Monohybrid Notes

Gregor Mendel• Mendel was an Austrian monk.

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

• He had 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.

• 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.

Genetics Vocabulary• Gene - the unit of heredity; a section of DNA

sequence encoding a single protein• Alleles - two genes that occupy the same

position on homologous chromosomes and cover the same trait

• Locus - a fixed location on a strand of DNA where a gene or one of its alleles is located

Genetics Vocabulary• Homozygous - having identical genes (one

from each parent) for a particular characteristic.

• Heterozygous - having two different genes for a particular characteristic.

Genetics Vocabulary

• Phenotype - appearance (description)• Genotype - genetic makeup (letter code)

Genetics Vocabulary

• Dominant - the trait that appears in the heterozygous condition.

• Recessive - the trait that is masked in the heterozygous condition.

Genetics Vocabulary• Incomplete Dominance - a form of

intermediate inheritance in which one allele for a specific trait is not completely dominant over the other. If the trait is incomplete it will be stated in the problem.

Genetics Vocabulary• Codominance - A condition in which

both alleles of a gene pair in a heterozygote are fully expressed, with neither one being dominant or recessive to the other. If the trait is codominant it will be stated in the problem.

RR RW WW

Genetics Vocabulary

• Monohybrid cross - cross involving a single pair of genes, one trait

• P – “parents”

• F1 – “kids”

• F2 – “grandkids”

Genetics Vocabulary

• Punnett squares - 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

2. Assign letters for the trait

Dominant trait gets the capital letter

Recessive traits gets the lower case letter

B = blackb = white

3. Determine genotype for parents

White male = bbHeterozygous 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 =

2Bb:2bb

7. Determine phenotype ratio

Phenotype = what they look like

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

*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 punnett 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 of F1?• What are the genotype ratio of F1?

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 or heterozygous?

Use a test cross: cross with a pure

breed recessive and look at the F1

generation

Possible Answer Formats

• % - chance out of total possible– 1 out of 4 = 25%

• Fraction– 1 out of 4 = 1/4

• Ratio – ratio adds up to total possible– 1 out of 4 = 1:3