7.1 - Inheritance + 7.2 - Populations

Question 1

In fruit flies, males have the sex chromosomes XY and the females have XX. In fruit flies, a gene for eye colour is carried on the X chromosome. The allele for red eyes, R, is dominant to the allele for white eyes, r. Male fruit flies are more likely than female fruit flies to have white eyes. Explain why. (2)

Answer 1

• Males have one allele
  ‘accept males only need one allele’
• Females need two recessive alleles / Females must be homozygous recessive /Females could have dominant and recessive alleles / Females could be heterozygous/carriers

Question 2

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

Answer 2

• Small sample size
• Fusion/fertilisation of gametes is random
• Linked Genes
  ‘accept crossing over / sex linkage’
• Epistasis
• Lethal genotypes

Question 3

Give two causes of genetic variation. (2)

Answer 3

• Mutation
• Crossing over
• Independent segregation/assortment (of homologous chromosomes)
  ‘accept independent assortment of alleles’
• Random fusion of gametes / random fertilisation

Question 4

When homozygous red-flowered plants were crossed with homozygous white-flowered plants, all the offspring had pink flowers. Name the relationship between the two alleles that code for flower colour. (1)

Answer 4

• Codominance

Question 5

What is meant by the term phenotype? (2)

Answer 5

• (Expression / appearance / characteristic due to) genetic constitution / genotype / allele(s)
• (Expression / appearance / characteristic due to) environment

Question 6

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

Answer 6

• (Genes / loci) on same chromosome

Question 7

Which statistical test could the scientist use to determine whether his observed results were significantly different from the expected results? Give the reason for your choice of statistical test. (2)

Answer 7

• Chi squared test
• Categorical data

Question 8

Define the term genotype. (1)

Answer 8

• The complete set of genes of an organism / the alleles of a gene possessed by an individual / the alleles that an organism carries on its chromosomes

Question 9

Define the term sex-linked in genetics. (1)

Answer 9

• A gene which is found on the X or Y chromosome / sex chromosome

Question 10

The allele which causes haemophilia is carried on the X-chromosome. Explain why a male can only inherit haemophilia from his mother. (2)

Answer 10

• Males (only) inherit the Y chromosome from their fathers (and the Y chromosome does not contain the haemophilia gene)
• The X chromosome is inherited from the maternal side/mother (and the allele for haemophilia is only found on the X chromosome)

Question 11

What is a gene pool? (1)

Answer 11

• All the alleles in a population
  ‘accept: the number of alleles in a population’

Question 12

State the two Hardy-Weinberg principle equations. (2)

Answer 12

Allele frequency:
p + q = 1
Genotype frequency:
p2+ 2pq + q2 = 1

• p = the frequency of the dominant allele (A)
• q = the frequency of the recessive allele (a)
• p^2 = frequency of homozygous dominant (AA)
• 2pq = frequency of heterozygous (Aa)
• q)^2 = frequency of homozygous recessive (aa)

Question 13

State the equation for chi squared?

Answer 13

∑(O - E) ^2 / E

• O = observed value
• E = expected value