Question

12. You have isolated two different mutations in squash plants. The first mutation causes fruit to be white instead of green. The second mutation causes fruit to be yellow instead of green. The two mutations affect different genes, w(hite) and g(reen) respectively. The allele symbols for w are w+ (wildtype) and w (mutant). The allele symbols for g are g+ (wildtype) and g (mutant). When you conduct a dihybrid cross (w+/w;g+/g x w+/w;g+/g) you obtain the following phenotypic ratio: White fruit (12) : Yellow fruit (3) : Green fruit (1)

a) Which allele(s) are dominant? (2 points) __ w+ _√_ w __ g+ __ g

b) Which gene is epistatic? (2 points) _√_ w(hite) __ g(reen) __ neither white nor green

(the check marks are my answers, wanted to confirm that my answers were correct, if not, why)

Answer 1

The first mutation causeses fruit to be white instead of green. And white color mutation is responsible for w allele ( mutated allele ) . And normal or wild type allele ( i.e non mutated allele ) w+ is responsible for green color .

On the other hand second mutation causeses fruit to be yellow instead of green . And yellow color mutation is responsible for g allele ( mutated ) and normal or wild type allele g+​​​​​​ is responsible for green color .

Here the cross occurs between ww+​​​​​​, gg+ and ww+​​​​​​,gg+

Gamets will be - w+g+​​​, w+g, wg+, wg from both the plants

	w+g+	w+g	wg+	wg
w+g+	w+w+g+g+  ( G)	w+w+g+g ( Y)	w+wg+g+ ( W )	w+wg+g ( W )
w+g	w+w+gg+ ( Y)	w+w+gg ( Y )	w+wgg+ ( W)	w+wgg ( W )
wg+	ww+g+g+ ( W)	ww+g+g ( W)	wwg+g+ ( W )	wwg+g ( W )
wg	ww+gg+ ( W)	ww+gg ( W )	wwgg+ ( W )	wwgg ( W )

In this cross we have got 12 white squash plant , 3 yellow squash plant , and only 1 green squash plant . Therefore it is clear that the gene for white color (w gene) is epistatic over gene for green (g gene ) thats why 12 offspring among 16 become white .

And g allele is dominant over g+  hence gg and gg+ both conditions can express yellow color . Only g+g+ condition can express green color..  

It is also clear from the phenotypes of offspring that w allele is dominant over w+ hence ww and ww+ both condition are able to express white color only w+w+ condition can express green color..

Hence only w+w+g+g+ condition can express green ( denoted by G in the punnett sq)  

Only w+w+gg , w+w+gg+ conditions can express yellow color ( denoted by Y in the punnett sq )  

Rest genotypic combination are responsible for white color squash ( denoted by W in punnett square )  

Hence w allele is dominant over other alleles i.e w+, g and g+ alleles .

w w+ g g+ and w gene g gene

There fore both of your answers are correct .