Sex Chromosomes | Definition, Types & Examples

By | October 29, 2019

Sex Chromosomes

Sex Chromosomes Definition

What are Sex Chromosomes: They are a kind of chromosome that involves sex determination. Humans and most of the other mammals have two chromosomes for sex; X chromosome and a Y chromosome. Female has two X chromosomes in their genetic cells, and the male has a pair of both X and Y chromosomes in their cells. The eggs from females have X chromosomes and sperms from males have X or Y chromosomes. It means that male chromosomes decide the sex of offspring in the occurrence of fertilization. In mammals, the gender of an individual is determined by the sex chromosomes.

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Types of Sex Chromosomes:

Sex chromosomes can take many forms and the number of ways in which sex determination takes place. The two main types which are used for the determination of sex are known as the XY and ZW systems.

  • X and Y Chromosomes:

In the XY system, the male has X and Y chromosomes in his sperm and the female has both X chromosomes in her egg. The male refer to heterogametic because he can produce two different gametes, depends on sperms. The female refer as homogametic because all her eggs carry one X chromosome. In this system, male or female develops based on the ration between X chromosomes and the numbers of Autosomes sets.

For example, is a diploid organism has two chromosomes, develop into the female, while male rise from diploid which has one X chromosome. The ration impact is important in the determination of sex in fruit flies Drosophila melanogaster and roundworms C. Elegans which has XXY or XXYY organisms of female and XO organisms of the male are lies. This lies in the case of humans where one Y chromosome presents confer maleness, irrespective of numbers of chromosomes or ratio between autosomes and sex chromosomes.

  • W and Z Chromosomes:

Reptiles, some fishes, birds, even some invertebrate underdo determination of sex by this method. In ZW method, males are homogametic as ZZ, while female contains two different sex chromosomes as ZW. Sometimes, the W chromosome can totally absent, as in some species of butterflies, and ZO produces into females. The presence of the W chromosome is not necessary for the development of the females.

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How many sex chromosomes do humans have :

Humans and most of the other mammals have two chromosomes for sex; X chromosome and a Y chromosome. Female has two X chromosomes in their genetic cells, and the male has a pair of both X and Y chromosomes in their cells. The eggs from females have X chromosomes and sperms from males have X or Y chromosomes.

How many sex chromosomes are in a human gamete

Human chromosomes typically exist in somatic cells as pairs. Normal somatic cells are diploid and contain a total of 23 pairs of chromosomes: 22 of these pairs are autosomes and one is a pair of sex chromosomes. Gametes are haploid, meaning their chromosomes do not exist in pairs.

Sex Chromosomes in Flowering Plants:

In most flowering plants, the sex of both male and female present on the same flower. Sometimes, a single plant can produce separate male and female flowers in order to enhance cross-fertilization. In the presence of the male and female plant is rare, and six percent angiosperms show this character, this is known as Dioecy. Those who have this sexual dimorphism is rise due to male-sterile or female sterile mutations, and different sex chromosomes are known in four plant families.

Genetic Disorders in Sex Chromosomes:

Heterogametic organisms are more susceptible to have a genetic disorder because they receive one copy of each gene. For example, if a mother has a recessive genetic disorder, she has a fifty percent chance of passing this disorder to her male baby, depending which allele gets retained in an egg. While, none of her daughters will be affected because they inherit another X chromosome from their father, which is a normal allele. Most cases of sex disorder discovered when the organisms show neurological symptoms, learning difficulties or infertility.

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Accumulation of Harmful Mutation on the Y-Chromosome:

The mutation of autosomes arises from the evaluation of sex chromosomes. At some stages, in the clustering of genes for determination of sex on one of the autosomes, there is a suppression of recombination to assure that the cluster is inherited. When this occurs, an incipient Y chromosome starts to develop with the accumulating transposable element, the chromosomal arrangement, and some harmful mutation.

During meiosis, the inability of the Y chromosome to auto-correct mutation by recombination makes it prone to accumulating errors. In other words, sperms formed in large numbers, all of which enhance the chance of error of accumulation.  Sperms also stored in a highly oxidative environment, and again increase the possibility of genetic mutation. These factors contribute to a situation where the Y chromosome lost most of the genes expects those which are crucial for the determination of sex and survival of the Fetus. This referred to homogametic females which have double numbers of genes of their sex chromosomes compared to heterogametic partners. In some other animals, where the males are XY gene expression on one of the X chromosomes in the female is silenced by the Heterochromatin formation.