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Describe the Control of Gene Expression

Question 1: Describe the control of gene expression

In the Andalusian fowl, the allele for black feathers (B) is incompletely dominant to that for white feathers (b) and the phenotype of the heterozygote is described as blue. The texture of the feathers is controlled by a second gene locus, silky feathers (f) being recessive to normal (F).

c. What is a heterozygote?
c. What phenotype results from Ff?

Crosses were made among blue birds which are heterozygous at the silky locus.

c. What phenotypic ratio would be expected for the offspring? (Show your working very clearly).

Expected phenotype ratio:

c. What does the term gene locus refer to?
c. Describe what linked genes are?
c. How are two genes recognised as being linked?
c. Using diagrams and words, explain how crossing over can separate linked genes and lead to new combinations of alleles, and consequently new combinations of phenotypes.

Words (Write about six- eight lines here):

Question 2: Describe the control of gene expression

The table below shows the effect of two genes in providing resistance to powdery mildew disease in peach trees.

Note that the observed phenotypic ratio did not agree with the expected phenotypic ratio.

F2 Classes
F2 Expected Phenotypic ratio
Reaction to powdery mildew disease


Highly resistant (HR)


Highly resistant (HR)


Fairly resistant (FR)


Susceptible (S)

Observed phenotypic ratio

12 HR : 3 FR : 1S

a. State one genotype represented by T_H_

b. If plants with the genotypes TThh and ttHH were crossed, what proportion of their offspring would be expected to be fairly resistant to powdery mildew disease? (Show your working clearly).

Proportion fairly resistant to powdery mildew:

c. How does the phenotypic effect of allele T differ from the effect of allele H?


Solution Summary

This solution-guide offers information on heterozygotes, phenotype results, the expected phenotypic ratio, gene locus, recognizing linked genes, genotype and the phenotypic effect in an attached Word document.