Question

77.
If only two segregating loci contribute to cob length, and we represent the parental cross as AABB
(9-in average cob length) x aabb (3-in average), the fraction of the F2 expected to be 4.5 in is
b. 1/4
d. 3/16
a. 1/8
C. 1/16
​

Answer 1

Answer:

Recombination fraction

II- Definition of the "lod score" of a family

III- Test for linkage

IV- Estimation of the recombination fraction

V- Recombination fraction for a disease locus and a marker locus

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Investigating the linked segregation of genes situated at different loci is a way of testing the independence of their transmission. This concept of independence is also reflected in the recombination fraction, q, which is the percentage of the gametes transmitted by the parents to be recombined. If they are transmitted independently, there will be the same number of recombined gametes as there are parental gametes, and so q = 1/2. If they are not transmitted independently, then the parenteral gametes are transmitted preferentially to the recombined gametes, and 0 q<1/2. In this case, there is said to be "linkage" between the two loci.

 

I- RECOMBINATION FRACTION

Let us consider the caseof two loci, A and B, with two codominant alleles at each of these loci, A1, A2 and B1, B2 respectively. Such an individual can produce four types of gamete:

A1B1

A2B1

A1B2

A2B2

Two situations are possible:

1- The loci A and B are on different chromosome pairs



Figure 1

In this case, the four gametes all have the same probability: 1/4.

2 —The loci A and B are on the same chromosome pairs

Here we have to distinguish between two possible situations: the alleles A1 and B1 may be on the same chromosome within the pair, in which case A1 and B1 are said to be "coupled"; or they may be on different chromosomes, in which case A1 and B1 are said to be in a state of "repulsion".



Figure 2

For instance, let us suppose that A1 and B1 are "coupled". Four types of gametes are still produced.



Figure3

Gametes A1B1 and A2B2 are said to be "parental". In the offspring, as in the parents, A1 is "coupled" with B1 (and A2 is "coupled" with B2).

The gametes A1B2 and A2 B1 are therefore described as being "recombined". An uneven number of recombination or "crossing-over" phenomena have occurred between the A and B loci.



Figure 4

Assuming that the crossing-over event for a pair of chromosomes follows Poisson’s law, and knowing that a parental gamete has zero or an even number of crossings-over, whereas a recombined gamete has an odd number, we can show that the frequency of recombined gametes is always equal to or lower than that of the parenteral gametes and so

0 q < 1/2

If q = 1/2, then all the gamete types have the same probability and the alleles at the loci A and B loci are transmitted independently. Loci A and B are therefore said not to enhibit genetic linkage. This is the situation if A and B are on different pairs of chromosomes, and also if A and B are one the same pair, but at some distance from each other.

However, if q < 1/2, then the two loci are genetically linked.

For a couple of which the genotypes at the A and B are known, the probability of observing the genotypes of the offspring depends on the value of q.

Explanation:

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