In mammals, the female is homogametic, with two X chromosomes ( XX ), while the male is the heterogametic sex, with one X and one Y chromosome ( XY ).
12.
However, in birds, and some reptiles, males have two Z sex chromosomes and so are the homogametic sex, while females, with one Z and one W chromosome, are the heterogametic sex.
13.
This happens because female birds have a single Z sexual chromosome, while males carry two, contrarily to what happens in humans, where the male is the heterogametic sex, as occurs in most diploid species.
14.
For example, in humans, males, with an Y sex chromosome, would be referred to as the heterogametic sex, and females having two X sex chromosomes would be referred to as the "'homogametic sex " '.
15.
Normal females are always of genotype " Dw /-", while dwarf females are always of genotype " dw /-", because female is the heterogametic sex having only one " Z " chromosome.
16.
Regardless of their ambiguous evolutionary history, platypuses have been empirically determined to follow an XY sex-determination system, with females possessing five pairs of X chromosomes as the homogametic sex, and males possessing five X and five Y chromosomes as the heterogametic sex.
17.
There has been debate on whether the MHM locus constitutes dosage compensation, however, since scientists claim that even if the MHM locus has been found to have significantly greater expression in females than in males, it could not even be considered to be a dosage compensation mechanism since it does not balance gene dose between the Z chromosome and autosomes in the heterogametic sex.
18.
If dioecious organisms are heterogametic and the gene locus is located on the X chromosome, it can be shown that if the allele frequencies are initially unequal in the two sexes [ " e . g " ., XX females and XY males, as in humans ], in the heterogametic sex chases in the homogametic sex of the previous generation, until an equilibrium is reached at the weighted average of the two initial frequencies.
19.
Since each crossover gives 50 % recombinants, the chiasma count implies a total male genetic map length of 2450 cM . The current version of the Location Database ( Collins et al ., 1996 ) suggests a total male map length of 2851 cM . Chiasmata are more frequent in female meiosis ( exemplifying Haldane's rule that the heterogametic sex has the lower chiasma count ), and the total female map length in the Location Database is 4296 cM ( excluding the X ).
