Outline 12: Intro to Genetics

Gene

Unit of hereditary information

Allele

Variant (form) of a gene

Character

Observable, heritable feature (flower color)

Trait (character state)

Specific variant of a character (purple or white flower)

Genotype

genetic makeup; alleles an organism carries

Phenotype

observable physical or physiological traits determined by genotype

Mendel Pea Plant Experiment: Advantages

1. Inexpensive and easy to obtain

2. Many identifiable, heritable traits

3. Easy to grow and short generation time

4. Easy to control pollination (self or cross fertilization)

5. Many varieties available

Mendel Pea Plant Experiment: True-breeding lines

• Always produce offspring with the same phenotype after self-fertilization

• Mendel developed true-breeding lines for 2 years before experiments

Blending Inheritance Hypothesis

• gametes contain fluids from parents that mix to form an intermediate offspring phenotype

• Prevailing idea in the mid-1800s 

• Mendel wanted to test this hypothesis

Mendel’s Experimental Design

• Cross-fertilization

  • crossed true breeding parents with contrasting traits (purple x white)

P generation

(parental) original parents

F1 generation

(first filial) offspring of P generation

F2 generation

(second filial) offspring of F1 (grandkids of P)

Mendel’s Experiment Predication

If blending hypothesis were true, F1 should show intermediate phenotypes between parents (lavender color)

Mendel’s Experiment Observations

• F1: resemble only one parent (not intermediate)

• The other parents trait disappeared in F1 

• F2: both parental phenotypes reappeared 

• Observed as a 3:1 phenotype ratio (dominant: recessive) in F2

Mendel’s Experiment Conclusion

• No intermediate (blended) traits appeared

• “Lost” traits reappeared → inheritance is particulate, not blending 

• Blending hypothesis rejected

Mendel’s Model of Inheritance: “Particulate Inheritance"

• Traits are determined by heritable factors (now called genes)

• Each character controlled by two factors — one from each parent

Four Concepts of Mendel’s Work

1. Alleles – alternative versions of a gene account for variations in traits 

2. Diploidy – individuals inherit two alleles for each gene (one per parent)

3. Dominance – 

   1. Dominant allele determines phenotype if present 

   2. Recessive allele expressed only if no dominant allele is present

4. Mendel’s Laws (Principles of Hereditary)

   1. Law of segregation

   2. Law of independent assortment

Law of segregation

the two alleles for a character separate during gamete formation

Heterozygous

two different alleles

Homozygous

two identical alleles

Law of independent assortment 

• Genes on different chromosomes assort independently during meiosis

• Caused by random orientation of tetrads in metaphase 1 

• Promotes genetic variation, especially when combined with crossing over

Genetic crosses: Purpose and Function

• predict offspring genotype

• tracks alleles to make and test predictions

Process of Genetic Crosses

1. determine possible gametes from each parent

2. determine possible offspring genotypes

3. calculate expected genotype and phenotype frequencies

Monohybrid

individual heterozygous for one character

Monohybrid Cross

Cross between heterozygotes

What tool do we use to track a genetic cross?

Punnett square — Visual method to show possible offspring genotypes and frequencies

Multiplication Rule

• Used for independent events (“and”)

• P(event 1 and event 2) = P(event 1) × P(event 2)

Addition Rule

• Used for mutually exclusive events (“or”)

• P(event 1 or event 2) = P(event 1) + P(event 2)