5 Types of Sex Determination in Animals
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5 Types of Sex Determination in Animals

5 types of sex determination (male or female) in animals. There are the XY sex-determination system, XO sex-determination system, ZW sex-determination system, haplodiploidy, and environmentally dependent sex-determination system.

Here is a list of interesting determination of sexes of animals (meaning multicellular organisms with some form of neurological function, reproductive function, and having specialized sense organs):

Just as a note, a typical human has one X chromosome from the mother, and either an X or a Y chromosome from the father.  Therefore, a human will have two chromosomes (either XX or XY) unless there is a mistake when the chromosomes separate during meiosis.  An XX genotype denotes a female human, whereas an XY genotype represents a male human.

1.   XY sex determination: In all mammals and some other animals, an individual is either XX (female) or XY (male).  This is what we typically learn in high school.

2.   XO sex determination: In animals such as grasshoppers, there is no Y chromosome.  Therefore, females are XX, whereas males are XO.  The “O” means that there is no chromosome—the male grasshoppers have only one X chromosome.  Therefore, the males' sperm can either carry an X chromosome or none whatsoever, with a 50% probability of either choice.

3.   ZW sex determination: In animals such as birds, the order of the “X” and “Y” chromosomes is switched.  Therefore, a female bird would be “XY” and a male bird would be “XX”.  But because it is confusing to call these chromosomes X and Y, Z and W are used instead.  As a result, males would be ZZ and females would be ZW.  Z chromosomes are larger than W chromosomes, which is much like the X chromosome being larger than the Y chromosome.  Also, in these cases, the egg decides the sex of the organism, and not the sperm.

4.   Haplodiploidy: In animals such as ants, the number of sets of chromosomes determines the individual’s sex.  When an egg is fertilized by a sperm, then the developed organism will be diploid, or 2n (two sets of chromosomes).  The organism will then develop as a female.  However, if the egg is not fertilized, then the organism will develop as a male.  The male will be haploid (1n, with only one set of chromosomes).

5.   Environmentally dependent sex-determination: In animals such as alligators, the temperature of the environment in which the eggs are raised in determines the sex of the individual.  This is because at higher temperatures, the embryos in the eggs develop at a faster rate, and as a result, the embryo develops into a female.  Contrastingly, in colder temperatures, the embryo develops at a slower rate and becomes male.  Temperature, however, is not the only environmental factor that controls sex determination.

As can be seen above, the presence of X and Y chromosomes do not absolutely define the sex of an organism.  With grasshoppers and ants, this is most certainly untrue; and with birds, it is the “opposite”.  With some other organisms that have X and Y chromosomes, the sex of the organism is neither determined by the sperm nor the egg, but by the environment the embryo lives in.  With that said, evolution has provided organisms a multitude of ways of determining whether an individual is male or female, with each method benefitting each species in unique ways. 

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Comments (1)

Good points, sometimes what we think we know... is wrong!

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