Patterns of Inheritance (OCR A Level Biology)

When studying variation, it is sometimes impractical to analyse DNA.

A student was investigating variation between a number of students in their school. They recorded the frequency of students that could and could not roll their tongue.

The results are shown in the table.

Represent the data in the table as a bar chart on the grid provided below.

• the total number of students who could roll their tongue = 171
• the total number of students who could not roll their tongue = 77.

proportion = .......................................................... [3]

The colour of onion bulbs is determined by two genes, A/a and B/b.

• A is a dominant allele and codes for the production of a red pigment.
• Onion bulbs that are homozygous for the recessive allele, a, produce no pigment and are white.
• B is a dominant allele that inhibits the expression of allele A.
• The recessive allele, b, allows the production of the red pigment.

A white onion plant was cross-pollinated with a red onion plant. All 15 offspring had the genotype AaBb.

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[2]

Agammaglobulinemia and Vici syndrome are both genetic diseases.

Agammaglobulinemia results in a lack of mature B lymphocytes in a person’s blood.

Vici syndrome is a genetic disease that shows a recessive inheritance pattern. The allele responsible for Vici syndrome is found on chromosome 18.

• have Vici syndrome
• be carriers of Vici syndrome.

Vici syndrome ...........................
Carriers ........................
[1]

Answer = ................... [1]

.......................................... fragments [2]

The sweet pea plant has been used to study inheritance since the nineteenth century. The seeds of the sweet pea can vary in colour and shape.

The gene that controls colour has two alleles:

• Y is dominant and produces yellow seeds.
• y is recessive and produces green seeds.

The gene that controls shape has two alleles:

• R is dominant and produces round seeds.
• r is recessive and produces wrinkled seeds.

In the nineteenth century, Gregor Mendel crossed a pea plant that was heterozygous for both seed colour and shape with a pea plant that had green and wrinkled seeds.

When Mendel crossed two pea plants that were heterozygous for both seed colour and shape, the ratio of phenotypes in the offspring was:

• 9 yellow round
• 3 green round
• 3 yellow wrinkled
• 1 green wrinkled

Some students tried to recreate this investigation using a modern variety of plant that showed the same phenotypic variation in seed colour and shape.a)

The students crossed two of the modern plants that were heterozygous for both seed colour and shape. The results of this cross were:

• 58 yellow and round
• 31 green and round
• 21 yellow and wrinkled
• 2 green and wrinkled

The students used the chi-squared test to compare their data to the expected 9:3:3:1 ratio.

(i)      Use the chi-squared formula χ² = to calculate the χ² value for these data.

χ² = .............................[3]

Table 17

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The yellow colour in peas is the result of an enzyme that breaks down chlorophyll, which is green.

• The Y allele codes for the production of an enzyme that breaks down chlorophyll.
• The y allele is the result of a mutation in the Y allele.
• The y allele codes for an inactive form of this enzyme.

Tigers, Panthera tigris, are predatory mammals. They have evolved striped patterns on their fur (as shown in Fig. 3.1a), which provide camouflage in their habitats.

Fig 3.1a

One subspecies of tiger is the Bengal tiger. One in 10000 Bengal tiger births results in a white Bengal tiger.

White Bengal tigers (shown in Fig. 3.1b) have black stripes but lack orange fur.

The allele that causes white fur is recessive and is a result of a mutation to a gene called SLC45A2.

According to the Hardy-Weinberg principle, the following equations can be used to estimate allele frequency within a population:

p² + 2pq + q² = 1

p + q = 1

Use the Hardy-Weinberg equations to calculate the percentage of Bengal tigers that are heterozygous for the SLC45A2 gene.

Give your answer to one significant figure.

Show your working.

Answer: .................... %

In domesticated, farmed pigs, the following two traits have been studied:

• The allele for curly tail, T, is dominant to the allele for straight tail, t.
• The allele for pink skin (dermis), D, is dominant to the allele for black skin, d.

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A pig farmer crossed one group of pigs, heterozygous for both traits, with another group homozygous recessive for both traits. The farmer expected to get roughly equal numbers of each of the four possible mixtures of tail and skin phenotype.
The results that actually occurred are shown in Table 17.2.

Table 17.2

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Table 17.3

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