BIOL 1081 Lecture Notes - Lecture 17: Dosage Compensation, X-Inactivation, Genetic Linkage
Ch. 13 LO
Sex Linkage and Sex Chromosomes Reading: Sections 13.1-13.2 in textbook
1. Describe how sex is determined in mammals, birds, grasshoppers, and honey bees.
• Mammals- F- XX, M-XY
• Birds-F- ZW, M- ZZ
• Grasshoppers-F- XX, M-XO
• Honey bees- F- diploid, M-haploid
o Y Generally shows maleness
2. Explain the patterns of inheritance displayed by genes on sex chromosomes, being able to
solve monohybrid and dihybrid crosses that involve sex chromosomes.
• Patterns of inheritance- for a single gene disorders are classified based on whether they are
autosomal or X linked and whether they have a dominant or recessive pattern of inheritance
• Because females have 2 copies of X, they will not be affected by inheriting a single recessive
mutation on an X gene
3. Explain X inactivation in mammals and its genetic consequences. Define dosage compensation and Barr
body.
• X inactivation balances the levels of gene expression
• Genetic mosaics- their individual cells may express different alleles, depending on which
chromosome is inactivated
• Dosage compensation- phenomenon which expression of genes carried on sex chromosomes is
kept the same in males and females, despite a different # of sex chromosomes
o Mammals- inactivation of one X in female cells accomplishes this
• Barr Body- deeply staining structure, seen in interphase nucleus of a cell of an individual with
more than one X chromosome, that is condensed and inactivated; only 1 X remains active in each
cell after early embryogenesis
4. Define what it means for genes to be linked and describe how that linkage is related to the principle of
independent assortment.
• When 2 genes are close together on the same chromosome pair, they do not assort
independently. The residing genes on the same chromosome pair is termed linkage.
• Two genes on the same chromosome pair are said to be linked
• Principle of Independent Assortment- describe show different genes independently separate from
one another when reproductive cells develop
5. Describe the impact of genetic linkage on genetic crosses and the mechanism that prevents
chromosomal linkage from being absolute.
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