| Dictionary: sex chromosome |
Either of a pair of chromosomes, usually designated X or Y, in the germ cells of most animals and some plants, that combine to determine the sex and sex-linked characteristics of an individual, with XX resulting in a female and XY in a male in mammals.
| Dental Dictionary: sex chromosomes |
Chromosomes responsible for sex classification— XX for female, XY for male.
| Britannica Concise Encyclopedia: sex chromosome |
For more information on sex chromosome, visit Britannica.com.
| Biology Q&A: What are sex chromosomes? |
Sex (X and Y in mammals) chromosomes are the nonmatching pair. The
Y chromosome, found only in males, has the sex-determining gene, SRY. The SRY determines
whether or not an individual will develop testis and produce appreciable
quantities of testosterone (the hormone that generates male characteristics).
There is no known correlate gene on the X chromosome for the formation of
ovaries. Therefore, it is the absence of the Y chromosome that determines a
female.
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| Science Dictionary: sex chromosomes |
The two chromosomes in each body cell of a living thing that determine what sex it is.
| Wikipedia: Sex-determination system |
A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most sexual organisms have two sexes. In many cases, sex determination is genetic: males and females have different alleles or even different genes that specify their sexual morphology. In animals, this is often accompanied by chromosomal differences. In other cases, sex is determined by environmental variables (such as temperature) or social variables (the size of an organism relative to other members of its population). The details of some sex-determination systems are not yet fully understood.
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The XX/XY sex-determination system is one of the most familiar sex-determination systems and is found in human beings and most other mammals, although at least one monotreme, the platypus, presents a particular sex determination scheme that in some ways resembles that of the ZW sex chromosomes of birds, and it also lacks the SRY gene. Several Arvicolinae (voles and lemmings) and some other rodents are also noted for their unusual sex determination systems.
In the XY sex-determination system, females have two of the same kind of sex chromosome (XX), while males have two distinct sex chromosomes (XY). Some species (including humans) have a gene SRY on the Y chromosome that determines maleness; others (such as the fruit fly) use the presence of two X chromosomes to determine femaleness. The XY sex chromosomes are different in shape and size from each other unlike the autosomes, and are termed allosomes.
In this variant of the XY system, females have two copies of the sex chromosome (XX) but males have only one (X0). The 0 denotes the absence of a second sex chromosome. This system is observed in a number of insects, including the grasshoppers and crickets of order Orthoptera and in cockroaches (order Blattodea).
The nematode C. elegans is male with one sex chromosome (X0); with a pair of chromosomes (XX) it is a hermaphrodite.
The ZW sex-determination system is found in birds and some insects and other organisms. The ZW sex-determination system is reversed compared to the XY system: females have two different kinds of chromosomes (ZW), and males have two of the same kind of chromosomes (ZZ).
Haplodiploidy is found in insects belonging to Hymenoptera, such as ants and bees. Unfertilized eggs develop into haploid individuals, which are the males. Diploid individuals are generally female but may be sterile males. Thus, if a queen bee mates with one drone, her daughters share ¾ of their genes with each other, not ½ as in the XY and ZW systems. This is believed to be significant for the development of eusociality, as it increases the significance of kin selection. This is common also in wasps that are parasitic and in the male greenflies.
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Many other sex-determination systems exist. In some species of reptiles, including (but not limited to) alligators, some turtles, and the tuatara, sex is determined by the temperature at which the egg is incubated. However, other species, such as some snails, practice sex change: adults start out male, then become female. In tropical clown fish, the dominant individual in a group becomes female while the other ones are male.
Some species, however, have no sex-determination system. Hermaphrodites include the common earthworm and certain species of snails. A few species of fish, reptiles, and insects reproduce by parthenogenesis and are female altogether.
In some arthropods, sex is determined by infection, as when Bacteria of the genus Wolbachia alter their sexuality; some species consist entirely of ZZ individuals, with sex determined by the presence of Wolbachia.
Other unusual systems (this section is still being researched):
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Sex chromosomes and sex determination in amniotes
Sex determination in insects
Sex determination in plants
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 | allosome (genetics) |
| autosome (genetics) |
| isogamety |
 | THe chromosome abnormality that occuswhen part of one chromosome breaks off and is add to a diffrent chromosome is? |
| What chromosome is the sex chromosome in fruit flies? |
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 | Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2007. Published by Houghton Mifflin Company. All rights reserved. Read more |
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| Dental Dictionary. Mosby's Dental Dictionary. Copyright © 2004 by Elsevier, Inc. All rights reserved. Read more |
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| Britannica Concise Encyclopedia. Britannica Concise Encyclopedia. © 2006 Encyclopædia Britannica, Inc. All rights reserved. Read more |
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| Biology Q&A. The Handy Biology Answer Book. 2004 ©Visible Ink Press. All rights reserved. Read more |
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| Science Dictionary. The New Dictionary of Cultural Literacy, Third Edition Edited by E.D. Hirsch, Jr., Joseph F. Kett, and James Trefil. Copyright © 2002 by Houghton Mifflin Company. Published by Houghton Mifflin. All rights reserved. Read more |
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| Wikipedia. This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Sex-determination system". Read more |
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