What phenotypic ratio is expected when two individuals heterozygous for two unlinked genes are crossed?

The expected ratio is 9 : 3 : 3 : 1. Nine show both dominant features, three and three show one dominant feature each, and one shows both recessive features.

What ratio is expected when AaBb is crossed with aabb, and why?

A ratio of 1 : 1 : 1 : 1 is expected. The aabb parent gives only ab gametes, so the four gametes of the AaBb parent each show directly in the offspring.

What is autosomal linkage?

Autosomal linkage occurs when two genes lie close together on the same non-sex chromosome. The alleles tend to be inherited together rather than independently.

How does autosomal linkage change the results of a dihybrid test cross?

Most offspring show the parental combinations of features, and few show new combinations. The expected 1 : 1 : 1 : 1 ratio is not obtained.

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Genetics, Stem Cell Research and Cloning

Dihybrid crosses, epistasis and the chi-squared test

3 things to know
A2 5

Overview

Dihybrid crosses and gametes, the 9:3:3:1 and test cross ratios, autosomal linkage and recombinants, epistasis in Labradors, and chi-squared tests on genetic crosses with worked values.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

How does epistasis explain the 9 : 3 : 4 ratio of black, chocolate and yellow Labrador puppies?

Key term

What is a dihybrid cross?

Key term

Which gametes can an individual with genotype AaBb produce if the two genes are on different chromosomes?

Questions from this subtopic

Each one with the model answer underneath.

1
What is a dihybrid cross?
Model answer
A dihybrid cross follows the inheritance of two genes at different loci at the same time. Each parent passes on one allele of each gene.
2
Which gametes can an individual with genotype AaBb produce if the two genes are on different chromosomes?
Model answer
The individual can produce four types of gamete, AB, Ab, aB and ab, in equal numbers. Independent segregation of the two chromosome pairs gives every combination.
3
What phenotypic ratio is expected when two individuals heterozygous for two unlinked genes are crossed?
Model answer
The expected ratio is 9 : 3 : 3 : 1. Nine show both dominant features, three and three show one dominant feature each, and one shows both recessive features.
4
What ratio is expected when AaBb is crossed with aabb, and why?
Model answer
A ratio of 1 : 1 : 1 : 1 is expected. The aabb parent gives only ab gametes, so the four gametes of the AaBb parent each show directly in the offspring.
5
What is autosomal linkage?
Model answer
Autosomal linkage occurs when two genes lie close together on the same non-sex chromosome. The alleles tend to be inherited together rather than independently.
6
How does autosomal linkage change the results of a dihybrid test cross?
Model answer
Most offspring show the parental combinations of features, and few show new combinations. The expected 1 : 1 : 1 : 1 ratio is not obtained.

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