Mendelian Genetics

Sophies, why don't you give this a try...

Solving Problems: Practicing Crosses

A wild population contains red-eyed and white-eyed flies. A scientist crosses two white-eyed flies and gets all white eyed progeny (cross 1). She crosses two red eyed flies and gets all red-eyed progeny (cross 2). When she crosses a different pair of red-eyed flies, she gets 22 white eyed progeny and 78 red-eyed progeny (cross 3). Explain her observations, giving the most probable genotypes of the parents and progeny of each cross.

- try making a model where white is dominant to red. Appropriate symbols would be:

W - dominant white allele

w - recessive red allele

White eyed flies are either Ww or WW, red eyed flies must be ww.

- cross 1: to get all white progeny, the parents must be WW and WW. The data are compatible with this model.

- cross 2: ww X ww would give all ww - red eyed. This is also compatible with the data.

- cross 3: ww X ww would give no white eyed progeny

This is not compatible with the data. Therefore white cannot be dominant.

- try making a model where red is dominant to white. Appropriate symbols would be:

R - dominant red allele

r - recessive white allele

White eyed flies are rr, red flies are RR or Rr.

- cross 1: to get all white progeny, the parents must be rr and rr. OK.

- cross 2: as above, if the parents were RR and RR, all the progeny would be RR - red. This is also OK.

- cross 3: RR X RR would give all red progeny

RR X Rr would give all red progeny

Rr X Rr would give 75% red, 25% white - this prediction matches the data.

Therefore, the model that red is dominant to white fits the cross data best.

cross 1: rr X rr gives only rr progeny

cross 2: RR X RR gives only RR progeny

cross 3: Rr X Rr gives 75% R_ (RR or Rr), 25% rr

For problems with more than one trait (if the problem involved red and white eyes and short and long wings, for instance), treat each trait independently (work with eye color alone by counting red-eyed/short-winged and red-eyed/long-winged as simply red-eyed) to break the problem into two smaller problems.