1. 1
   A gardener is investigating stem height in pea plants. The allele for tall stems (T) is dominant over the allele for short stems (t). The gardener crosses a tall pea plant with the genotype Tt with a short pea plant with the genotype tt.

   (a)

   Which term describes an allele that is only expressed in the phenotype when two copies of it are present in the genotype?

   [1 mark]

   • ARecessive
   • BDominant
   • CHomozygous
   • DHeterozygous

   (b)

   The short pea plant used in the cross has the genotype tt. What term describes this genotype?

   [1 mark]

   • AHomozygous dominant
   • BCodominant
   • CHeterozygous
   • DHomozygous recessive

   (c)

   The gardener crosses the tall plant (Tt) with the short plant (tt). Using the alleles T and t, describe the possible genotypes of the offspring and state the ratio of tall to short offspring you would expect.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Cystic fibrosis is an inherited disorder caused by a recessive allele, f. A man and a woman are both unaffected by cystic fibrosis, but genetic testing shows they are both carriers, with the genotype Ff.

   (a)

   What is the probability that a child of this couple will be born with cystic fibrosis?

   [1 mark]

   • A0%
   • B25%
   • C50%
   • D100%

   (b)

   Both parents are described as carriers of cystic fibrosis. What does the term carrier mean in this context?

   [1 mark]

   • AAn individual with two recessive alleles who shows the disorder
   • BAn individual with two dominant alleles
   • CAn individual with one dominant and one recessive allele who does not show the disorder
   • DAn individual whose alleles are unknown

   (c)

   Explain why cystic fibrosis does not appear in the phenotype of either parent, even though they can each pass the recessive allele to their children.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A family is discussing two different inherited characteristics. The first is biological sex, which is determined by one pair of chromosomes. The second is polydactyly (having extra fingers or toes), which is caused by a dominant allele, D, with the normal number of digits caused by the recessive allele, d. In this family, the father has the genotype Dd for polydactyly and the mother has the genotype dd.

   (a)

   Describe how the biological sex of a baby is determined, including the chromosomes involved in the egg and sperm cells and the possible combinations.

   [3 marks]

   (b)

   The father has the genotype Dd for polydactyly and the mother has the genotype dd. Predict the possible genotypes and phenotypes of their children for this characteristic, and state the expected ratio.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Scientists have now studied the entire human genome, which is the entire genetic material of an organism. Researchers are using this genome data to search for genes linked to inherited disease, to understand inherited disorders, and to trace human migration patterns. A change (mutation) in the DNA sequence of a gene can sometimes alter the protein it codes for.

   (a)

   Explain how the structure of DNA allows genetic information to be stored, and explain how a mutation in a gene could affect the phenotype of an organism.

   [6 marks]

   (b)

   Evaluate the potential benefits and risks of studying the human genome to search for genes linked to inherited disease.

   [6 marks]

   Total for question 4: 12 marks

End of questions