Sex is the most conspicuous phenotype in the animal kingdom. Animals with distinct males and females are sexually dimorphic. Sometimes this dimorphism is established by environmental factors. For example, in one species of turtles, sex is determined by temperature. Eggs that have been incubated above 30°C hatch into females. The eggs that are incubated at a lower temperature hatch into males. In many other species, sexual dimorphism is established by genetic factors. It involves pair of sex chromosomes X and Y.
Sex Chromosomes
The fruit fly (Drosophila melanogaster) has eight chromosomes in the form of four homologous pairs. The chromosomes of the three homologous pairs are similar in both sexes, but the fourth pair is very different. The female had two similar rod shaped X-chromosomes in the fourth pair, while the male has one rod shaped X-chromosome and one J-shaped Y-chromosome in the fourth heteromorphic pair. X and Y chromosomes are called sex chromosomes because these have gene for determination of sex. All chromosomes other than sex chromosomes are called autosomes. Autosomes do not carry any sex determining gene.
Humans have 46 chromosomes in the form of 23 pairs. 22 pairs are of autosomes and one pair is of sex chromosomes. Autosome pairs are common in both the sexes but the 23rd sex chromosome pair is very different in males and females. The 23rd pair in man is heteromorphic. She is XX but he is XY.
SRY is the male determining gene. It is located at the tip of short arm of Y-chromosome. Its name SRY stands for “Sex determining regions of Y”.
In some grasshoppers males and females have different number of chromosomes. The female has 24 chromosomes in the form of 11 pairs of autosomes and one pair of X-chromosomes. But the male has 23 chromosomes. He has 11 pairs of autosomes and only one X-chromosome. The other member for sex chromosome is entirely missing in male. Thus male is XO and female is XX.
SEX DETERMINATION IN ANIMALS
1. XY-XX System (Sex Determination in Human and Drosophila)
In both Drosophila and humans, males produce two kinds of gametes. These gametes are X-bearing and Y-bearing. Hence it is called heterogametic sex. Males produce two types of sex-determining sperms. Half the sperms carry X-chromosome and the other half carry Y-chromosome. Chances for both types of sperms are equal.
In these species females are the homogametic sex. Females produce only one type of eggs, each with an X chromosome. Sex of the offspring is determined by the type of sperm. If an X-carrying sperm fertilizes the egg, the zygote will be XX, and a female offspring will be produced. If a Y-carrying sperm fertilizes the egg, the zygote will be XY, and a male offspring will be produced. The sex-ratio between male and female offspring is 1:1.
2. ZZ-ZW System (Sex Determination in Birds, Butterflies, Moths and Reptiles):
This type of sex-determination is common in birds, butterflies, moths and some reptiles. It was discovered by J. Seiler in 1914 in moth. It is the reverse of XY-XX system. Here the female is heterogametic ZW but the male is homogametic ZZ. Female produces two kinds of eggs Z and W in equal proportions. All sperms are alike, each carrying a Z-chromosome. It is the kind of egg that determines the sex of offspring. Sex ratio is 1:1.
3. XO-XX (Haplo-Diplo) System (Sex Determination in Honeybees):
In honeybees, sex is determined by whether the animal is haploid or diploid. It is called haplo-diplo system of sex determination.
Diploid embryos develop from fertilized eggs. It becomes females (worker bees). The haploid embryos develop from unfertilized eggs. It becomes males (drones). The maturing of female into a reproductive form (queen) depends on how she was nourished as a larva.
In this system, a queen can control the ratio of males to females. She regulates the proportion of unfertilized eggs that she lays. Thus most of the progeny are female. These are sterile and serve as workers for the hive.
In this system eggs are produced through meiosis in the queen. But the sperms are produced through mitosis in the male. This system ensures that fertilized eggs will have diploid chromosome number. But the unfertilized eggs will have the haploid number.
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