Genetics Notes Week 3
Genetics Notes Week 3 BIO 235
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This 2 page Class Notes was uploaded by Joseph Notetaker on Friday September 9, 2016. The Class Notes belongs to BIO 235 at Missouri State University taught by Lazlo Kovacs in Summer 2015. Since its upload, it has received 21 views. For similar materials see Genetics in Biology at Missouri State University.
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Date Created: 09/09/16
Genetics Notes Week 3 Sex determination in other animals: o Insects: In fruit flies (Drosophila melanogaster) sex determination is also by the XX-XY system Same as humans Males are XY and Females are XX Some other insects use the X0-XX system Males are X0 and Females are XX o Birds In birds the sex chromosomes are ZW instead of XY Males are Homogametic (ZZ) Females are Heterogametic (ZW) o Temperature-Dependent Sex determination (TSD) Common in reptile species This happens because temperature influences the expression of aromatase genes which convert androgens to estrogens Variation in the number of X chromosomes: o In each female cell, only one copy of the X-chromosome is expressed even though 2 copies are present o When the female embryo contains 500-1000 cells, one copy of the X chromosome is inactivated and remain forever inactivated o The particular X chromosome that is inactivated is randomly determined o Females are mosaics of cells with either the maternal or the paternal X chromosome inactivated The inactive X chromosome becomes heterochromatin for the purpose of dosage compensation This chromosome appears in interphase cells as a dark mass. Incomplete dominance o Neither parental allele is dominant, instead both contribute to the F1 phenotype o Phenotypic ratios are same as genotypic ratios Codominance o F1 hybrids express phenotype of both parents equally o Phenotypic ratios are the same as genotypic ratios Genotype vs Phenotype o The same genotype doesn’t always produce the same phenotype Penetrance The number of individuals in a population with a particular genotype show the expected phenotype. For example, only a percentage of people with the gene that predicts heart disease actually develop heart disease Expressivity The intensity with which a particular genotype is expressed in a phenotype For example, a man who inherited the gene that predisposes him to atherosclerosis may only develop slightly higher blood pressure Environment can affect the phenotypic expression of a genotype. Temperature is a common environmental factor Coat color in Siamese cats is lighter on its torso because of a mutation that renders an enzyme involved in melanin o synthesis is inactive at 37 C The enzyme is functional in the extremities because the temperature of those parts is below 37 C and the enzyme is still functional at that temperature Height is a continuous trait in humans o Continuous variation can be explained by Mendalian analysis