Genetics week 2 notes
Genetics week 2 notes Biology 215
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This 4 page Class Notes was uploaded by Kayla Notetaker on Thursday April 14, 2016. The Class Notes belongs to Biology 215 at Northwestern University taught by Professor peterson in Spring 2016. Since its upload, it has received 11 views. For similar materials see Genetics in Biology at Northwestern University.
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Date Created: 04/14/16
Week two Day 4 Sex-linked inheritance and human pedigree analysis Aneuploidy and chromosome abnormalities Chromosome segregation through meiosis accounts for mendels equal segregation law of parent alleles This creates 4 haploid gametes Aneuploidy is a nondisjunction that results in chromosome number abnormalities So half of the gametes may end up with more or less chromosome than normal Down syndrome is an example x-linked inheritance How does this trait behave? In an experiment on fly eye color (w+ for red eyes [dominant] and w for white eyes) A pure gene white eyed male fly crossed with a pure red eyed female F1 all heterozygous for red eyes Intercross F2 observed 50% red eyed females 25% red eyed males 25% white eyed males 0% white eyed females If this was on individual alleles we would expect 75% red eyes total and 25% white eyed total (male and female) This is because it is sex linked and males receive the Y chromosome from their father and an X chromosome from their mother Males are heterogametic sex (XY) and females are homogametic sex (XX) Y has very few genes on it and males only carry 1 copy of x linked genes so males expressed x-linked alleles even if they are recessive (AKA only need 1 recessive allele to be expressed) To confirm breed the F2 white eyed males with a pure breeding red eyed female Should observe 25% red eyed females, 25% red eyed males, 25% white eyed males, and 25% white eyed females (1:1:1:1 ratio) Examples of human x-linked traits are Colorblindness, hemophilia, Duchenne muscular dystrophy, etc Dosage compensation in mammals Need an equal dose of proteins on x linked genes in males and females so in females one x is inactivated (at random) in each cell The inactive x is called a barr body This type of X-inactivation can result in mosaic phenotypes in mammalian females (and example would be calico cats where they can express 3+ fur patch colors) X-inactivation allows aneuploidies of sex chromosome Single X is phenotypically female but causes turner syndrome XXY phenotypically male causes klinefelter syndrome XXX normal female XYY normal male DAY 5 Y linked inheritance Only males will have it never females Independent assortment Separate genes for separate traits are passed independently of each other from parents to offspring but this is only true for genes that are on different chromosomes How to figure out what proportion of progeny will have a specific genotype Measure out each allele separately for frequency of that specific allele that you are looking for then multiply all the fractions together Example Looking for genotype a/a b/b c/c d/d e/e Breed A/a b/b C/c D/d E/e and A/a B/b C/c d/d E/e together Take frequency of finding a/a from cross of A/a x A/a This would be ¼ Do the same with b/b x B/b etc Multiply frequencies together in this case it would be ¼ x ½ x ¼ x ½ x ¼ which equal 1/256 chance of finding a genotype of a/a b/b c/c d/d e/e What sample size do you need to be 95% confident that you will find 1 individual of interest Take the fraction you just found and subtract from 1 1-1/256 = 255/256 Multiply 256 by 3 = 765 therefore 765<N N being the sample size This means that you need AT LEAST about 765 progeny to find at least 1 genotype of interest DAY 6 Linkage and gene mapping Crossovers hold homologous chromosome pairs together in metaphase 1 There is usually at least 1 crossover at randomly selected positions along the chromosome This results in an exchange of DNA between chromosomes This results in a new combination of Double stranded DNA sequences Independent assortment occurs if 2 genes are on different chromosomes If 2 genes do not assort independently then they are linked (like eye color and wing size) Genes tend to be thought of as linked if they are under 50% away from each other The genes with the highest number of progeny tend to be “parental” so these are the genes that come from the female parent and show up in the offspring. The genes with the lowest number of progeny tend to be single recombinant (crossover of parent genes to create new combination) Crossovers will occur in meiosis 1 at random positions The likelihood of crossovers between two genes depends on their distance apart from one another In meiosis 1 is when homologs separate and meiosis 2 is when sister chromatids separate Frequency of recombinants is a measurement of distance (# of recombinant progeny/total # progeny)x100=distance between 2 genes This is good to know for next notes on 3-point crosses
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