Chapter 13 Mendelian Genetics The theory of chromosome inheritance Learning Goals By the end of class you should be able to 1 Explain how chromosomes can cross over 2 Describe how crossing over leads to genetic variance 3 Comprehend what makes an allele dominant or recessive 4 Explain how Mendel developed the concept chromosomal adherence 5 Complete a Punnett square Mendel first set of experiments consisted of crossing pure bred lines that differed in just one trait He self crossed the offspring of the first mating and discovered an interesting trend To confirm this trend he looked at 6 additional traits In each case he obtained similar results one form was always and the trait didn t show up until the F2 generation How could these patterns be explained The Principle of Segregation To explain the 3 1 ratio of phenotypes in the F2 generation Mendel reasoned that each gene pair must during the formation of gamete cells He called this the principle of segregation For pure lines he drew diagrams called Punnett squares to represent the different alleles such as RR for dominate round and rr for recessive wrinkled Homozygotes are individuals with two of the same alleles for a given gene The Principle of Segregation This could then explain how the F2 generation would yield a 3 1 ratio if the parents were each heterozygotes which means individuals with two different alleles for each gene Dihybrid Cross Which color is dominant Are the two alleles separated and sorted into gametes independently Based on these data Mendel accepted the principle of independent assortment hypothesis that alleles of different genes are transmitted of one another Meiosis Explains Mendel s Principles Sutton and Boveri discovered in 1902 that meiosis not only reduces chromosome number in but also explains the principle of segregation and independent assortment The physical separation of alleles during anaphase of meiosis I is responsible for Mendel s principle of Meiosis Explains Mendel s Principles If the alleles for different genes are located on different non homologous chromosomes they will sort of one another at meiosis I the basis for the principle of independent assortment Drosophila melanogaster At one point Thomas Morgan discovered a male fly that had white eyes rather than the wildtype red eyes To explore how the white eye is inherited he mated a red eyed female with a mutant white eyed male The Discovery of Sex Chromosomes These results suggested that there was a relationship between the sex of the progeny and the of eye color Sex Chromosomes In flies male fruit flies carry an X and a Y chromosome while females have two X chromosomes Morgan realized that the pattern of the X chromosome in males and females explained the results from his crosses This is called sex linked inheritance Extending Mendel s Rules Mendel studied the simplest possible genetic system There was no sex linkage were influenced by just two alleles and each allele was completely dominant or recessive Once Mendel s work was re discovered researchers began to analyze traits and alleles whose inheritance was more complicated They found some exceptions and extensions to Mendel s Rules 1 Linkage Genes on the Same Chromosome The association between genes on the same chromosome is called linkage Because linked genes are physically part of the same chromosome they violate the principle of independent assortment Linked genes are inherited together unless crossing over occurs between them Genetic Recombination 2 Do Heterozygotes Always Have a Dominant or Recessive Phenotype Dominance is not necessarily an all or none phenomenon In the case of four o clock flowers neither purple nor white alleles Instead the progeny show an intermediate phenotype between the parents Incomplete Dominance Blood Type and Co dominance Many alleles show a relationship called where heterozygotes have the phenotype associated with each individual allele Different alleles of the I gene lead to the production of membrane glycoproteins with different polysaccharides attached to the glycoprotein in the plasma membrane of blood cells 3 Polygenic Inheritance of Quantitative Traits Many traits don t fall into categories Many exhibit quantitative variation meaning individuals by degree These are called quantitative traits rather than discrete traits Polygenic inheritance where each gene adds a small amount to the value of the phenotype Human Eye Color Human eye color is an example of inheritance producing different phenotypes determined by the amount and type of pigment found in the eye s iris The genetics of eye color are complicated and are determined by multiple genes Generally speaking brown eyes are considered dominant over both blue and green eyes Green eyes are thought to be over blue eyes Applying Mendel s Rule to Humans Sickle cell anemia Autosomal Recessive Individuals with the trait must be For an individual to be affected both parents must carry the allele responsible Autosomal Dominant Disease Huntington s disease Autosomal Dominant Individuals with the trait must have one of the parents who show the trait as well X Linked Recessive Hemophilia If males are more likely to have the trait than females the allele is likely to be recessive and found on the X chromosome Get in your groups Suppose that in garden peas the genes for seed color and seed pod shape are linked You cross YYII parents which produce yellow seeds and inflated pods with yyii parents which produce green seeds and constricted pods A Draw the F1 Punnett square and predict the genotypes and phenotype s if no crossing over occurs B Draw the F2 Punnett square if no crossing over occurs and predict the genotypes and phenotypes
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