Mendel and the Gene

5.2 Mendel and the Gene

Idea

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Root Words

  • Pheno-: From Greek, meaning “observe”, “shining”, and “appearance”.

    • phenotype: The physical appearance or observed behavior of an organism.

    • phenomenon: “That which appears”, referring to situations lacking explanation.

  • -gen: From Latin, meaning “birth” or “origin”.

    • generation: A group of organisms born around the same time.

  • Polygenic inheritance: Traits formed from multiple genes.

Key Terms

  • Blending hypothesis: Early idea suggesting that offspring are an intermediate blend of traits from parents. Particulate hypothesis: Suggests that traits are inherited as discrete units, leading to the development of the field of genetics.

  • Trait: A specific characteristic of an organism.

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

  • P generation: Parent generation in a genetic cross.

  • F1 generation: First offspring generation resulting from the P generation.

  • F2 generation: Offspring generation resulting from self-fertilizing the F1 generation.

  • Law of segregation: Two alleles for a heritable character segregate during gamete formation.

  • Dominant: An allele that determines the organism's appearance.

  • Recessive: An allele that does not affect appearance when paired with a dominant allele.

  • Heritable factor: Now known as a gene.

  • Gene: The basic unit of heredity.

  • Locus: Specific location of a gene on a chromosome.

  • Allele: Different versions of a gene.

  • Homologous chromosomes: Chromosomes that are similar in shape, size, and gene content; one from each parent.

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

  • Homozygous: Organisms with two identical alleles for a character.

  • Heterozygous: Organisms with two different alleles.

  • Phenotype: The observable traits of an organism.

  • Genotype: The genetic makeup of an organism.

  • Testcross: A breeding experiment used to determine an organism’s genotype.

  • Monohybrid cross: A genetic cross between parents that differ in a single trait.

  • Dihybrid cross: A cross between parents differing in two traits.

  • Law of independent assortment: The alleles of different genes assort independently of one another during gamete formation.

  • Product “and” rule: The probability of two independent events occurring together.

  • Sum “or” rule: The probability of any one of two or more exclusive events occurring.

  • Complete dominance: A form of dominance where the dominant allele completely masks the effect of the recessive allele.

  • Incomplete dominance: A form of dominance in which the heterozygous phenotype is intermediate between both parents.

  • Codominance: A condition in which both alleles in a heterozygote are fully expressed.

  • Polygenic inheritance: Inheritance of traits controlled by multiple genes.

  • Norm of reaction: The range of phenotypes produced by a single genotype in varying environments.

  • Pedigree: A family tree that tracks the inheritance of traits.

  • Carriers: Individuals who carry one copy of a recessive allele but do not express the trait.

  • Huntington’s disease: A genetic disorder caused by a dominant allele, characterized by progressive degeneration of nerve cells in the brain.

Overview: Drawing from the Deck of Genes

  • Mendel's Genetic Principles: Investigated how traits are passed from parents to offspring.

    • The “blending” hypothesis suggests genetic material from both parents blends together (analogous to mixing paint).

Particulate hypothesis

  • The “particulate” hypothesis states parents pass on discrete heritable units (genes).

  • Mendel provided evidence supporting this through his experiments with garden peas.

Concept 1: Mendel's Scientific Approach

Mendel's Laws of Inheritance

  • Mendel utilized careful breeding of garden peas to discover the basic principles of heredity.

Mendel’s Experimental, Quantitative Approach

Advantages of Pea Plants for Study

  • Distinct Varieties: Many varieties exist with unique observable traits, such as flower color.

  • Controlled Mating: Ability to control which plants mate.

  • Reproductive Organs: Each pea plant has both sperm-producing (stamens) and egg-producing organs (carpels).

  • Cross-Pollination: Achieved by manually dusting one plant with pollen from another.

Mendelian Experimental Design

  • Mendel focused on tracking traits that exhibited distinct variations (e.g., flower color).

  • He used true-breeding varieties in his experiments, which produce offspring identical to themselves when self-pollinated.

Hybridization Process

  • Mendel performed hybridizations by mating two contrasting, true-breeding varieties, with true-breeding parents referred to as the P generation and their offspring as the F1 generation.

  • Upon self-pollination of the F1 hybrids, the F2 generation is produced, evidenced in the following experiment:

    • P Generation: Purple flowers and White flowers.

    • F1 Generation: All F1 plants exhibited purple flowers.

    • F2 Generation: Resulting in 705 purple-flowered plants to 224 white-flowered plants, showing a 3:1 ratio of purple to white flowers.

The Law of Segregation

Mendel's Observations

  • When crossing true-breeding white and purple flowered pea plants, all F1 hybrids were purple.

  • In the F2 generation, the observed ratio of purple to white flowers was approximately 3:1.

  • Mendel concluded that the purple flower factor was dominant over the white flower factor, categorizing purple as a dominant trait and white as a recessive trait.

Table of Observations

  • Mendel noted the consistent inheritance across six additional pea plant characters, which he termed heritable factors (now understood as genes).

Mendel’s Model of Inheritance

Four Key Concepts

  1. Gene Variants: Alternative versions of genes account for variations in inherited characters (e.g., alleles for flower color: purple or white).

  2. Inheritance of Alleles: Each organism inherits two alleles for each trait, one from each parent. The alleles at a locus may be identical (true-breeding) or different (hybrid, e.g., F1 generation).

  3. Dominance of Alleles: If the two alleles at a locus differ, the dominant allele dictates the observed phenotype while the recessive allele does not manifest.

  4. Law of Segregation: During gamete formation, alleles segregate so that each gamete carries only one allele for each gene, corresponding to the distribution of homologous chromosomes in meiosis.

Mendel’s Observations in F2 Generation

  • Mendel identified a 3:1 ratio consistent with the law of segregation in the F2 generation across various crosses. Visual representation using Punnett squares can illustrate potential combinations of alleles in offspring.

  • Capital letters are used to indicate dominant alleles, while lowercase letters indicate recessive alleles.

Mendelian Phenotypic and Genotypic Ratios

Definitions

  • Homozygous: An organism possessing two identical alleles (e.g., PP or pp).

  • Heterozygous: An organism holding two different alleles (e.g., Pp).

  • Phenotypic Ratio: Ratios highlighting physical characteristic ratios (e.g., 3 purple: 1 white).

  • Genotypic Ratio: Ratios emphasizing genetic combinations (e.g., 1 PP: 2 Pp: 1 pp).

The Testcross Method

  • To determine the genotype of an individual demonstrating a dominant phenotype, a testcross is performed by mating the mystery individual with a homozygous recessive individual.

  • The results indicated:

    • If all offspring exhibit dominant phenotype, the mystery individual is homozygous recessive.

    • If there's a mix of phenotypes, the mystery parent is heterozygous (e.g., PP or Pp crossed with pp).

The Law of Independent Assortment

  • Mendel's second law, derived from observing two characters simultaneously, states that alleles of different genes assort independently during gamete formation.

  • This rule applies strictly to genes located on different, non-homologous chromosomes, while genes positioned close to each other on the same chromosome exhibit linked inheritance.

Concept 2: Laws of Probability in Mendelian Inheritance

Application of Probability

  • Mendel’s laws reflect rules of probability, exemplified in coin toss outcomes being independent of one another. The same principle applies to the segregation of alleles.

  • Multiplicative Rule: The probability of two independent events occurring together equals the product of their probabilities.

  • Additive Rule: The probability of any one of two exclusive events occurring equals the sum of their individual probabilities.

Complex Genetics Problems

Solving with Probability Rules

  • The rules of probability can resolve complex genetic crosses by considering each character separately before multiplying their corresponding probabilities.

Variations in Mendelian Patterns

Inheritance Complexities

  • Many heritable traits deviate from simple Mendelian inheritance patterns due to the involvement of multiple alleles or the interactions of genes.

Incomplete Dominance and Codominance

Degrees of Dominance

  • Complete Dominance: Heterozygous phenotypes appear identical to homozygous dominant phenotypes.

  • Incomplete Dominance: Heterozygous phenotypes display an intermediate character (e.g., red and white flowers producing pink offspring).

  • Codominance: Both alleles in a heterozygote express their phenotypes distinctly (e.g., AB blood group).

Multiallelic Traits

  • Most genes have more than two alleles present in a population, illustrated by the ABO blood group in humans governed by three alleles: IA, IB, and i.

  • Pleiotropy: Refers to a gene influencing multiple phenotypic traits; for instance, Tay-Sachs disease causes various symptoms due to one gene affecting multiple functions.

Epistasis

  • A gene at one locus can affect the expression of a gene at a second locus; this is seen in coat color in mice dictated by two genes controlling pigment production and deposition.

Polygenic Inheritance

  • Character traits show quantitative variation due to polygenic inheritance, an accumulation of effects from several genes (e.g., skin color).

Nature vs. Nurture

  • Environmental factors can also influence phenotypic expression, shown by the norm of reaction, which defines how genotypes respond to environmental factors.

Pedigree Analysis in Human Genetics

  • Pedigrees elucidate family inheritance patterns for specific traits across generations, utilizing symbols to denote gender and affected status.

Recessive and Dominant Disorders

  • Disorders caused by recessive alleles appear only in homozygous individuals. Carriers (heterozygous) remain phenotypically normal. Examples include albinism. Dominant disorders, like Huntington’s disease, manifest phenotypically much later in life.