Chapter 12: Patterns of Inheritance

The Mystery of Heredity

Before the 20th century, scientists held two main ideas about heredity: that heredity occurs within species and that traits are transmitted directly from parent to offspring. The prevailing belief was that traits were borne through fluids (such as blood) and blended in offspring, leading to the concept of "bloodlines." However, this blending theory could not explain why all individuals do not look alike.

• Heredity: The passing of traits from parents to offspring.

• Blending inheritance: The outdated idea that parental traits mix in offspring.

• Paradox: If blending occurs, genetic diversity should decrease over generations, but this is not observed.

Early Plant Biologists and Hybridization

Experiments by early plant biologists, such as Joseph Kölreuter in 1760, challenged the blending theory. By crossing tobacco strains, Kölreuter found that hybrid offspring differed from both parents and that additional variation appeared in the second generation, contradicting direct transmission of traits.

• Hybrid offspring: Offspring resulting from crosses between different varieties or species.

• Variation in F2 generation: New traits can appear, not simply blends of parental traits.

Mendel’s Experiments with Pea Plants

Gregor Mendel used pea plants (Pisum sativum) to study inheritance. Peas were ideal due to their many varieties, ease of growth, and ability to self- or cross-fertilize. Mendel’s method involved producing true-breeding strains, cross-fertilizing them, and analyzing offspring over several generations.

• True-breeding strains: Plants that produce offspring identical to themselves when self-fertilized.

• Reciprocal crosses: Crosses where the source of pollen is alternated to test inheritance patterns.

Monohybrid Crosses: The Principle of Segregation

Monohybrid crosses study two variations of a single trait. Mendel produced true-breeding strains for seven traits, each with two variants (e.g., flower color, seed color, seed texture).

• F1 Generation: Offspring of two true-breeding parents; all resembled one parent (dominant trait).

• F2 Generation: Offspring from self-fertilized F1 plants; recessive traits reappeared in a 3:1 ratio.

• Genotypic ratio: 1 (homozygous dominant): 2 (heterozygous): 1 (homozygous recessive).

Key Terms:

• Allele: Alternative form of a gene.

• Homozygous: Two identical alleles for a gene.

• Heterozygous: Two different alleles for a gene.

• Genotype: The genetic makeup of an individual.

• Phenotype: The observable traits of an individual.

Mendel’s Five-Element Model:

• Parents transmit discrete factors (genes).

• Each individual receives one copy of a gene from each parent.

• Not all copies are identical (alleles).

• Alleles remain discrete; no blending.

• Presence of an allele does not guarantee expression (dominant vs. recessive).

Principle of Segregation

Alleles for a gene segregate during gamete formation and are rejoined at random during fertilization. The physical basis is the movement of chromosomes during meiosis (metaphase I).

• Punnett Square: A diagram used to predict the outcome of genetic crosses.

Dihybrid Crosses: The Principle of Independent Assortment

Dihybrid crosses study two traits simultaneously. Mendel’s dihybrid crosses produced a 9:3:3:1 phenotypic ratio in the F2 generation, showing that alleles of different genes assort independently.

• Principle of Independent Assortment: Alleles of different genes segregate independently during gamete formation.

• Example: Seed shape is independent of seed color.

Testcrosses

A testcross determines the genotype of an individual with a dominant phenotype by crossing it with a homozygous recessive individual. The offspring ratios reveal the unknown genotype.

• Testcross: Cross between an individual of unknown genotype and a homozygous recessive individual.

Probability in Genetics

Probability rules help predict genetic outcomes.

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

• Rule of Addition: Probability of either of two mutually exclusive events occurring is the sum of their individual probabilities.

Extensions to Mendel

Not all traits follow Mendel’s simple rules. Some genes have more than two alleles, show incomplete dominance, codominance, or are influenced by the environment.

• Environmental Influence: Phenotypic plasticity allows different phenotypes from the same genotype due to environmental conditions (e.g., coat color in Himalayan rabbits).

• Continuous Variation: Traits show a range of phenotypes due to multiple genes (quantitative traits).

• Heritability: Fraction of phenotypic variation due to genetic variation. - Broad sense heritability (): Total genetic contribution. - Narrow sense heritability (): Additive genetic variance.

• Pleiotropy: One allele affects multiple traits (e.g., cystic fibrosis).

• Multiple Alleles: More than two alleles exist for a gene in a population (e.g., ABO blood types).

• Incomplete Dominance: Heterozygote is intermediate between two homozygotes (e.g., pink flowers from red and white parents).

• Codominance: Heterozygote shows aspects of both homozygotes (e.g., AB blood type).

• Epistasis: One gene’s action obscures the effects of another gene (e.g., Labrador retriever coat color).

ABO Blood Group System

The ABO blood group system demonstrates both multiple alleles and codominance. Three alleles (IA, IB, i) determine blood type. IA and IB are codominant to each other and dominant over i.

Epistasis Example: Labrador Retriever Coat Color

Coat color in Labrador retrievers is controlled by two genes: Brown and Extension. Interaction between these genes results in only three coat colors (black, brown, yellow), rather than the four expected from independent assortment.

• Epistasis: The effect of one gene masks or modifies the effect of another gene.

Summary Table: Mendelian vs. Non-Mendelian Inheritance

Additional info: These notes expand on the brief points in the slides and text, providing definitions, examples, and context for key genetic concepts relevant to a General Biology course.