In: Biology
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?
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.