1. In a three-point cross, why does the sum of the two short lengths not add up
ID: 212946 • Letter: 1
Question
1. In a three-point cross, why does the sum of the two short lengths not add up to the long one (that is, for the map a-b-c, why do the recombination freguencies a-b plus b-c not equal a-c)? 2. Can you do a three-point cross in which one parent is recessive for one gene (a-B-C) and the other parent is recessive for the two other genes (A-b-c)? 3. In a three-point cross in which two genes are close together and the third gene far away, how can you tell what side of the two genes the third gene is on (a-b-c or c-a-b)? CONCEPT ONE: ALLELES 1996 Times Mirror Higher Education GroupExplanation / Answer
1- Crossover between the long one is the result of double crossover whereas cross between the short one is the result of single crossover. It is well known that the frequency of single crossover is higher than that of doublecross over.
The expected frequency of double cross is given by - the distance between the one short lenght/100* distance between the second short lenght/100.
suppose distance between one short length is 5 map unit
the distance between the second short length is 10 map unit
expected the frequency of double cross will be 5/100*10/100
= 50/10000 = 0.005
2- Following will be the progenies
aBC
Abc
ABc
abC
AbC
aBc
abc
ABC
3- In the three-point cross, the maximum number of progenies are of the parental type and the least number of progenies are the result of double crossover.
Suppose the parental genotype is ABC and abc
and the genotype of double crossover is ABc and abC
In the double crossover, there is the exchange of the middle gene.
if we compare that parental and double crossover progeny, there is the difference in only C/c alleles.
So by comparing parental genotype the and genotypes which has the least number of progeny, we can easily make out which gene is present in the middle.
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