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

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

3. 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?
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Let B = brown
Let b = black
Parents : Bb X Bb
Genotype: Phenotype:
BB 25% brown fur 75%
Bb 50% black fur %
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

4. 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?

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

6. 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

7. 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

8. Let W = white wool
Let w = black wool

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

10. 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?