Problem - Week 8
1. Red-green color blindness in humans is caused by a recessive gene on the X chromosome
(rare trait). A man and woman both have normal vision, although both of their fathers are
red-green colourblind. What is the probability that their first child will be:
(a) a son with normal vision?
The man does not carry the gene for color blindness because he inherited his normal
X chromosome from his mother. However, the woman is a carrier of the
colorblindness gene because she inherited one normal X chromosome from her
mother and one colorblind X chromosome from her father who is colorblind.
Therefore, the probability of their first child being a son with normal vision is 50%, as
the son can inherit either the normal X chromosome from his father or the normal X
chromosome from his mother.
(b) a daughter with normal vision?
The probability of their first child being a daughter with normal vision is also 50%.
The daughter can inherit either the normal X chromosome from her father or the
normal X chromosome from her mother.
(c) a color blind son?
The probability of their first child being a colorblind son is 25%. The son can only
inherit the colorblind X chromosome from his mother, as the father does not carry the
gene for color blindness.
(d) a color blind daughter?
The probability of their first child being a colorblind daughter is also 25%. The
daughter can inherit the colorblind X chromosome from both her mother and father,
but this is less likely than inheriting one normal X chromosome from either parent.
2. What are the sexual phenotypes of the following genotypes XX, XY, XXY, XO in:
(a) Drosophila?
XX individuals are females, while XY individuals are males. XXY individuals are
intersex (also known as "metafemales") with female external genitalia and some male
traits such as smaller testes. XO individuals are also intersex (also known as
"metamales") with male external genitalia but are sterile and have underdeveloped
gonads.
(b) Humans?
In humans, XX individuals are females, while XY individuals are males. XXY
individuals have Klinefelter syndrome, which results in male characteristics but often
includes reduced testosterone levels and infertility. XO individuals have Turner
1. Red-green color blindness in humans is caused by a recessive gene on the X chromosome
(rare trait). A man and woman both have normal vision, although both of their fathers are
red-green colourblind. What is the probability that their first child will be:
(a) a son with normal vision?
The man does not carry the gene for color blindness because he inherited his normal
X chromosome from his mother. However, the woman is a carrier of the
colorblindness gene because she inherited one normal X chromosome from her
mother and one colorblind X chromosome from her father who is colorblind.
Therefore, the probability of their first child being a son with normal vision is 50%, as
the son can inherit either the normal X chromosome from his father or the normal X
chromosome from his mother.
(b) a daughter with normal vision?
The probability of their first child being a daughter with normal vision is also 50%.
The daughter can inherit either the normal X chromosome from her father or the
normal X chromosome from her mother.
(c) a color blind son?
The probability of their first child being a colorblind son is 25%. The son can only
inherit the colorblind X chromosome from his mother, as the father does not carry the
gene for color blindness.
(d) a color blind daughter?
The probability of their first child being a colorblind daughter is also 25%. The
daughter can inherit the colorblind X chromosome from both her mother and father,
but this is less likely than inheriting one normal X chromosome from either parent.
2. What are the sexual phenotypes of the following genotypes XX, XY, XXY, XO in:
(a) Drosophila?
XX individuals are females, while XY individuals are males. XXY individuals are
intersex (also known as "metafemales") with female external genitalia and some male
traits such as smaller testes. XO individuals are also intersex (also known as
"metamales") with male external genitalia but are sterile and have underdeveloped
gonads.
(b) Humans?
In humans, XX individuals are females, while XY individuals are males. XXY
individuals have Klinefelter syndrome, which results in male characteristics but often
includes reduced testosterone levels and infertility. XO individuals have Turner
