Chapter 17 Complex Patterns of Inheritance PDF

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Chapter 17: Complex Patterns of Inheritance -

Inheritance Patterns: o Mendelian: single gene affects single trait and alleles obey law of seg. Includes:  simple Mendelian inheritance  X- linked inheritance  incomplete dominance  codominance o epistasis: alleles of one gene mask effects of alleles of other gene o continuous variation: offspring display cont. range of phenotypes o linkage: two or more genes close together on same chromosome (not assort. Ind.) o extranuclear inheritance: genes found in genomes of mitochondria + chloroplast (mom!)  endosymbiosis o X-chromosome inactivation: (females) one X chrom inactivated in every somatic cell  produces mosaic phenotype o genomic imprinting: allele from one parent is silenced in somatic cells of offspring, but allele from other parent expressed

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Gene Interaction: single character is controlled by two or more different genes, each of which has two or more alleles o epistasis: alleles of one gene mask the expression of the alleles of another gene  TB purple and TB white crossed  F1: all purple  F2: 3:1 purple to white  two different varieties of TB white crossed  F1: all purple  F2: (self-fertilize) 9:7 purple to white  **two different genes involved  C (purple), c (white)  P (purple), c (white)  cc masks P, pp masks C o discrete traits: phenotypes do not overlap, clearly defined (round or wrinkled) o quantitative traits: continuous variation over range of phenotypes (skin color)  polygenic: (common) multiple genes contribute to outcome of trait

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Gene on the Same Chromosome: Linkage, Recombination, and Mapping o linkage: two genes are close together on same chromosome, transmitted as one unit  linkage group: group of genes stay together during meiosis  Bateson and Punnett, 1911: two factor cross of sweet peas (flower color and pollen shape) o PP (purple), LL (long pollen) + pp (red), ll (round pollen)  F1: all purple, long pollen  F2: **all 4 phenotypes displayed, but not in 9:3:3:1 ratio  law of independent assortment rejected

o nonrecombinant: offspring whose combination of traits unchanged from parents o recombinant: offspring with combination of traits different from parental gen  Morgan’s Drosophila  when different genes are located on same chromosome, traits determine by genes are more likely to be inherited together (violates law of in.assort)  homologous chromosomes exchange pieces of chromosomes and create new combos of alleles b/c of crossing over during meiosis  likelihood of crossing depends on distance b/w genes o genetic mapping: study of arrangement of genes in a species’ genome o genetic map: chart showing linear arrangement of genes along chromosome  estimates relative distance b/w linked genes based on likelihood of crossover  recombination frequency: frequency of crossing over to find distance b/w genes  determined by testcross  map distance: defined as # of recombinants divided by total offspring x100 o in map units -

Extranuclear Inheritance: Organelle Genomes o extranuclear inheritance (cytoplasmic): transmission of genes outside cell nucleus  can be biparental, maternal, paternal  chloroplast  maternal inheritance: offspring phenotype depends on female parent o Carl Correns, 1901: four o’ clock plant: offspring pigmentation follows female plant o reciprocal cross: sexes and phenotype reversed o in plants, egg cell provides most of zygote’s cytoplasm, male gamete provides nucleus  chloroplasts inherited via egg  biparental inheritance: both gametes contribute to offspring  paternal inheritance: male gamete contributes only  mitochondria  maternally inherited  mitochondrial diseases o Leber’s hereditary optic neuropathy (LHON): mutation in mito genes encoding electron transport proteins  blindness o neurogenic muscle weakness: mutation in mito gene encoding subunit of mitochondrial ATP synthase (for ATP synth)  abnormalities in nervous system affecting eyes + muscles o maternal myopathy + cardiomyopathy: mito mutation in gene encoding tRNA for leucine  muscle abnormalities **heart o myoclonic epilepsy + ragged-red muscle fibers: mito mutation in gene encoding tRNA for lysine  epilepsy, dementia, blindness, deafness, heart + kidney problems

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Epigenetic Inheritance: modification of gene/chrom during egg/sperm formation or early embryonic growth stages that alters gene expression (fixed during life) o X-chromosome inactivation: (Mary Lyon) two copies of X chromosome in somatic cells of female mammals inactivated (genes unexpressed)  Barr body: (Murray Barr + Ewart Bertram) highly condensed structure in cells  Susumu Ohno: highly condensed X chromosome  mosaic: somatic cells genetically different  dosage compensation: equalizes expression of X-linked genes in male + females  X inactivation center (Xic): (Eeva Therman + Klaus Patau) short region on X chrom to play critical role in X inactivation o genomic imprinting: segment of DNA imprinted/marked so that gene expression occurs only from the DNA inherited from one parent  can involve single gene, part or all of chromosome, or all chrom from parent  permanent in somatic cells  Igf2: insulin-growth factor for proper growth (on autosome, NOT sex chrom)  if functional copy of gene unexpressed, mouse will be dwarf  mice are diploid, possess two copies of gene (from each parent)  Igf2- allele mutation (blocks functional allele)  same genotype, different phenotype (Igf2 Igf2-) o Igf2 must be from paternal gene to be functional o paternal allele silences maternal allele o expressed b/c allele is transcribed into mRNA (maternal not)  DNA methylation silences gene expression by inhibiting the initiation of transcription or causing chromatin in region to become more compact  can also enhance by attracting activator proteins to promoter or preventing binding of receptor proteins...