Inheritance Genes and Chromosomes

Chapter 12: Inheritance, Genes, and Chromosomes

12.1 Inheritance of Genes Follows Mendelian Laws

• Historical Context:

  • Humans have bred plants and animals for millennia, leading to two main hypotheses regarding inheritance:

    • Blending inheritance: Suggests that hereditary determinants blend in the zygote.

    • Particulate inheritance: Suggests that hereditary determinants are distinct and maintain their identity.

• Key Definitions:

  • Character: Observable physical feature (e.g., seed shape).

  • Trait: Specific form of a character (e.g., round vs. wrinkled seeds).

  • Phenotype: Observable properties of an individual determined by genetic and environmental factors.

• Mendel's Experiments:

  • Conducted with true-breeding varieties of peas.

  • Parental Generation (P): Plants he cross-pollinated.

  • First Filial Generation (F1): Seeds and offspring resulting from the P generation.

  • Second Filial Generation (F2): Produced by self-pollination of F1 plants.

• Monohybrid Crosses:

  • Crossed parental varieties differing in one character.

  • Results supported the particulate inheritance hypothesis; dominant traits appeared in F1, while recessive traits reappeared in F2.

• Key Conclusions:

  • Each gamete contains one copy of each gene; zygote contains two copies (diploid).

  • Homozygous: Two identical alleles (e.g., RR or rr).

  • Heterozygous: Two different alleles (e.g., Rr).

• Law of Segregation:

  • During gamete formation, the two copies of a gene separate, with each gamete receiving one copy.

• Chromosomal Basis of Inheritance:

  • Genes reside on chromosomes; alleles segregate as chromosomes separate during meiosis.

12.2 Alleles Can Produce Multiple Phenotypes

• Mutations: New alleles arise through mutations, which are stable inherited changes in genetic material.

• Wild Type and Mutant Alleles:

  • Wild type: Most common allele in the population; mutation results in a variant allele.

  • Polymorphic: Less than 99% of the time the wild-type allele is present.

• Examples of Inheritance Patterns:

  • Incomplete Dominance: Phenotype is a blend of both alleles, as observed in flower color crosses.

  • Pleiotropy: A single allele can affect multiple phenotypes (e.g., PKU affects multiple traits).

12.3 Genes Can Interact to Produce a Phenotype

• Gene Interaction: Genes can influence each other’s expression, leading to variations in traits.

• Inbreeding: Mating among close relatives can lead to offspring with reduced fitness (inbreeding depression).

• Hybrid Vigor: Hybrids may show superior traits compared to their parents, possibly due to overdominance.

• Influence of Environment: Gene expression can be modified by environmental factors, leading to varied phenotypes.

• Penetrance vs. Expressivity:

  • Penetrance: Proportion of individuals with a specific genotype that express the phenotype.

  • Expressivity: Degree to which a genotype is expressed in an individual.

12.4 Genes Are Carried on Chromosomes

• Historical Studies: Thomas Hunt Morgan used fruit flies (Drosophila melanogaster) to study inheritance.

• Linkage: Alleles can be linked on the same chromosome and may not assort independently.

• Crossing Over and Recombination: During meiosis, homologous chromosomes can exchange segments, resulting in genetic recombination.

• Sex Determination:

  • Many organisms (e.g., mammals, birds) have a sex determination system based on sex chromosomes (X and Y).

  • Nondisjunction during meiosis can lead to abnormalities such as Turner syndrome (XO) or Klinefelter syndrome (XXY).

12.5 Some Eukaryotic Genes Are Outside the Nucleus

• Organelle DNA: Mitochondria and plastids contain genes in circular DNA molecules, which mutate faster than nuclear genes.

• Bacterial Gene Transfer:

  • Conjugation: Bacteria can transmit genes through mating or plasmid exchange.

  • This method allows for increased genetic diversity and adaptation in prokaryotic populations.