Genetic terms and monohybrid inheritanceAQA A-Level Biology: Subtopic test

1. 1
   In shorthorn cattle, coat colour is controlled by a single gene with two alleles, $C^R$ (red hairs) and $C^W$ (white hairs). The alleles are codominant. Animals with both alleles have a roan coat, with a mixture of red and white hairs.

   (a)

   What is the genotype of a roan animal?

   [1 mark]

   • A$C^RC^R$
   • B$C^WC^W$
   • C$C^RC^W$
   • D$C^RC^RC^W$

   (b)

   Two roan cattle are crossed. What proportion of their calves is expected to be roan?

   [1 mark]

   • A25%
   • B50%
   • C75%
   • D100%

   (c)

   A red bull is crossed with a white cow. Predict the phenotype of the calves and explain your answer.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Cystic fibrosis is caused by a recessive allele, $f$, of a gene on chromosome 7. The dominant allele, $F$, codes for a functioning chloride ion channel. Two healthy parents, who have both already had one child with cystic fibrosis, plan to have more children.

   (a)

   What is the probability that the next child of this couple has cystic fibrosis?

   [1 mark]

   • A0
   • B1/2
   • C3/4
   • D1/4

   (b)

   Which term describes the genotype of each parent at this locus?

   [1 mark]

   • AHeterozygous
   • BHomozygous dominant
   • CHomozygous recessive
   • DCodominant

   (c)

   The couple go on to have two more children. Calculate the probability that both children have cystic fibrosis.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   In pea plants, tall stems are produced by the dominant allele $T$ and dwarf stems by the recessive allele $t$. A plant breeder crosses a tall plant of unknown genotype with a dwarf plant. The offspring are 48 tall plants and 52 dwarf plants.

   (a)

   Deduce the genotype of the tall parent and explain how you reached your answer.

   [3 marks]

   (b)

   Complete a fully labelled genetic diagram for this cross, showing the parental phenotypes and genotypes, the gametes, the genotypes of the offspring and their phenotypic ratio.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   In humans, the gene for the beta chain of haemoglobin has two alleles, $Hb^A$ and $Hb^S$. Both alleles are expressed in heterozygous people, who make both types of haemoglobin and are healthy carriers. People who are $Hb^SHb^S$ have sickle cell anaemia. A genetic counsellor advises a couple who are both carriers.

   (a)

   Complete a fully labelled genetic diagram for a cross between two carriers. Use it to state the probability that their child (i) has sickle cell anaemia, and (ii) is a carrier. Explain why a carrier shows a different phenotype from a person who is $Hb^AHb^A$.

   [6 marks]

   (b)

   The couple want two children. Calculate the probability that (i) neither child has sickle cell anaemia, (ii) exactly one child has sickle cell anaemia, and (iii) both children have it. Show that your answers are consistent.

   [6 marks]

   Total for question 4: 12 marks

End of questions