Most animals and many plants show sexual dimorphism; in other words, individuals are either male or female. In most of these cases, sex is determined by a special pair of sex chromosomes. Let’s look at humans as an example. Human body cells have 46 chromosomes: 22 homologous pairs of autosomes plus 2 sex chromosomes.
In females, there is a pair of identical sex chromosomes called the X chromosomes. In males, there is a nonidentical pair, consisting of one X and one Y.The Y chromosome is considerably shorter than the X. At meiosis in females, the two X chromosomes pair and segregate like autosomes so that each egg receives one X chromosome. Hence with regard to sex chromosomes the gametes are of only one type, and the female is said to be the homogametic sex. At meiosis in males, the X and the Y chromosomes pair over a short region, which ensures that the X and Y separate to opposite ends of the meiotic cell,creating two types of sperm,half with an X and the other half with a Y.Therefore the male is called the heterogametic sex.
The inheritance patterns of genes on the sex chromosomes are different from those of autosomal genes. These sex chromosome inheritance patterns were ﬁrst investigated in the fruit ﬂy Drosophila melanogaster.
This insect has been one of the most important research organisms in genetics;its short,simple life cycle contributes to its usefulness in this regard. Fruit ﬂies have three pairs of autosomes plus a pair of sex chromosomes again referred to as X and Y.As in mammals, Drosophila females have the constitution XX and males are XY. However, the mechanism of sex determination in Drosophila differs from that in mammals. In Drosophila, the number of X chromosomes determines sex: two X’s result in a female and one X results in a male. In mammals, the presence of the Y determines maleness and the absence of a Y determines femaleness.This difference is demonstrated by the sexes of the abnormal chromosome types XXY and XO.
Vascular plants show a variety of sexual arrangements. Dioecious species are those showing animal-like sexual dimorphism, with female plants bearing ﬂowers containing only ovaries and male plants bearing ﬂowers containing only anthers