Sex Chromosomes

Related to Sex Chromosomes: DNA, Down syndrome, Multiple alleles

Sex Chromosomes


a special pair of chromosomes containing the genes that determine the sex found in the chromosome bank of dioecious organisms. Differences were discovered in the chromosome banks of male and female insects and unusual sex chromosomes were described by the German researcher H. Henking in 1891 and by C. McClung and E. Wilson in the early 20th century. It was later discovered that many dioecious organisms have sex chromosomes containing sex-determining elements.

Usually, chromosomes constitute an unequal pair. The larger chromosome contains the female sex factors and is called the X chromosome; the smaller chromosome is called the Y chromosome. In mammals, including man, the factors that determine the male sex are localized in the Y chromosome. In other organisms, for example, in Drosophila, the Y chromosome is contained in autosomes. In many species the Y chromosome is absent when the male sex is determined by autosomes.

Females usually have two identical sex chromosomes (XX type) and males have either two unequal chromosomes (XY type) or a single sex chromosome (X0 type). Females are said to be homogametic because, as a result of meiosis, each egg contains one X chromosome. Males are heterogametic because two types of sperm, containing either the X chromosome or the Y chromosome, are formed from the XY pair. The chance union of sex cells, or gametes, during fertilization results, on a large scale, in an equal number of females (XX) and males (XY).

The opposite relationships are true in butterflies, birds, some reptiles, and amphibians. The males are homogametic and possess the XX chromosome. Accordingly, a single type of sperm with an X chromosome is formed. The females are heterogametic and have XY chromosomes. As a result, two types of eggs are formed: one with an X chromosome and the other with a Y chromosome. Chromosomes are generally designated by Z and W when females are heterogametic.

In addition to the genes that determine sex, sex chromosomes also contain sex-linked genes that determine an organism’s different characters and are inherited with sex. There are many sex-linked genes in an X chromosome and only a few in a Y chromosome. In man the recessive genes of hemophilia and albinism are examples of sex-linked genes. These genes manifest themselves exclusively in males if they are present in only one of the X chromosomes of the female. Females are then the carriers of latent sex-linked diseases.

Deviations from the normal number of sex chromosomes in human cells result in developmental anomalies. Shereshevskii-Turner’s syndrome occurs in women with an XO chromosome and is characterized by short stature, infertility, and mental retardation. Klinefelter’s syndrome occurs in men with an XXY chromosome and is characterized by excessive height, long extremities, abnormal development of sex characters, infertility, and mental retardation. As many as four X chromosomes can be paired with a Y chromosome (XXXXY) in those with this syndrome. Trisomy syndrome occurs in women with an extra X chromosome (XXX) and is characterized by mental retardation and underdevelopment of the ovaries.

The Y chromosome is easily visible in cells because part of it can be selectively stained with fluorescent dyes of the quinacrine hydrochloride variety; this procedure is used for diagnostic purposes.

Sex chromosomes are found in some diclinous plants; for example, in strawberries. Hermaphroditic animals and monocli-nous plants do not have sex chromosomes.


Morgan, T. H. “Ogranichennaia polom nasledstvennost’ u drozofily.” In his book Izbrannye raboty po genetike. Moscow-Leningrad, 1937. (Translated from English.)
Osnovy tsitogenetiki cheloveka. Moscow, 1969.
Lektsii po meditsinskoi genetike. Edited by A. A. Prokof’eva-Bel’govskaia and V. P. Efroimson. Moscow, 1974.


References in periodicals archive ?
While only a small percentage of teleosts have morphologically distinguishable sex chromosomes, those that do display a variety of kinds (Kitano and Peichel, 2012) with the X1X2Y-X1X1X2X2 system being the most prevalent.
The presence of sex chromatin in this species is a useful parameter for monitoring the effect of radiation, as its morphology and quantity can be used as a marker to identify sex chromosome aberrations (Makee & Tafesh 2006).
She urges researchers to move beyond studying the sex chromosomes when carrying out research into the genetics of sex differences--even going so far as to suggest adopting "sex-neutral alternatives" for the X and Y chromosomes (p.
Working from the early twentieth century to the current "post-genomic" age, Richardson elegantly lays out how the sex chromosomes were first discovered and then identified in such a way that dominant meanings of male and female were reproduced.
Types of sex chromosomes at Morrell Creek and the Swan and Yaak rivers are similar to those observed at the Clearwater River (Shields et al.
Importantly, many of these cases had underlying biological reasons for the excess sequences detected, such as confined placental mosaicism (CPM), maternal sex chromosome aneuploidy, maternal malignancy, low-level fetal mosaicism, and cotwin demise (Table 2).
Canis familiaris female's karyotype has a chromosome number of 78, consisting of 39 chromosome pairs of which 38 are autosomes (acrocentric) and the remaining pair are sex chromosomes XX (metacentric) (Figure 1A).
Of the four pesticide studies that investigated aneuploidy in the sex chromosomes (Recio et al.
In this issue, Bidau and Martf (2010) examine the history of orthopteran cytogenetics from McClung's classic paper (1899) to the current day, highlighting Michael White's contributions to the field in such diverse subjects as sex chromosomes, anomalous meiotic systems and population and evolutionary cytogenetics.
Most species have non-distinguishable sex chromosomes, the XX/XY system (female/male) was found in several species of Gagrellinae and Leiobuninae (Sclerosomatidae, Eupnoi) and in Sabacon makinoi (Sabaconidae, Dyspnoi), while the ZW/ZZ system (female/male) was only observed in Mitopus morio (Fabricius) and Odiellus aspersus (Karsch) (Phalangiidae, Eupnoi) (Tsurusaki, 1985; Tsurusaki & Cokendolpher, 1990; Tsurusaki, 2007).
Silver-staining analysis revealed the activity of nucleolar organizer regions (NORs) on the short arms of both acrocentric chromosomes of autosomal pair number 6; the presumptive sex chromosomes did not show NOR activity (Fig.
Chromosomes can be one of two types: sex chromosomes or autosomes.