Lecture 28 Sex determination

Lecture 28 Sex determination - Sex Linkage and Deter...

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Unformatted text preview: Sex Linkage and Deter mination CH APTER 15 pp 289-292 and mater ial in these lectur e notes. Lear ning objectives • Explain how sex is genetically determined in humans. • Explain why sex-linked diseases are more common in human males. • Understand the significance of the SRY gene in determining maleness. • Explain the physical differences between the human X and Y chromosome and how these differences are used to establish paternal lineages. • Be able to predict the genotypic and phenotypic frequencies of inheritance of sex-linked disorders. • Describe the process of X inactivation in female mammals and understand its consequences on X-linked phenotypes. • Some organisms have both male and female organs and produce both types of gametes ( her maphr odites/ monoecious) [I n Greek monoecious means "one house" so this means both sexes live in the same "house" or individual.] • Most animals (including humans) and some plants are dioecious. [I n Greek that means "two houses" so this means each sex lives in a different "house" or individual.] • Dioecious or ganisms come in two sexes, male and female, and each individual will pr oduce only one type of gamete. The chr omosomal basis of sex var ies with the or ganism I n human and other mammals, the chr omosomal basis of sex is r ather simple. I f its got a Y, it’s a guy! • An individual who inherits two X chromosomes usually develops as a female. • An individual who inherits an X and a Y chromosome usually develops as a male. • This X-Y system of mammals, in which the sex of an offspring depends on whether the sperm contains an X or Y chromosome, is not the only chromosomal mechanism of determining sex. • Other options include the X-0 system (e.g. grasshoppers crickets, roaches), the Z-W system (birds, fish, butterflies, moths), and the haplo-diploid system (bees, ants). • I n some organisms, envir onment plays a critical role in sex determination (crocodiles, some turtles, limpets, and fish). female male N ote nomenclatur e for female ver ses male. • I n both testes (XY) and ovaries (XX), the two sex chromosomes segregate during meiosis and each gamete receives one. • Every haplod egg carries an X chromosome since mothers produce eggs and mothers are XX. • Half the sperm carry an X chromosome and half a Y chromosome since fathers are XY and 1/ 2 the gametes get the X and 1/ 2 get the Y. • Because of this, each conception has about a fifty-fifty chance of producing a particular sex ( that is XX or XY). I n or ganisms with the X-Y system, i.e. humans: I n the X-Y system, Y and X chr omosomes behave as homologous chr omosomes dur ing meiosis despite their differ ences....
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This note was uploaded on 11/14/2010 for the course CHE 131 taught by Professor Kerber during the Spring '08 term at SUNY Stony Brook.

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Lecture 28 Sex determination - Sex Linkage and Deter...

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