I know for instance some cells are sexual, so, this got me wondering, do the males of all species that have distinct sexes have Y chromosomes?


2 Answers 2


Very short answer

No, not all males of all sexual species have Y chromosomes. You might want to have a look to the Wikipedia page on sex-determination systems.

Long answer

Diversity among the species that reproduce sexually

Not all species that have sexual reproduction have sexes. Yeasts, for example, have mating types but no sex.

Diversity among the species that have sexes

Sex is determined by both genetic and environmental factors. In some species, genetic factors are more important than environmental factors in other species it is the reverse. Species which sex is mostly determined by the genetics are said to have GSD (Genetic Sex Determination). For example, humans are GSD, as the female is XX and the male is XY. The species where sex is mostly determined by the environment are called ESD (Environmental Sex Determination). For examples, crocodiles are ESD as sex is determined by temperature. It is important to understand however that there is a whole continuum between these two extremes.

Diversity among the species that are GSD

Among the species that are GSD, some have sexual chromosomes some don't. Some have one locus (locus=position on a chromosome) that determines the sex, some have many loci (loci=plural of locus). Humans, for example, have sexual chromosomes (X and Y) and have only one locus which determines the sex. This locus is called SRY and it codes for a protein called TDF.

Now you can split GSD with sexual chromosomes into two more categories (it is a bit more complex in reality): XY and ZW. XY are those species where the male is heterogametic (XY), while the female is homogametic (XX). In ZW systems, the male is homogametic (ZZ) and the female is heterogametic (ZW). Birds and some plants have ZW systems for example, while mammals (except "basal" mammals) and Drosophila have XY system.

See also the post What determines sex in birds?

Extra Information

Dosage Compensation

In species that have sex chromosomes, there is a difference in the number of copy of genes between the sexes. In eutherian mammals, for example, females have two copies of all the genes on the X chromosomes, while males only have one copy of most of these genes (plus a few Y chromosome genes). The set of methods to deal with this issue is called Dosage Compensation and there is also an impressive diversity dosage compensations.

Comments on this diversity of sexual systems

The diversity in a sex-determination system, dosage compensation and other things related to sex are impressive. It is even more impressive when we look at how many independent origins there are. Below are some other examples.

The Amazon molly (a fish) is a species that have sexual reproduction but there are no males. The females have to seek for sperm in a sister species in order to activate the development of the eggs but the genes of the father from the sister species are not used. (see this article)

There are also hermaphrodites including sequential hermaphrodites (first male, then females or the opposite) in plants and animals. There are also species where populations are made of hermaphrodites and females and others where there are hermaphrodites and males (very uncommon).

In some species, the sex is determined by social factors. In clownfish, the sex is determined by comparing its own size with the size of the other fishers living in the same anemone.

In an ant species (or two species actually), males and females can both reproduce by parthenogenesis (some kind of cloning but with meiosis and cross-over) but they the meet they reproduce together and their offsprings are sterile workers. So males and females are just like two sister species that reproduce sexually to create an army to protect and feed them. See more information in this paper

Here is a nice figure from Bachtrog et al. 2014 that offers an idea of the diversity of sex determination system (thank to @rg225 for pointing out this figure).

enter image description here Book suggestion

The Evolution of Sex-Determination is a great book that may interest you.


No. There are many sex-determination systems. Mammals and fruitflies use the XX/XY sex-determination system — except for the platypus, which has 10 sex chromosomes.

ZW sex-determination system is used by birds and some reptiles. It's similar but with the male having two of the same chromosome (ZZ) and the female being the heterogametic sex (ZW). There's several other variations, such as X0 (XX for females, X for males, with no Y).

There are also animals with temperature-dependent sex determination and others use reverse their sex.

  • 4
    $\begingroup$ another fun fact is that the bird sex ZZ/WZ and the mammalian XY/XX evolved independatly without common origin... also the genes involved in sex determination is mostly the same but the order of the activation of genes is different. For example: birds are capable of sex reversal on stimulation/inhibition of estrogen, unlike mammals but like reptiles... $\endgroup$ Commented Jan 9, 2015 at 21:06

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .