Section 7 - Unit 17: Inherited change

Question 1

Explain what is meant by the term phenotype (2 marks)

Answer 1

• Characteristic due to genotype

- Characteristic due to environment

Question 2

In fruit flies, the genes for body colour and wing length are linked. Explain what this means (1 mark)

Answer 2

• Genes at same loci

Question 3

Explain what is meant by codominant alleles (1 mark)

Answer 3

• Both alleles are expressed in the phenotype

Question 4

Explain why recessive, sex-linked characteristics are more common in human males than females (2 marks)

Answer 4

• Males have XY chromosomes

- Recessive alleles are always expressed in males

Question 5

Explain what is meant by a dominant allele (1 mark)

Answer 5

• Is always expressed / shown

Question 6

Explain what is meant by a recessive allele (1 mark)

Answer 6

• Only expressed when two alleles are present

Question 7

State the null hypothesis for chi squared (1 mark)

Answer 7

• There is no significant difference between observed and expected data, the difference is due to chance

Question 8

How do multiple alleles of a gene arise (2 marks)

Answer 8

• Mutations

- At different positions in the gene

Question 9

Explain why during meiosis, when the X and Y chromosomes pair up, they do not form a typical bivalents like other chromosomes (2 marks)

Answer 9

• X and Y chromosomes are different sizes / shapes

- Chromatids unable to line up and form bivalent

Question 10

What is meant by a sex-linked gene (1 mark)

Answer 10

• Gene located on one sex chromosome

Question 11

Suggest two reasons why in genetic crosses, the observed phenotypic ratios obtained in the offspring are often box not the same as the expected ratios (2 marks)

Answer 11

• Fertilisation of gametes is random
• Linked Genes
• Epistasis

Question 12

Ratio for dihybrid linkage (1 mark)

Answer 12

9:3:3:1

Question 13

Ratio for autosomal linkage (1 mark)

Answer 13

3:1