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Understanding Sex Chromosomes and Dosage Compensation in Mammals

This document provides essential information about sex chromosomes, specifically the X and Y chromosomes, and their role in sex determination and dosage compensation in mammals. Key topics include how these chromosomes segregate during meiosis, the significance of the SRY gene on the Y chromosome, and the process of X inactivation in female mammals. Important insights into genetic mosaics, such as tortoiseshell and calico coat patterns, are also discussed, alongside implications for sex chromosome aberrations. Prepare for the upcoming exam with these crucial concepts.

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Understanding Sex Chromosomes and Dosage Compensation in Mammals

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  1. Announcements Homework due 9:00 Thursday • only your first grade will count! Review Session Sunday 3:00 to 4:00 PM 124 Burrill Hall Exam Tuesday 9:00 AM, in Lecture room

  2. Sex Chromosomes and Sex Determination

  3. X and Y are clearly not homologous chromsomes. How do they segregate normally at Meiosis? During what part of Meiosis do they pair and then segregate?

  4. Human Y chromosome • Yellow: pseuoautosomal regions • Red: genes with homologs on the X chromosome • Blue: genes without homologus on the X

  5. Genetically, what determines gender in mammals? Presence of a single gene (SRY) that usually, but not always, occurs on the Y chromosome. If the Y chromosome is missing (this gene deleted) or has a non-functional mutation in the gene, an XY individual can be a perfectly normal female. If the SRY gene becomes translocated to another chromosome, an XX individual can be a phenotypically normal (but sterile) male.

  6. XX (+SRY) mice

  7. Dosage Compensation Do males have half as much of the products of genes on the X as do females?

  8. X Inactivation Barr Body: Inactive X Interphase: Chromomes can’t be stained, but a dark-staining body is visible in the nuclei of cells of female mammals

  9. Which X gets inactivated? Mary Lyon & Lianne Russell (1961) proposed that one or other of X becomes inactivated at a particular time in early development. Within each cell,which X becomes inactivated is ____________. As development proceeds, all cells arising by cell division after than time have _______ _________________.

  10. In 64-cell embryos Adult female mammals are ___________ for genes on the X chromosome.

  11. Calico cats are almost always ____________. One X chromosome carries the allele for black coat color The other X chromosome carries the allele for orange coat color In 64-cell embryos, one of each pair of X chromosomes and its genes are _____________ ________________. Daughter cells inherit active or inactive X chromosomes, creating a cat with patches of coat color Genetic Mosaics XOXo

  12. Tortoiseshell (calico) coat color allele allele

  13. Tortoiseshell (calico) coat color

  14. Tortoiseshell (calico) coat color

  15. Carbon Copy

  16. X inactivation is different in marsupials kangaroos, wombats, bandicoots, opossums

  17. Sex chromosome aberrations

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