Assuming a single, diallelic gene, coding for antennae length, the offspring end up with traits that are different from the traits of their parents because the mode of inheritance is by incomplete dominance.
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Available data:
We will assume that this trait is coded by a single, autosomic, diallelic gene.
The expression of three phenotypes suggests that the mode of inheritance is by incomplete dominance.
Incomplete dominance refers to the cases in which the heter0zyg0us individual expresses an intermediate phenotype between the h0m0zyg0us dominant and the recessive phenotypes. Neither of the involved alleles completely dominates over the other one.
In this example the expression of the trait would be as follows,
Genotype Phenotype
AA Long anteannae
Aa Medium length antennae
aa Short antennae
According to this information, the cross would be
Cross: Between two medium length antennae individuals
Parentals) Aa x Aa
Gametes) A a A a
Punnett square) A a
A AA Aa
a Aa aa
F1) 1/4 = 25% of the progeny is expected to be h0m0zyg0us dominant expressing long antennae
2/4 = 1/2 = 50% of the progeny is expected to be heter0zyg0us expressing the medium length phenotype
1/4 = 25% of the progeny is expected to be h0m0zyg0us recessive expressing short antennae.
So, while the parents were heter0zyg0us for the trait, Aa, one of the offspring was h0m0zyg0us dominant, AA, and the other one h0m0zyg0us recessive, aa.
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Genes are the hereditary units, which carry information from the parent to offspring. The variant form of the gene, an allele codes for different traits, but is present at the same locus of the chromosome.
Given that:
The phenotypes of crawfish are long, medium, and short.
Parents have medium antennae.
The progeny have long and short antennae.
Let us assume the genotype as:
Let allele A be dominant that codes for long antennae.
Allele a is recessive and codes for short antennae.
The expression of three phenotypes suggests that the alleles are inherited through incomplete dominance.
Incomplete dominance is the phenomenon in which both the alleles will be expressed incompletely. The alleles for long and short will not be expressed properly, leading to medium-length antennae.
The expression can be given as:
AA - Long antennae
Aa - Medium Length antennae
aa - Short Length antennae
The cross for the given alleles is given in the attachment below.
Thus, F1 progeny will have 25 % purebred dominant for long antennae, 50% of the progeny will have heterozygous medium antennae, and 25% of the progeny will have purebred recessive short antennae.
To know more about the cross, refer to the following link:
