Probability & Genetics Punnett Squares

Punnett Squares To help explain how alleles were separated into gametes and rejoined, Reginald Punnett developed a visual tool to depict test crosses and show the offspring’s possible genotypes and phenotypes A Punnett square looks like 

Setting Up a Punnett Square In Punnett squares, letters represent dominant and recessive alleles The alleles from the mother will be placed along the top of the square and the alleles from the father will be placed along the side

Punnett Squares An uppercase letter stands for a dominant allele. ALWAYS use the letter of the Dominant trait Tall height – T Short height – t A lowercase letter stands for a recessive allele. This will be the lowercase letter of whichever letter was used for the dominant trait/allele

Using a Punnett Square Find the genotypes of the parents TT x tt Draw the Punnett Square Write the alleles from one parent on the top of the square and the other alleles on the left side Fill in the inside of the Square with alleles Write the Genotype and Phenotype ratios and percentages from the cross

Facets of Genetics Many inherited traits for humans are influenced by genes. Traits such as hair color, eye color, height, body build, and intelligence are difficult, if not impossible to predict. Each of these traits has wide variations and involves many genes, some of which interact with each other. There are some human traits, however, that often follow a simple dominant or recessive inheritance pattern. The inheritance of these traits are fairly easy to determine

Examples of Simple Inheritance Patterns Dominant Recessive Lacks dimples Cannot roll tongue Attached ear lobes Short eyelashes Smooth chin Thin lips Flat feet Straight hairline Normal vision Has dimples Can roll tongue Unattached ear lobes Long eyelashes Cleft chin Widow’s peak hairline Near/far-sightedness Arched feet

Monohybrid Cross A monohybrid cross is a test mating of only ONE trait Example: The kernel color on a corn cob is either purple (dominant) or yellow (recessive) Each parent plant will contribute 1 allele for the trait so that the offspring has a total of 2 alleles for kernel color

Monohybrid Cross Cross a homozygous tall plant with a homozygous short plant T – Tall t - short

Monohybrid Cross In humans, having a Widow’s peak hairline is dominant to a smooth hairline. Cross a heterozygous man with a homozygous female.

Punnett Square WS#1