8.2.4. Sex Determination

We have discussed the idea that the two sexes participating in sexual reproduction must be somewhat different from each other for a number of reasons. How is the sex of a newborn individual determined? Different species use very different strategies for this.

In sexual reproduction, males and females exhibit differences. The determination of sex in newborns varies among species. Some organisms use environmental factors to decide sex, while others rely mainly on genetics. For instance, certain reptiles may have their sex determined by the temperature of the eggs they are in, while in some snails, individuals can change their sex based on circumstances. However, in humans, sex is typically established through genetics.

Imagine a baby sea turtle that will become a male or female based on the warmth of the sand it was born in. If the sand is warm, it will become female, and if it's cooler, it will become male. This shows how in some animals, the environment plays a critical role in determining sex.

However, in human beings, the sex of the individual is largely genetically determined. In other words, the genes inherited from our parents decide whether we will be boys or girls.

In humans, genetic inheritance is the primary factor in determining sex. Each person receives a set of chromosomes from their parents. Humans have 23 pairs of chromosomes, but the last pair, the sex chromosomes, are unique. Women have two X chromosomes (XX), while men have one X and one Y chromosome (XY). This distinction is key to understanding how sex is determined at conception.

Think of chromosomes as pairs of shoes. A woman has two pairs of identical shoes (XX), and a man has one pair of regular shoes (X) and one smaller pair (Y). When they have a child, the child will always get one shoe from each pair. The type of shoe from the father will ultimately decide whether the child is a boy or a girl.

Now, can we work out what the inheritance pattern of X and Y will be? As Fig. 8.6 shows, half the children will be boys and half will be girls. All children will inherit an X chromosome from their mother regardless of whether they are boys or girls.

When a child is conceived, they inherit one sex chromosome from their mother and one from their father. Mothers will always contribute an X chromosome. The father can either contribute another X chromosome, resulting in a girl (XX), or a Y chromosome, leading to a boy (XY). Therefore, theoretically, it results in a 50% chance of having a boy and a 50% chance of having a girl.

It's like flipping a coin. If you flip a coin twice, you might get heads (girl) one time and tails (boy) another. With each flip, there's an equal chance of getting either result, just like the way parents determine the sex of their child through chromosome contribution.

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