CHAPTER 14 CONTINUED Mendel s interpretation o Heritable factor now called genes are responsible for character inheritance o Inheritance is particulate do not blend o Mendel worked with simple system o Peas best candidate genetically simple o Most traits controlled by a single gene o Each gene has 2 alleles 1 dominant over other Mendel s laws ratio 1 2 1 o Law of segregation based on monohybrid cross Phenotype ratio 3 1 and Genotype o Law of independent assortment based on dihybrid cross and assuming genes on non homologous chromosomes Phenotype ratio 9 3 3 1 Complete dominance one allele dominates phenotype heterozygote and homozygote dominant are indistinguishable Incomplete dominance the traits appear to blend at least in the F1 generation producing an intermediate phenotype one allele shows partial dominance F2 homozygous phenotypes reappear o Hair texture in humans exhibits incomplete dominance HH curly Hh wavy hh straight Multiple alleles of a single gene o Codominant and multiple alleles 2 alleles affect the phenotype equally and separately Those involved in blood groups are both expressed at the phenotypic level Has six possible genotypes Universal recipient AB universal donor O Rh factor genes are inherited in dom recess fashion Rh dominant Rh recessive Multiple gene affecting phenotype of one character o Polygenic inheritance additive effect of two or more genes on a single phenotypic o Blood character Quantitative characters that vary continuously throughout a population such as skin color and height in humans are usually encoded by many independent genes o Epistasis one gene altering or controlling phenotypic expression of another gene Form of gene interaction in which one gene effects the expression of the other Alleles that are masking the effect epistatic alleles Alleles whos effect is being masked hypostatic alleles No new phenotypes produced by this type of gene interaction Can be described as either recessive epistasis or dominant epistasis Other genes on other chromosomes contribute to severity of Cystic Fibrosis symptoms o modifier genes TGFB1 allele on chromosome 19 influences immune response to infection more infection more scarring of lung tissue inherited as recessive allele Sickle cell disease affects one out of 400 blacks o Pleiotropic one gene affects more than one phenotypic character Another example of pleiotropy is phenylketonuria reduced hair pigmentation retardation Achondroplasia type of dwarfism inherited by rare dominant allele
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