inheritance

Question 1

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

Answer 1

1. Small sample size;
2. Fusion/fertilisation of gametes is random;
   Ignore breeding is random
3. Linked Genes;
   Accept crossing over / sex linkage
4. Epistasis;
5. Lethal genotypes;

Question 2

Lactose is the main sugar in milk and is hydrolysed by the enzyme lactase. Lactase is essential to newborn mammals as milk is their only source of food. Most mammals stop producing lactase when they start feeding on other food sources. Humans are an exception to this because some continue to produce lactase as adults. The ability to continue producing lactase is known as lactase persistence (LP) and is controlled by a dominant allele. A number of hypotheses based on different selection pressures have been put forward to explain LP in humans.
(a) One hypothesis for LP in humans suggests that the selective pressure was related to some human populations farming cattle as a source of milk.
Describe how farming cattle as a source of milk could have led to an increase in LP.

Answer 2

1. LP due to mutation
   OR
   Allele due to mutation;
   Reject mutation caused by drinking milk.
   Reject (LP) gene
2. Milk provides named nutrient;
   Accept any correct named nutrient e.g. glucose, galactose, protein
   Ignore ‘sugar’ ‘lactose’ as named nutrient
3. Individuals with LP more likely to survive and reproduce
   OR
   Individuals with advantageous allele more likely to survive and reproduce;
   Reject (LP) gene
   Accept ‘individuals who produce lactase’ for ‘LP individuals’
   Accept ‘pass on allele/LP/characteristic’ for reproduce.
4. Directional selection;
5. Frequency of allele increases (in the offspring/next generation);

Question 3

Use the information provided to explain why the number of people showing LP would rapidly increase once selection for this condition had been established.

Answer 3

1. Dominant allele;
2. (Always) expressed/shown (when present in phenotype/offspring)
   OR
   Expressed when only one (dominant allele) present;

Question 4

Lactase persistence is caused by a mutation in DNA. This mutation does not occur in the gene coding for lactase.
Suggest and explain how this mutation causes LP.

Answer 4

Mutation in promoter (DNA/gene) for transcription factor
OR
Mutation in promoter (region/DNA) for the gene
OR
Mutation in gene for transcription factor;
Accept mutation in an epistatic gene
2. Lactase gene continues to be transcribed/active;

Question 5

What is meant by a genome?

Answer 5

(All) the DNA in a cell/organism

Question 6

Explain why the antibody binds to the transcription factor.

Answer 6

1. (Transcriptional factor/antibody) has a specific/tertiary structure/shape;
   Accept (antibody) has a specific variable region
   Accept (transcription factor/antibody) has a specific binding site
   Reject active site but only once.
2. Complementary (shape/structure);

Question 7

What is meant by the term phenotype?

Answer 7

1. (Expression / appearance / characteristic due to) genetic constitution / genotype / allele(s);
2. (Expression / appearance / characteristic due to) environment;

Question 8

What is a gene pool?

Answer 8

All the alleles in a population;