Lecture 5 BIO 311D 2nd EditionOutline of Last Lecture I. Interactions: gene loci traitII. Linked genesIII. Sex-linked traitsOutline of Current Lecture I. Linked genesII. Mechanisms of sex determination III. XX mammals are genetic mosaicsIV. Intro to population geneticsCurrent LectureI. 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? – ½II. Mechanisms of sex determinationA. 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 patchesD. 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 frequenciesAllele frequencies: p+q = 1Genotype 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 =
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