Chapter 4: Sex Determination And Sex-Linked Characteristics

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Sex

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Male or female.

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Sex Determination

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Specification of sex (male or female). Sex-determining mechanisms include chromosomal, genic, and environmental sex-determining systems.

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Hermaphroditism

Back 3

Condition in which an individual organism possesses both male and female reproductive structures. True hermaphrodites produce both male and female gametes.

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Monoecious Organism

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Individual organism that has both male and female reproductive structures.

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Dioecious Organism

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Belongs to a species whose members have either male or female reproductive structures.

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Sex Chromosome

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Chromosomes that differ morphologically or in number in males and females.

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Autosome

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Chromosome that is the same in males and females; nonsex chromosome.

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Heterogametic Sex

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The sex (male or female) that produces two types of gametes with respect to sex chromosomes. For example, in the XX-XY sex-determining system, the male produces both X-bearing and Y-bearing gametes.

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Homogametic Sex

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The sex (male or female) that produces gametes that are all alike with regard to sex chromosomes. For example, in the XX-XY sex-determining system, the female produces only X-bearing gametes

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Pseudoautosomal Region

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Small region of the X and Y chromosomes that contains homologous gene sequences.

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Genic Sex Determination

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Sex determination in which the sexual phenotype is specified by genes at one or more loci, but there are no obvious differences in the chromosomes of males and females.

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Sequential Hermaphroditism

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Phenomenon in which the sex of an individual organism changes in the course of its lifetime; the organism is male at one age or developmental stage and female at a different age or stage.

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Genic Balance System

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Sex-determining system in which sexual phenotype is controlled by a balance between genes on the X chromosome and genes on the autosomes.

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X:A Ratio

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Ratio of the number of X chromosomes to the number of haploid autosomal sets of chromosomes; determines sex in fruit flies

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Turner Syndrome

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Human condition in which cells contain a single X chromosome and no Y chromosome (XO). Persons with Turner syndrome are female in appearance but do not undergo puberty and have poorly developed female secondary sex characteristics; most are sterile but have normal intelligence.

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Klinefelter Syndrome

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Human condition in which cells contain one or more Y chromosomes along with multiple X chromosomes (most commonly XXY but may also be XXXY, XXXXY, or XXYY). Persons with Klinefelter syndrome are male in appearance but frequently possess small testes, some breast enlargement, and reduced facial and pubic hair; often taller than normal and sterile, most have normal intelligence.

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Triplo-X Syndrome

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Human condition in which cells contain three X chromosomes. A person with triplo-X syndrome has a female phenotype without distinctive features other than a tendency to be tall and thin; a few such women are sterile, but many menstruate regularly and are fertile.

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SRY Gene

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On the Y chromosome, a gene that triggers male development.

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Sex-Linked Characteristic

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Characteristic determined by a gene or genes on sex chromosomes.

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X-linked Characteristic

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Characteristic determined by a gene or genes on the X chromosome.

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Y-linked Characteristic

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Characteristic determined by a gene or genes on the Y chromosome.

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Hemizygosity

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Possession of a single allele at a locus. Males of organisms with XX-XY sex determination are hemizygous for X-linked loci because their cells possess a single X chromosome.

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Nondisjunction

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Failure of homologous chromosomes or sister chromatids to separate in meiosis or mitosis.

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Dosage Compensation

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Equalization in males and females of the amount of protein produced by X-linked genes. In placental mammals, dosage compensation is accomplished by the random inactivation of one X chromosome in the cells of females.

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Barr Body

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Condensed, darkly staining structure that is found in most cells of female placental mammals and is an inactivated X chromosome.

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Lyon Hypothesis

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Proposed by Mary Lyon in 1961, this hypothesis proposes that one X chromosome in each female cell becomes inactivated (a Barr body) and suggests that which of the X chromosomes becomes inactivated is random and varies from cell to cell.