Inheritance and Genetics: Question 3
Syllabus 17.4
In a species of field cricket, wing length is controlled by a single gene with two alleles. The dominant allele, L, produces long wings; the recessive allele, l, produces short (vestigial) wings. Two long-winged crickets, both heterozygous for this gene, are bred together.
(a) State the genotype of each parent cricket, and explain what is meant by the term heterozygous. [2]
(b) Use a genetic diagram (Punnett square) to work out the possible genotypes of the offspring of this cross, and use it to give the expected phenotypic ratio of long-winged to short-winged offspring. [4]
(c) One of the offspring has short wings. State the genotype of this cricket, and explain how two long-winged parents can produce a short-winged offspring. [2]
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Worked solution
Part (a): Parent genotypes and heterozygous
Both parent crickets are long-winged and are stated to be heterozygous for the wing-length gene, so each has the genotype Ll: one dominant allele (L) and one recessive allele (l). “Heterozygous” means an organism has two different alleles of a particular gene, rather than two identical alleles (which would make it homozygous).
Part (b): Punnett square for the cross
Each Ll parent produces two types of gamete, L and l, in equal proportions. Setting out a Punnett square with one parent’s gametes across the top and the other’s down the side:
| L | l | |
|---|---|---|
| L | LL | Ll |
| l | Ll | ll |
This gives four equally likely offspring genotypes: 1 LL : 2 Ll : 1 ll.
LL crickets and Ll crickets both have long wings, because the dominant allele L is present in both. Only the ll crickets have short (vestigial) wings, because they have no dominant allele to mask the recessive allele. Combining LL and Ll as “long-winged” gives an expected phenotypic ratio of 3 long-winged : 1 short-winged.
Part (c): Explaining the short-winged offspring
The short-winged offspring must have the genotype ll. Although both parents are long-winged, each one is heterozygous (Ll) and carries the recessive l allele hidden alongside the dominant L allele. The l allele is not destroyed or lost, it is simply not expressed in the parents’ phenotype because the dominant L allele is present. During reproduction, each parent can pass on either its L allele or its l allele to a given offspring. When, by chance, both parents pass on their l allele to the same offspring, that offspring is ll, has no dominant allele present, and so shows the recessive short-wing phenotype.
Final answers
- (a) Both parents: Ll. Heterozygous means having two different alleles of a gene.
- (b) Offspring genotypes 1 LL : 2 Ll : 1 ll; expected phenotypic ratio 3 long-winged : 1 short-winged.
- (c) Genotype ll; both parents were heterozygous carriers of the hidden recessive l allele, and both happened to pass it on to this offspring.