1. Mendel and Punnett Squares
Father of Genetics and Predicting Outcomes

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Gregor Mendel’s Peas
Genetics is the scientific study of heredity.
Gregor Mendel was an Austrian monk. His work was important to the understanding of heredity.
Considered to be “Father of Genetics”
Mendel carried out his work with ordinary garden peas.
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3. Important Vocabulary
A trait is a specific characteristic that varies from one individual to another.
Alleles – different forms of the same gene
ex: blue and brown eye colors

4. Genotypes
Genotype – combination of alleles it has for a particular trait
A pair of letters (ex: Bb, bb, BB) is used for each trait because one gene (one letter) comes from mama and one comes from daddy.

5. Genotypes
If there are two capital letters or two lowercase letters in the same genotype (BB or bb) then the trait is called homozygous or pure. The alleles for the trait are the same
If there is one capital and one lower case letter (Bb) then then genotype is called heterozygous or hybrid. The alleles for the trait are different.

6. Phenotypes
The phenotype is the physical characteristics of the organism – what it looks like. The phenotype is the expression of genes for a particular trait.
Ex: phenotypes are brown or blonde hair color, freckles or no freckles, white or purple flowers, right handed or left handed

7. Example:
In guinea pigs, brown fur is dominant (B), and the allele for white is b. What are the possible genotypes of guinea pigs regarding fur color? What are the hair color phenotypes for guinea pigs?
Genotype Definition Phenotype
BB homozygous dominant brown fur
Bb heterozygous brown fur
bb homozygous recessive white fur

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Punnett Squares
The gene combinations that might result from a genetic cross can be determined by drawing a diagram known as a Punnett square.
Punnett squares can be used to predict and compare the genetic variations that will result from a cross.
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Punnett Squares
A capital letter represents the dominant allele for tall.
A lowercase letter represents the recessive allele for short.
In this example,
T = tall
t = short
The principles of probability can be used to predict the outcomes of genetic crosses. This Punnett square shows the probability of each possible outcome of a cross between hybrid tall (Tt) pea plants.
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10. All of the tall plants have the same phenotype, or physical characteristics.
The tall plants do not have the same genotype, or genetic makeup.
One third of the tall plants are TT, while two thirds of the tall plants are Tt.

11. Probabilities Predict Averages
Probabilities predict the average outcome of a large number of events.
Probability cannot predict the precise outcome of an individual event.
In genetics, the larger the number of offspring, the closer the resulting numbers will get to expected values.
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