SINGLE TRAIT INHERITANCE and TEST CROSS BIOLOGY 12 SINGLE TRAIT INHERITANCE and TEST CROSS

Single Trait Inheritance solving problems that involve just one gene with two alleles use a 2x2 Punnett Square have up to 4 possible outcomes

Single Trait Inheritance continued example: In a particular species of mice, fur colour is either black or brown. Brown fur is dominant to black fur. A heterozygous brown mouse is crossed with another heterozygous brown mouse. What is the probability that their offspring will be homozygous for fur colour? ________________________________________________ Let B = brown Let b = black Parents : Bb X Bb Genotype: Phenotype: BB 25% brown fur 75% Bb 50% black fur 25% bb 25% There is a 50% [BB (25%) + bb(25%)] chance that the offspring will be homozygous for fur colour. B b BB Bb bb B b

Single Trait Inheritance practice example In guinea pigs, the allele for short hair is dominant to long hair. A pure breeding short haired guinea pig is crossed with a hybrid short haired guinea pig. What is the probability that the offspring will be a short haired hybrid?

Test Cross used to determine the genotype of an individual either homozygous dominant or heterozygous

Test Cross example example: a black sheep is not preferred because the wool is brittle and difficult to dye black wool is recessive to ensure you will have no black sheep, you must have at least one homozygous dominant parent

Test Cross example continued use a test cross to determine the genotype of an individual with a dominant phenotype cross homozygous recessive with the unknown individual

Let W = white wool Let w = black wool

Test Cross example continued if any of the offspring show the recessive trait, then the parent must be heterozygous

Test Cross practice example In Labrador retrievers, black fur colour is dominant over yellow. A yellow female mates with a male with an unknown genotype, if the pups all have black fur, what is the genotype and phenotype of the male?