How Chromosomes Determine Sex

Human Karyotype
Credit: PASIEKA/SPL/Getty Images

Chromosomes are long segments of genes that carry hereditary information. They are composed of DNA and proteins and are located within the nucleus of our cells. Chromosomes determine everything from hair color and eye color to sex. Whether you are a male or female depends on the presence or absence of certain chromosomes. Human cells contain 23 pairs of chromosomes for a total of 46. There are 22 pairs of autosomes (non-sex chromosomes) and one pair of sex chromosomes.

The sex chromosomes are the X chromosome and the Y chromosome.

Sex Chromosomes

In human sexual reproduction, two distinct gametes fuse to form a zygote. Gametes are reproductive cells produced by a type of cell division called meiosis. Gametes are also called sex cells. They contain only one set of chromosomes and are thus said to be haploid.

The male gamete, called the spermatozoan, is relatively motile and usually has a flagellum. The female gamete, called the ovum, is nonmotile and relatively large in comparison to the male gamete. When the haploid male and female gametes unite in a process called fertilization, they develop into what is called a zygote. The zygote is diploid, meaning that it contains two sets of chromosomes.

Sex Chromosomes X-Y

The male gametes, or sperm cells, in humans and other mammals are heterogametic and contain one of two types of sex chromosomes. Sperm cells carry either an X or Y sex chromosome.

Female gametes, or eggs, however, contain only the X sex chromosome and are homogametic. The sperm cell determines the sex of an individual in this case. If a sperm cell containing an X chromosome fertilizes an egg, the resulting zygote will be XX, or female. If the sperm cell contains a Y chromosome, then the resulting zygote will be XY, or male.

Y chromosomes carry the necessary genes for the development of male gonads, or testes. Individuals that lack a Y chromosome (XO or XX) develop female gonads, or ovaries. Two X chromosomes are needed for the development of fully functioning ovaries.

Genes located on the X chromosome are called X-linked genes, and these genes determine X sex-linked traits. A mutation occurring in one of these genes could lead to the development of an altered trait. Because males have only one X chromosome, the altered trait would always be expressed in males. In females, however, the trait may not always be expressed. Because females have two X chromosomes, the altered trait could be masked if only one X chromosome has the mutation and the trait is recessive. An example of an X-linked gene is red-green colorblindness in humans. 

Sex Chromosomes X-O

Grasshoppers, roaches, and other insects have a similar system for determining the sex of an individual. Adult males lack the Y sex chromosome that humans have and have only an X chromosome. They produce sperm cells that contain either an X chromosome or no sex chromosome, which is designated as O. The females are XX and produce egg cells that contain an X chromosome.

If an X sperm cell fertilizes an egg, the resulting zygote will be XX, or female. If a sperm cell containing no sex chromosome fertilizes an egg, the resulting zygote will be XO, or male.

Sex Chromosomes Z-W

Birds; some insects such as butterflies; frogs; snakes; and some species of fish have a different system for determining sex. In these animals, it is the female gamete that determines the sex of an individual. Female gametes can either contain a Z chromosome or a W chromosome. Male gametes contain only the Z chromosome. Females of these species are ZW, and males are ZZ.

Parthenogenesis

What about animals like most kinds of wasps, bees, and ants that have no sex chromosomes? In these species, fertilization determines sex. If an egg becomes fertilized, it will develop into a female. A non-fertilized egg may develop into a male.

The female is diploid and contains two sets of chromosomes, while the male is haploid. This development of an unfertilized egg into a male and a fertilized egg into a female is a type of parthenogenesis known as arrhenotokous parthenogenesis.

Environmental Sex Determination

In turtles and crocodiles, sex is determined by the temperature of the surrounding environment at a specific period in the development of a fertilized egg. Eggs that are incubated above a certain temperature develop into one sex, while eggs incubated below a certain temperature develop into the other sex. Both males and females develop when eggs are incubated at temperatures ranging between those that induce only single-sex development.