Question

Let’s say you are looking at three genes in elephants: A, B, and C. All are on the same chromosome. There are two alleles of each, and all three show complete dominance. You cross 2 heterozygous individuals, and find the following:

ABc 18

aBc 112

abc 308

ABc 66

abC 59

ABC 321

aBC15

AbC 102

total 1000

What would be the possible order of genes?

What would happen if one of these genes was on a different chromosome that the other two?

Answer 1

In the elephant, all 3 gene A,B and C present on same chromosome so crossing over will give progenies. First we cross AABBCC with aabbcc which give F1 AaBbCc that self cross and produce progenies.

Here, parent genotype always in hieghest frequency so parent and double cross over always in lowest frequency.

ABC. 321

abc parent genotype. 308

Abc. 18

aBC. 15. Double cross over

ABc. 66

abC. 59. Cross over between A and C

aBc. 112

AbC. 102 cross over between B and C

Double cross over move middle allele from one sister chromatid to other. aBC and Abc are the double cross over genotype. We can see from the table that A gene in the middle because the recessive allele a is now on the same chromosome as the B and C allele and dominant A allele on the same chromosome as recessive b and c allele. So the order og gene will be BAC.

Distance between A and C

100* (66+59+15+18)/1000 = 15.8cM

Distance between B and C gene is

100* (112+102+15+18)/1000

= 24.7cM

So the correct order will be B-------A---------C.