STUDYING HEREDITY Punnett Squares – a diagram that predicts the outcome of a genetic cross by considering all possible combinations of gametes in the cross; named after its inventor, Reginald Punnett.

Monohybrid Cross - a cross that involves one pair of contrasting traits; Mendel’s studies looked at one contrasting trait at a time

Setting Up Punnett Squares: The possible gametes that one parent can produce are written along the top of the square. The possible gametes that the other parent can produce are written along the left side of the square. Each box inside the square is filled in with two letters obtained by combining the allele along the top of the box with the allele along the side of the box. The letters in the boxes indicate the possible genotypes of the offspring.

Punnett Squares

Probability – the likelihood that a specific event will occur; probability calculations can be used to predict the results of genetic crosses probability = number of 1 kind of possible outcome total number of all possible outcomes (%)

Setting Up A Punnett Square Red flowers are dominant to white flowers. A homozygous red flower is crossed with a white flower. What percentage will be red? R R 100% r Rr Rr Rr Rr

Setting Up A Punnett Square Red flowers are dominant to white flowers. A heterozygous red is crossed with a homozygous red. What percentage will be white? R r 0% RR Rr R RR Rr

Setting Up A Punnett Square B b Black hair is dominant to brown hair. Calculate the probability of heterozygous offspring from a cross between two homozygous dominant parents. 0% B B BB BB B BB BB

Genotypic & Phenotypic Ratios T t Tall is dominant to short. T TT Tt t Tt tt Genotypic Ratio – Phenotypic Ratio – 1 TT: 2 Tt: 1 tt 3 tall: 1 short

Genotypic & Phenotypic Ratios T t Tall is dominant to short. t Tt tt t Tt tt Genotypic Ratio – Phenotypic Ratio – 2 Tt: 2 tt 2 tall: 2 short

Genotypic & Phenotypic Ratios T T Tall is dominant to short. T TT TT t Tt Tt Genotypic Ratio – Phenotypic Ratio – 2 TT: 2 Tt 4 tall

Determining Unknown Genotypes: *Animal breeders, horticulturists, and others involved in breeding organisms often need to know whether an organism with a dominant phenotype is heterozygous or homozygous for a trait. In this case, they could perform a test cross. In a test cross, an individual whose phenotype is dominant, but whose genotype is unknow is crossed with a homozygous recessive individual.

What is a test cross? In a test cross, the unknown genotype If the offspring show that half are dominant and half are recessive, then the unknown parent was heterozygous for the trait. In a test cross, the unknown genotype is crossed with a known homozygous recessive genotype. ? If all offspring show the dominant trait, then the unknown parent was homozygous for the trai.t