Sample 1Medium
Two karyotype sketches of a species are shown. Individual P has two identical sex chromosomes labelled Z and Z. Individual Q has two different sex chromosomes labelled Z and W, and produces two kinds of gametes. Which statement is correct?
- A
This is male heterogamety, as in humans, and Q is the male
- B
This is the XO type, as in grasshoppers, and Q is the male
- C
This is female heterogamety, as in Drosophila
- D
This is female heterogamety, as in birds, and Q is the female
Show the answer
The answer is D.In birds the female has one Z and one W chromosome and produces two kinds of eggs, while the male has two Z chromosomes. This is female heterogamety.
- A.
Humans use XX-XY; the ZW-ZZ system shows female heterogamety.
- B.
In the XO type one sex lacks a chromosome; here both individuals have two sex chromosomes.
- C.
Drosophila has the XY type with male heterogamety.
NCERT: Class 12 Biology, Chapter 4, 4.6
Sample 2NEET level
Consider the following statements about sex determination: 1. In grasshopper, males have one chromosome fewer than females. 2. In birds, the female is heterogametic, with Z and W sex chromosomes. 3. In human beings, the sex of the child is decided by the sperm. 4. In the XO type, all the sperms produced by the male carry an X chromosome. Which of the statements above are correct?
- A
1, 2 and 3 only
- B
2 and 3 only
- C
1, 3 and 4 only
- D
1, 2, 3 and 4
Show the answer
The answer is A.In grasshopper (XO type) the male has only one X, so half its sperms carry the X and half do not, and males have one chromosome fewer than females. Birds show ZW-ZZ determination with the female heterogametic, and in humans the father's sperm, carrying either X or Y, decides the sex of the child.
- B.
Leaves out statement 1, which is correct: XO males have one chromosome fewer than XX females.
- C.
Includes statement 4, which is false: only half the sperms of an XO male carry the X, and leaves out the correct statement 2.
- D.
Includes statement 4, which is false: an XO male makes sperms with and without the X.
NCERT: Class 12 Biology, Chapter 4 (Principles of Inheritance and Variation), Sex determination