Inheritance

Inheritance Concepts

  • Inheritance

    • Definition: Gene segment of DNA that codes for a polypeptide.

    • Loci: The specific location of a gene on a chromosome.

    • Alleles: Genes located at the same position (locus) on each member of the homologous chromosomes that code for the same characteristic.

Allele and Chromosomes

  • Allele for Purple Flowers

  • Locus for Flower-Color Gene

  • Allele for White Flowers

  • Homologous Pair of Chromosomes: Structures that consist of two sister chromatids.

Terminology

  • Segregation: The separation of the maternal and paternal alleles into the gametes.

  • Independent Assortment: Different loci randomly assort into the gametes without affecting each other.

  • Homozygous: Having two of the same allele at a locus (e.g., RR, rr).

  • Heterozygous: Having one of each type of allele at a locus (e.g., Rr).

  • Special Case - Hemizygous: Having no corresponding allele, for example, in sex chromosomes in males.

  • Dominant: The allele in a heterozygous state that is expressed.

  • Recessive: The allele in a heterozygous state that is masked.

Gregor Mendel

  • Title: Father of Classical Genetics.

  • Contributions: Established the principles of single gene Mendelian inheritance through his Garden Pea experiments.

    • P: Parental Generation in a mating experiment.

    • F1: First generation of offspring (first filial generation).

    • F2: Second generation of offspring.

  • True Breeding: Will always produce the same phenotype in offspring from mating these plants with each other.

  • Types of Crosses

    • Monohybrid Cross: Mating in which the inheritance of 1 locus is followed to offspring.

    • Dihybrid Cross: Mating in which the inheritance of 2 loci is followed to offspring.

Mendel's Peas

  • Conducted experiments with garden peas.

  • Traits determined by a single locus include:

    • Seed Form: Round (R) / Wrinkled (r)

    • Seed Color: Yellow (Y) / Green (y)

    • Pod Form: Inflated (V) / Restricted (v)

    • Pod Color: Green (G) / Yellow (g)

    • Flower Position: Axial (F) / Terminal (f)

    • Stem Length: Tall (L) / Short (l)

Mendel’s Assumptions (Hypothesis)

  1. Phenotype is determined by the combination of 2 alleles at a particular locus (in monohybrid cross).

    • Alleles separate and segregate into gametes; each gamete has 1 allele per locus.

    • Alleles come together at fertilization; offspring have 2 alleles per locus.

    • Genes do not chemically alter (no gene blending).

    • Phenotype is the result of the combination of alleles, where one is dominant and the other is recessive.

  2. Independent random assortment of different loci into gametes (in dihybrid cross).

Punnett Square

  • A statistical test to predict outcomes for two independent and random events.

  • Example: Flipping two coins yields a probability outcome.

Observed Phenotypes from Punnett Square

  • P Generation: Purple flowers (PP) and white flowers (pp) - true breeding.

  • F1: All offspring will be purple (all heterozygous).

  • F2: Phenotype ratio of 3:1 (purple to white), indicating purple is dominant.

The Scientific Method

  • Mendel formulated his hypotheses and used Punnett squares to predict offspring appearance and proportions of different phenotypes.

Experiment and Observed Results

  • F1 Generation: All purple, indicating purple allele is dominant to white.

  • F2 Generation: 3:1 ratio of phenotypes (purple to white).

Overall Experimental Results

  • Mendel conducted many similar experiments across pea plant traits, counting results and employing statistical tests to validate observations (using Punnett squares for predictions and Chi Square analysis for validation).

Mendel’s Conclusions

  1. Alleles segregate during gamete production, containing only one allele per locus.

  2. Alleles recombine during fertilization in offspring.

  3. Alleles are either dominant or recessive.

  4. The structure of alleles does not change (no gene blending).

Mendel's Additional Hypothesis

  • Different loci segregate independently of each other when gametes are formed.

    • It took a dihybrid cross experiment to support this hypothesis

Dihybrid Cross Predictions

  • Pea Texture & Color:

    • Locus 1: R = Round (dominant), r = Wrinkled (recessive).

    • Locus 2: Y = Yellow (dominant), y = Green (recessive).

    • Example Cross: RrYy x RrYy

Dihybrid Cross Observed Results

  • Results:

    • P: RRYY x rryy

    • F1: All Round Yellow

    • F2: 9:3:3:1 ratio of four different phenotypes.

Mendel's Conclusion on Dihybrid Cross

  • Loci assort into gametes independently of all other loci. One loci does not affect other loci.

    • Meiosis eventually supported why this is correctby demonstrating how alleles segregate into gametes during the formation of sperm and egg cells, thereby reinforcing the principle of independent assortment.

Statistical Testing by Mendel

  • Mendel utilized statistics to assess if observed numbers were close to expected values through the Chi Square test.

Chi Square Test

  • Observed Values are actual counts from experiments.

  • Expected Values are numbers hypothesized based on Mendelian inheritance.

  • Tests the "null" hypothesis (Ho).

  • Rejecting Ho means rejecting the underlying hypothesis, and accepting Ho indicates data support for that hypothesis.

Degrees of Freedom

  • Defined as the number of categories (n) minus one (df = n - 1).

  • A common level of significance chosen by biologists is 0.05 (95% accuracy).

Chi Square Test Procedure

  1. Calculate expected values based on your hypothesis (for 3:1 in monohybrid crosses, and 9:3:3:1 in dihybrid crosses).

  2. Conduct experiments and calculate observed values.

  3. Use Chi Square formula: x2=∑(o−e)2ex^2 = \sum \frac{(o-e)^2}{e}

Comparing Chi Square Values

  • Compare calculated Chi Square value against values in a Chi Square table for a p-value of 0.05 and the given degrees of freedom.

Variations on Simple Mendelian Genetics

  • Multiple Alleles: Some loci have more than 2 possible alternatives in a population.

  • Example: ABO blood type:

    • IA: Allele for A blood type

    • IB: Allele for B blood type

    • Io: Allele for O blood type.

Codominance and Incomplete Dominance

  • Codominance: Both alleles express themselves completely when heterozygous (e.g., AB blood type).

    • AB blood type, with two dominant alleles and one recessive allele

  • Antigens and Antibodies Present:

    • Group A: Antigen A, Anti-B antibodies.

    • Group B: Antigen B, Anti-A antibodies.

    • Group AB: Antigen A and B, No Antibodies

    • Group O: No Antigens, Anti-A and Anti-B Antibodies.

  • Incomplete Dominance: Heterozygous alleles show an intermediate phenotype, distinguishable from codominance.