Bio 321 F13 lecture 2 v2

Division 4n a a a 2nd a division 25 a 2nd division

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Unformatted text preview: 339513/2_P riests_large.jpg What about phenotypic ra5os e.g. cross between two Da heterozygotes in Peas (dihybrid cross); D is dominant to d; DD = tall; Dd = tall; dd = short EGGS DD, Dd, dd 50% D 50% d DD, Dd, dd 50% D 25% Dd 50% d SPERM 25% DD 25% Dd 25% dd Punne= square Ra5o of genotypes in progeny: 1 : 2 : 1 DD: Dd : dd tall plants (DD, Dd) short plants (dd) Ratio of phenotypes: 3 tall : 1 short When to use a Punnet square •  When there is random ma7ng –  Gametes are uni7ng to form zygotes at random, with respect to the gene under considera7on •  Many marine invertebrates mate by broadcas7ng gametes, which unite to form zygotes in the water –  Zygote forma7on not random with respect to some genes •  When calcula7ng genotype/phenotypes in a human- controlled cross the problem with dominance •  DD and Dd have the same phenotype = can’t tell them apart •  need to do a test cross to a homozygous recessive individual dd (short) –  D? x dd •  if DD, then genotype ra7o = 100% Dd, phenotype ra7o = 100% tall •  if Dd, then genotype ra7o = 50% Dd, 50% dd, phenotype ra7o = 50% tall, 50% short in class exercise 1 •  What if a gene H had two alleles with intermediate dominance, also called addi7ve effects, such that: –  HH = tall –  Hh = intermediate height –  hh = short •  do a PunneR square to determine the genotypic and phenotypic ra7os of a monohybrid cross th postulate Mendel’s 4 •  Independent assortment – alleles at different loci segregate independently of each other •  Mendel’s loci – were all unlinked – not close to each other on the same chromosome –  alleles at linked loci do not segregate independently – see future lecture Independent Assortment two loci illustrated on two different chromosomes (in this case, A/a; B/b double heterozygote; “dihibrid cross”) A a division A replication B 2nd division 1st a a A pre-meiotic cell (2N) a b b A or 1st division 4N a A B B A A b B b 2nd division A A a b b b 2nd division B 2nd division B a a b A a B 4N B b A B B a B b or a b Independent Assortment two loci illustrated on two different chromosomes (in this case, A/a; B/b double heterozygote) nd A a A replication B 2 1st division a a A pre-meiotic cell (2N) a b or b A or 1st division 4N a A B B A A b B b division A A b 2nd division B 2nd division B a a b b A A 25% B 25% b 2nd a b B a B 4N B division a B b a 25% b 25% the outcome of independ...
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