Cambridge A Level Biology A2 Genetic Diagrams Questions

[Maximum number: 10]

The tortoise beetle, Chelymorpha alternans, is an insect found in Panama that has several different colour patterns. Fig. 3.1 shows a tortoise beetle.

Fig. 3.1

Researchers have identified a gene, L, that controls colour pattern in the pronotum and elytra. Gene L has four different alleles: $\mathbf{L}^{\mathbf{V}}, \mathbf{L}^{\mathbf{T}}, \mathbf{L}^{\mathbf{R}}$ and $\mathbf{L}^{\mathbf{r}}$ .

Table 3.1 shows five different colour pattern phenotypes of tortoise beetles and their genotypes.

Table 3.1

Figure for Question CAIE A-Level Biology A2 16.2.2 Genetic Diagrams Questions question 1 — CAIE A-Level Biology A2

(a)

Explain why the inheritance of colour pattern in tortoise beetles can be described as involving multiple alleles.

[ 1 ]

(gene L has) four alleles / $\mathbf{L}^{\mathbf{V}}, \mathbf{L}^{\mathbf{T}}, \mathbf{L}^{\mathbf{R}}$ and $\mathbf{L}^{\mathbf{r}}$ ;

A a gene which has more than two alleles

(b)

A tortoise beetle with dfm-b phenotype was crossed with another tortoise beetle with dfm-b phenotype.

Construct a genetic diagram to show the results of this cross, including the ratio of offspring phenotypes.
parental phenotypes:
dfm-b
dfm-b
parental genotypes:
gametes:
ratio of offspring phenotypes:

[ 4 ]

Genetic diagram:
- parental genotypes and gametes are shown in the attached official markscheme figures ;
- offspring genotypes:
$L^T L^T$ , $L^T L^R$ , $L^T L^R$ , $L^R L^R$ ;
- offspring phenotypes linked to genotypes:
dfm-a, dfm-b, dfm-b / rufipennis ;
- ratio of offspring phenotypes: 1 : 2 : 1 ;

Figure for Answer to Question (b) — CAIE A-Level Biology A2

Accept correct phenotype descriptions.

(c)

Colour pattern phenotype involves alleles that show codominance. There is also an order of dominance of alleles (dominance hierarchy).

Use the information in Table 3.1 to:
- identify the codominant alleles
- list the dominance hierarchy with alleles from the most dominant to the least dominant.
codominant alleles
dominance hierarchy

[ 2 ]

$\mathbf{L}^{\boldsymbol{\top}}$ and $\mathbf{L}^{\mathbf{R}}$ ;

$\mathbf{L}^{\boldsymbol{\top}}$ and $\mathbf{L}^{\mathbf{R}} \boldsymbol{>} \mathbf{L}^{\mathbf{V}} \boldsymbol{>} \mathbf{L}^{\mathbf{r}}$ ;

(d)

Researchers carried out two crosses.

Cross 1: female veraguensis tortoise beetles were crossed with male metallic tortoise beetles.
The results are shown in Table 3.2.

Table 3.2

Cross 2: female veraguensis tortoise beetles were crossed with male veraguensis tortoise beetles.

The results are shown in Table 3.3.

Table 3.3

[ 3 ]

(i)

Using Table 3.1, deduce the genotypes of each of the parental beetles used in cross 1 and cross 2.
cross 1
cross 2

[ 2 ]

(female) $\mathbf{L}^{\mathbf{V}} \mathbf{L}^{\mathbf{r}}$ and (male) $\mathbf{L}^{\mathbf{r}} \mathbf{L}^{\mathbf{r}}$ ;

$\mathbf{L}^{\mathbf{V}} \mathbf{L}^{\mathbf{r}}\left(\right.$ and $\left.\mathbf{L}^{\mathbf{V}} \mathbf{L}^{\mathbf{r}}\right)$ ;

(ii)

An assumption was made that female tortoise beetles have X X chromosomes and males have XY chromosomes. Gene L is not located on the X chromosome. It was concluded that colour pattern phenotype followed autosomal inheritance.

Explain how the evidence in Table 3.3 supports this conclusion.

[ 1 ]

CAIE A-Level Biology A2 16.2.2 Genetic Diagrams Questions question 1

Question (a)

Question (b)

Question (c)

Question (d)

Question (i)

Question (ii)