10.2 Inheritance

When investigating variation scientists often study sets of twins, both identical and non-identical. 

Suggest the advantages of studying twins when investigating variation.

A scientific study in 2013 investigated whether academic achievement was influenced by genetics or the environment. 

11,117 pairs of twins participated in the study, which found that around 55% of the outcomes in core GCSE subjects (English, maths, and science) were explained by genetic influence, 25% by shared environmental influences, such as parental support, and the remaining 20% by environmental influences that were not shared between the two twins, such as teacher quality and class grouping. 

Using this information, evaluate the statement "intelligence is caused by genetics". 

Twin studies tend to be run using two main assumptions:

• That the identical twins in the study share 100% of their DNA
• That all twins are raised in exactly equal environments at home

Suggest whether it is correct to make these assumptions about twin studies.

The diagram below shows two homologous chromosomes of the fruit fly Drosophila melanogaster. 

The white regions are the loci of seven genes involved in different phenotypic traits. 

The letters A-F and a-f represent the alleles present at each locus. 

Discuss the relative chances of this fly's gametes containing these combinations of alleles:

• A and F
• A and f
• c and d
• c and D

In a potential scenario where crossing over does not occur between the two genes, the fly in part (a) reproduces with another fly who is homozygous for the dominant A and B alleles. 

Predict the genotypes of the offspring, with relation to these two genes. 

Drosophila melanogaster is a useful organism to use in studies on inheritance patterns. Female fruit flies can lay up to 400 eggs, developing into adults between 7 and 14 days. They have simple nutrient requirements.

Using the information provided, explain two reasons why Drosophila melanogaster is a useful organism to use in studies of inheritance patterns

Mendel studied many characteristics of pea plants during his time investigating inheritance. Two such characteristics were seed colour and flower colour. Seed colour can either be yellow, encoded by the dominant allele Y, or green encoded by the recessive allele y. Flower colour can either be purple, encoded by the dominant allele P, or white encoded by the recessive allele p. 

A student is unsure whether the two genes are linked. In order to try and work it out they decided to do some breeding experiments. 

Outline a method they could use to determine whether the two genes are linked. 

As part of this process, the student bred together two plants that were heterozygous for both traits. 

Draw a genetic diagram of the expected results from this cross, using the correct notation as if the two genes are located on the same chromosome. 

On the genetic diagram you produced in part (b), highlight all the recombinants. 

Upon further research, the student found that the two genes are located on the same chromosome, chromosome one of the pea plant. 

When Mendel was researching these traits he found that all of his crosses produced a 9:3:3:1 ratio of phenotypes, and there was no evidence of any 'non-Mendelian' inheritance ratios. 

Suggest a possible reason for this. 

One mark is available for clarity of communication throughout this question. 

When looking at buildings built over 100 years ago it is often a notable feature that the door frames are a lot shorter than modern day doors. 

A student observed this and made the conclusion that humans must have evolved to become taller over the years since the door frames were built. 

Suggest some reasons explaining why the student is most likely incorrect with their conclusion.

Describe how Morgan and his associates were able to produce a genetic map of a chromosome using breeding experiments in Drosophila. 

Pollen from a pure-bred tomato plant with white flowers and yellow fruit was transferred to the stigmas of a pure-bred plant with yellow flowers and red fruit. All the F₁ generation had yellow flowers and red fruit. 

Pollen from the F₁ generation was transferred to pure-bred plants with white flowers and yellow fruit. The ratio of phenotypes expected among the offspring of a dihybrid test cross such as this is 1:1:1:1.

Seeds from the plants were collected and grown, giving plants with the following phenotypes:

• Yellow flowers and red fruit: 59
• Yellow flowers and yellow fruit: 56
• White flowers and red fruit: 40
• White flowers and yellow fruit: 45
• Total number of plants in the four categories: 200

A chi-squared (χ²) test can be carried out to check whether the numbers of each phenotype of offspring resulting from the test cross are in agreement with a 1:1:1:1 ratio. 

The chi-squared (χ²) equation and distribution is as follows:

Use this information to produce a statistical conclusion about the difference between the expected and actual results in the tomato breeding experiment.

You must show your working in your answer.

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.

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.

Calculate the value of χ² by completing the following table:

Phenotype of offspring	Observed (O)	Expected (E)
Grey body, long wings	1 650
Grey body, vestigial wings	600
Ebony body, long wings	690
Ebony body, vestigial wings	260

=

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.

One mark is available for clarity of communication throughout this question.

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.