Objectives for Exam 2 PCB 3063 Summer 2012 Maternal Inheritance Know what the term maternal inheritance refers to know that mitochondria and chloroplasts contain a circular DNA molecule that codes for some of the proteins needed for respiration and photosynthesis Be able to identify maternal inheritance in a human pedigree Linkage and Mapping Chapter 7 Q Important Terms concepts to know Linkage genes that are part of the same chromosome and thus can t undergo independent assortment Genes on the same chromosome are said to be linked Linkage is usually less than 100 because of crossing over during meiosis Crossing over reciprocal exchange of chromosome segments when homologs are paired Parental or noncrossover gametes complete linkage 2 parental gametes are formed in equal proportions noncrossover or nonrecombinant progeny are most frequent Recombinant or crossover gametes when exchange occurs between nonsister chromatids can never be more than 50 of the total Single crossover double crossover when a single crossover occurs between two nonsister chromatids the other two chromatids of the tetrad are not involved in this exchange are enter the gamete unchanged When a double crossover occurs double exchanges of genetic material have happened Map unit is equal to 1 recombination between 2 genes centimorgans Genetic map physical map Genetic maps are generated by using the recombination process as we have been learning about physical maps are calculated by using physical distances along the chromosome as the number of base pairs Q Essential to chapter 7 is to be able to do problems involving two point and three point crosses leading to construction of a genetic map As part of this you should understand why recombination frequencies can never exceed 50 Genes on the same chromosome that are more than 50 map units apart undergo recombination so frequently that they assort independently they are inherited as if they were on different chromosomes The frequency with which crossing over occurs between any two linked genes is generally proportional to the distance separating the respective loci along the chromosomes As the distance between the two genes increases the proportion of recombinant gametes increases and that of the parental gametes decreases The farther apart the better the chances of recombination Q You should know the relationship between genetic maps produced by studying recombination frequencies and physical maps of chromosomes that are produced by DNA sequencing they are co linear but the distances between genes can vary because some areas on chromosomes are hot spots for recombination and in some areas like centromeres recombination is suppressed The closer 2 loci reside along the axis of the chromosome the less likely any single crossover event will occur between them Quantitative Genetics Polygenic Inheritance Chapter 22 Q Important Terms concepts to know Polygenic inheritance also known as quantitative inheritance refers to inheritance of a phenotypic characteristic trait that is attributable to two or more genes or the interaction with the environment or both Unlike monogenic traits polygenic traits do not follow patterns of Medel s inheritance separated traits Instead their phenotypes typically vary along a continuous gradient depicted by a bell curve Trait is determined by 3 gene pairs which contribute additively to the phenotype each pair has an additive and a non additive allele phenotype reflects the total number of additive alleles Discontinuous Traits individuals can be grouped into distinct phenotypes Continuous traits individuals display a continuous range of phenotypes shaped distribution Multifactorial traits polygenic traits that are influenced by the environment ex height eye color skin color blood pressure IQ behavioral traits bell Quantitative traits Additive and nonadditive alleles Complicated of simple Twin studies Concordance When a pair of twins display the same phenotype Monozygotic twins should be 100 concordant Dizygotic twins less than 100 concordant You should understand the genetic basis for traits that display continuous variation polygenic inheritance involving additive or nonadditive alleles and have a general understanding of the types of traits that have this genetic basis You should be able to do simple problems that require determining the number of genes that control a polygenic trait and the amount each allele contributes to the phenotype similar to those in homework 2 Q You should also be able to analyze human twin study data and identify traits that are determined primarily by the environment primarily by genes or by a mixture of both genes and environment Many polygenic traits are also influenced by the environment multifactorial traits additive alleles Polygenic traits may be more complicated that simple additive and non When a pair of twins display the same phenotype Concordant If the trait is controlled completely by genes monozygotic twins should be 100 concordant and Dizygotic twins should be 100 concordant Can use this basis to determine if a trait is entirely environmental or gene controlled Chromosome Abnormalities Chapter 6 Q Know the following terms and their significance Chromosome mutations or chromosome abnormalities change in total of chromosomes the deletion or duplication of genes or segments of a chromosome and rearrangements of the genetic material either within or among chromosomes PASSED TO OFF SPRING Nondisjunction leads to monosomy and trisomy where paired homologs fail to separate during segregation ABNORMAL DISTRIBUTION OF CHROMOSOMES Aneuploidy general condition where an organism loses or gains one or more chromosomes but not a complete set 2n or X chromosome s Monosomy loss of one chromosome monosomy for the X chromosome occurs in humans monosomy for any of the autosomes is usually not viable 2n 1 ex Turner syndrome 45 X Trisomy addition of an extra chromosome produces more viable individuals in animal and plant species than does the loss of a chromosome 2n 1 Down Syndrome trisomy 21 the only human trisomy autosomal in which a significant number of individuals can survive longer than a year The frequency of Down syndrome is related to the mothers age Amniocentesis chorionic villus sampling fetal cells are obtained from the amniotic fluid or the chorion of the placenta after the cells are obtained the karyotype can be determined Triploid three sets of chromosomes are present 3n Tetraploid four sets of
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