Skip navigation. That year, Morgan was breeding Drosophila , or fruit flies. After observing thousands of fruit fly offspring with red eyes, he obtained one that had white eyes. Morgan began breeding the white-eyed mutant fly and found that in one generation of flies, the trait was only present in males.
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Disorders of the sex chromosome like disorders of the autosomes can be either numerical or structural, and can be present in all cells or in a mosaic form. Clinical indications that should raise suspicions of a sex chromosome abnormality are:. These disorders are considered very common with incidence of about — Phenotypes associated with sex chromosome disorders are less severe than autosomal defects and this is mainly due to X chromosome inactivation , as well as the fact that Y chromosomes have a low gene content. X chromosome inactivation is the the process by which most genes on one of the two X chromosomes in females are silenced epigenetically and randomly, and so fail to produce any product. In somatic cells in normal females but not in normal males , one X chromosome is inactivated early in development, thus equalizing the expression of X-linked genes in the 2 sexes. Thus females are mosaic with respect to X-linked gene expression.
How Chromosomes Determine Sex
A sex-determination system is a biological system that determines the development of sexual characteristics in an organism. Most organisms that create their offspring using sexual reproduction have two sexes. Occasionally, there are hermaphrodites in place of one or both sexes. There are also some species that are only one sex due to parthenogenesis , the act of a female reproducing without fertilization. In many species, sex determination is genetic: males and females have different alleles or even different genes that specify their sexual morphology.
Alex Quinn, a Ph. Sex-determining mechanisms in reptiles are broadly divided into two main categories: genotypic sex determination GSD and temperature-dependent sex determination TSD. Species in the genotypic group, like mammals and birds, have sex chromosomes, which in reptiles come in two major types.