CAIE A-Level Biology A2 16.2 The Roles of Genes in Determining Phenotype Questions

Practise 16.2 inheritance by interpreting crosses, assigning genotypes from offspring data and linking named alleles to protein effects and phenotype.

Exam points

Use gene, allele, locus, genotype and phenotype terms to interpret inheritance data.
Use Punnett squares to predict genotypes and phenotype ratios in mono- and dihybrid crosses.
Use homozygous-recessive test crosses and offspring phenotypes to determine an unknown genotype.
Infer parental genotypes or gene linkage from offspring ratios and recombinant phenotypes.
Trace TYR, HBB, F8 or HTT alleles through protein changes to their disease phenotypes.
Link Le/le enzyme function to active gibberellin and stem elongation in pea plants.

Covered syllabus points

16.2.1—Core genetic terms 16.2.2—Genetic diagrams 16.2.3—Chi-squared test in genetics 16.2.4—Genes, proteins and phenotype 16.2.5—Gibberellin role in stem elongation

[Maximum number: 2]

Cystic fibrosis is an autosomal recessive genetic disease. People with cystic fibrosis have a homozygous recessive genotype.

Explain the meaning of the terms homozygous and recessive.
homozygous
recessive

[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 2 — 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 ]

[Maximum number: 8]

Genes have a role in determining the overall phenotype of an organism.

(a)

Using haemophilia as an example, explain the relationship between a gene, a protein and a phenotype.

[ 5 ]

Any five from:
1. a gene codes for a protein which determines the phenotype ;
2. F8 gene ;
3. located on X chromosome / sex-linked ;
4. reference to recessive allele ;
5. non-functioning factor VIII protein, or less factor VIII protein, or no factor VIII protein ;
6. so blood does not clot quickly enough / excessive bleeding occurs after an injury ;
7. link the phenotype to the genotype, e.g.:
- $X^F X^F$ = normal female ;
- $X^F X^f$ = normal female ;
- $X^f X^f$ = affected female ;
- $X^F Y$ = normal male ;
- $X^f Y$ = affected male ;
8. AVP, e.g. prevents activation of thrombin / fibrinogen not converted to fibrin / mostly males affected / different mutations cause a range of severity of haemophilia.

(b)

In the pea plant, Pisum sativum, the genotype lele results in plants with short stems (dwarf plants).

Explain how the lele genotype and its gene product results in dwarf plants.

[ 3 ]

any three from:

1 lele is, homozygous recessive / two recessive alleles ;

2 le (allele)/lele, codes for a non-functional enzyme ;

3 no, active gibberellin / $\mathrm{GA}_{1}$

or

ref. to gibberellin inactive ;

4 no / less, cell elongation

or no / less, stem elongation / increase in internodal length ;

5 AVP ;
e.g. ref. to GA $3 \beta$ -hydroxylase / GA3-oxidase / last enzyme in the pathway that produces, active gibberellin / $\mathrm{GA}_{1}$ le allele codes for alanine instead of threonine (in the enzyme)

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Covered syllabus points

Question 1

Question 2

Question (a)

Question (b)

Question (c)

Question (d)

Question (i)

Question (ii)

Question 3

Question (a)

Question (b)