Chapter 11 - Mendel & Genetics

Gregor Mendel: "father of genetics"

Blending Theory of Inheritance - offspring of two parents "blend" the traits of both parents
Particulate Theory of Inheritance - traits are inherited as "particles", offspring receive a "particle" from each parent.

Evidence for Particulate Theory of Inheritance: A plant with purple flowers is crossed with another plant that has purple flowers. Some of the offspring have white flowers (wow!). Mendel set out to discover how this could happen.

Some stuff on Mendel

Mendel's Experiments

Mendel chose pea plants as his experimental subjects, mainly because they were easy to cross and showed a variety of contrasting traits (purple vs white flowers, tall vs short stems, round vs wrinkled seeds)


1. Mendel chose true-breeding lines of each plant/trait he studied (true breeding lines always produced offspring of the same type)

2. He crossed a true breeding plant with a plant of the opposite trait (purple x white). He called this the Parental (P) generation. (In this case, he cross-pollinated the plants)

3. He recorded data on the offspring of this cross (First Filial, F1)

4. He self pollinated the F1 offspring

5. He recorded data on the offspring of the second generation, calling it the Second Filial generation (F2)


flowersMendel established three principles (or Laws) from his research

1. The Principle of Dominance and Recessiveness - one trait is masked or covered up by another trait

2. Principle of Segregation - the two factors (alleles) for a trait separate during gamete formation

3. Principle of Independent Assortment - factors of a trait separate independently of one another during gamete formation; another way to look at this is, whether a flower is purple has nothing to do with the length of the plants stems - each trait is independently inherited

Modern Genetics

Mendel's factors are now called ALLELES. For every trait a person have, two alleles determine how that trait is expressed.

We use letters to denote alleles, since every gene has two alleles, all genes can be represented by a pair of letters.

PP = purple, Pp = purple, pp = white

Homozogyous: when the alleles are the same, the individual is said to be homozygous, or true breeding. Letters designating a homozgyous individual could be capital or lowercase, as long as they are the same. Ex. AA, bb, EE, dd

Heterozygous: when the alleles are different, in this case the DOMINANT allele is expressed. Ex. Pp, Aa

Monohybrid cross = a cross involving one pair of contrasting traits. Ex. Pp x Pp

Punnet Square: used to determine the PROBABILITY of having a certain type of offspring given the alleles of the parents

Genotype: letters used to denote alleles (BB, Pp..etc)
Phenotype: what an organism looks like (brown, purple..)

How to Use Punnett Squares to Determine Probability

In pea plants, round seeds are dominant to wrinkled.

Assign genotypes:

RR = round

R r = round

r r = wrinkled

If two heterozygous plants are crossed (R r x R r ), set up the square as shown below.

Punnett Square


In this case 25% of the offspring will be wrinkled, and 75 % will be red.