A 3point testcross was made in corn The results are shown in

A 3-point testcross was made in corn. The results are shown in the table below, where p^+ = green leaves, p = purple leaves, v = virus-resistant seedlings, v^+ = virus-sensitive seedlings, b^+ = plain seed, b = brown. Draw a genetic map of the chromosome carrying these 3 genes.

Solution

Maximum number of individuals are parental ones (P) and Minimum number of individuals are Double cross overs (DCO): By comparing P with DCO we can find the middle gene:

P+ V+ B+ ---------------> P

P V+ B+ ----------------> DCO

The different gene in P & DCO are considered as Middle Gene. So P is the middle gene. So the order of the genes is either V - P - B or B - P - V

Distance between V-P = [(690+678) + (72+ 60)] X 100 / 10000

= 15%

= 15 mU or cM

= 15 X 10⁶ Base pairs

= 3.4 x 15 x 10⁶ A^o

⁼ 51 x 106 A^o

Distance between P-B = [(1024+1044) + (72+ 60)] X 100 / 10000

= 22%

= 22 mU or cM

= 22 X 10⁶ Base pairs

= 3.4 x 22 x 10⁶ A^o

⁼ 74.8 x 106 A^o

So the Mapping of the genes and order is

V----------------------15cM---------------P------------------------------------------------22cM--------------------------------------B