Inheritance (A Level only) (AQA A Level Biology)

Aubergine (Solanum melongena L.) belongs to the same family as potatoes and tomatoes. They are a widely grown vegetable crop across Southeast Asia, Africa and the Mediterranean. Due to their importance as a food crop, scientists have been studying the inheritance of two genes (stem prickliness and fruit shape) that can have an influence on the profitability of the crop.  

The scientists examined 3000 offspring produced from the crosses between parent plants heterozygous for both genes. The allele P, for a non-prickly stem, is dominant to the allele p, for a prickly stem. The allele R, for round fruit, is dominant to the allele r, for linear fruit. 

Complete Table 1 below to show the corresponding phenotype or genotype.

Table 1

Complete Table 2 to show the expected ratio for each phenotype if the genes were located on different homologous pairs of chromosomes.

Table 2

In Table 3 are the phenotypes and number of offspring observed by the scientists.  

Table 3

Suggest two reasons why the observed ratios do not reflect the scientists expected ratios.

Draw a pair of chromosomes below to show the genotype that would obtain the observed numbers of offspring for each of the phenotypic ratios in Table 3.

In summer squash, two genes (A and B) interact to form the colour of the fruit. Figure 1 shows a drawing of a summer squash.

         Figure 1

         Summer Squash

Gene A controls whether the fruit has pigmentation or not. The dominant allele of this gene, A, results in no pigmentation, the squash are white. The recessive allele, a, results in pigmentation to the fruit. Gene B controls which pigmentation (yellow or green) the fruit has. The dominant allele, B, causes the fruit to have yellow pigmentation and the recessive allele, b, results in the fruit having green pigmentation. This gene has no effect on squash that have no pigmentation; white squash do not have any yellow or green pigmentation, even if they have the dominant B allele.

Scientists performed 4000 crosses in which white squash, heterozygous for both genes were crossed with yellow squash, heterozygous for gene B. They expected white, yellow and green squash in the offspring in a 4 : 3 : 1 phenotypic ratio.

Complete the genetic diagram to show how this ratio of phenotypes would be produced.

The actual numbers of offspring with each phenotype were

The scientists carried out a   test on the data collected. Suggest the null hypothesis that was tested. 

Complete the table to calculate the value of 2for these results.

The table shows values for ² 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. 

Hypophosphatemia is a sex-linked inherited condition which results in abnormally low levels of phosphate in the blood which can cause the disease rickets. It is caused by a dominant allele.

Figure 1 shows the inheritance of hypophosphatemia in one family.

Figure 1

State the evidence from Figure 1 that suggests that hypophosphatemia is a sex-linked, dominant inherited disease.

Using the following symbols,

X^H = an X chromosome carrying the allele for hypophosphatemia 

X^h = an X chromosome carrying the normal allele

Y = a Y chromosome

Give all the possible genotypes of each of the following persons,

 1  :

 4  :

 5  :

13 :

Person 20 is pregnant for the fourth time. As the family has a history of hypophosphatemia, a test was carried out to discover the sex of the embryo.

Explain how observation of the chromosomes from an embryo cell could enable the sex to be determined.

State the probability that the child Person 20 is pregnant with will be a male with hypophosphatemia. Explain your answer by drawing a genetic diagram, using the following symbols

X^H = an X chromosome carrying the allele for hypophosphatemia 

X^h = an X chromosome carrying the normal allele

Y = a Y chromosome

A horticulturist investigated the monohybrid inheritance of flower colour in snapdragons. Two snapdragons with pink flowers were crossed. Of the offspring, 230 had pink flowers, 86 had red flowers and 84 had white flowers.

Using suitable symbols, give the genotypes of the parents. Explain your answer.

When the horticulturist undertook the monohybrid cross, they had observed pink snapdragon flowers and white snapdragon flowers in their greenhouses. Therefore the ratio of flower colours obtained was unexpected. State and explain the expected ratio.

As the horticulturist discovered the snapdragons were codominant for the flower colour gene, the phenotypic ratio was expected to be 1 white : 2 pink : 1 red. Suggest two reasons why the actual ratios observed are not the same as the expected ratios.

To determine if the results obtained occurred due to chance the horticulturist undertook a statistical test. State which test was used and give the null hypothesis.

When most people cross their legs, they either always have their right leg on top, R, or always have their left leg on top, L. A scientist investigated this characteristic on four small islands, M, N, O and P. Their results are shown in Figure 1.

Figure 1

On one of the islands, they recorded the leg-folding characteristics of parents and their children. These results are shown in Table 1.

 Table 1

The scientist concluded that leg-folding is not determined by a single gene with a dominant allele and a recessive allele. Using information from Figure 1, describe the results the scientist obtained.

Suggest why it was advantageous for the scientist to use island populations in this investigation.

The scientist concluded that leg-folding is not determined by a single gene with a dominant allele and a recessive allele.

Use information from Table 1 to explain why they reached this conclusion.

In another study, the scientist investigated leg-folding in genetically identical twins. Data from this study supported their conclusion from the island study.

Suggest the evidence they found that supported their conclusion.

Fur colour in rabbits is controlled by a gene inherited on the X chromosome. Some rabbits can be either brown or white. Female rabbits can be described as ‘roan’ if they have patches of brown and white fur. 

Explain why male rabbits cannot be roan. 

A roan female rabbit is crossed with a white male rabbit. 

Use a genetic diagram to state the ratio of phenotypes expected in the offspring of this cross.

The effects of the fur colour alleles can be further modified by a second gene which is not sex-linked. The allele r changes the brown colour to tan, and white colour to grey. Allele R has no effect on fur colour in rabbits. 

A grey coloured male rabbit was crossed with a roan female who is heterozygous for the second colour modifying gene. 

State the genotypes of both parents. 

A grey coloured male rabbit was crossed with a roan female who is heterozygous for the second colour modifying gene. 

Use a genetic diagram and your answers to part c), to show the ratio of expected  phenotypes of the male offspring of this cross.

An investigation on fruit flies, Drosophila melanogaster, was carried out to determine the relationship between body colour and wing length. Scientists found the genes controlling these characteristics are inherited on different autosomal chromosomes. Fruit flies are either black or grey and have either long or short wings. 

Two homozygous fruit flies were crossed: a fruit fly with a black body and long wings was crossed with a grey fruit fly with short wings. All of the offspring produced had black bodies with long wings. 

Using this information and a genetic diagram, show how these offspring were produced. Use B/b and L/l to represent the alleles. 

The offspring from part a) were crossed with grey bodied fruit flies with short wings. Use a genetic diagram to show the expected ratios of the phenotypes expected from this cross.

The scientists determined the offspring phenotypes from the cross in part b) and the data collected is shown in Table 1. 

Table 1

Table 1 showing the observed number of offspring produced for each phenotype

Complete Table 1 to show the expected number of offspring for each phenotype.

The scientists claimed that independent segregation had taken place. Chi² can be used to determine the significance of the data. The critical value for this data is 7.82. Use the following formula to calculate the value of Chi squared (χ²) and comment on the scientists claim. 

A scientist crossed a grey male cat with a grey female cat multiple times. There were 12 kittens born of which 8 were grey and 4 were white. The trait is not sex-linked. 

Explain how the information provides evidence that white fur is recessive. 

Two cats were crossed, producing 11 kittens. Five were grey and six were white. Use this information to determine the genotype of the parents.

There are three alleles that control fur colour in cats. The allele G gives grey fur, g gives black fur, and the allele gi gives ginger fur. Allele G is dominant to both of the other alleles, and gi is recessive to g. 

Scientists crossed a black male several times with a grey female. The offspring were as follows:

• 10 grey kittens

• 3 black kittens

• 4 ginger kittens

Draw a genetic diagram to show how these offspring were produced.

Scientists expected an equal number of black and ginger kittens. Explain why the numbers of actual black and ginger kittens are different to expected. 

Muscular dystrophy is a recessive genetic condition that causes muscles to weaken over time. There are multiple forms of muscular dystrophy that can be inherited. One of these forms is inherited on the X chromosome. The dominant allele results in normal muscle formation. 

Figure 1 shows the pattern of inheritance of these alleles in one family. 

Figure 1

Use evidence from the diagram to explain that muscular dystrophy is caused by the recessive allele and inherited on the X chromosome.  

Use a genetic diagram to show the probability of the next child born to parents 5 and 6 of having muscular dystrophy.  

Males are more likely than females to express the phenotype for muscular dystrophy.

Explain why. 

Individual 3 and a heterozygous female had a female child who had muscular dystrophy.

Explain how this could happen.

Drosophila melanogaster is a fruit fly and is a useful organism to study patterns of inheritance. 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 study patterns of inheritance.

In Drosophila melanogaster the genes for eye colour and antenna shape are linked.

Explain what this means. 

Scientists investigated patterns of inheritance of eye colour and antenna shape in fruit flies. They carried out multiple crosses between fruit flies homozygous for both red eyes and bent antennae, and fruit flies homozygous for both white eyes and straight antennae. They found that all of the offspring produced in the F1 generation were heterozygous for both characteristics, having red eyes and bent antennae.

• R represents the dominant allele for red eyes and r represents the recessive allele for white eyes

• B represents the dominant allele for bent antenna and b represents recessive allele for straight antennae

The offspring from the F1 generation were crossed with fruit flies homozygous for both white eyes and straight antennae.

The results of these crosses are shown below in Table 1. 

Table 1

Table 1 shows the number of offspring produced for each phenotype

Explain the results from Table 1.

To determine if there is a significant difference between the observed results and expected results a statistical test can be used. State which statistical test should be used and explain your reasoning.