Question

1. who will males always inherit their x chromosome from?

1. who will males always pass on their x chromosome to?

1. why would a recessive x - linked trait always be expressed in males, but not in females?

1. under what circumstances will recessive x - linked traits be expressed in females?

1. look at the figure showing the unaffected father (xy) and the carrier mother (xx^c). let c represent the recessive allele for color blindness. use the colors in the diagram to help you fill in the punnett square that represents the children you see.

	x	y
x^c

Explanation:

1. Males have the sex - chromosome combination \(XY\). Since the \(Y\) chromosome comes from the father, the \(X\) chromosome must come from the mother.
2. When males pass on their chromosomes, they can pass either an \(X\) or a \(Y\) chromosome. Females have \(XX\) chromosomes. So, males always pass their \(X\) chromosome to their daughters.
3. Males have only one \(X\) chromosome (\(XY\)). If a recessive allele is on that single \(X\) chromosome, there is no second \(X\) chromosome with a dominant allele to mask it. Females have two \(X\) chromosomes (\(XX\)). A recessive trait on one \(X\) can be masked by a dominant allele on the other \(X\).
4. For a recessive \(X -\)linked trait to be expressed in females, they must inherit the recessive allele on both of their \(X\) chromosomes (i.e., be homozygous recessive \(X^{c}X^{c}\)).
5. In a Punnett square, we combine the gametes. The father has gametes \(X\) and \(Y\), and the mother has gametes \(X\) and \(X^{c}\).

• When we combine \(X\) (from mother) and \(X\) (from father), we get \(XX\).
• When we combine \(X\) (from mother) and \(Y\) (from father), we get \(XY\).
• When we combine \(X^{c}\) (from mother) and \(X\) (from father), we get \(X^{c}X\).
• When we combine \(X^{c}\) (from mother) and \(Y\) (from father), we get \(X^{c}Y\).

Answer:

1. Mother
2. Daughters
3. Males have one \(X\) chromosome; no dominant allele to mask recessive. Females have two \(X\) chromosomes; dominant can mask recessive.
4. When homozygous recessive (\(X^{c}X^{c}\))

17.

	\(X\)	\(Y\)
\(X^{c}\)	\(X^{c}X\)	\(X^{c}Y\)