In ZW sex-determination system used by birds the opposite is true : the male is the homogametic sex ( ZZ ), and the female is heterogametic ( ZW ).
12.
Alternatively, birds might show decreased transcription of the two Z chromosomes present in the male heterogametic sex, similar to the system observed in the two male X chromosomes of Drosophila.
13.
This system is in contrast with the ZW sex-determination system found in birds, some insects, many reptiles, and other animals, in which the heterogametic sex is female.
14.
The heterogametic sex ( males in humans ) only receives one of each sex chromosome and will therefore show recessive trait even if they have only inherited one copy of a recessive allele.
15.
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 ).
16.
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.
17.
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.
18.
Birds and many insects have a similar system of sex determination ( " ZW sex-determination system " ), in which it is the females that are heterogametic ( ZW ), while males are homogametic ( ZZ ).
19.
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 " '.
20.
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.
