In human genetics, the sex (male or female) of a newborn baby is determined by the type of sex chromosome contributed by which parent at fertilisation?

Introduction:
In humans, sex determination follows an XY chromosome mechanism. Understanding which parent contributes which type of sex chromosome helps explain how the sex of a baby is decided. This concept is frequently tested in school biology and competitive exams. The question here asks you to identify which parent's chromosome contribution determines whether a child will be male or female.

Given Data / Assumptions:
- Human females have two X chromosomes (XX). - Human males have one X and one Y chromosome (XY). - Each gamete carries one sex chromosome. - The options mention the father, the mother, the father's father and both parents.

Concept / Approach:
During gamete formation, the mother (XX) produces only eggs carrying an X chromosome. The father (XY) produces two kinds of sperm: half carry X and half carry Y. At fertilisation, if an X bearing sperm fertilises the X bearing egg, the resulting zygote is XX and develops as female. If a Y bearing sperm fertilises the egg, the zygote becomes XY and develops as male. Since the mother always contributes an X and the father contributes either X or Y, it is the chromosome from the father that actually decides the sex of the baby.

Step-by-Step Solution:
Step 1: Note that the mother's genotype is XX, so all eggs contain only X chromosomes. Step 2: Recognise that the father's genotype is XY, so he produces X bearing and Y bearing sperm in roughly equal numbers. Step 3: Combine the gametes: egg (X) with sperm (X) gives XX (female), and egg (X) with sperm (Y) gives XY (male). Step 4: Observe that the difference comes solely from whether the sperm brings X or Y, and this depends on the father. Step 5: Conclude that the sex of the newborn is determined by the chromosome inherited from the father.

Verification / Alternative check:
Genetics diagrams such as Punnett squares for a cross between XX (mother) and XY (father) show that the probability of XX and XY offspring is 50% each, based on random fertilisation by X or Y sperm. Medical genetics textbooks also explain that blaming the mother for the sex of the child is scientifically incorrect, because the variation arises from the father's sperm. This consensus strongly supports the father as the determining parent.

Why Other Options Are Wrong:
The mother: She always contributes an X chromosome, so she does not decide whether the combination will be XX or XY. The father's father: The grandfather's genes are already incorporated into the father, but he does not directly provide the gamete that fertilises the egg. Both parents equally: While both contribute one sex chromosome, only the father contributes the variable X or Y type, so the determining factor is not equal in this specific mechanism.

Common Pitfalls:
Cultural misconceptions sometimes lead people to blame mothers for not producing sons or daughters, which is scientifically wrong. Another confusion is thinking that both parents jointly determine sex in a 50-50 manner, without recognising that the variability actually resides in the sperm. Remembering that eggs are always X and that only sperm can be X or Y helps clarify that the father's contribution is decisive in human XY sex determination.

Final Answer:
The sex of a newborn baby is determined by the sex chromosome inherited from The father.