Lecture 5 BIO 311D 2nd Edition Outline of Last Lecture I Interactions gene loci trait II Linked genes III Sex linked traits Outline of Current Lecture I Linked genes II Mechanisms of sex determination III XX mammals are genetic mosaics IV Intro to population genetics Current Lecture I Linked genes A Inherited together because they are located near each other on the same chromosome B Crossing over can unlink these genes C A woman is heterozygous for the recessive X linked gene for hemophilia Her husband is not affected by hemophilia What proportion of their daughters will be carriers for the trait II Mechanisms of sex determination A Environmental temperature cues hormone environment Fish steroid hormones affects the development of gonads Temperature in turtles above a certain temperature the sex is determined however this could be a great disadvantage cause the turtles could be all male or female B Chromosomal genetic presence of sex determination factor number of sex chromosomes In groups with chromosomal sex determination the two sexes differ in their six chromosomes Primary sex determination is genetic In humans and insects the two chromosomes in the female are the same length and fully homologous In birds the male has two similar chromosomes and the females differ Primary sex determination in mammals sry gene on Y chromosome Secondary sex characteristics depend on sex hormones affecting gene expression on other chromosomes XY individuals lack a small portion of the Y chromosome sry gene Female XX individuals have a small piece of the Y chromosome attached to another chromosome male C XX mammals are genetic mosaics In human female embryos one X chromosome in each cell becomes inactivated Initial determination of which X is inactivated is totally random Barr body is the inactive X heavily methylated DNA Because of X chromosome inactivation heterozygous mammalian females are genetic mosaics for X linked traits Some cells express genes on one of their X chromosome and some cells the other Inactive X stays highly condensed the Barr body The phenotype of a heterozygous female shows random patches D Intro to population genetics Gene pool sum of all alleles in a population Allele frequency the proportion of a certain allele at a given locus in all individuals within a population Genotype frequency the proportion of a given genotype within a population Hardy Weinberg equation relates allele and genotype frequencies Allele frequencies p q 1 Genotype frequencies p2 2pq q2 1 Example problem there are two alleles for a certain gene locus A and a 70 of the alleles in a particular gene pool are A What is the allele frequency of a 1 0 7 0 30
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