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Genetics Lab 12 Genes & Alleles Gene – Basic unit of heredity. A short sequence of DNA that codes for a polypeptide. Example – the gene that codes for flower color in peas. Allele – one of two or more forms of a gene. Example – purple or white for flower color in peas. Dominant vs. Recessive Dominant – allele that controls the appearance of heterozygotes. Example – an individual with alleles for purple and white will appear purple. Recessive – allele that is masked in the presence of a dominant allele. Example – white flowers only appear when only white alleles are present. Homozygous vs. Heterozygous Homozygous – both alleles are the same. Hom. Recessive – shows recessive trait. Hom. Dominant – shows dominant trait. pp – pea flowers appear white. PP – pea flowers appear purple. Heterozygous – two different alleles. Pp – pea flowers appear purple. Genotype vs. Phenotype Genotype – which ALLELES are actually present. PP, Pp, pp Phenotype – what the organisms LOOKS like. Purple, white Punnett Square A purple flowered plant PP is crossed with a white flowered plant pp. PP x pp. PP can pass on only one of the two alleles – P or P. pp can pass on only one of the two alleles – p or p. Punnett Square PP x pp P P p Pp Pp p Pp Pp Genotype – All Pp Phenotype – all Purple Monohybrid Cross Monohybrid cross – examines how one trait is passed on when both parents are heterozygous for the trait. Monohybrid Cross – Punnett Square Pp x Pp P P PP p Pp p Pp Genotype – 1PP:2Pp:1pp pp Phenotype – 3 purple : 1 white Dihybrid Cross Dihybrid cross – examines how two traits are passed on when both parents are heterozygous for both traits. Flower color in peas: P allele is dominant for purple flowers. p allele is recessive for white flowers. Plant height in peas: T allele is dominant for tall plants. t allele is recessive for short plants. Dihybrid Cross - PpTt x PpTt Each parent is the same and can donate the same alleles. PpTt – P with T – PT PpTt – P with t – Pt PpTt – p with T – pT PpTt – p with t – pt Dihybrid Cross - PpTt x PpTt PT Pt pT pt PT PPTT PPTt PpTT PpTt Pt PPTt PpTt Pptt PpTT PpTt ppTT ppTt PpTt ppTt pptt pT pt PPtt Pptt Genotypes Dihybrid Cross - PpTt x PpTt Phenotypes 9 purple, tall (P_T_) 3 purple, short (P_tt) 3 white, tall (ppT_) 1 white, short (pptt)
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