Question

The following Drosophila crosses were performed Cross 1 Parents: white-eyed straight winged male x red-eyed; curly winged female Progeny: all red-eyed flies, half of which are curly winged Cross 2 (reciprocal cross) Parents: red-eyed curly winged male x white-eyed straight winged female Progeny: white-eyed males and red-eyed females, half of each have curly wings. Test crosses with the curly winged progeny from cross 1 and 2 to homozygous straight winged flies resulted in all half curly and half straight winged flies. Cross 3 (progeny_of cross 1) Parents: red-eyed curly winged females x red-eyed curly winged males Predict the ratios and phenotypes of the progeny from cross 3:

Answer 1

It is given that when curly winged fly (Cc) mated with homozygous straight winged (cc) fly they deliver 50% curly winged (Cc) and 50% straight winged (CC) progeny. This means curly winged offsprings from cross 1 and 2 are heterozygous (Cc).

From cross 1 and cross 2 it is understandable that the eye colour is sex-linked (X chromosome-linked) recessive trait.

Let me discuss the crosses in brief:

1^st white eye X_w (recessive)

Red-eye X (Dominant)

Male sex chromosome Y

Curly wing Cc

Straight wing CC

Now the 1^st cross :

White-eyed straight winged male X_wYCC x                              Red-eyed curly winged female XXCc

	Male	XwC	YC
Female
XC		Xw XCC	XYCC
Xc		Xw XCc	XYCc

Progeny: All red-eyed but 50% Curly winged Cc and 50% straight winged CC.

Now the 2^nd cross :

Red-eyed curly winged male XYCc            x              White eyed straight winged female X_wX_wCC

	Male	XC	Xc	YC	Yc
Female
XwC		XwXCC	XXwCc	XwYCC	XwYCc

Progeny: All males are white eyed. All females are red-eyed. Half of each has a curly or straight wing.

Now, when curly winged (Cc) crossed with homozygous straight winged (CC) the cross is:

	Curly	C	c
Straight
C		CC	Cc

50% curly and 50% straight winged offspring

Cross 3 :

Red-eyed curly winged male XYCc            x              Red-eyed curly winged female X_wXCc

	Male	XC	Xc	YC	Yc
Female
XwC		XwXCC	XwXCc	XwYCC	XwYCc
Xwc		XwXCc	XXwcc	XwYCc	XwYcc
XC		XXCC	XXCc	XYCC	XYCc
Xc		XXCc	XXcc	XYCc	XYcc

Phenotype:

Red-eyed curly winged male XYCc                            2

White-eyed curly winged male X_wYCc 2

Red-eyed straight winged male XYCC 1

White-eyed straight winged male X_wYCC 1

Red-eyed curly winged female X_wXCc/XXCc 4

Red-eyed straight winged female X_wXCC/XXCC 2

The bold marking indicates the lethal effect of the gene.

If the ratios of the offspring are like above then the previous question illustrate recessive lethality (for wings) and the sex-linked inheritance (for eye colour)

But if the progeny show any other form (marked as bold) of a wing (cc) other than curly or straight, then its an example of incomplete dominance and sex linkage.

Please revert back if have any query