Question

drosophila eye color is an x - linked trait. red eye color is do...ant, and white eye color is recessive. which punnett square shows a cross in which the probability of offspring w...ve red eyes is 100 percent?

Explanation:

Step1: Recall X - linked inheritance rules

For X - linked traits, females have two X chromosomes (\(X\) and \(X\)) and males have one X and one Y chromosome (\(X\) and \(Y\)). Red eye color (\(R\)) is dominant, white (\(r\)) is recessive. To have 100% red - eyed offspring, all offspring must have at least one dominant \(R\) allele.

Step2: Analyze the first Punnett square

In the first Punnett square, the female has \(X^{R}X^{R}\) (wait, no, looking at the first square, the female gametes are \(X^{R}\) and \(X^{R}\)? Wait, no, the first square's female is \(X^{R}X^{R}\)? Wait, no, the first square's female gametes are \(X^{R}\) (top left and bottom left), and the male gametes are \(X^{r}\) and \(Y\). Wait, no, the first square: male has \(X^{r}\) and \(Y\), female has \(X^{R}\) and \(X^{R}\). The offspring are \(X^{R}X^{r}\) (female, red - eyed, since \(R\) is dominant), \(X^{R}Y\) (male, red - eyed), \(X^{R}X^{r}\) (female, red - eyed), \(X^{R}Y\) (male, red - eyed). Wait, but the second square: male has \(X^{R}\) and \(Y\), female has \(X^{R}\) and \(X^{R}\)? Wait, no, the second square's male gametes are \(X^{R}\) and \(Y\), female gametes are \(X^{R}\) and \(X^{R}\)? Wait, no, the second square: female is \(X^{R}X^{R}\) (gametes \(X^{R}\) and \(X^{R}\)) and male is \(X^{R}Y\) (gametes \(X^{R}\) and \(Y\))? Wait, no, let's re - examine.

Wait, the key is: for 100% red eyes, all offspring must have \(R\) allele. Let's check the genotypes:

- In the first Punnett square (the one with \(X^{r}\) and \(Y\) as male gametes, \(X^{R}\) and \(X^{R}\) as female gametes):
- Female offspring: \(X^{R}X^{r}\) (red, since \(R\) is dominant)
- Male offspring: \(X^{R}Y\) (red, since \(R\) is on X)
- In the second Punnett square (the one with \(X^{R}\) and \(Y\) as male gametes, \(X^{R}\) as female gamete? Wait, the second square's female is \(X^{R}X^{R}\) (gametes \(X^{R}\)) and male is \(X^{R}Y\) (gametes \(X^{R}\) and \(Y\))? Wait, no, the second square: female gamete is \(X^{R}\) (only one? No, Punnett square has two rows for female? Wait, maybe the second square is a 2x2 square with female \(X^{R}X^{R}\) (gametes \(X^{R}\) and \(X^{R}\)) and male \(X^{R}Y\) (gametes \(X^{R}\) and \(Y\)). Then offspring:
- \(X^{R}X^{R}\) (female, red)
- \(X^{R}Y\) (male, red)
- \(X^{R}X^{R}\) (female, red)
- \(X^{R}Y\) (male, red)

Wait, but the first square has a recessive allele (\(r\)) in the male's X. But in the first square, the female is \(X^{R}X^{R}\)? No, wait the first square's female gametes are \(X^{R}\) (two of them), male gametes are \(X^{r}\) and \(Y\). So offspring are \(X^{R}X^{r}\) (red), \(X^{R}Y\) (red), \(X^{R}X^{r}\) (red), \(X^{R}Y\) (red). Wait, but the second square: if the male is \(X^{R}Y\) (gametes \(X^{R}\) and \(Y\)) and female is \(X^{R}X^{R}\) (gametes \(X^{R}\) and \(X^{R}\)), then all offspring are \(X^{R}X^{R}\) (female) or \(X^{R}Y\) (male), all red - eyed. But wait, the first square has a male with \(X^{r}\), but the female is homozygous dominant (\(X^{R}X^{R}\)), so even with \(X^{r}\) from male, the female offspring get \(X^{R}\) from female and \(X^{r}\) from male, but since \(R\) is dominant, they are red. Male offspring get \(X^{R}\) from female and \(Y\) from male, so red. Wait, but the question is which Punnett square shows 100% red - eyed offspring.

Wait, maybe the second square (the one with \(X^{R}\) and \(Y\) as male gametes, \(X^{R}\) as female gametes) is the one where both parents are homozygous dominant for red eyes. Let's assume the second square is:

Female: \(X^{R}X^{R}\) (gametes \(X^{R}\), \(X^{R}\))

Male: \(X^{R}Y\) (gametes \(X^{R}\), \(Y\))

Then the Punnett square would be:

| | \(X^{R}\) | \(Y\) |
|---|---|---|
| \(X^{R}\) | \(X^{R}X^{R}\) | \(X^{R}Y\) |
| \(X^{R}\) | \(X^{R}X^{R}\) | \(X^{R}Y\) |

All offspring have at least one \(R\) allele, so 100% red - eyed.

In the first square, male has \(X^{r}\), but female is \(X^{R}X^{R}\), so offspring are \(X^{R}X^{r}\) (female, red) and \(X^{R}Y\) (male, red). Wait, but \(X^{R}X^{r}\) is still red - eyed. Wait, maybe the first square is also 100% red - eyed? No, wait the problem says "which Punnett square shows a cross in which the probability of offspring with red eyes is 100 percent".

Wait, maybe the second square is the one where the male is \(X^{R}Y\) and female is \(X^{R}X^{R}\), so all offspring are homozygous or hemizygous for \(R\), so 100% red. The first square has a male with \(X^{r}\), but since female is \(X^{R}X^{R}\), the female offspring are heterozygous (\(X^{R}X^{r}\)) but still red, and male offspring are \(X^{R}Y\) (red). So both? No, maybe the second square is the one with male \(X^{R}Y\) and female \(X^{R}X^{R}\), so all offspring have \(R\) allele.

Assuming the second Punnett square (the one with \(X^{R}\) and \(Y\) as male gametes, \(X^{R}\) as female gametes) is the correct one. Let's confirm:

- Genotype of offspring in second square: \(X^{R}X^{R}\) (female, red), \(X^{R}Y\) (male, red), \(X^{R}X^{R}\) (female, red), \(X^{R}Y\) (male, red). All have red eyes.

In the first square, offspring are \(X^{R}X^{r}\) (female, red), \(X^{R}Y\) (male, red), \(X^{R}X^{r}\) (female, red), \(X^{R}Y\) (male, red). Also red. Wait, maybe the difference is in the male's genotype. If the male in the first square is \(X^{r}Y\) (white - eyed male) and female is \(X^{R}X^{R}\) (red - eyed female), then offspring are red - eyed. If the male in the second square is \(X^{R}Y\) (red - eyed male) and female is \(X^{R}X^{R}\) (red - eyed female), then offspring are also red - eyed. But the question is which one has 100% red eyes. Both? But maybe the second square is the one with male \(X^{R}Y\) and female \(X^{R}X^{R}\), so all offspring are homozygous for \(R\) on X (females) or have \(R\) on X (males).

Wait, maybe the first square has a typo, but based on the given, the second Punnett square (the one with \(X^{R}\) and \(Y\) as male gametes, \(X^{R}\) as female gametes) is the one where all offspring have red eyes.

Answer:

The Punnett square with male gametes \(X^{R}\) and \(Y\), and female gametes \(X^{R}\) (resulting in offspring \(X^{R}X^{R}\) and \(X^{R}Y\) for all four cells) shows a cross with 100% red - eyed offspring. (Assuming the second Punnett square in the given options is this one)