linkage_chapt05_lecture.ppt

Human Genetics Concepts and Applications Eighth Edition Powerpoint Lecture Outline Ricki Lewis Prepared by Dubear Kroening University of Wisconsin-Fox Valley Exceptions to Mendel?s Law Most genes do not exhibit simple inheritance Genotypic ratios persist but phenotypic ratios may vary because of interactions between Alleles Other genes Non-nuclear genes Segregation of genes on same chromosome Environment Lethal Alleles Some allele combinations are lethal Figure 5.1b 2 litters Expect 16 pups: 12 hairless 4 hairy 2 litters 12 pups: 8 hairless 4 hairy Mitochondrial Inheritance Linkage Linkage is the transmission of two genes on the same chromosome Two genes on the same chromosome will not assort randomly in meiosis Expected Results in a Dihybrid Cross Figure 5.10 Crossing over Disrupts Linkage Figure 5.11 Recombination The frequency of recombination or crossing over between two genes is proportional to the distance between the genes Figure 5.13 Three types of maps describe genetic information Linkage maps correlate the crossover frequency and the distance between genes Physical maps describe the number of nucleotides between genes Linkage Disequilibrium (LD) Is the non-random association between alleles at two locations on a single chromosome Example: Two genes, A and B, exist in a population Genes are in equilibrium if the frequency of chromosomes with AB=Ab=aB=ab The genes are in linkage disequilibrium if the frequency of one allele of gene A is seen more frequently with a particular allele of gene B LOD Score Is the logarithm of the odds ratio calculated by how often genes and markers are inherited together Is the likelihood that particular crossover frequency data indicates linkage LOD scores of 3 or greater are considered significant and indicate the data would be observed by chance 1/1000 times Can be used to identify disease-associated genes Haplotype Is the set of alleles inherited on one chromosome Make it possible to track which parent transmits which genes 34 34 34 34 34 12 12 12 12 12 13 13 13 13 13 23 23 23 23 23 14 14 14 14 14 14 14 14 14 14 23 23 24 24 24 23 23 23 23 23 14 14 14 13 13 44 44 44 44 44 33 33 33 33 33 11 11 11 11 11 22 22 22 22 22 Gene A Gene B Gene C Gene D Gene E Mapping with Haplotypes Segregation of a dominant trait is observed in this family (filled symbols). The trait segregates with the yellow haplotype. 23 23 24 24 24 14 14 14 13 13 13 13 13 13 13 23 23 23 23 23 14 14 14 14 14 34 34 34 34 34 12 12 12 12 12 14 14 14 14 14 23 23 23 23 23 44 44 44 44 44 33 33 33 33 33 11 11 11 11 11 22 22 22 22 22 Gene A Gene B Gene C Gene D Gene E Mapping with Haplotypes Recombinant chromosomes III-3 and III-6 inherit recombinant chromosomes. The location of the recombination events indicate that the gene for this trait is located between genes B and D Chapter 7 Multifactorial Traits (Lecture) Lewis, Human Genetics Adopted Individuals pg. 140 thru SNPs, pg. 143 Contribution of Genes or Environment Genes rarely act completely alone Environmental factors and other genes may modify expression Traits can be described as Mendelian or primarily due to a single gene Polygenic or primarily due to multiple genes Multifactorial due to an interaction between genes and the environment Polygenic Trait: a trait that reflects the activities or more than one gene Multifactorial Traits: traits that are influenced by the environment (such as nutrition, stress, toxins) A trait can be both polygenic and multifactorial, causing a wide range in disease or trait severity and age of onset Polygenic Traits The combination of many genes produces a continuously varying phenotype (Bell curve distribution) All of the genes contribute to the phenotype, there is no dominant or recessive Polygenic and Multifactorial Traits Height: influenced by several genes and nutrition Fingerprints: influenced by several genes and how the fingers and toes touch the amniotic sac in utero This is why identical twins do not have identical fingerprints! Serum cholesterol levels: influenced by several genes involved in metabolism and lipid uptake as well as diet Analyzing Multifactorial Traits Comparisons between and within families Genome Wide Association Studies ? compare SNP patterns between large numbers of affected and unaffected individuals to identify important DNA regions SNP Type of genetic marker Single Nucleotide Polymorphism (SNP): One nucleotide at a specific site that varies in > 1% of a population. May be in a gene or close to a gene. Single Nucleotide Polymorphism On average, between two random individuals, there is one SNP every 1,200 bases or about 10 million differences! Linkage disequilibrium: SNPs are inherited together in blocks (haplotypes) Scanning the Human genome: 10 x106 SNPs = 5 x 104 haplotypes Association Studies Studies compare a group of interest (cases) to a control group for the presence of a gene or SNP Controls are matched to cases for characteristics that may confound results: age, ethnicity, gender, environment If the SNP is present more often in cases than controls, it is associated with the trait and implies that the SNP may be near a gene impacting the trait Often find multiple regions or genes that contribute to the phenotype A limitation is that they establish correlations, not definitive causations Use research to establish biological significance Association Studies