X-linked genetics problems - Name genetics: x linked genes ****In ... - Free Printable

Let’s go through each question step by step to make sure we get the right answers.

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Question 1: What are the sexes and eye colors of flies with the following genotypes?

We’re told:
- Red is dominant (R)
- White is recessive (r)
- This is X-linked, so males have one X and one Y; females have two X chromosomes.

Genotypes:

- X^R X^r → Female (two X’s), has at least one R → Red eyes
- X^R Y → Male (X and Y), has R on X → Red eyes
- X^r X^r → Female, both alleles r → White eyes
- X^R X^R → Female, both R → Red eyes
- X^r Y → Male, only X has r → White eyes

✔ All match what’s written in the image — correct!

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Question 2: What are the genotypes of these flies?

- White eyed, male → must be X^r Y (only way for male to have white eyes)
- Red eyed female (heterozygous) → has one R and one r → X^R X^r
- White eyed, female → must have two r’s → X^r X^r
- Red eyed, male → must have R on his single X → X^R Y

✔ Again, matches the image — correct!

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Question 3: Cross a white-eyed female (X^r X^r) with a red-eyed male (X^R Y)

Punnett Square:

Female gametes: all X^r
Male gametes: X^R or Y

So offspring:

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

Offspring:
- Females: all X^R X^r → red-eyed females
- Males: all X^r Y → white-eyed males

So 50% red-eyed females, 50% white-eyed males.

The Punnett square in the image shows this correctly.

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Question 4: Cross pure red-eyed female (X^R X^R) with white-eyed male (X^r Y)

Parents: X^R X^R × X^r Y

Gametes:
- Female: all X^R
- Male: X^r or Y

Offspring:

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

All offspring:
- Females: X^R X^r → red-eyed
- Males: X^R Y → red-eyed

So:
- White eyed, male → 0
- White eyed, female → 0
- Red eyed, male → 50%
- Red eyed, female → 50%

✔ Matches image — correct!

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Question 5: Cross heterozygous red-eyed female (X^R X^r) with red-eyed male (X^R Y)

Parents: X^R X^r × X^R Y

Gametes:
- Female: X^R or X^r
- Male: X^R or 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 → red
- X^R X^r → red
→ So 100% of females are red-eyed

Males:
- X^R Y → red
- X^r Y → white
→ So 50% red males, 50% white males

Total breakdown (each box = 25%):

- X^R X^R → red female → 25%
- X^R X^r → red female → 25%
→ Total red females = 50%

- X^R Y → red male → 25%
- X^r Y → white male → 25%

So:

- White eyed, male → 25%
- White eyed, female → 0%
- Red eyed, male → 25%
- Red eyed, female → 50%

✔ Matches image — correct!

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Math Question: If in the above cross (Q5), 100 males and 200 females were produced, how many total red-eyed flies would there be?

From Q5 results:

Among males:
- 50% red → 50% of 100 = 50 red-eyed males
- 50% white → 50 white-eyed males

Among females:
- 100% red → 100% of 200 = 200 red-eyed females

Total red-eyed flies = 50 (males) + 200 (females) = 250

✔ The answer “250” in the image is correct!

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Final Answer:
250