Biology · Principles of Inheritance and Variation · NEET
The human female is XX. She has two X chromosomes as her sex chromosomes, plus 22 pairs of autosomes, so her full karyotype is 44 autosomes + XX. Because both her sex chromosomes are the same (X and X), all her eggs carry only an X chromosome. This is why the female is called homogametic (she makes only one kind of gamete for sex).
The male is XY. He has one X and one Y chromosome. When he makes sperm, half of the sperm carry the X chromosome and half carry the Y chromosome. Since he makes two different types of gametes, he is called heterogametic. 'Hetero' means different. The female is homogametic because all her eggs are the same (only X).
The father decides the sex of the baby. The mother's egg always has an X. If a sperm carrying X joins the egg, the baby is XX (a girl). If a sperm carrying Y joins the egg, the baby is XY (a boy). So the type of sperm from the father sets the sex. It is scientifically wrong to blame the mother for the sex of the child.
There is a 50% chance of a boy and a 50% chance of a girl in each pregnancy. This is because the father makes X sperm and Y sperm in equal numbers (1:1). When you draw a Punnett square of XX (mother) x XY (father), you get 2 XX (girls) and 2 XY (boys), which is a 1:1 ratio.
XX and XY name the sex chromosomes a person has. X and Y are two special chromosomes. A person with two X chromosomes (XX) develops as a female. A person with one X and one Y (XY) develops as a male. The Y chromosome carries genes that push development toward the male sex, so having a Y usually makes the person male.
Human sex determination is male heterogamety of the XX-XY type. The male (XY) is heterogametic because he makes two kinds of sperm (X and Y). The female (XX) is homogametic. Female heterogamety is the ZW-ZZ type seen in birds, not in humans. So for humans, always write: XX-XY type, male heterogamety.
Match the items of Column I with Column II: Column I: (a) XX-XO method of sex determination (b) XX-XY method of sex determination (c) Karyotype-45 (d) ZW-ZZ method of sex determination Column II: (i) Turner's syndrome (ii) Female heterogametic (iii) Grasshopper (iv) Female homogametic Select the correct option:
In which genetic condition, each cell in the affected person has three sex chromosomes XXY?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A human body cell has 46 chromosomes, that is 23 pairs. Of these, 22 pairs (44 chromosomes) are autosomes and 1 pair is the sex chromosomes. In a female the sex pair is XX and in a male it is XY.
A normal human female is written as 44 + XX (or 46, XX). A normal human male is written as 44 + XY (or 46, XY). The 44 stands for the 22 pairs of autosomes.
No. Every egg from the mother carries an X chromosome, because the mother is XX. So the egg cannot change the sex. The sex is decided only by whether the sperm carries an X or a Y.
The Y chromosome carries genes (such as the SRY gene) that direct the embryo to develop as a male. If a Y is present, the person usually develops male features. Without a Y (as in XX), the person develops as a female.
In humans, both males and females are diploid and sex depends on the XX/XY sex chromosomes. In the honey bee, sex depends on the number of chromosome sets: females (queen, workers) are diploid from fertilised eggs, while males (drones) are haploid from unfertilised eggs. This is called haplodiploidy.
