Question

Suppose the ear length of two populations of jerboas is controlled by one gene. To determine the mode of inheritance, a homozygous short‑eared female is crossed with a homozygous long‑eared male. Then, siblings from the F1 are crossed, and the number of short‑ and long‑eared animals are counted to determine the phenotype ratio. Match the phenotypic ratio anticipated in the offspring from a cross of two heterozygous individuals to the appropriate mode of inheritance for the short‑ear allele.

Answer 1

Crossing two heterozygous jerboas for ear length, with short ears as the recessive trait, will result in a F2 phenotypic ratio of 3:1, with three quarters having long ears and one quarter having short ears.

Step-by-step explanation:

When true-breeding or homozygous individuals that differ for a certain trait are crossed, the resulting F1 offspring are all heterozygotes for that trait, exhibiting the phenotype of the homozygous dominant parent. A cross between two heterozygous individuals, known as a monohybrid cross, gives rise to F2 offspring with a phenotypic ratio of 3 dominant to 1 recessive. This ratio, 3:1, indicates a Mendelian pattern of inheritance for a single gene with two alleles - one dominant and one recessive.

If we apply this understanding to the specific case of jerboa ear length, where short ears are recessive and long ears are dominant, crossing two heterozygous jerboas would result in approximately one quarter of the offspring having short ears (homozygous recessive) and three quarters having long ears (homozygous dominant and heterozygous combined).

Answer 2

The short‑ear allele phenotypic ratio is 25%.

Step-by-step explanation:

Genotype for homozygous short‑eared progenitor: ll

Genotype for homozygous long-eared progenitor: LL

Genotype for heterozygous: Ll

P) LL x ll

Gametes) L L l l

F1) LlLl

All of the individuals of the first progeny are long-eared heterozygote.

The cross of two heterozygous individuals:

Ll x Ll

Gametes) L l L l

F2) 25% Homozygote for the long ear: LL

25% Homozygote for the short ear: ll

50% Heterozygote: Ll

75% of progeny are Longed-ear animals

25% of progeny are Shorted-ear animals