BIOL 2030 Lecture Notes - Lecture 6: Turner Syndrome, Homologous Chromosome, Sister Chromatids
6 – Sex Determination
January 15th, 2016
1. Mechanisms of sex determination
a. Chromosomal
b. Genetic
c. Environemtnal
2. Sex-linked Characteristics
3. Dosage competition
Sexual reproduction: process of organism reproduction that results in offspring
Monoecious: male and female reproductive structure are found on same individual
Dioecious: individuals have either male or female reproductive structures
• Chromosomal sex determination
o Sex chromosomes and autosomes
o XX – homogametic
o XY – hetergametic
o XX/XO
o ZZ/ZW **females are heterogametic
• Genetic sex determination
o Sexual phenotype is determined by genes at one or more loci carried on
autosomes *no sex chromosomes
• Environmental sex determination
o Sexual phenotype is determined in part (or in full) by environmental factors
• Haplodiploidy
o No sex chromosome
o Males – haploid
o Female – diploid
o Ex. Bees, wasps, ants
Sex Determination in Mammals: Y chromosome mechanism
• Early evidence for this mechanism came from studied of abnormal sex chromosome
complement due to non-disjunction in humans
• Failure of homologous chromosome or sister chromatids to move to opposite poles in
anaphase
• Better tolerated for sex chromosomes
• Ex. Turner Syndrome only one X chromosome; no Y
• Klinefelter syndrome: too many sex chromosomes
• TRIPLO X XXX
Role of Sex Chromosomes:
• Gene on Y determines if individual is male
• X chromosome carries genes essential for both sexes
• Both X and Y carry genes essential for fertility
• Sex reversed XX males have SRY gene while sex reversed XY females lack it
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Document Summary
January 15th, 2016: mechanisms of sex determination, chromosomal, genetic, environemtnal, sex-linked characteristics, dosage competition. Sexual reproduction: process of organism reproduction that results in offspring. Monoecious: male and female reproductive structure are found on same individual. Turner syndrome only one x chromosome; no y: klinefelter syndrome: too many sex chromosomes, triplo x xxx. Sex determination in humans: sexual development depends on sex chromosomes and autosomes, most secondary sex characteristics depend on autosomal genes, sexual phenotypes does not depend on the presence of genes but on correct control of gene expression. Dosage compensation: mechanism that equalizes the amount of protein produced by x-linked genes between heterogametic and homogametic sexes. In mammals: xx- 1 barr body (highly condensed mass of chromatin, xy- no barr body. Some patches of cells have paternally derived active x-linked genes while others have maternally- derived x-linked genes: ex.
