Understanding InheritanceWhat controls traits? Genes on a chromosomes

Gene: a section of DNA on a chromosome that

has genetic information for one trait.Alleles: the different forms of a gene.How many alleles controlled flower color in Mendel’s experiment? Two

How are alleles represented? By letters

Letters are chosen by Determining what the dominant trait is.

The letter chosen is usually the first letter of that trait.

Upper case letters represent dominant alleles.Lower case letters represent recessive alleles.The dominant allele of seed shape is round; therefore the symbol will be

R.The recessive allele of seed shape is wrinkled; therefore the symbol will be

r.

According to Mendel, an organisms receives two alleles for each gene. One allele comes from the egg, and the other from the sperm. Therefore an organism has a combination of two alleles.

Genotype: the two alleles that control the phenotype of a trait (letter combination)

Phenotype: how a trait appears, or is expressed

RR

Rr

rr

Let’s try another example:In Mendel’s experiment, purple flowers are dominant over white flowers. What are the possible genotypes for purple

flowers? PP or Pp

What is the possible genotype for white flowers?

pp

REMEMBER, each parent contributes one allele, so there should be two in total!!!

When the 2 alleles of a gene are the same, its genotype is homozygous

If the 2 alleles of a gene are different, its genotype is heterozygous

Label the following genotypes as either heterozygous or homozygous.

RR (round) = homozygous Rr (round) = heterozygous rr (wrinkled) = homozygous

A Punnett Square is a model used to predict possible genotypes and phenotypes of offspring.

Example: Tall is dominant over short. What possible genotypes could we

have if the parent were tall? TT or Tt What possible genotype could we

have if the parent were short? tt

Let’s say the mother is homozygous tall. Her genotype would be ___________

And let’s say the father is heterozygous tall. His genotype would be _____________

Mom’s alleles

Dad’s alleles

Offspring = __________________________________

T T

T

t

Write the parent’s genotypes in the correct location in the Punnett Square below.

Genotypes and Probability

Genotype Probability

You can use the Punnett square to determine the probability that offspring with a certain combination of alleles will result. (Use the Punnett square to help you fill in the charts)

Phenotype Probability

With this information, you can determine the probability of appearance in the offspring.

Heredity is Complicated… Unfortunately, it is not

always so black and white. There are times that neither allele is dominant or recessive.

Codominance is when both alleles can be observed in a phenotype.

Punnett Squares

And Sometimes Even More Complicated…

Sometimes there can be a gray area where neither allele dominates the other.

Incomplete dominance is when the offspring’s phenotype is a combination of the parents’ phenotypes.

Punnett Squares