# Mendel and Monohybrid Crosses Notes

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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 Experiments A 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

Monohybrid Crosses (only one trait)
1. Determine the dominant and Recessive trait Will be stated in the problem ie. Black is dominant to white in bunnies

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

Punnett square holds offspring genotypes
B b b Genotype = letter code 5. Determine genotype of offspring 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