Ln Drosophila, dominant Bar is located on the X chromosome, while spineless bris

Question

Ln Drosophila, dominant Bar is located on the X chromosome, while spineless bristles and curved wings are located on chromosome 3, separated by 18 map units. A homozygous spineless female is crossed to a homozygous male expressing Bar eyes and curved wings. The F1 files are INBRED. What is the probability of observing a fly of either sex in the F2 expressing Bar eyes, but not curved wings or spineless bristles? a. 0.0625 b. 0.125 c. 0.1025 d.025 e. 0.0225 f. 0.205 What is the probability of observing a male fly expressing only Bar eyes? a. 0.0625 b. 0.125 c. 0.1025 d.025 c.0.0225 f.0.205 If the F1 females from the cross above were test-crossed, what is the probability of observing a male fly expressing Bar and spineless, but not curved in the F2? a. 0.0675 b.0.125 c. 0.1025 d. 0.045 e 0.0225 f.0.3075

Explanation / Answer

Let us denote X chromosome with bar with the symbol XB; spineless with “s” and curved wings with “c”. This notation is taken because spineless and curved are autosomal recessive.

Homozygous spineless (ssCC XX) female is crossed to homozygous curved male with bar eyes (SScc XBO)

F1 result: SsCc XBX females and SsCc XO males. All females and males will be curved and spineless. All females will have bar eyes all males will be without bar eye.

F2 Cross is SsCc XBX females and SsCc XO

Answer 23. The correct answer is a.

Probability of having a fly without curved and spineless = (¼* ¼ ) = 0.25

Answer 24. The correct option is d.

Probability of male having bar eyes: ½ * ½ = ¼

Answer 25. The correct option is a (not ‘c’; as marked).

F1 females Ss Cc XBX are test crossed with ss cc XO

We have to find out the probability of having a male with bar and spineless but not curved. Please note that ¼ males are having bar eyes. Out of these ½ will be spineless and half will not be curved. So, probability of having spineless without curves is ¼ . ¼ of these ¼ are males with bar. So, the probability becomes 1/16.

So, the probability is = ¼* ½* ½ * = (1/16) = 0.0625

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