6.4.4 Sex Determinator

Since the mechanism of sex determination in human completely depends upon the father instead of mother, therefore, the male individual is called Sex determinator.

Simple explanation: The father determines the sex of the baby because he can contribute either an X or a Y chromosome. The mother always contributes an X chromosome.

Summary Table

Intermediate Level

Sex determination in humans is controlled by sex chromosomes:

Key points:

• Humans have 23 pairs of chromosomes.
• 22 pairs are autosomes.
• 1 pair is sex chromosomes.
• Females are XX; males are XY.
• Mother contributes X; father contributes X or Y.
• XX = female; XY = male.
• Probability of son or daughter = 50%.
• Male is the sex determinator.

Important Points

• - Sex determination means that a newborn would be a male baby like father or female baby like the mother.
• - The mechanism of sex determination in humans is controlled by genetic factors, specifically the sex chromosomes.
• - Humans have two types of sex chromosomes: X and Y.
• - The combination of these chromosomes inherited from the parents determines the biological sex of the individual.
• - Humans have 46 chromosomes in total, arranged in 23 homologous pairs.
• - Out of these 23 pairs, 22 pairs are autosomes.
• - 1 pair is sex chromosome pair.
• - In females, the sex chromosome pair consists of two completely homologous chromosomes called XX.
• - In males, the sex chromosome pair consists of two partially homologous chromosomes called X and Y.
• - During reproduction, each parent contributes one sex chromosome to the offspring.
• - The mother can only contribute an X chromosome, as she is XX.
• - Since mother can produce only same type of gametes, therefore, the human female is called homogametic sex.
• - The father can contribute either an X or a Y chromosome as he is XY.
• - Since father can produce two different types of gametes, therefore, the human male is called heterogametic sex.
• - When male and female gametes are fused, the combination of these chromosomes determines the sex of the baby.
• - If the baby inherits an X chromosome from the father, the combination will be XX and the baby will be female.
• - If the baby inherits a Y chromosome from the father, the combination will be XY and the baby will be male.
• - The probability for a couple to have a son or a daughter during each pregnancy is 50%.
• - Since the mechanism of sex determination in human completely depends upon the father instead of mother, therefore, the male individual is called Sex determinator.

Remember

• - Sex chromosomes = X and Y
• - Female = XX (homogametic)
• - Male = XY (heterogametic)
• - Mother contributes X only
• - Father contributes X or Y
• - XX = female
• - XY = male
• - Probability = 50% son, 50% daughter
• - Male = sex determinator
• - SRY gene on Y chromosome triggers male development

Common Mistakes

• 1. Thinking the mother determines the sex of the baby.
• - The father determines the sex because he can contribute either X or Y. The mother always contributes X.
• 2. Confusing homogametic and heterogametic.
• - Homogametic = female (XX), produces only X gametes.
• - Heterogametic = male (XY), produces X and Y gametes.
• 3. Mixing up XX and XY.
• - XX = female.
• - XY = male.
• 4. Forgetting the role of the SRY gene.
• - The SRY gene on the Y chromosome triggers male development.
• 5. Thinking the probability of a son or daughter is not 50%.
• - Each pregnancy has a 50% chance of a son and 50% chance of a daughter.
• 6. Believing autosomes determine sex.
• - Autosomes do not determine sex. Sex is determined by sex chromosomes.
• 7. Confusing the number of chromosomes.
• - Humans have 46 chromosomes (23 pairs): 22 pairs autosomes + 1 pair sex chromosomes.

Quick Revision

60-Second Revision

• **Sex chromosomes** = X and Y
• **Female** = XX (homogametic)
• **Male** = XY (heterogametic)
• Mother contributes X only
• Father contributes X or Y
• **XX** = female
• **XY** = male
• **Probability** = 50% son, 50% daughter
• **Male** = sex determinator
• SRY gene on Y chromosome triggers male development
• Warning: Do not think the mother determines the sex of the baby. The father determines the sex because he can contribute either X or Y. The mother always contributes X.