a.
To determine:
The resulting progeny that will be obtained from the pure breeding line of a sinistral hermaphrodite with the males of pure breeding dextral line.
Introduction:
The Punnett square is the tool used in genetics to represent the gametes and the progeny that will be obtained from the cross of the parental gametes. The square is expressed as gametes on one side, and the squares are filled with the possible progeny.
Explanation of Solution
The genotypic expression for the homozygous dextral snails can be taken as “S+S+”, and the genotypic composition of the homozygous hermaphrodite sinistral snails can be taken as “S-S-“. When a pure breeding line of the snails Limnaea that are sinistral are crossed with the pure breeding dextral snails the resultant progeny will be as follows:
Gametes |
S+ |
S+ |
S- |
S+S- (heterozygous dextral) |
S+S- (heterozygous dextral) |
S- |
S+S- (heterozygous dextral) |
S+S- (heterozygous dextral) |
b.
To determine:
The resulting progeny that will be obtained from the pure breeding line of dextral hermaphrodite with the males of pure breeding sinistral line.
Introduction:
The genotype is the genetic constitution of the organism, while the
Explanation of Solution
The genotypic expression for the homozygous hermaphrodite dextral snails can be taken as “S+S+” and the genotypic composition of the homozygous sinistral male snails can be taken as “S-S-“. When a pure dextral hermaphrodite is crossed with the male pure sinistral line the progeny obtained will be as follows:
Gametes |
S+ |
S+ |
S- |
S+S- (heterozygous dextral) |
S+S- (heterozygous dextral) |
S- |
S+S- (heterozygous dextral) |
S+S- (heterozygous dextral) |
c.
To determine:
The resulting progeny from the self-cross made between the dextral and sinistral snails.
Introduction:
The alleles are inherited from the parents into the offspring. The genotypic ratio represents the alleles that constitute the offspring, while the phenotypic ratio represents the external features of the offsprings.
Explanation of Solution
The progeny obtained from either of the crosses that is the cross between dextral hermaphrodite and sinistral male or sinistral hermaphrodite and dextral male is the same that is heterozygous dextral. Hence the self-cross of the heterozygous dextral progeny are as follows:
Gametes |
S+ |
S- |
S+ |
S+S+ (homozygous dextral) |
S+S- (heterozygous dextral) |
S- |
S+S- (heterozygous dextral) |
S-S- (homozygous sinistral) |
d.
To determine:
The resulting progeny that will be obtained in the F3 generation from the self-cross of the dextral and sinistral snails.
Introduction:
The dominant allele masks the expression of the recessive allele. Therefore the dominant allele is expressed in homozygous and heterozygous genotype while the recessive phenotype is only expressed under homozygous condition.
Explanation of Solution
The resulting F2 generation has homozygous sinistral snails with the genotype “S-S-“. After self-crossing the F2 progeny, the organisms obtained will be of the following genotype:
Gametes |
S- |
S- |
S- |
S-S- (homozygous sinistral) |
S-S- (homozygous sinistral) |
S- |
S-S- (homozygous sinistral) |
S-S- (homozygous sinistral) |
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Chapter 18 Solutions
EBK GENETICS: FROM GENES TO GENOMES
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