Mendel and Monohybrid Crosses Notes

Gregor Mendel• Mendel was an Austrian monk.

• Mendel formulated two fundamental laws of heredity in the early 1860's.

• He had previously studied science and mathematics (including Statistics) at the University of Vienna. Mendel’s knowledge of statistics later proved valuable in his research on Heredity – the transmission of characteristics from Parent to Offspring.

• Mendel's work was unrecognized until 1900.

Mendel’s ExperimentsA hybrid is the product of parent organisms that are True-

Breeding or pure-breeding for different forms of One Trait. Plants that are PURE for a trait always produce offspring with that Trait. The term STRAIN denotes plants that are Pure for a Specific Trait.

1. Mendel produced strains by allowing the plants to Self-Pollinate for several generations.

2. He eventually obtained 14 strains, one for each of the 14 traits he observed. He called each strain a Parental Generation or  P1 generation. This is the parental generation in a breeding experiment.

3. He used Cross-pollination between two strains and tracked each trait through two generations. (Monohybrid Cross)

4. F1 generation is the first-

generation offspring in a breeding experiment. The First Filial Generation.

5. F2 generation is the second-generation offspring in a breeding experiment. The Second Filial Generation.

6. He also performed reciprocal

crosses of pollen on stigmas (e.g. tall-with-short and short-with tall).

Mendel’s Conclusions

• In his crosses each trait had a Dominant Factor which masked, or dominated the other factor for a specific trait; the other we call the Recessive Factor.

• In a true-breeding cross (BBxbb) the recessive factor is masked in F1 but appears in F2

• Law of Segregation: Each organism contains two factors for each trait; factors segregate in the formation of gametes. When two gametes combine during fertilization, the offspring have two factors controlling a specific trait.

• Law of Independent Assortment states that factors for different characteristics are distributed to gametes independently.

Punnett Squares

• Punnett sqaures are used to aid in predicting the probability that certain traits will be inherited by offspring

• Probability is the likelihood that a specific event will occur or is the likely outcome a given event will occur from random chance

1. Determine the dominant and Recessive trait

Will be stated in the problem

ie. Black is dominant to white in bunnies

Monohybrid Crosses (only one trait)

2. Assign letters for the trait

Dominant trait gets the capital letter

Recessive traits gets the lower case letter

B = black

b = white

3. Determine genotype for parents

White male = bb

Heterozygous black female = Bb

4. Put parents on the square

Bb x bb

B b

b

b

5. Determine genotype of offspring

Punnett square holds offspring genotypes

B b

b

b

Genotype = letter code

Bb bb

Bb bb

6. Determine genotype ratio

Count offspring in the Punnett square genotype ratio = 2:2

0/4 BB

2/4 Bb

2/4 bb

7. Determine phenotype ratio

Phenotype = what they look like

Count offspring in the Punnett square phenotype ratio = 2:2

*If ratio doesn’t add up to total number of boxes in the punnett square you’ve made a mistake

Practice Problems

A pure-breed white flower crosses with a pure-breed purple flower. Purple is dominant to white.

• Show the punnet square

• What is the genotype of the F1 generation

• What is the phenotype of the F1 generation

Genotype = Pp

Phenotype = Purple

More practice

A homozygous black rat is crossed with a heterozygous black rat.

B = black b = brown

• Show the punnett square

• What are the genotypes?

• What are the genotype ratio?

Genotypes = BB and Bb

Genotype Ratio = 2BB:2Bb

More practice

If you had a black rat how could you tell it’s genotype was homozygous and heterozygous?

Use a test cross: cross with a pure breed recessive and look at the F1 generation