Mendelian Genetics Gregor Mendel 1822 1884 father of genetics Charles Darwin 1809 1882 not founder of evolution but how evolution occured Genotype The actual genetic make up of the organism Phenotype A physical characteristic or function that is observable SO physical characteristics phenotypes are dependent upon genetic make up genotype Allele alternative forms of a single gene Homozygous identical alleles of a gene yellow yellow Heterozygous different alleles of a gene yellow green Dominant allele upper case in a heterozygote the allele that determines the phenotype Recessive allele lower case in a heterozygote the allele that has no noticeable effect on the phenotype in a heterozygous blend the dominant allele will determine the phenotype Homozygous dominant two alleles for a dominant trait Homozygous recessive two alleles for a recessive trait Crossing homozygous dominants result in pure bred offspring always have the same genetic characteristics of the parent Crossing a homozygous dominant and a homozygous recessive reduction through meiosis F 1 generation yields all heterozygous offspring peas that are all Yy genotype is Yy phenotype is yellow Crossing F 1 generations with each other results in an F 2 generation pea genotype occurs in a 1 2 1 proportion 1YY 2Yy 1yy The point being green seeds disappeared in the F1 generation but re appeared in the F2 generation Phenotype green seeds re appeared in the F2 generation in a proportion of 3 1 dominant trait recessive trait Two phenotypes yellow and green Three genotypes YY Yy and yy Mendel concluded Inheritance does not involve a blending of characteristics if not true we would loose the recessive gene and all start to look alike Therefore the F1 generation must have held the element responsible for green seeds in order for it to have been passed on to the F2 generation Thus a single gene for seed color occurs in two alleles green yellow residing on separate but homologous chromosomes To obtain an F 3 generation TWO F 2 generations are crossed Homozygous alleles are restored recessive genes don t just go away Mendel s Law of segregation Organisms have two genetic elements that separate when gametes are formed what we know today as the separation of homologous chromosomes Dihybrid crosses Involves crossing of two characteristics F 1 is all heterozygous Ss Yy F 2 ratio 9 3 3 1 Still represents a 3 1 ratio referring to phenotype Mendel s Law of Independent Assortment During gamete formation gene pairs assort independently of one another Seed color and seed shape are transmitted independently of each other Co dominance Heterozygous state where both alleles are dominant both alleles are expressed combination of the two alleles red flowers with white flowers pink flowers 1 2 1 ratio Sex linked inheritance Female sex chromosomes are homologous XX Male sex chromosomes are not XY Therefore males do not have a second homologous chromosome with a possible dominant allele with which to mask the effect of a recessive allele x link
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