Putting genes on a map-1998

Using three factor crosses to place a new X-linked

gene on an existing genetic map.

3-factor cross data for the X-linked markers: crossveinless wings, vermilion eyes and garnet eyes cv, v, and g. vermilion is known to map between crossveinless and garnet. cv maps at 14 cM and g at 44 cM.

What is the map position of vermilion (v)?

male progeny
cv + g + v +	1015 1370	parentals
cv v + + + g	249 254	cv - v recombinants
cv + + + v g	185 159	v - g recombinants
cv v g + + +	8 9	double recombinants

In the cv - v region there are: 249 + 245 + 8 + 9 = 520 recombination events.

In the v - g region there are: 185 + 159 + 8 + 9 = 361 recombination events.

In the cv- g region there are: 249+ 254 + 185 + 159 + 2 (8) + 2 (9) = 881 rec. events.

To map v with respect to the standard map positions of cv (14 cM) and g (44 cM), you must consider the 881 recombination events that occurred between cv and g. Of those, 520 were between cv and v (the cv-v interval), and 361 between v and g (the v-g interval). The data place v in the middle and closer to g than to cv because there are fewer crossovers between v and g than between cv and v. The standard map distance from either marker can be determined by considering that the cv-v distance accounts for 59% (520 / 881) of the distance between cv and g (30 cM) and that the v-g distance accounts for 41% (361/881) of the cv-g distance. Since cv is at 14 cM, v is located at 14 cM + 0.59(30 cM) = 31.7 cM. Using g as the standard, the calculation is 44 cM - 0.41(30 cM) = 31.7 cM.

Study questions.

1) Why are the female progeny ignored in this example?

2) Is there any need to count the total number of male flies that emerge from the cross?

3) If the relative map position of v was not given at the start, how would that affect your approach to this problem?

Note for the afficionado. vermilion is one of the bedrock standards of the Drosophila X chromosome map. Its agreed upon standard position is 33.00 cM.