Chapter 6 & 7 - Lecture notes 5 PDF

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Grace Goschke...

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Chapter 6 & 7 Tuesday, January 29, 2019

9:18 AM

Aneuploidy - Humans have a 10x higher rate of aneuploidy - 1 in 2 conceptions are aneuploidy - 5 - 7% of early childhood deaths are related to aneuploidy Nondisjunction - Failure of ○ Homologous chromosomes don’t separate in meiosis 1 ○ Sister chromatids separate in meiosis 2 or mitosis ○ Produces abnormal gametes

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All abnormal cells

- In meiosis 2, 2 cells are normal and the other 2 are abnormal Autosomal Monosomy - Lethal condition - Rarely seen in spontaneous abortions and live births because the majority are lose very early in development Effects of autosomal Trisomy - Most are lethal - 50% of cases that cause fetal death are autosomal - Varies by chromosome (some more common) Trisomy 21, 18, 13 screening - Trisomy 21 - down syndrome ○ Only autosomal trisomy that allows survival into adulthood ○ 1/800 births - Trisomy 18 - Edwards syndrome (lethal) - Trisomy 13 - Patau syndrome (lethal) Maternal Age and Trisomy - The older the mom, the higher risk for down syndrome ○ Long meiosis - older eggs ○ Maternal selection is less effective - Usually a mistake in meiosis 1 - After 35, down syndrome - 1/111 births Frequency of Aneuploidy during development - Almost all chromosomally abnormal embryos and fetuses are eliminated as pregnancy progresses

Sex Determination - Chromosomal sex determined at fertilization - Sexual differentiation begins in the 7th week and is influenced by a combination of genetic and environmental factors Chromosomal Sex Determination - X = 155 million base pairs with 1500 protein coding genes ○ Essential for life - Y = 30 million base pairs with 231 protein coding genes Aneuploidy of Sex Chromosomes - More common than autosomal - Can involve both X and Y - Balance needed for normal development ○ At least one copy of X required for development

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If they're sterile, sperm probably doesn't function correctly ○ Homologous chromosomes don't pair up correctly ○ Would not be allowed to undergo meiosis

Turner Syndrome (45, XO) - 1/10,000 births - Females - short, wide chest, rudimentary ovaries and abnormal sexual development - Puffiness of hands and feet - Aorta abnormalities - No mental dysfunction - Monozygotic twins ○ Nondisjunction in mitosis Kleinfelter Syndrome (47, XXY) - Males with fertility problems and maybe some mental dysfunction - Older moms - 60% due to maternal nondisjunction XYY Syndrome (47, XXY) - Usually taller than normal - Where did the nondisjunction happen ○ dad Sexual development begins in 7th week of gestation - Affected by ○ Gene action ○ Interactions within the embryo ○ Interactions with other embryos in the uterus ○ Interactions with the maternal environment Sex determination - Lots of opportunities for epistasis - S(ex determining) R(egion)Y gene ○ Found on Y chromosome ○ Turn on genes that promote development ○ If turned on, person will be male

Both develop from the genital tubercle Androgens - testosterone - Dihydotesterone - SRY gene signals development of testes Mutations can Uncouple Chromosome Sex from phenotypic Sex - Someone with XY but cant make testosterone are female Androgen Insensitivity - Can produce testosterone but cant receive signal so they are female XY with Androgen Insensitivity Mutations can Cause Sexual Phenotypes to Change at Puberty - Pseudohermaphoditism ○ An autosomal genetic condition that causes XY individuals to develop the phenotype of females ○ Caused by mutations in several different genes ○ Have both penis and vag at different times of life ○ At puberty, females change into males Crossing Over - Swap information just at the tips - Abnormal crossing over ○ The SRY gene can make its way onto the X chromosome

X Inactivation - Cat pigmentation on X chromosome - Calico cats are all female X Chromosomes and Barr Bodies - Barr body - inactive X chromosome - Only 1 active X in each Mosaic Type - Sweat glands - some active and some inactive

Random X-chromosome Inactivation - X inactivation begins and is regulated from the X inactivation center (Xic) of the X chromosome 1. Counting the number of X chromosomes present by pairing X chromosomes 2. Selecting which X chromosome to inactivate by expression of XIST gene (inactivated chromosome coated with XIST RNA) a. This is reversed in germ line cells destines to become oocytes b. Why fertilized ovum does not have an inactive X chromosome - Can cause twins with identical genotypes to have different phenotypes ○ Twins inherited allele for color blindness from dad

Sex-Related Phenotypic Effects - In sex influenced and sex-limited inheritance, the sex of the individual affects 1. Whether the trait is expressed

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The degree to which the trait is expressed a. True for both autosomal and sex-linked genes A Sex-Influenced Trait - Expressed in both sexes - Male pattern baldness A Sex-Limited Traits - Inherited by both sexes, but expressed in only one ○ Genes that produce a phenotype in only one sex because gene products have different functions ○ Traits expressed only in females because males die before birth Genomic Imprinting - Parent of Origin Effects - Lion, Tiger, Tigon, and Liger Imprinting - Epigenetic alteration: ○ One copy of a gene is inactivated - depending on whether it comes from the father or mother - Imprinting ○ a phenomenon in which expression of a gene depends on whether it is inherited from the father or mother Genomic Imprinting - Required for normal development - Mistakes I imprinting are called epimutations - Usually paternal expressed and maternal silenced - prader willi - Usually maternal expressed and paternal silenced - angleman - Mono-allelic expression ○ Abnormal imprinting...