Question

The frequencies of gametes of different genotypes, determined by test crossing a triple heterozygote, are as shown in the following table:

	Gamete Genotype	Number
Set 1	+ + +	129
Set 2	a b c	139
Set 3	+ + c	65
Set 4	a b +	69
Set 5	+ b c	264
Set 6	a + +	272
Set 7	a + c	31
Set 8	+ b +	35
	Total	1000

a) Which gametes types are known to have been involved in double crossovers?

b) Single cross overs?
c) Which gamete types have not been involved in any exchanges? What are they called?
d) The order shown is not necessarily correct. Which gene locus is in the middle? Show the order

Answer 1

In a three point cross the gamete genotype which is most abundant is the parental type and least abundant is the double cross over.

So ans-A is set 7 and 8 with genotype ( ac+) and (++b) are the double cross overs.

By comparing the double cross over with parental type it is found that c gene is present in the middle so the order of genes is a c b or b a c. Both the orders are correct .

Ans-b set 1, 2,3 and 4 are the single cross overs. Set 1 ( +++ =129) and set 2 ( acb= 139) are the single cross overs between a and c.

Set 3 ( +c+ =65) and set 4 (a+b=69) are single cross overs between c and b.

Ans - c) set 5 (+cb=264) and set 6 (a++ =272) are not involved in any crosses and called as parental types or non recombinant gametes.

Ans_d the order of the genes is predicted by comparison of double cross over with the parental. In case of three point cross the middle genes changes its position from one sister chromatid to another sister chromatid. So here by comparison we found c gene is present in the middle and correct order is( a c b) or (b c a).