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BSC 219 (22)
mortimer (22)
Lecture 10

BSC 219 Lecture 10: Lecture

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Biological Sciences
BSC 219

Tuesday, March 28, 2017 X linked genes X-Linked Genes - Most genes on X chromosome do not have a copy on Y - Genes on X chromosome will have modified ratios - Thomas Hunt Morgan discovered this in flies using white gene • Mutations in white gene prevent eye pigment from making it to the surface of the eye • Red eye female (homo) x White eye male - F 1: Females- W+W- Males- W+Y - F 2: Females- W+W+ & W+W- Males- W+Y & W-Y • White eye female (homo rec) x Red eye male - F 1: Females- W+W- Males- W-Y - F 2: Females- W+W- & W-W- Males- W+Y & W-Y Other sex chromosome properties - Hemizygous: an individual who has only 1 member of a chromosome pair or chromosome segment rather than usual two (MALES) • Males cannot be homozygous for X chromosome - Mutations in linked genes can be dominant or recessive • Rhett Syndrome (brain disorder-almost exclusively girls) - Males die early if having Rhett syndrome - Lose the skills they developed (after 6 months) - Eventually can't use muscles that control movement 1 Tuesday, March 28, 2017 - Caused by more than 620 mutations in MeCP2 gene • Plays a role in chromatin remodeling (wrapping around histones) • May also play a role in mRNA splicing • May control expression of genes that are important for maintaining the synapse and how neurons communicate • X-Linked Dominant Gene - Female #1 passes it to male and female children - If man #7 has it he only passes it to female children - Duchenne Muscular Dystrophy • Leads to muscle degeneration and weakness • More common in males • 3-5 years it begins- death by age of 20 • Caused by loss of function mutations in dystrophin • Dystrophin acts like a shock absorber - When you lose this you are more susceptible to wear and tear • X-Linked recessive gene - Female # 1 passes onto sons - Only women who are homozygous will have trait - Woman #4 who is carrier will only pass to sons Sex Determination - Gonadal ridge: areas in developing fetus that can become testes or ovaries - Tissues called the cortex (ovary) and medulla (testes) on each ridge - In humans, 5th week of development sex is not yet determined 2 Tuesday, March 28, 2017 - Sex determination: initial event that determines whether gonadial ridge will develop testes or ovaries - Sexual differentiation: Subsequent events that produce either male or female phenotype • Primary differentiation: full development of gonads • Secondary differentiation: differentiation of the internal and external genitalia - Development of facial hair in men and breasts in women - How is sex determined? • Environmental factors (ex- reptiles use temperature) • Hormones (ex- fish use levels of estrogen) • Sex chromosomes (ex- in some species they only have 1 sex chrom XX or XO) - Homogametic sex of a species produces identical gametes - Heterogametic sex of species is 2 different (Many have 2 sex chromosomes ) • Some species use the ZZ/ZW notation when the female of the species is the heterogametic sex • Is it the presence of two X's or an X and Y that determines sex? - Example 1: genes on Y chromosome determine male development - Example 2: Two X chromosomes determine female development - Human Sex Determination • Does Y chromosome determine maleness? - Klinefelter syndrome • Have a Y chromosome and more than 1 X • Appear male- infertile, sometimes gynecomastia • This indicates Y determines maleness 3 Tuesday, March 28, 2017 - Turner syndrome • Have X and no Y so XO • Phenotypically female • Indicates Y determines maleness
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