1. Understanding Inheritance
What 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

3. 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.

4. According to Mendel, an organisms receives two alleles for each gene
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

5. 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!!!

6. 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) =
Rr (round) =
rr (wrinkled) =

7. 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

8. Let’s say the mother is homozygous tall
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 _____________

9. T T T t Offspring = __________________________________
Write the parent’s genotypes in the correct location in the Punnett Square below. T T
Mom’s alleles T t
Dad’s alleles
Offspring = __________________________________

10. Genotypes and 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)
Genotypes and Probability
Genotype
Probability

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

12. 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.

13. Punnett Squares

14. 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.

15. Punnett Squares