantity observed minus expected, squared, divided by expected, for each phenotype class.

Step 5 - When to use the chi-squared test

Compare your calculated value with the critical value at the five percent significance level with three degrees of freedom, which is seven point eight one five.