Punnett Squares JH-KEADLE

Punnett Square When Gregor Mendel studied pea plants, he noticed patterns in some of the traits. In some cases, you can predict what traits offspring will have based on the parent’s genetics. A Punnett Square is a useful tool for predicting traits.

Punnett Square Punnett square – a graphic used to predict the possible genotypes of offspring in a given cross. Each parent has two alleles for a particular gene. An offspring receives one allele from each parent.

Punnett Square To set up a Punnett Square, decide which trait is dominant. dominant trait = capital letter recessive trait = lower case letter One parent’s alleles are placed at the top. The other parent’s alleles are placed along the side.

Results of the Punnett Square Each individual from the Punnett Square will have a genotype and a phenotype. genotype – what the genes are (the letters). phenotype – what the trait looks like.

homozygous- both alleles are dominant or both are recessive homozygous- both alleles are dominant or both are recessive. (so you see either two uppercase or two lower case letters. heterozygous – each allele is different. (You will see one capital and one lower case).

P P p p Pp Pp Pp Pp Punnett Square Use P for purple flower color Use p for white flower color homozygous P P p Pp Pp homozygous p Pp Pp

P p P p PP Pp pp Pp Punnett Square Cross two heterozygous plants

Results of the Punnett Square Punnett Results Genotype Phenotype PP Homozygous dominant Purple Flowers Pp Heterozygous pp Homozygous recessive White Flowers

Results of the Punnett Square The results of a Punnett square will give you the probability of certain outcomes. Results are usually written as a ratio or a percent.

Complete dominance complete dominance – the dominant allele hides the recessive. The phenotype will be the dominant characteristic. You can use a Punnett square and will have two genotypes. RR rr Rr X =

Punnett Square Practice A dog breeder is mating a brown heterozygous lab to a black homozygous lab. Brown is dominant. Set up a Punnett Square and decide what percent of the offspring will be black and brown. ? X =

b B b Bb bb Bb bb b Bb One parent is heterozygous brown B = brown b = black Bb One parent is heterozygous brown One parent is homozygous black bb b B b Bb bb Bb bb b

Bb Bb bb bb 50% are brown, 50% are black brown brown black black Phenotype Genotype Bb brown Bb brown bb black bb black 50% are brown, 50% are black

Incomplete Dominance = X Some traits are not dominant or recessive incomplete dominance – each allele can affect the phenotype. These traits will blend when heterozygous. You can use a Punnett square and will have 3 genotypes instead of two RR rr Rr = X

Punnett Square Practice In purple people eaters one eye is homozygous dominant, two eyes is homozygous recessive, and three eyes is heterozygous. The number of eyes in an incomplete dominant trait. What would the offspring look like from a homozygous recessive purple people eater and a homozygous dominant purple people eater? Set up a Punnett Square and find the genotypes and phenotypes of the offspring.

E E e Ee Ee Ee Ee e EE = one eye ee = two eyes Ee = three eyes One parent is homozygous dominant One parent is homozygous recessive EE ee E E e Ee Ee Ee Ee e

Ee Ee Ee Ee 100% three eyed 3 eyes 3 eyes 3 eyes 3 eyes Phenotype Genotype Ee 3 eyes Ee 3 eyes Ee 3 eyes Ee 3 eyes 100% three eyed

Codominance = X = Some traits are not dominant or recessive codominance – both of the alleles contribute to the phenotype. The individual will have traits of both phenotypes. You can use a Punnett square and will have 3 genotypes instead of two. = X =

Punnett Square Practice In snakes, certain colors can be codominant. For our example, we will look at the yellow color in boas. Yellow and White are co-dominant. What results would you get with a homozygous yellow parent and a homozygous white parent? ? = X

Y Y y Yy Yy Yy Yy y YY yy Y = yellow y = white One parent is homozygous yellow One parent is homozygous white yy Y Y y Yy Yy Yy Yy y

Phenotype Genotype Yy pied Yy pied Yy pied Yy pied 100% are pied

(blue is dominant to yellow) Color in and complete the following chart based on the type of inheritance. Assume parents are homozygous. X complete dominance (blue is dominant to yellow) incomplete dominance codominance