10.2 Inheritance

The gene for body colour and antennal shape in the fruit fly (Drosophila melanogaster) are close together on the same chromosome. These genes are therefore said to be linked.

Allele E for a striped body is dominant over allele e for an ebony body. Allele A codes for the dominant normal antennae, whereas allele a codes for an aristopedia antennae. Aristopedia antennae resemble a Drosophila leg rather than an antennae.

A male that is heterozygous for striped body and normal antennae is crossed with female that has an ebony body and aristopedia antennae.

State the possible allele combinations in the gametes of these flies. Use the correct notation in your answer.

State the possible phenotypes of the offspring of this genetic cross, along with the predicted ratios of each.

Explain why it may still be possible for recombinant offspring to appear in this genetic cross, even though the genes are linked.

Describe the idea that Thomas Hunt Morgan proposed to explain the variation observed in the number of recombinants that resulted from crossing linked genes.

Describe the early events that lead scientists to discover exceptions to Mendel’s predicted phenotypic ratios for dihybrid crosses.

Recombinant offspring are those that have a different allele combination to their parents.
 

Explain how test crosses can be used to identify recombinant individuals in offspring.

A set of identical twins were raised in similar conditions by the same parents. One of the twins fell off a swing, which left a permanent scar on his chin.

State the impact this would have on the phenotypic variation between the twins.

Shepherd's purse (Capsella bursa) is a flowering plant which belongs to the mustard family. Fruit shape in this plant is determined by two alleles, namely allele T for a triangular fruit shape, which is dominant over allele t for top-shaped fruit.
A plant with triangular shaped fruit was crossed with a plant that has top-shaped fruit.
All 30 offspring of this cross had triangular shaped fruit.

State, with a reason, whether there can be certainty that the original parent plant with triangular shaped fruit had a genotype of TT.

In fruit flies (Drosophila melanogaster) wing length and body colour are each controlled by a single gene with two alleles. Allele L for long wings is dominant over allele l for vestigial wings, while allele G for grey body colour is dominant over allele g coding for ebony body colour.

Two homozygous fruit flies were crossed, one had a grey body colour and long wings while the other had an ebony body colour and vestigial wings.

State, with a reason, the number of offspring that would display a grey body colour and vestigial wings if 350 offspring were produced from this cross.

Two fruit flies from this cross, heterozygous for both body colour and wing length, were crossed and 3 200 offspring were produced.

Calculate the expected number of offspring that would display the following phenotypes, assuming that the genes for body colour and wing length are not linked.

Phenotype	Expected number of offspring
Grey body, long wings
Grey body, vestigial wings
Ebony body, long wings
Ebony body, vestigial wings

The results for the cross were different from what was expected. Scientists decide to perform a chi-squared test to determine if the difference is significant.

The table shows values for² at different levels of probability and for different degrees of freedom.

State the conclusion the scientists should make about the significance of their results and explain your answer.

Contrast continuous and discontinuous variation.

Explain the use of a chi-squared test on data from a dihybrid cross.

Describe sex linkage and autosomal linkage in genes.