Mendel & Transmission Genetics

Blending Model

Parents pass traits onto offspring, not in equal amounts (paint mixing)

Particulate Model

Each parent passes traits in equal amounts, quality of the traits inherited by offspring varies

Model Organisms

Must be good stand ins for humans, needs to apply to humans, must accurately affect the way humans’ genetics work

Mendel Details

• Mendel used seven visible features; each with two contrasting traits; true breeding strains

• Mendel kept quantitative records

• used peas for controlled matting

Seven Mendel Traits

1. Seed shape

2. Seed color

3. Pod shape

4. Pod color

5. Flower color

6. Flower position

7. Stem height

Mendel’s Legacy

• Mendel was not appreciated during his lifetime

• His work was rediscovered

• Geneticists recognized Mendel had discovered the basics for the transmission of hereditary traits

Monohybrid Crosses

• Looking at a single trait

• True-breeding-line that will always produce offspring identical to the parents

• Involve a single pair of contrasting traits

• P1 Generation: original parents

• F1 Generation: offspring

• F2 Generation: Offspring of F1 generation crossed

• Found 3-1 dominant (purple) to recessive (white) ratio

• Recessive came from P1 gen

• All 7 traits exhibited this 3-1 ratio in F2 gen

• All F1s had one dominant allele

P1 Generation

Original parents

F1 Generation

Offspring

F2 Generation

Offspring of F1 generation crossed (Self-Fertilizing)

Phenotype

• Physical expression of a trait

• The outward expression of an allele combination

Gene

• Unit of inheritance

• Encoded a trait

• recipe

Allele

Alternative form of a single gene

Genotype

Combination of genes that produces the phenotype

• genetic makeup of individuals

• Alleles written in pairs

Mutant Phenotype

A variant of a gene’s expression that arises when the gene undergoes a change

Organisms Inherit

Two copies of each gene

• one from each parent

• Called maternal and paternal copy

Homozygous/Homozygote

Both alleles are the same (DD,dd)

Heterozygous/Heterozygote

Alleles are different (Dd)

Testcross

Determines if individual displaying dominant phenotype is homozygous or heterozygous for that trait

Cross between dominant phenotype and homozygous recessive

Particulate Unit Factors (genes)

• Mendel viewed genes as little beads (Packets). Offspring inherit one from each parent

• basic units of heredity

• Are passed unchanged from generation to generation

• Determine various traits expressed by each individual plant

Unit factors exist in pairs

Genetic characteristics controlled by unit factors

Dominance/Excessiveness

In a pair unit factor, one units factor is dominant, the other is recessive

Segregation

Paired unit factors segregate (separate) independently during gamete formation

Dihybrid Cross

Looking at Two Traits

• Two pairs of contrasting traits

• Generates unique F2 generation

• Always saw a 9:3:3:1 Ratio

Independent Assortment

• Unit factors (traits) assort independently during gamete formation

• All possible gamete combinations form with equal frequency

Mendel’s Work Rediscovered

• Rediscovered in early 20th century

• Mendel suggested heredity resulted in Discontinuous Variation: a dominance- recessive relationship

• Laws of probability can be used to accurately predict genotypic ratios of offspring in Mendel’s system