MadSci Network: Genetics |
Not all animals possess the "traditional" XY chromosomal sex determination mechanisms. Mammals including humans, Drosophila and many other species have an X-Y mechanism that determines sex at fertilization. The male is the heterogametic sex, containing an X chromosome and a Y chromosome. The female is the homogametic sex carrying two X chromosomes. However, the mechanism that determines maleness may vary among these animals. For example, in mammals it is the presence of the Y chromosome, but in Drosophila, maleness is not determined by the presence of Y but by the ratio of X chromosomes to autosomes. Other animals like grasshoppers, crickets, roaches and some other insects have an X-O sex detrmination mechanism.Like the X-Y system, the male is the heterogametic sex because males have two types of gametes: X and O (absence of sex chromosome). Females have two X chromosomes. A third sex determination mechanism is Z-W found in birds, some fish and some insects such as butterflies and moths.The female is the heterogametic sex; males are ZZ while females are possess two different sex chromosomes, ZW. Finally, some animals exhibit haplo-diploidy. Some species of bees and ants have no sex chromosomes at all, yet sex determination has a chromosomal basis.Females develop from fertilized eggs and are diploid (have two copies of each chromosome).Males develop from unfertilized eggs (parthenogenetically) and are haploid (have only one copy of each chromosome).
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