Punnett squares are how we work out if the offspring will express which trait.

Recap of Dominant and Recessive traits

Dominant and Recessive traits

• Dominant traits are represented with capital letters (e.g. A)
• Recessive traits are lower case (e.g. a)
• Dominant will always be expressed over recessive traits (so if brown eyes were dominant, then if there was a brown eye allele then the child would have brown eyes. The blue eyes can only be expressed if there are 2 blue eyes and no brown eyes alleles. The blue eyes would be recessive in this case)

Punnett Square

• They are used to check the probability of a child inheriting a trait
• Firstly, genotype is AA, aa or Aa, and phenotype is the trait that the people are expressing
• For example, eye colours…blue eyes genotype is aa etc., and phenotype would be blue eyes

Laying out a punnett square:

First, we have to write ‘Let [lowercase/capital] be ___ trait (dominant/recessive)

• E.g. Let A = brown eyes dominant, and let a = blue eyes dominant

Then we have to list the parent’s genotypes and phenotypes (if they are given)

• E.g. BB x bb; brown eyes x blue eyes

Then we simply draw a 3x3 square with the parents genotypes on each side, and add the two genotypes into the box they correspond to. Then you look at the 4 inside genotypes to determine the probability that the child has the trait, and the genotype of the offspring.

When you have figured out the answer, you have to list the % of offspring, the genotype and phenotype

• E.g. 100% of children are heterozygous (Aa) and will have brown eyes

Summary of Layout

give summary of genotype, then phenotype

[x]% of offspring are [dominant/recessive/none] [homozygous/heterozygous]

• Repeat and list all if there is more than one possibility (if not 100% of offspring)

[x]% of offspring have/will express [trait]

• list all other possibilities [if there are 3: AA, Aa, aa, then homozygous dominant AND heterozygous will lead to the dominant trait being expressed]

Cross Testing

This is where you breed a animal with an unknown genotype with a homozygous recessive animal (e.g. a mouse that you want to find the genotype of and a white mouse which is homozygous recessive). You check the offspring and look at the patterns of inheritence to determine which is the most likely genotype of the mouse.