Mendelian Genetics

Part A: Mendel and Genetics Basics

Genetics Basics Genetics = the study of how traits are passed from parents to offspring (children) Gregor Mendel = an Austrian monk who worked with garden peas and was the “father of modern genetics”

Mendel Studied inheritance of traits in pea plants Lived from … His work wasn’t recognized until early 1900’s

From , Mendel grew and tested 28,000 pea plants He found that the offspring plants showed some parent traits Mendel’s garden In the Czech Republic

Mendel stated that physical traits (ex: eye color, hair color) were inherited as particles Today we know that particles are actually chromosomes (super- coiled DNA)

Some Genetics Vocab Trait: any characteristic that can be passed from parents to offspring Heredity: passing of traits from parents to offspring Genetics: study of heredity

What are genes? Genes are short sections of chromosomes (DNA) that “code” for a certain trait Different forms of a gene are called alleles… Ex: Flower color gene Allele Type 1  Purple Flowers Allele Type 2  White Flowers

Inheritance of Alleles Because we have homologous chromosomes (1 from each parent)… we inherit 2 copies of each gene If we inherit two types of alleles…one may be dominant over the other… What does this mean?

Dominant Allele: stronger of the two alleles, “masks” the other allele…usually represented by an uppercase letter Ex: R stand for purple flower color Recessive Allele: weaker of the two alleles…usually represented by a lowercase letter Ex: r stands for white flower color

Say you have a plant with the following allele combinations for flower color. What flower color would you see? Remember… R(Purple) and r(white) RR  ________? Rr  ________? rr  ________?

More Vocab Genotype = gene combination for a trait (ex: RR, Rr, rr) Phenotype = the physical trait that results from the genotype (ex: purple and white flower color)

Examples GenotypePhenotype RRPurple Flowers RrPurple Flowers rrWhite Flowers Remember: Dominant Alleles  capital letters (Example: P) Recessive Alleles  lowercase letters (Example: p)

Different Genotypes Homozygous = when an organism has two ALIKE alleles for a trait RR = homozygous dominant rr = homozygous recessive Heterozygous = when an organism has two UNLIKE alleles for a trait Rr = heterozygous

So how did we figure all this out?... Mendel’s Experiments!

Mendel’s Experiments CharacteristicsTrait 1Trait 2 Plant HeightTallShort Flower PositionAxialTerminal Pod ColorGreenYellow Pod Appearance InflatedConstricted Seed TextureSmoothWrinkled Seed ColorYellowGreen Flower ColorPurpleWhite Mendel looked at several different pea plant traits: Ex: Plant height, seed color, flower color

Mendel’s Methods Carefully controlled flower pollination Pollination = occurs when pollen grains are transferred from the anthers (male) to stigma (female)

Pollination Self-pollination = pollen transferred from anthers to stigma of same flower or another flower on same plant Cross-pollination = involves flowers of two separate plants

Mendel’s Observations Parents with purple flowers  offspring with purple OR white flowers Tall parents  tall OR short offspring

P1 = parental generation F1 = offspring of P1 generation F2 = offspring of F1 generation

Mendel’s Results

Mendel’s Conclusions Some “factor” must be passed down from parents to give offspring certain traits Scientists later discovered that “factors” were alleles Ex: Purple and white flower alleles

Both purple and white Flower alleles are Passed down, but The purple allele is DOMINANT!

Mendel’s Laws Law of Dominance = one allele can “mask” another Law of Segregation = homologous chromosomes separate when gametes are made Law of Independent Assortment = chromosome pairs can each arrange themselves differently before separating

Remember… during meiosis homologous chromosomes separate!

Part B: Genetic Crosses

Genetic Crosses Vocab Genetic Cross = mating two parents to see inherited traits in their offspring Monohybrid Cross = a cross between parents that involves one trait (ex: flower color) Punnett Square = a tool used to predict offspring genotypes and phenotypes

Steps to Completing a Punnett Square Problem 1) Write out the parent genotypes 2) Write possible gametes for each parent on the axes of the square 3) Fill in the boxes of the square 4) Find offspring genotype and phenotype ratios

Example 1: Homozygous x Homozygous Situation: One parent is homozygous for green pods(GG) and the other parent is homozygous for yellow pods(gg). Parent Genotypes: Offspring Ratios -Genotype: -Phenotype: G G g g Gg

Example 2: Homozygous X Heterozygous Situation: One parent is homozygous for green pods, and the other parent is heterozygous. Parent Genotypes: Offspring Ratios -Genotype: -Phenotype: GG G g GG Gg

Example 3: Heterozygous X Heterozygous Situation: Both parents are heterozygous for pod color Parent Genotypes: Offspring Ratios -Genotype: -Phenotype: GGGg gg G g G g

Example 4: Testcross Testcross = cross between an unknown dominant genotype (could be homozygous or heterozygous) and a homozygous recessive genotype Result: find out whether the dominant genotype is homozygous or heterozygous

Example 4: Testcross Situation: a green-podded plant with an unknown genotype is crossed with a yellow-podded plant. The offspring genotype ratios are given below. Genotype Ratio: 50% Gg, 50% gg Question: What was the genotype of the parent green-podded plant? g g gg Gg ??