chapter 12

Life

  • The Science of Biology
  • TWELFTH EDITION
  • Chapter 12: Inheritance, Genes and Chromosomes
  • Authors: Hillis, Heller, Hacker, Hall, Laskowski, Sadava

Sexual Reproduction

  • Definition: A process in which offspring are produced that are NOT genetically identical to parents.
  • Requirements:
    • Involves gametes created by meiosis.
    • Each parent contributes one gamete to the offspring.
  • Characteristics:
    • Gametes and offspring differ genetically from each other and from the parents.

Meiosis and Genetic Diversity

  • Mechanisms:
    • Homologous chromosomes separate during meiosis.
    • Independent assortment/segregation of paternal and maternal chromosomes occurs.
    • Alleles separate during gamete formation.
    • Sister chromatids separate, which were originally exact copies. Crossing over (recombination) results in slightly different chromatids.
  • Result: Eggs and sperm are genetically different from parents, contributing to genetic variation.

Mendelian Laws of Inheritance

  • Gregor Mendel: An Austrian monk whose studies in physics and mathematics influenced his use of quantitative experimental methods in genetics.
    • Conducted experiments over seven years with 24,034 pea plants.

Theories Prior to Mendel

  • Blending hypothesis: Current thinking at the time.
    • Suggests hereditary materials from each parent are blended in the offspring.
    • Offspring exhibit intermediate forms (e.g., red + blue = purple).
    • Once blended, hereditary material is lost and cannot be separated.
  • Mendel’s work contradicted this hypothesis, published in 1866 but mostly ignored until 1900 when meiosis was observed.
Choice of Organism: Pisum sativum (Garden Pea)
  • Reason for Selection:
    • Rapid growth and abundant seed production.
    • Requires little physical space.
    • Naturally self-pollinating, allowing controlled fertilization.
    • Exhibits distinct characters (observable physical features such as seed shape).
Mendel’s Characteristics and Traits
  • Focused on true-breeding traits, which are traits present in many generations and are homozygous.
  • True-breeding strains isolated through inbreeding and selection.
  • Concentrated on seven characters, each existing in two distinct forms (e.g., round vs. wrinkled seeds).
Mendel’s Terminology
  • P = Parental generation.
  • F1 = First filial generation (offspring of P).
  • F2 = Second filial generation (offspring of F1).
  • A cross of plants differing in one trait results in F1 generation being termed monohybrids.
Mendel’s Findings
  • Observation: One trait of each pair disappears in F1 generation, reappears in F2.
    • Traits in F1 termed dominant; those that disappear termed recessive.
  • Dominant to recessive ratio in F2 approximately 3:1.
    • Proposed the particulate theory of inheritance: heritable units are discrete and each plant carries two units for each character (diploid).
    • During gamete formation, only one unit is contributed (haploid).
    • Two alleles for a character segregate during gamete formation.

Monohybrid Experiment

  • Hypothesis: Characteristics of two contrasting traits irreversibly blended in succeeding generations.
  • Method: Cross true-breeding round seeds with true-breeding wrinkled seeds.
    • Heterozygous outcome observed in offspring: Round seeds dominate.
  • Results: In self-pollination of F1:
    • Distribution of genotypes in F2 is 3:1 (dominant to recessive).

Key Terminology

  • Alleles: Different forms of a gene (e.g., gene for starch branching enzyme creating round vs. wrinkled).
  • Homozygous: Individual with two identical alleles (e.g., RR or rr).
  • Heterozygous: Individual with two different alleles (e.g., Rr).
Dominance and Recessiveness
  • Dominant allele: Preferentially expressed.
  • Recessive allele: Not expressed unless homozygous.
  • Representation: Capital letters for dominant alleles (e.g., R) and lowercase for recessive alleles (e.g., r).

Genotype vs. Phenotype

  • Phenotype: Physical appearance (e.g., round or wrinkled seeds).
  • Genotype: Genetic makeup (e.g., Rr).
  • Organisms with identical phenotype can have different genotypes (e.g., RR vs. Rr).

Sigificance of Mendel’s Work

  • True-breeding plants in the P generation denote distinct copies of the gene.
    • E.g., Round (RR) and wrinkled (rr) yields offspring (F1) with Rr genotype, showing round phenotype.
    • Results in varied genotypes (F2) and phenotypes (3:1 ratio).

Mendel’s First Law: The Law of Segregation

  • Two copies of a gene separate during gamete formation.
  • Each gamete inherits only one allele per trait, leading to segregation during meiosis I.
  • F1 self-pollinates yielding a 3:1 ratio in F2.

The Role of Meiosis

  • Meiosis accounts for the segregation of alleles.
  • Different alleles located at the same locus in homologous chromosomes.
  • Alleles separate during meiosis, contributing to genetic variation.

Punnett Square

  • A tool for predicting and displaying allelic combinations.
  • Example of a monohybrid cross using a Punnett Square illustrates potential outcomes in F1 and F2 generations.

Test Crossing

  • Utilized to identify a “mystery” plant's genotype.
    • Cross round-pea plant (unknown genotype: RR or Rr) with wrinkled-pea plant (rr).
    • Outcome determines possible genotype of the round-seed parent.

Mendel’s Dihybrid Cross

  • Experiment involved true-breeding parents for two characters (e.g., seed shape and color).
    • True-breeding parents crossed: RRYY (round yellow) × rryy (wrinkled green).
    • F1 generation all RrYy (round yellow). Crossing these in a dihybrid cross observed segregation.
    • Phenotypic ratio in F2 observed as 9:3:3:1.

Mendel’s Second Law: The Law of Independent Assortment

  • States that alleles of different genes sort independently during gamete formation.
  • Applicable only to genes on nonhomologous chromosomes.
  • Exceptions exist for genes on the same chromosome (linkage).

Key to Mendel’s Success

  • Large sample sizes allowed detection of clear patterns.
  • Employed mathematical approaches for results analysis.
  • Probability principles:
    • Certain event: Probability = 1.
    • Impossible event: Probability = 0.

Probability Rules

  • Multiplication Rule: For two independent events, probabilities multiply.
    • E.g., probability both coins land heads as 12imes12=14\frac{1}{2} imes \frac{1}{2} = \frac{1}{4}.
  • Addition Rule: Probability of two events occurring in different ways sums individual probabilities.
    • E.g., two ways to get genotype Rr: 14+14=12\frac{1}{4} + \frac{1}{4} = \frac{1}{2}.

Application of Mendel’s Laws

  • Pedigree analysis in humans demonstrates Mendel’s principles despite fewer offspring.
  • Examples of genetic traits:
    • Huntington’s Disease (dominant): Affected individuals have affected parents, approx. half of offspring affected.
    • Albinism (recessive): Often not exhibited by affected parents; approximately one-quarter of offspring may be affected.

Genetic Variation and Alleles

  • Alleles arise from mutations, with populations harboring more than two alleles for a gene, while each individual carries two.
    • Wild type: Predominant allele; other forms termed mutant alleles.
  • Penetrance: The proportion of a genotype expressing the expected phenotype.
  • Expressivity: Degree to which a genotype is phenotypically expressed.

Beyond Mendelism

  • Mendelism is incomplete; gene interactions exist, and dominance/recessiveness isn't absolute.
  • Polymorphism: Genes can have multiple alleles.
  • Incomplete dominance: Some alleles exhibit intermediate phenotypes.
  • Epigenesis: Environmental and other non-genetic factors influence phenotypes.
  • Many phenotypes show continuous variation influenced by multiple genes.