Question

You own 2 fish. The 1sthas Wild Type scales. The 2nd has no scales – a Phenotype called Naked. Your Wild Type fish is True Breeding, but you are unsure about your Naked fish? If you cross these fish you observe the following among the F1: ½ Linear & ½ Wild Type. If you cross 2 Linear fish from the F1 you observe the following: 56% Linear, 25% Naked, & 19% Wild Type.

Explain this pattern of Inheritance? What are the Genotypes of the P generation fish?

-What would you observe if you crossed an F1 Linear x F1 Wild Type fish? and also 2 F1 Wild Type fish?

Answer 1

Since it is known from the question that the wild type is true breeding, we can assign the genotype as SS (for scales-wildtype). We will assign "s"allele to naked phenotype. Since we don't know if it is heterozygous or homozygous we shall assign s_ as the genotype of the naked phenotype.

In the first cross SS ^_X s__

Progeny: 50% "Linear" (new phenotype) and 50% wild type (parental).

This shows that the trait may be controlled by three alleles or it may be the case of incomplete dominance.

However, since it is provided that crossing two Linear phenotype fish leads to a wild type phenotype also, it is likely this is a case of incomplete dominance.

  "Linear" self cross: Ss  x Ss  

Progeny: 56% Linear (Ss), 25% Naked (ss), & 19% Wild Type (SS)

If we do a chi-square goodness of fit test for the phenotypic ratio of 1:2:1 (which is the case for incomplete dominance), these results fit the ratio.

Hence, we can now assign the genotype of the "naked" parent as ss. It is seen that the "S" (wildtype) scales allele is incompletely dominant over the s allele (naked), and hence a third phenotype (linear) appears.

If we cross F1 Linear (Ss) x F1 Wild Type fish (SS) we would get 50% wild type (SS) and 50% Linear fish (Ss).

If we cross 2 F1 Wild Type fish: SS x SS we would get all (100%) wildtype scaled fish (SS).