Question

In fish of the species Perissodus microlepis, some individuals have mouths that open to the right and some individuals have mouths that open to the left.The direction of the mouth opening is a genetic trait controlled by a single gene. The allele for aright-opening mouth® is dominant to the allele fora left-opening mouth®.If two fish heterozygous for the mouth trait are crossed, what is the expected ratio of phenotypesin the offspring?

A 1 right-opening mouth : 3 left- opening mouth
B 2 right-opening mouth : 2 left- opening mouth
C 3 right-opening mouth : 1 left- opening mouth
D 4 right-opening mouth : 0 left- opening mouth

Answer 1

C
When crossing two heterozygous individuals for a dominant and recessive gene, the phenotypes (phenotypes!!!) are almost always 3 dominant to 1 recessive.
If ur not sure, make a Puneet square. You'll get.
P p
P PP Pp
P Pp pp

Answer 2

Answer:

C (3 right-opening mouth: 1 left-opening mouth)

Explanation:

This is a typical monohybrid cross involving a single gene coding for direction of moouth opening in Perissodus microlepis fish. According to the question, the allele for right mouth opening is dominant over the allele for left mouth opening i.e. The right mouth opening allele will always mask the expression of the left mouth opening allele in a combined state.

Based on this information, let's assume that R is the single gene coding for this trait. Hence, allele R is for right-opening mouth while allele r is for left-opening mouth.

Since allele R is dominant over allele r, in an heterozygous state (Rr), the fish will have a phenotypical right-opening mouth. Therefore in a cross between two heterozygous (Rr) Perissodus fishes, each will produce gametes (R and r) according to Mendel's law of segregation. These gametes fuse randomly and produce four possible F1 offsprings (see attached image).

According to the cross as depicted on the image, 3 offsprings will have phenotypical right-opening mouth (RR, Rr and Rr) while 1 will have a left-opening mouth (rr).

Phenotypic ratio= 3:1