In fruit flies (*Drosophila melanogaster*), grey body color (B) is dominant over black bod… — JAMB UTME

In fruit flies (*Drosophila melanogaster*), grey body color ( $B$ ) is dominant over black body color ( $b$ ), and normal wing length ( $V$ ) is dominant over vestigial wings ( $v$ ). If two heterozygous grey-bodied, normal-winged flies ( $BbVv$ ) are crossed and produce a total of 320 offspring, how many of these offspring are expected to exhibit a black body with normal wings?

60Answer
B
180
C
20
D
240

Answer

60 offspring are expected to have a black body with normal wings.

In a dihybrid cross of two heterozygous individuals (

BbVv \times BbVv

), the independent assortment of alleles yields an F2 phenotypic ratio of

9 : 3 : 3 : 1

. The proportion of offspring displaying the recessive trait for the first gene (black body,

bb

) and the dominant trait for the second gene (normal wings,

V\_

) is

\frac{3}{16}

. Multiplying

\frac{3}{16}

by the total offspring count of

320

gives exactly

60

Step-by-Step Solution

Determine parental genotypes and set up the dihybrid cross.

Parental cross is

BbVv \times BbVv

, where both parents are heterozygous for both traits.

Mendel's Second Law states that alleles of different genes assort independently during gamete formation.

Identify the expected phenotypic ratio for the offspring.

The phenotypic ratio for a dihybrid cross between two double heterozygotes (

BbVv \times BbVv

) is

9 : 3 : 3 : 1

.
- Grey body, normal wings (

B\_V\_

) =

\frac{9}{16}

- Grey body, vestigial wings (

B\_vv

) =

\frac{3}{16}

- Black body, normal wings (

bbV\_

) =

\frac{3}{16}

- Black body, vestigial wings (

bbvv

) =

\frac{1}{16}

The probability of recessive body color (

bb = \frac{1}{4}

) combined with dominant wing length (

V\_ = \frac{3}{4}

) is calculated as

\frac{1}{4} \times \frac{3}{4} = \frac{3}{16}

Calculate the expected number of offspring with black body and normal wings.

\frac{3}{16} \times 320 = 60

Multiplying the expected phenotypic fraction by the total offspring count yields the specific expected count.

Key Concept

Mendel's Law of Independent Assortment and Dihybrid Phenotypic Ratios

Estimated Time:1m 30s

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