Linkage destroys the beauty of prediction inherent in genetics. A. H Sturtevant and T. H. Morgan developed mapping to allow prediction. The following calculations allow us to predict assortment outcomes from crosses.
Using map data to predict the progeny of the cross a+b+/ab with ab/ab
Let f =the expected frequency of cross overs
| Description | genotype | P | f=.1 | N=1000 |
|
Parental type one |
a+ b+ | (1-f)/2 | .45 | 450 |
| Parental type two | a b | (1-f)/2 | .45 | 450 |
|
Recombinant |
a+ b | f/2 | .05 | 50 |
| Recombinant | a b+ | f/2 | .05 | 50 |
Example: If genes a and b are twenty map units apart (ie. f= .2) what is the expected progeny from a cross between an organism that is heterozygous for a and b is test crossed to an organism that is ab/ab. Assume that one parent was a+ b/a+ b. What is the probability that the double recessive will be found in the progeny?
| Description | genotype | P | f=.2 | N=1000 |
|
Parental type one |
a+ b | (1-f)/2 | .40 | 400 |
| Parental type two | a b+ | (1-f)/2 | .40 | 400 |
|
Recombinant |
a b | f/2 | .10 | 100 |
| Recombinant | a+ b+ | f/2 | .10 | 100 |
The answer is P = 0.10
How does this problem change for three linked loci?
Text iGenetics by Peter J. Russell
This web site is provided for instruction in Botany and Zoology 342
by Kenneth G. Wilson,
Professor of Botany
Miami University
wilsonkg@muohio.edu