Human Genetics Questions and Answers

Answer: A

Red green colour blindness is an X-linked recessive disorder.

The genes for the red and green cone opsins lie on the X chromosome. A male has one X, so a single defective allele makes him colour blind; a female needs two copies and is usually an unaffected carrier. That is why about 8% of men of European descent but far fewer women are affected, and why the trait passes from a carrier mother to half her sons.

An X-linked dominant trait would appear in every daughter of an affected father, which is not seen. Y-linked traits pass only from father to son, and there are no colour vision genes on the Y.

Answer: C

Trisomy 21 is Down syndrome.

An extra copy of chromosome 21, usually from non-disjunction in the mother's meiosis, gives a karyotype of 47,+21. Features include intellectual disability, a flat facial profile, upward-slanting eyes, a single palmar crease and congenital heart defects; the risk rises with maternal age.

Edwards syndrome is trisomy 18 and Patau syndrome trisomy 13. Klinefelter syndrome is a sex-chromosome condition, 47,XXY.

Answer: A

The insensitivity is to androgen.

In male pseudohermaphroditism due to androgen insensitivity syndrome, a 46,XY individual has testes that make testosterone normally, but the androgen receptor does not respond to it. External genitalia develop along female lines and, in the complete form, the person appears female, though without a uterus.

Insensitivity to oestrogen, progesterone or corticosterone does not change the development of the genitalia in this way, since male differentiation depends on androgen action.