Concept explainers
The Punnett square in Fig. 5.4 on p. 131 shows how Mendel’s dihybrid cross results would have been altered had the two genes (A and B) been linked and had the P generation cross been AB/AB × ab/ab
a. | What would be the frequency of each F2 |
b. | Answer part (a) assuming that the original, P generation cross was Ab/Ab × aB/aB. |
a.
To determine:
The frequency of each F2 phenotypic class if the parents are present at a frequency of 80% in the given question.
Introduction:
Recombination frequency is used to measure the genetic linkage and for the creation of a genetic linkage map. The frequency at which a single chromosome crossover takes place between two genes during meiosis is termed as recombination frequency.
Explanation of Solution
Given:
A_B_ (Parental)
aabb (Parental)
A_bb (Recombinant)
aaB_ (Recombinant)
From the given data, since the parental frequency is 0.80 (80 cM), so the total offsprings (x) can be calculated from the given parental frequency:
Thus, the number of total offsprings is about 78.
Number of recombination progeny is as follows:
The frequency of each phenotypic class is as follows:
b.
To determine:
The frequency of each F2 phenotypic class if the parentas are present at a frequency of 80% in the given question.
Introduction:
The first set of parents crossed is termed as the parental generation (P). All the offsprings from the parents constitute the F1 (first filial) generation. All the offspring obtained when the F1 individuals are interbred constitute the F2 (second filial) generation.
Explanation of Solution
The cross is as follows:
Male/Female gametes | AB | Ab | aB | ab |
AB | AABB | AABb | AaBB | AaBb |
Ab | AABb | AAbb | AaBb | Aabb |
aB | AaBB | AaBb | aaBB | aaBb |
ab | AaBb | Aabb | aaBb | aabb |
A_bb (Parental)
aaB_ (Parental)
A_B_ (recombinant)
aabb (recombinant)
Here, the offspring’s showing A and B gene linkage are more in number. They are about 80% of the F2 phenotype.
Frequency of each phenotypic class is as follows:
This cross produces the same offsprings as the previous cross indicating that A and B genes are linked, a and b genes are linked.
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Chapter 5 Solutions
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