Chromosomal Inheritance II Flashcards
Insights from GWAS
- On average, the proportion of variance explained by each individual variant is small
- Each individual carries several alleles that increase and decrease the trait or disease risk
Factors that affect success of GWAS
- how many loci affecting the trait segregate in the population
- joint distribution of effect size and allele frequency at those loci (I.e. genetic architecture)
- experimental sample size
Missing heritability mystery
We know that 80-90% of variance in height is explained by genetics but the alleles identified that determine height only explain 20% (same is true for many other genetically complex traits)
Nondisjunction
pairs of homologous chromosomes do not separate normally during meiosis, resulting in one gamete receives two of the same type of chromosome and another gamete receives no copy
Nondisjunction in meiosis I
n+1 (2)
n-1 (2)
Nondisjunction in meiosis II
n+1
n-1
n (2)
Aneuploidy
Results from the fertilization of gametes in which nondisjunction occurred and results in monosomic (2n-1) and trisomic (2n+1) individuals
Down Syndrome (Trisomy 21)
-aneuploid condition that results from three copies of chromosome 21
-1/830 children
-the frequency of Down Syndrome increases w/ the age of the mother
Patau Syndrome (Trisomy 13)
Symptoms: serious eye, brain, circulatory defects as well as cleft palate
-children rarely live more than a few months
Edward’s Syndrome (Trisomy 18)
Almost every organ system affected
-children generally do not live more than a few months
Trisomy X (XXX)
healthy and fertile, usually cannot be distinguished from normal female except by karyotype, slightly taller than average
Klinefelter Syndrome (XXY)
unusually small testes, sterile, breast enlargement, other feminine body characteristics
Monosomy X (singular X chromosome)
-only known viable monosomy in humans
-phenotypically female but sterile due to lack of female organ maturation
XYY
healthy and fertile, tend to be taller than average
Accessory chromosomes
chromosomes that are not essential for survival
-show high mutation rates, the acquisition of foreign genes, copy number polymorphisms, and frequent rearrangements (evolve fast)
Core chromosomes
chromosomes that carry essential genes that encode housekeeping functions such as cellular metabolism and reproduction (evolve slowly)
Polyploidy
a condition in which an organism has more than two complete sets of chromosomes
-common in plants
Triploidy
3 complete sets of chromosomes, either miscarriage or death within the first year of life
Developmentally Programmed Polyploidy
-polyploidy increases the metabolic capacity of a tissue
-Hepatocytes and placental trophoblast giant cells (TGCs): provide nutrients and metabolites
-Megakaryocytes: stockpile of components needed to generate platelets
Chromosome Structure Alterations
-Deletion: deletes a segment
-Duplication: repeats a segment
-Inversion: reverses a segment within a chromosome
-Translocation: moves a segment from one chromosome to a non-homologous chromosome
Cri du Chat
-specific deletion in chromosome 5
-small head w/ unusual facial features, severe intellectual disabilities, and a cry that sounds like the mewing of a distressed cat
-individuals usually die in infancy or early childhood
Chronic Myelogenous Leukemia
-translocation between chromosome 9 and 22
-translocated chromosome 22 = Philadelphia chromosome
-causes cancer by activating a gene involved in control of cell cycle
Genomic Imprinting
-silencing genes through DNA methylation
-only affects a small fraction of mammalian genes
-most imprinted genes are critical for embryonic development
-not expressed early in fetal development
-reprogrammed in the germ line of the adult, who could be either male or female
What is an exception to standard Mendelian Inheritance?
Mitochondria and plastids
Extranuclear/Cytoplasmic genes
genes in mitochondria or plastid genomes
What is inheritance controlled by in mitochondria and plastids?
depends on the maternal parents bc the zygote’s cytoplasm comes from the egg
Evidence of extranuclear genes
yellow or white patches on leaves of an otherwise green plant
-phenotype of offspring only matched the maternal parent
