In one kind of cattle, coat colour is set by a single gene. The gene has two alleles. The allele for a black coat is dominant. We show it as B. The allele f...
Question 1 Report
In one kind of cattle, coat colour is set by a single gene. The gene has two alleles. The allele for a black coat is dominant. We show it as B. The allele for a red coat is recessive. We show it as b. A farmer kept a herd of cattle. The farmer wanted many black calves. Black cattle sold for more money. The farmer crossed a black bull with red cows. Some calves were black. Some calves were red. This surprised the farmer. The farmer thought a black bull would give only black calves. A student used a genetic diagram to explain the result.
(a) State what is meant by the term recessive allele. [1]
(b) State the genotype of a red cow. [1]
(c) The black bull produced both black and red calves when crossed with red cows. State the genotype of the black bull and explain your answer. [2]
(d) Draw a genetic diagram to show this cross and the expected ratio of black to red calves. [4]
(e) Explain why the actual numbers of black and red calves may not be exactly in the expected ratio. [2]
Answer Details
(a) A recessive allele is one whose characteristic is only shown in the phenotype when two copies are present (i.e. when no dominant allele is present) [1]. Here a red coat only appears in a bb animal.
(b) Red is recessive, so a red cow must carry two red alleles: its genotype is bb [1].
(c) The black bull must be Bb (heterozygous) [1]. This is because when he was crossed with red (bb) cows some calves were red (bb); a red calf can only inherit a b allele from each parent, so the bull must carry a b allele as well as the B allele that makes him black [1]. A pure-breeding BB bull crossed with bb cows would give only black (Bb) calves.
(d) The genetic diagram for the cross Bb (black bull) x bb (red cow) is set out below [parents 1; gametes 1; offspring genotypes 1; ratio 1]:
| Black bull | Red cow | |
|---|---|---|
| Parent phenotypes | black | red |
| Parent genotypes | Bb | bb |
| Gametes | B or b | b or b |
Punnett square (bull's gametes across the top, cow's gametes down the side):
| B | b | |
|---|---|---|
| b | Bb (black) | bb (red) |
| b | Bb (black) | bb (red) |
Offspring genotypes are Bb, Bb, bb, bb, giving an expected ratio of 1 black : 1 red \(( = 2\ \text{Bb} : 2\ \text{bb})\) [ratio 1].
(e) The 1:1 ratio is only a prediction of probabilities. Fertilisation is random: which sperm (carrying B or b) fuses with each egg is a matter of chance [1]. So with a real, especially small, number of calves the actual numbers vary from the predicted ratio; the ratio only approaches 1:1 when very many offspring are produced [1].
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