Practice problems on sex-linked genes in fruit flies, illustrating eye color inheritance with red (dominant) and white (recessive) traits.

Let’s go through each problem step by step. We’re working with fruit flies and eye color, which is a sex-linked trait — meaning the gene is on the X chromosome. Red (R) is dominant over white (r). Males are XY, females are XX.

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Problem 1: What are the sexes and eye colours of flies with the following genotypes?

- X^R X^r → Female (two X chromosomes), has one red allele and one white allele → since red is dominant, she has red eyes.
- X^R Y → Male (has Y), has red allele on his only X → red eyes.
- X^r X^r → Female, both alleles white → white eyes.
- X^R X^R → Female, both alleles red → red eyes.
- X^r Y → Male, has white allele on his X → white eyes.

✔ So answers:
- X^R X^r → female, red
- X^R Y → male, red
- X^r X^r → female, white
- X^R X^R → female, red
- X^r Y → male, white

*(Note: The student wrote “X^R X^R ? red” and “X^r Y ? white” — that’s correct!)*

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Problem 2: What are the genotypes of these flies?

- white eyed, male → must be X^r Y (males get their X from mom; if he has white eyes, his X must carry r)
- red eyed female (heterozygous) → heterozygous means different alleles → X^R X^r
- white eyed, female → must have two recessive alleles → X^r X^r
- red eyed, male → must have R on his X → X^R Y

✔ Answers:
- white eyed, male → X^r Y
- red eyed female (heterozygous) → X^R X^r
- white eyed, female → X^r X^r
- red eyed, male → X^R Y

*(Student got all these right too!)*

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Problem 3: Cross of white eyed female (X^r X^r) and red-eyed male (X^R Y)

Set up Punnett square:

Mom’s gametes: X^r and X^r
Dad’s gametes: X^R and Y

Offspring:

| | X^R | Y |
|-------|---------|---------|
| X^r | X^R X^r | X^r Y |
| X^r | X^R X^r | X^r Y |

So we get:
- 2 females: X^R X^r → red eyed (since they have R)
- 2 males: X^r Y → white eyed

Total = 4 offspring

→ white eyed, male: 2 out of 4 = 50%
→ white eyed, female: 0 → 0%
→ red eyed, male: 0 → 0%
→ red eyed, female: 2 out of 4 = 50%

✔ Student’s answer is correct!

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Problem 4: Cross between pure red eyed female and white eyed male

Pure red eyed female = homozygous dominant → X^R X^R
White eyed male → X^r Y

Gametes:
- Mom: X^R, X^R
- Dad: X^r, Y

Punnett Square:

| | X^r | Y |
|-------|---------|---------|
| X^R | X^R X^r | X^R Y |
| X^R | X^R X^r | X^R Y |

Offspring:
- Females: X^R X^r → all red eyed (heterozygous)
- Males: X^R Y → all red eyed

So:
- white eyed, male: 0 → 0%
- white eyed, female: 0 → 0%
- red eyed, male: 2 out of 4 = 50%
- red eyed, female: 2 out of 4 = 50%

✔ Student’s answer is correct!

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Problem 5: Cross of red eyed female (heterozygous) and red eyed male

Heterozygous red female → X^R X^r
Red eyed male → X^R Y

Gametes:
- Mom: X^R, X^r
- Dad: X^R, Y

Punnett Square:

| | X^R | Y |
|-------|---------|---------|
| X^R | X^R X^R | X^R Y |
| X^r | X^R X^r | X^r Y |

Offspring:
- X^R X^R → female, red
- X^R X^r → female, red
- X^R Y → male, red
- X^r Y → male, white

So:
- white eyed, male: 1 out of 4 = 25%
- white eyed, female: 0 → 0%
- red eyed, male: 1 out of 4 = 25%
- red eyed, female: 2 out of 4 = 50%

✔ Student’s answer is correct!

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Math Question: If in the above cross, 100 males were produced and 200 females. How many total red-eyed flies would there be?

From Problem 5 cross:

Among males:
- 25% are red eyed → 25% of 100 = 25 red eyed males
- 75% are white eyed? Wait — no! From our Punnett square, half the males are red, half are white? Let’s check again.

Wait — let’s recheck the ratios for males specifically.

In the cross: X^R X^r (mom) × X^R Y (dad)

Male offspring come from:
- Mom gives X^R → son gets X^R Y → red
- Mom gives X^r → son gets X^r Y → white

Each happens 50% of the time.

So among males, 50% are red, 50% are white.

But earlier I said 25% overall — that was across ALL offspring. But now the question says: “100 males were produced and 200 females.”

So we need to calculate based on actual numbers given.

From the cross:

For females:
All females get X^R from dad (because dad is X^R Y, so he gives X^R to daughters).
Mom can give X^R or X^r → so daughters are either X^R X^R or X^R X^r → BOTH are red eyed.

So ALL 200 females are red eyed.

For males:
They get Y from dad, and X from mom.
Mom is X^R X^r → so 50% chance she gives X^R → red eyed male
50% chance she gives X^r → white eyed male

So out of 100 males:
- 50 are red eyed
- 50 are white eyed

Therefore, total red-eyed flies = red females + red males = 200 + 50 = 250

Wait — but the student wrote “350”. That’s incorrect.

Let me double-check.

Is it possible the student misread the percentages?

In the Punnett square for Problem 5, we had 4 offspring:

- 2 red females (X^R X^R and X^R X^r)
- 1 red male (X^R Y)
- 1 white male (X^r Y)

So ratio among offspring:
- Red females: 2/4 = 50%
- Red males: 1/4 = 25%
- White males: 1/4 = 25%

BUT — when the question says “100 males were produced and 200 females”, it’s telling us the actual number born, not assuming equal numbers.

So we don’t use the 25%/50% from the Punnett square as proportions of total — instead, we apply the genetic ratios *within each sex*.

As established:

→ All females are red eyed → 200 red females

→ Among males: 50% red, 50% white → 50 red males

Total red-eyed = 200 + 50 = 250

The student wrote 350 — probably added 200 females + 100 males + 50 something? Or maybe thought 75% of males are red? But no — from genetics, it’s 50% of males are red.

Alternatively, maybe they used the overall percentage: 75% of total offspring are red-eyed? Total offspring = 300 (100 males + 200 females). 75% of 300 = 225 — still not 350.

Or perhaps they did: 200 females (all red) + 100 males (thinking all are red?) → 300, then added 50 more? Doesn’t make sense.

Actually, looking back at the student’s work: they wrote “350” under the math question.

But according to correct genetics:

Females: 200 → all red → 200

Males: 100 → half red → 50

Total red = 250

So the correct answer should be 250

I think the student made a mistake here.

Let me confirm once more.

Cross: X^R X^r (female) × X^R Y (male)

Female offspring: always get X^R from father → so genotype is X^R X^? where ? is from mother → either X^R or X^r → both result in red eyes → 100% of females red.

Male offspring: get Y from father, X from mother → mother gives X^R (50%) → red male; X^r (50%) → white male → so 50% of males red.

Given: 200 females → 200 red

100 males → 50 red

Total red = 250

✔ Final Answer for Math Question: 250

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Final Answer:
250