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.  pp – pea flowers appear white. Hom. Dominant – shows dominant trait.  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 p p Pp PP x 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 Pp pp P P p p Pp x Pp Genotype – 1PP:2Pp:1pp 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 PPTTPPTtPpTTPpTt PPTt PPtt PpTtPptt PpTT PpTt ppTT ppTt PpTtPpttppTtpptt 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)