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An allosome (also referred to as a sex chromosome, heterotypical chromosome, heterochromosome, or idiochromosome) is a chromosome that differs from an ordinary autosome in form, size, and behavior. The human sex chromosomes, a typical pair of mammal allosomes, determine the sex of an individual created in sexual reproduction. Autosomes differ from allosomes because autosomes appear in pairs whose members have the same form but differ from other pairs in a diploid cell, whereas members of an allosome pair may differ from one another and thereby determine sex.
In humans, each cell nucleus contains 23 pairs of chromosomes a total of 46 chromosomes. The first 22 pairs are called autosomes which look exactly the same in both males and females. The 23rd pair of chromosomes is called an allosome. These sex chromosomes usually differ between males and females. females have two copies of the X chromosome, while males have one X chromosome and one Y chromosome. The X chromosome is always present as the 23rd chromosome in the ovum, while either X or Y chromosomes can be present in an individual sperm.
All diploid organisms with allosome-determined genders get half of their allosomes from each of their parents. In mammals, females are XX, they can pass along either of their X’s, and since the males are XY they can pass along either an X or a Y. For a mammal to be considered a female, the individual must receive an X chromosome from both parents, whereas to be considered a male, the individual must receive a X chromosome from their mother and a Y chromosome from their father. It is thus the male’s sperm that determines the sex of each offspring in humans. There is about a 51 percent chance of producing a male offspring and 49 percent a female, with Fisher's principle determining this sex ratio.
Allosomes not only carry the genes that determine male and female traits, but also those for some other characteristics as well. Genes that are carried by either sex chromosome are said to be sex linked. Sex-linked diseases are passed down through families through one of the X or Y chromosomes. Since only men inherit Y chromosomes, they are the only ones to inherit Y-linked traits. Men and women can get the X-linked ones since both inherit X chromosomes.
EDITOR, NOTE: The following section contains factual errors regarding the meaning of dominance and recessivity. It must be rewritten. Here is the corrective information: In the field of Genetics (Genomics, molecular biology, etc.) the terms dominance and recessivity characterize phenotypes, not genes. Genes do not have the dominance and recessivity character. Also in error is the statement that “Dominant inheritance” is the result of abnormality. In this erroneous section, it might be intended that the medical community uses these terms informally as a convenient short-hand. In that case, a specific reference should be included and informal usage delineated from the strict meanings of the terms in biology explained.  For example, the β-globin gene HBB is involved in two phenotypes, a dominant phenotype and the other a recessive phenotype. Sickle cell low oxygen trait (phenotype) is dominant, but sickle cell anemia phenotype is recessive; both controlled by the same gene. Also note that not all genes and their related phenotypes follow Mendelian rules. Non-Mendellian phenotypes (and their underlying genes) are neither dominant or recessive (e.g. multi-factorial). Finally, whether a phenotype is dominant or recessive is independent of (unrelated to) whether the phenotype is considered a medical abnormality. The following contains factual errors:
A gene is either said to be dominant or recessive. Dominant inheritance occurs when an abnormal gene from one parent causes disease even though the matching gene from the other parent is normal. The abnormal gene dominates. Recessive inheritance is when both matching genes must be abnormal to cause disease. If only one gene in the pair is abnormal, the disease does not occur, or is mild. Someone who has one abnormal gene (but no symptoms) is called a carrier. A carrier can pass this abnormal gene to his or her children. X chromosome carry about 1500 genes, more than any other chromosome in the human body. Most of them code for something other than female anatomical traits. Many of the non-sex determining X-linked genes are responsible for abnormal conditions. The Y chromosome carries about 78 genes. Most of the Y chromosome genes are involved with essential cell house-keeping activities and sperm production. Only one of the Y chromosome genes, the SRY gene, is responsible for male anatomical traits. When any of the 9 genes involved in sperm production are missing or defective the result is usually very low sperm counts and infertility. Examples of mutations on the X chromosome include more common diseases such as color blindness, hemophilia, and fragile-X syndrome.
- Color blindness or color vision deficiency is the inability or decreased ability to see color, or perceive color differences, under normal lighting conditions. Color blindness affects many individuals in the population. There is no actual blindness, but there is a deficiency of color vision. The most usual cause is a fault in the development of one or more sets of retinal cones that perceive color in light and transmit that information to the optic nerve. This type of color blindness is usually a sex-linked condition. The genes that produce photopigments are carried on the X chromosome; if some of these genes are missing or damaged, color blindness will be expressed in males with a higher probability than in females because males only have one X chromosome.
- Hemophilia refers to a group of bleeding disorders in which it takes a long time for the blood to clot. This is referred to as X-Linked recessive.  Hemophilia is much more common in males than females because males are hemizygous. They only have one copy of the gene in question and therefore express the trait when they inherit one mutant allele. In contrast, a female must inherit two mutant alleles, a less frequent event since the mutant allele is rare in the population. X-linked traits are maternally inherited from carrier mothers or from an affected father. Each son born to a carrier mother has a 50% probability of inheriting the X-chromosome carrying the mutant allele.
- Queen Victoria was a carrier of the gene for hemophilia. She passed on the harmful allele to one of her four sons and at least two of her five daughters. Her son Leopold had the disease and died at age 30. As a result of marrying into other European royal families, the princesses Alice and Beatrice spread hemophilia to Russia, Germany, and Spain. By the early 20th century, ten of Victoria's descendents had hemophilia. All of them were men, as expected.
- Fragile X syndrome is a genetic condition involving changes in part of the X chromosome. It is the most common form of inherited intellectual disability (mental retardation) in males. It is caused by a change in a gene called FMR1. A small part of the gene code is repeated on a fragile area of the X chromosome. The more repeats, the more likely there is to be a problem. Males and females can both be affected, but because males have only one X chromosome, a single fragile X is likely to affect them more. Most fragile-X males have large testes, big ears, narrow faces, and sensory processing disorders that result in learning disabilities.
Other complications include:
- 46,XX testicular disorder of sex development, also called XX male syndrome, is a condition in which individuals with two X chromosomes in each cell, the pattern normally found in females, have a male appearance. People with this disorder have male external genitalia. In most people with 46,XX testicular disorder of sex development, the condition results from an abnormal exchange of genetic material between chromosomes (translocation). This exchange occurs as a random event during the formation of sperm cells in the affected person's father. The SRY gene (which is on the Y chromosome) is misplaced in this disorder, almost always onto an X chromosome. Anyone with an X chromosome that carries the SRY gene will develop male characteristics despite not having a Y chromosome.
- Fisher's principle
- Haldane's rule
- XY sex-determination system
- ZW sex-determination system
- X0 sex-determination system
- How many chromosomes do people have? - Genetics Home Reference
- Biological Basis of Heredity: Sex Linked Genes
- MITx: 7.00x Introduction to Biology - The Secret of Life
- Sex-linked recessive - National Library of Medicine - PubMed Health
- Sex-Linked Traits | Heredity and genetics | Khan Academy
- Hemophilia - National Library of Medicine - PubMed Health
- Fragile X Syndrome - Symptoms, Diagnosis, Treatment of Fragile X Syndrome - NY Times Health Information
- 46,XX testicular disorder of sex development - Genetics Home Reference
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