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II. Mendelism and the Chromosomal Theory
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5. Sex Determination, Sex Linkage, and Pedigree Analysis
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that actively determine male- owering plants. The other system is similar to that found in fruit ies, in which the X:A ratio determines sex. In the mammalian-type system, the Y chromosome carries genes needed for the development of male ower parts while suppressing the development of female parts. An example of this is in the white campion (Silene latifolia). In the Drosophila-type system, found in the sorrel (Rumex acetosa), the ratios determine sex exactly as in the ies. That is, an X:A ratio of 0.5 or lower results in a male; a ratio of 1.0 or higher results in a female; and an intermediate ratio results in a plant with hermaphroditic owers. It seems that all owers have the potential to be hermaphroditic. That is, ower primordia for hermaphroditic, male, and female owers look identical during early development. The simplest mechanism of sex determination would involve repressing the development of the female ower parts in male owers and repressing the male ower parts in female owers. Current research indicates that this repression of one component or another is probably involved in most ower sex determination and is under genetic and hormonal control. (We discuss further the genetic control of ower development in chapter 16.)
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Mary F. Lyon (1925 ).
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(Courtesy of Dr. Mary F. Lyon.)
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chromosomes have one fewer Barr body than they have X chromosomes per cell: XXX females have two Barr bodies and XXXX females have three.
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Proof of the Lyon Hypothesis
Direct proof of the Lyon hypothesis came when cytologists identi ed the Barr body in normal females as an X chromosome. Genetic evidence also supports the Lyon hypothesis: Females heterozygous for a locus on the X chromosome show a unique pattern of phenotypic expression. We now know that in human females, an X chromosome is inactivated in each cell on about the twelfth day of embryonic life; we also know that the inactivated X is randomly determined in a given cell. From that point on, the same X remains a Barr body for future cell generations. Thus, heterozygous females show mosaicism at the cellular level for X-linked traits. Instead of being typically heterozygous, they express only one or the other of the X chromosomal alleles in each cell. Glucose-6-phosphate dehydrogenase (G-6-PD) is an enzyme that a locus on the X chromosome controls. The
D O S A G E C O M P E N S AT I O N
In the XY chromosomal system of sex determination, males have only one X chromosome, whereas females have two. Thus, disregarding pseudoautosomal regions, males have half the number of X-linked alleles as females for genes that are not primarily related to gender. A question arises: How does the organism compensate for this dosage difference between the sexes, given the potential for serious abnormality In general, an incorrect number of autosomes is usually highly deleterious to an organism (see chapter 8). In human beings and other mammals, the necessary dosage compensation is accomplished by the inactivation of one of the X chromosomes in females so that both males and females have only one functional X chromosome per cell. In 1949, M. Barr and E. Bertram rst observed a condensed body in the nucleus that was not the nucleolus. Noting that normal female cats show a single condensed body, while males show none, these researchers referred to the body as sex chromatin, since known as a Barr body ( g 5.7). Mary Lyon then suggested that this Barr body represented an inactive X chromosome, which in females becomes tightly coiled into heterochromatin, a condensed, and therefore visible, form of chromatin. Various lines of evidence support the Lyon hypothesis that only one X chromosome is active in any cell. First, XXY males have a Barr body, whereas X0 females have none. Second, persons with abnormal numbers of X
Barr body (arrow) in the nucleus of a cheek mucosal cell of a normal woman. This visible mass of heterochromatin is an inactivated X chromosome. (Thomas G. Brewster
and Park S. Gerald, Chromosome disorders associated with mental retardation, Pediatric Annals, 7, no. 2, 1978. Reproduced courtesy of Dr. Thomas G. Brewster, Foundation for Blood Research, Scarborough, Maine.)
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