Chapter 14 - Mendel and the Gene Idea

Learning Objectives

  • Students should be able to:

    • Explain conclusions about inheritance and the principle of segregation from monohybrid crosses.

    • Explain conclusions about the principle of independent assortment from dihybrid crosses.

    • Explain how chromosome movement during meiosis accounts for the principles of segregation and independent assortment.

    • Define common genetics terms and provide examples:

    • Multiple alleles: Variations of a gene that exist within a population.

    • Codominance: A situation where both alleles in a heterozygous organism contribute to the phenotype.

    • Incomplete dominance: A scenario where the phenotype of the heterozygote is intermediate to the phenotypes of the two homozygotes.

    • Pleiotropic genes: Genes that influence multiple phenotypic traits.

    • Environmental influences on phenotype: How environmental factors can affect the expression of traits.

    • Interactions between genes: How different genes can affect each other's expression and the resulting phenotype.

    • Quantitative traits: Traits that show continuous variation and are typically influenced by multiple genes.

    • Describe how gene linkage affects the assortment of alleles at meiosis.

    • Solve patterns of inheritance of human traits through pedigree analysis.

Gregor Mendel

  • Background:

    • Austrian monk (1822-1884) known as the father of genetics.

    • Conducted experiments on pea plants (approximately 29,000) from 1856 to 1863.

    • His meticulous observations laid the foundation for the field of genetics.

Genes and Alleles

  • Key Concepts:

    • Mendel introduced the idea that hereditary determinants for traits are called genes.

    • Each individual possesses two versions of each gene, known as alleles, which can vary among different individuals.

    • Types of Alleles:

    • Dominant alleles: Exert their effects when present; mask the effect of recessive alleles.

    • Recessive alleles: Their effects are masked in the presence of a dominant allele.

    • Definitions:

    • Genotype: The genetic makeup of an individual (combination of alleles).

    • Phenotype: The observable characteristics or traits of an individual.

Homologous Chromosomes

  • Definition:

    • Homologous pairs of chromosomes are similar in shape, size, and genetic content. Each pair consists of one chromosome from each parent.

Dominant and Recessive Traits

  • Mendel's findings led to the understanding that:

    • Individuals with two copies of the same allele are termed homozygous (can be dominant or recessive).

    • Individuals with two different alleles are termed heterozygous.

Mendel's Monohybrid Crosses

  • Phenotypic Ratios from Crosses:

    • Seed shape: 5474 round vs. 1850 wrinkled (Ratio: 2.96:1)

    • Seed color: 6022 yellow vs. 2001 green (Ratio: 3.01:1)

    • Pod shape: 882 inflated vs. 299 constricted (Ratio: 2.95:1)

    • Pod color: 428 green vs. 152 yellow (Ratio: 2.82:1)

    • Flower color: 705 purple vs. 224 white (Ratio: 3.15:1)

    • Flower and pod position: 651 axial vs. 207 terminal (Ratio: 3.14:1)

    • Stem length: 787 tall vs. 266 short (Ratio: 2.96:1)

Principle of Segregation

  • Mendel proposed:

    • During gamete formation, the two members of each gene pair separate so that each gamete contains only one member of the pair.

    • Meiosis Explanation:

    • Chromosomes replicate, and during Meiosis I, alleles segregate into separate gametes.

Mendel’s Dihybrid Crosses

  • Hypotheses Tested:

    • Independent assortment: Alleles of different genes are transmitted independently.

    • Dependent assortment: The transmission of one allele depends on the transmission of another.

  • Observations:

    • Parental genotypes:

    • Female: rr (wrinkled) and yy (green)

    • Male: RR (round) and YY (yellow)

    • All F1 offspring genotypes are RrYy.

  • F2 Generation Ratios:

    • 9/16 RY (round, yellow) : 3/16 R_yy (round, green) : 3/16 rrY (wrinkled, yellow) : 1/16 rryy (wrinkled, green)

    • Possible gametes produced: {RY, Ry, rY, ry}.

Gamete Formation and Independent Assortment

  • Meiosis involves interactions between pairs of homologous chromosomes, leading to diverse configurations and ultimately contributing to the independent assortment of alleles.

Testing the Theory of Inheritance

  • Thomas Hunt Morgan's Research:

    • Utilized fruit flies (Drosophila melanogaster) for genetic studies, classifying phenotypes into wild types and mutants.

    • Explored how traits like eye color exhibit relationships between sex and inheritance.

Sex-linked Inheritance

  • Definitions:

    • Genes located on non-sex chromosomes exhibit autosomal inheritance.

    • X-linked inheritance pertains to genes found on sex chromosomes:

    • Males require only one allele (hemizygous) to express the trait, while females require two alleles (homozygous).

    • Conditions such as color blindness and hemophilia are often X-linked recessive traits.

Gene Linkage and Crossing Over

  • Concept:

    • Linkage refers to the tendency of genes located on the same chromosome to be inherited together.

    • Crossing over during meiosis allows for the separation of linked genes.

  • Measuring Linkage:

    • Recombination frequency is calculated using:
      extRecombinationfrequency=racextNumberofrecombinantoffspringextTotaloffspringimes100ext{Recombination frequency} = rac{ ext{Number of recombinant offspring}}{ ext{Total offspring}} imes 100

    • Example: 391 recombinants from 2300 total offspring yields a recombination frequency of 17%.

Extensions to Mendel’s Rules

  • Polygenic Inheritance: Many traits are influenced by multiple genes.

  • Genetic Terms:

    • Multiple alleles: Seen in examples such as human ABO blood groups.

    • Codominance and incomplete dominance: Examples can be found in various phenotypes.

    • Pleiotropy: A situation where a single gene affects multiple traits, such as in Marfan syndrome.

    • Epistasis: Interaction between genes where one gene's effect is modified by the presence of 'modifier genes'.

Human Inheritance and Pedigree Analysis

  • Purpose: Pedigrees enable researchers to trace inheritance patterns in humans, as experimental crosses are infeasible.

  • Traits in Pedigrees:

    • Autosomal traits appear equally across both sexes, though their inheritance patterns vary.

    • Autosomal recessive traits: Require homozygous recessive individuals to express the trait.

    • Autosomal dominant traits: Manifest in homozygous dominant or heterozygous individuals.

    • X-linked traits: May display biased ratios in male and female offspring.

Traits Classification in Pedigrees

  • Autosomal Recessive: Traits can skip generations; carriers are heterozygous.

  • Autosomal Dominant: Affected individuals have an affected parent, and the trait does not skip generations.

  • X-Linked Recessive: The trait is more common in males and often skips generations; female carriers can pass it on.

  • X-Linked Dominant: Both sexes can be affected, the trait does not skip generations, and all female offspring of an affected male will also be affected.

Example Questions for Analysis

  • Analyze phenotypes in pedigrees to determine if they are autosomal or sex-linked, dominant or recessive, and deduce possible genotypes for individuals based on family data.