Genetics Part 2

Two-Trait Inheritance

Overview of Mendel's Crosses

  • Mendel conducted experiments with plants exhibiting two differing traits.

  • Example: A cross between tall plants with green pods (homozygous dominant, TTGG) and short plants with yellow pods (homozygous recessive, ttgg).

F1 and F2 Generations

  • The F1 generation resulted from the initial cross, self-pollinating to form the F2 generation.

  • Two potential outcomes expected in the F2 generation:

    • If dominant traits (T and G) segregate together: all progeny will show tall stature with green pods.

    • If traits segregate independently: four phenotypic expressions possible.

Observed Phenotypes in F2 Generation

  • Results of the cross produced four distinct phenotypes:

    • Tall plants with green pods

    • Tall plants with yellow pods

    • Short plants with green pods

    • Short plants with yellow pods

  • Demonstrated that the factors segregated independently.

Law of Independent Assortment

  • Based on findings, Mendel formulated the law of independent assortment:

    • Alleles for two or more genes are sorted into gametes independently.

    • All combinations of factors can occur in the gametes.

Genetic Cross Diagrams

P Generation and F1 Cross

  • P generation: Cross of TTGG (tall green) and ttgg (short yellow).

    • Gametes produced: TG from TTGG and tg from ttgg.

    • F1 generation comprised entirely of tall green pod plants (TtGg).

F2 Generation Analysis

  • F1 gametes fertilize, giving rise to the F2 generation with variations:

    • Possible combinations: TG, Tg, tG, tg.

  • Phenotypic ratio established:

    • 9 tall green, 3 tall yellow, 3 short green, 1 short yellow.

Mendel's Laws and Probability

Application of Probability in Genetics

  • Mendel noted genetic results follow probability rules:

    • Multiplication Rule: Probability of two independent events occurring together is the product of their individual probabilities.

    • Example: Chance of two tails in coin flipping: ½ X ½ = ¼.

Probability of Genotypes

  • Calculating probabilities in genetic crosses:

    • For ll X ll: Probability for ll = ½.

    • For Gg X gg: Probability for gg = ½.

    • Combined probability for llgg genotype: ½ X ½ = ¼.

Relationship Between Mendel’s Laws and Meiosis

Meiosis and Genetic Laws

  • Mendel's laws are linked to meiosis events:

    • Segregation law relates to the separation of homologous chromosomes during Meiosis I.

Inheritance Beyond Mendel's Laws

Complex Patterns of Inheritance

  • Mendel's observations applied to simple, independently-segregating traits.

  • There are more inheritance patterns beyond dominance/recessive relations.

  • Environmental factors can also influence phenotypes significantly.

Incomplete Dominance

  • Incomplete dominance: offspring display an intermediate phenotype.

    • Example: Four-o'clock flowers show pink flowers from red and white parents.

    • In humans, wavy hair exemplifies incomplete dominance between curly and straight hair.

Environment and Phenotype

  • Environmental factors may surpass genetic influence on phenotype.

    • Effects observed in temperature-dependent traits such as flower color in primroses and coat color in Himalayan rabbits.

  • Nature versus nurture concept arises from genetic and environmental interaction.

Sex-Linked Inheritance

Understanding Sex Chromosomes

  • Human sex chromosomes (X and Y) dictate biological sex and other genetic traits.

  • Y chromosome carries 26 genes, primarily sex-related.

  • X chromosome harbors genes pertaining to both sex and various other traits (X-linked genes).

  • XY Combination: Typically males; presence of Y chromosome, specifically the SRY gene, leads to male characteristics.

  • XX Combination: Typically females; absence of Y activates pathways for female development. The X chromosome also carries X-linked traits influencing various characteristics.

Example of X-Linked Inheritance

  • Color vision gene inheritance:

    • X chromosomes convey normal vision or colorblindness.

    • Possible offspring genotypes include normal vision and carriers of colorblindness.