sex chromosomes

(noun)

A chromosome involved with determining the sex of an organism, typically one of two kinds.

Related Terms

Examples of sex chromosomes in the following topics:

  • Chromosomes and Genes

    • Human chromosomes are divided into two types—autosomes and sex chromosomes.
    • Some genetic traits are linked to a person's sex and therefore passed on by the sex chromosomes.
    • All human beings have 23 pairs of chromosomes by which genetic material is developed and characteristically demonstrated; 22 of these are autosomes, while the remaining pair (either XX, female, or XY, male) represents a person's sex chromosomes.
    • Common abnormalities include Down syndrome (caused by an extra chromosome #21), Klinefelter syndrome (caused by an extra X chromosome), and Turner syndrome (caused by a missing X chromosome).
    • Chromosomes are made up of a variety of gene sequences.

  • Sex-Linked Traits

    • A gene present on one of the sex chromosomes (X or Y in mammals) is a sex-linked trait because its expression depends on the sex of the individual.
    • In humans, as well as in many other animals and some plants, the sex of the individual is determined by sex chromosomes.
    • The sex chromosomes are one pair of non-homologous chromosomes.
    • Until now, we have only considered inheritance patterns among non-sex chromosomes, or autosomes.
    • Insects also follow an XY sex-determination pattern and like humans, Drosophila males have an XY chromosome pair and females are XX.

  • X-Inactivation

    • The presence of extra X chromosomes in a cell is compensated for by X-inactivation in which all but one X chromosome are silenced.
    • Several errors in sex chromosome number have been characterized.
    • Turner syndrome, characterized as an X0 genotype (i.e., only a single sex chromosome), corresponds to a phenotypically female individual with short stature, webbed skin in the neck region, hearing and cardiac impairments, and sterility.
    • In addition to the loss or gain of an entire chromosome, a chromosomal segment may be duplicated or lost.
    • Duplicated chromosomal segments may fuse to existing chromosomes or may be free in the nucleus.

  • Sex Determination

    • Mammalian sex is determined genetically by the presence of X and Y chromosomes .
    • Avian sex determination is dependent on the presence of Z and W chromosomes.
    • The W appears to be essential in determining the sex of the individual, similar to the Y chromosome in mammals.
    • The presence of X and Y chromosomes are one of the factors responsible for sex determination in mammals, with males being the heterozygous sex.
    • In birds, Z and W chromosomes determine sex, with females being the heterozygous sex.

  • Identification of Chromosomes and Karyotypes

    • A karyotype is the number and appearance of chromosomes.
    • In a human karyotype, autosomes or "body chromosomes" (all of the non–sex chromosomes) are generally organized in approximate order of size from largest (chromosome 1) to smallest (chromosome 22).
    • However, chromosome 21 is actually shorter than chromosome 22.
    • The X and Y chromosomes are not autosomes and are referred to as the sex chromosomes.
    • Finally, the karyotype can pinpoint translocations, which occur when a segment of genetic material breaks from one chromosome and reattaches to another chromosome or to a different part of the same chromosome.

  • Chromosomal Theory of Inheritance

    • The Chromosomal Theory of Inheritance identified chromosomes as the genetic material responsible for Mendelian inheritance.
    • During meiosis, homologous chromosome pairs migrate as discrete structures that are independent of other chromosome pairs.
    • The gametic chromosomes combine during fertilization to produce offspring with the same chromosome number as their parents.
    • Despite compelling correlations between the behavior of chromosomes during meiosis and Mendel's abstract laws, the Chromosomal Theory of Inheritance was proposed long before there was any direct evidence that traits were carried on chromosomes.
    • They have three pair of autosomes and a pair of sex chromosomes.

  • Genomic DNA and Chromosomes

    • Human body cells have 46 chromosomes, while human gametes (sperm or eggs) have 23 chromosomes each.
    • Human cells that contain one set of chromosomes are called gametes, or sex cells; these are eggs and sperm, and are designated 1n, or haploid.
    • Matched pairs of chromosomes in a diploid organism are called homologous ("same knowledge") chromosomes.
    • The sex chromosomes, X and Y, are the single exception to the rule of homologous chromosome uniformity.
    • In this image, the chromosomes were exposed to fluorescent stains for differentiation of the different chromosomes.

  • Sex and Physiology

    • Sex is biologically determined based on chromosomes, hormones, gonads, internal reproductive anatomy, and external genitalia.
    • A person's sex includes both primary sex characteristics (those that are related to the reproductive system) and secondary sex characteristics (those unrelated to the reproductive system, such as breasts and facial hair).
    • Intersex, in humans and other animals, is a variation in sex characteristics (including chromosomes, gonads, or genitals) that does not allow an individual to be distinctly identified as male or female.
    • Such variation may involve genital ambiguity and/or combinations of chromosomes other than XY (typically found in males) and XX (typically found in females).
    • In addition to the most common XX and XY chromosomal sexes, there are several other possible combinations commonly known as intersex, such as Turner syndrome (XO), Triple X syndrome (XXX), Klinefelter syndrome (XXY), de la Chapelle syndrome (XX male), and Swyer syndrome (XY female).

  • The Role of Biology

    • It is important to distinguish between sex and gender.
    • Males are defined as having an XY 23rd chromosome, while females are defined as having an XX 23rd chromosome (though tests have revealed variations in chromosomes, including XXY, XYY, and XXX).
    • Scientists have linked a person's 23rd chromosome to the development of a sexed phenotype.
    • Sex is largely constant across different cultures; in virtually any country, a person with XY chromosomes and male reproductive organs is considered male.
    • Is this a question of sex, gender or simply sexism?

  • Defining Sex, Gender, and Sexuality

    • Sex includes both primary sex characteristics (those related to the reproductive system) and secondary sex characteristics (those that are not directly related to the reproductive system, such as breasts and facial hair).
    • In humans, the biological sex of a child is determined at birth based on several factors, including chromosomes, gonads, hormones, internal reproductive anatomy, and genitalia.
    • However, anywhere from 1.0 to 1.7% of children are born intersex, having a variation in sex characteristics (including chromosomes, gonads, or genitals) that do not allow them to be distinctly identified as male or female.
    • Typically, babies born with male sex characteristics (sex) are assigned as boys (gender); babies born with female sex characteristics (sex) are assigned as girls (gender). 
    • In humans, sex is typically divided into male, female, or intersex (i.e., having some combination of male and female sex characteristics).