Sex Determination - Reproduction
Sex, whether an individual will be a male or a female is determined at fertilisation, and this directs and controls all the later processes involved in male-female differentiation of the genital system. The genetic determination however is not final and irrevocable. Many internal and external environmental factors come into operation during the developmental process and modify or completely reverse the sexual constitution of the individual. Sex which is established at fertilisation, depends upon the "sex" chromosomes contributed by the parents. You are aware that cells of multicellular organisms contain two types of chromosomes; autosomes and sex chromosomes. In human for example, there are twenty two pairs of autosomes and one pair of sex chromosomes.
In mammals, most frogs, some fishes, and dipterus insects there are two types of sex chromosomes, X and Y. Y chromosome is strongly "male determining". The zygote bearing XY chromosomes gives rise to male and it is called heterozygote, where as zygote of XX chromosomes develops into a female, and it is homozygote. Since a mammalian male is a heterogamete, containing XY chromosomes, therefore, half of the spermatozoa bear X chromosomes and the other half Y chromosomes. A mammalian female is a homogamete containing XX chromosomes, therefore ova contain only the X chromosomes. The union of the spermatozoon bearing X chromosome with the egg gives rise to a female and union of the spermatozoon bearing Y chromosome with the egg will be the genetic male. In birds, female is the heterogametic sex. The small chromosome, equalent to the Y in mammals, is designated by the letter W, and the X chromosome is designated in this case as , Z. Half of the eggs carry a W chromosome and the other half a Z chromosome. All of the sperms carry a Z chromosome. The homozygous (ZZ) condition produces males, and the heterozygous (ZW) produces females. This is the type of mechanism that operates in birds, most reptiles, salamanders, some fishes and insects. XX-XY type of sex determination is called mammalian type of sex determination and ZZ-ZW type is called avian type of sex determination.