Probability and Punnett Squares Genotype: the combination of alleles for a give trait (ex. Tt) Phenotype: the appearance of a trait as determined by its genotype (ex. Tall) Homozygous: two of the same alleles (TT, tt) Heterozygous: two different alleles (Tt)
Independent Assortment Genes from different traits can segregate independently during the formation of gametes
2 Trait Cross Dihybrid “The Double Cross” Let’s examine the following pea plant traits: T=tall; t=short F=purple; f=white Cross a plant that is homozygous dominant for both traits with a plant that is homozygous recessive for both traits What are the percentages of genotypes? Phenotypes?
TT FF x tt ff TF tf TtFf
Cross 2 plants that are both heterozygous for both traits.
Cross 2 plants that are both heterozygous for both traits. What are the genotypes of the parents? Heterozygous for both traits. Tt Ff x Tt Ff What are the possible combination of genes the parent can pass on to the offspring? (FOIL) TF, Tf, tF, tf
Cross 2 plants that are both heterozygous for both traits.
Cross 2 plants that are both heterozygous for both traits. TF Tf tF tf
Cross 2 plants that are both heterozygous for both traits. TF Tf tF tf TTFF TTFf TtFF TtFf TTff Ttff ttFF ttFf ttff
Cross 2 plants that are both heterozygous for both traits. TF Tf tF tf TTFF TTFf TtFF TtFf TTff Ttff ttFF ttFf ttff
Cross a male who is heterozygous for both traits with a female who is homozygous recessive for fur color and heterozygous for coat texture. B=black; b=white and R=rough; r=smooth
Beyond Dominant and Recessive Alleles
Incomplete Dominance One allele is not dominant over another. The heterozygous phenotype is somewhere in between the two homozygous phenotypes. Example: CRCR = red CWCW = white CRCW = pink
Codominance Both alleles contribute to the phenotype. Example: B = black W = white BB = black WW = white BW = speckled
Multiple Alleles More than 2 alleles exist in a population
Human Blood Type
Polygenic Trait Traits controlled by 2 or more genes. Example human eye color and skin color