genetic map units question

[299678]

---

Members don't see this ad.

The results of a F1 testcross are: 250 bald head, four ears : 247 hairy head, six ears : 21 bald head, six ears : 19 hairy head, four ears. How many map units apart are the head and ear genes?

3.5
3.9
7.5
50.5
92.6

Book answer said, 21+19/total units x 100
but I wonder why we choose 21 and 19?

 

[zestypeas]

You choose 21 and 19 because those are the recombinant classes of offspring. Generally, to calculate map distances, you will choose the recombinant classes divided by the total number of offspring. I hope this helps.

 

[299678]

Thanks you for your answer, but
How we know 19 and 21 is recombinant classes?

 

[299678]

Thanks you for your answer, but
How we know 19 and 21 is recombinant classes?

 

[zestypeas]

Sorry for the late response - I forgot I had posted here.

You know 21 and 19 are the recombinant classes because recombinants do not occur as often as the parental classes. Therefore, your parental classes (250, 247) should have the most offspring, and your recombinant classes (19, 21) fewer offspring.