What is the gene that determines the sex of an offspring?

Answer 1

With people, girls have XX and guys have XY chromosomes.

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This mechanism of how this genetic sex determination works varies:-

With many animals, females have two of the same kind of sex chromosome (XX), while males have two distinct sex chromosomes (XY). Some species (including humans) have a gene SRY on the Y chromosome that determines maleness; others (such as the fruit fly) use the presence of two X chromosomes to determine femaleness.

There are variants to this mechanism to be found in insects which use an XX/X0 mechanism:-

In this variant of the XY system, females have two copies of the sex chromosome (XX) but males have only one (X0). The 0denotes the absence of a second sex chromosome.

Another variation found in birds uses the ZW chromosome set:-

The ZW sex-determination system is reversed compared to the XY system: females have two different kinds of chromosomes (ZW), and males have two of the same kind of chromosomes (ZZ)

One must also consider the way animals such as bees determine sex, this is called Haplodiploidy:-

In this mechanism, unfertilized eggs develop into haploid individuals, which are the males. Diploid individuals are generally female but may be sterile males. Thus, if a queen bee mates with one drone, her daughters share ¾ of their genes with each other, not ½ as in the XY and ZW systems.

With other animals genes play no part in the determination of sex, for instance in some species of reptiles, including alligators, some turtles, and the tuatara, sex is determined by the temperature at which the egg is incubated

While other species, such as some snails, practice sex change: adults start out male, then become female. In many tropical reef fish, the dominant individual in a group becomes female while the other ones remain male,

Also in some species individuals have no real sex differentiation at all, for instance Earthworms are hermaphrodites.

Answer 2

The gene that determines the sex of an offspring is the sex-determining gene, specifically the SRY gene located on the Y chromosome in humans. The presence of this gene leads to the development of male characteristics, while its absence results in female development.