12: Intro to Genetics

Define Gene

Gene: A unit of hereditary information.

define allele

Allele: An alternative variant of a gene (e.g., the "blue eye" allele vs. the "brown eye" allele for the eye-color gene).

define genotype

Genotype: The specific genetic makeup of an individual; the alleles they possess (e.g., Bb, BB, or bb).

define phenotype

Phenotype: The observable physical or physiological traits of an individual (e.g., brown eyes).

• Genotype determines phenotype.

define character

Character: An observable, heritable feature (e.t., "eye color," "hair color," "seed shape").

define trait

Trait: A detectable variant of a character (e.g., "blue eyes," "red hair," "wrinkled seeds")

define homozygous

Homozygous: Having two of the same allele at a specific gene locus.

• Homozygous dominant: YY

• Homozygous recessive: yy

define heterozygous

Heterozygous: Having two different alleles at a specific gene locus.

• Heterozygous: Yy

What were the major advantages of using pea plants for Mendel's experiments?

• Many identifiable, heritable traits (e.g., flower color, seed shape).

• Easy to grow and have a short generation time.

• Easy to control pollination (could force self- or cross-pollination).

• Many true-breeding varieties were available.

What does "true-breeding" mean?

A "true-breeding" organism, when self-fertilized, will always produce offspring with the same phenotype as the parent, for generations. This means the parent line is homozygous for that trait (e.g., YY or yy).

Compare and contrast the "Blending Inheritance" hypothesis with Mendel's actual observations.

• Blending" Hypothesis (Prediction): Offspring are an intermediate blend of the parents. A cross between a purple-flowered plant and a white-flowered plant should produce all light-purple (intermediate) F1 offspring.

• Mendel's Observations (Result):

  1. The F1 generation was not an intermediate blend; they all resembled one of the parents (e.g., all purple).

  2. The "lost" trait (white) reappeared in the F2 generation.

  3. This proved inheritance is "particulate," not "blending."

Sequence Mendel's typical monohybrid cross experiment. Define the P, F1, and F2 generations.

• P (Parental) Generation: Cross two true-breeding organisms with contrasting traits (e.g., YY - yellow seeds × yy - green seeds).

• F1 (First Filial) Generation: Observe the offspring. All were heterozygous (Yy) and showed the dominant phenotype (all yellow).

• F2 (Second Filial) Generation: Cross the F1 generation with itself (Yy × Yy).

• Result: The F2 generation showed a mix of both P-generation phenotypes, in a consistent 3:1 ratio (3 yellow: 1 green).

What are the 4 components of Mendel's Model of Inheritance?

• Alleles: Alternative versions of genes (factors) account for variations in characters.

• Diploidy: For each character, an organism inherits two "factors" (alleles), one from each parent.

• Dominance: If the two alleles differ, the dominant allele determines the phenotype, and the recessive allele has no noticeable effect.

• Two Principles (Laws): The Law of Segregation and the Law of Independent Assortment.

Compare and contrast Mendel's two laws of heredity:

• Law of Segregation

• Law of Independent Assortment

• Law of Segregation: States that the two alleles for a single character (e.g., Y and y) separate (segregate) from each other during gamete formation so that each gamete gets only one allele.

• Law of Independent Assortment: States that the alleles for different characters (e.g., seed color, Y/y, and seed shape, R/r) assort independently of each other. The inheritance of one character does not affect the inheritance of another.

During which specific phases of meiosis do Mendel's two laws occur?

1. Law of Segregation

2. Law of Independent Assortment

• Law of Segregation (Anaphase I): The two alleles for a character are on homologous chromosomes. When homologous chromosomes separate during Anaphase I, the alleles are segregated.

• Law of Independent Assortment (Metaphase I): The inheritance of different genes is independent because of the random orientation of tetrads on the metaphase plate during Metaphase I. How the "seed color" tetrad lines up does not affect how the "flower color" tetrad lines up.

What is the phenotypic and genotypic ratio of a monohybrid cross (e.g., Yy × Yy)? (Assume Y=Yellow, y=green)

• Genotypic Ratio: 1 YY : 2 Yy : 1 yy

  • (1 homozygous dominant : 2 heterozygous : 1 homozygous recessive)

• Phenotypic Ratio: 3 Yellow : 1 Green

  • (3 dominant phenotype : 1 recessive phenotype)

What is the Multiplication Rule of probability?

• What is the key word?

• When do you use it?

• Apply it: In a cross Aa × Aa, what is the probability of an aa offspring?

• Rule: Predicts the combined probability of independent events.

• Key Word: "AND"

• Use: P(Event 1 and Event 2) = P(Event 1) × P(Event 2)

• Application (P of aa):

  • This requires P(getting a from mom) AND P(getting a from dad).

  • P(a from mom) = 0.5

  • P(a from dad) = 0.5

  • P(aa) = 0.5 × 0.5 = 0.25 (or 1/4)

What is the Addition Rule of probability?

• What is the key word?

• When do you use it?

• Apply it: In a cross Ee × Ee, what is the probability of a heterozygous (Ee) offspring?

• Rule: Predicts the combined probability of mutually exclusive events.

• Key Word: "OR"

• Use: P(Event 1 or Event 2) = P(Event 1) + P(Event 2)

• Application (P of Ee):

  • There are two mutually exclusive ways to get a heterozygote:

    1. Get E from mom AND e from dad

    2. OR

    3. Get e from mom AND E from dad

  • P(Way 1): P(E mom) × P(e dad) = 0.5 × 0.5 = 0.25

  • P(Way 2): P(e mom) × P(E dad) = 0.5 × 0.5 = 0.25

  • P(Ee) = P(Way 1) OR P(Way 2) = 0.25 + 0.25 = 0.5 (or 1/2)

Why are probability rules more useful in genetics than Punnett squares?

Punnett squares are simple for a monohybrid cross (4 squares). They become complex for a dihybrid cross (16 squares) and almost impossible for a trihybrid cross (64 squares). Probability rules let you calculate the expected frequencies for complex crosses (dihybrid, trihybrid, etc.) quickly and easily.