Biological Beginnings: Principles of Hereditary Transmission

Biological Beginnings: Chapter 3

Basic Principles and Processes of Hereditary Transmission

Problems that Arise When Genes and Chromosomes Develop Abnormally

Principles of Hereditary Transmission

Some Basic Definitions

  • Genotype: The genetic constitution of an individual organism.

  • Phenotype: The observable physical or biochemical characteristics of an organism, as determined by both genetic makeup and environmental influences.

  • Genes: Segments of DNA that serve as the instructions for the construction of proteins, which govern the traits of organisms.

  • Chromosomes: Structures located within the nucleus of cells that contain genes. Each chromosome comprises a single molecule of DNA, tightly coiled and compacted.

Cell Structures Related to Hereditary Transmission

  • Cell: Basic structural, functional, and biological unit of all living organisms.

  • Chromosome: Repository of genetic information, exists in pairs in human cells.

  • Nucleus: Membrane-bound organelle housing chromosomes and controlling cellular activities.

  • DNA (Deoxyribonucleic Acid): Molecule that carries genetic instructions, organized as genes along chromosomes.

Cell Division and Chromosome Duplication

Mitosis

  • Process of cell division that results in two genetically identical daughter cells.

  • Steps in Mitosis:

    • DNA Replication: Prior to cell division, genetic material is duplicated.

    • Cell Division: The cell divides into two genetically identical cells, ensuring genetic continuity.

Meiosis

  • Specialized form of cell division that produces gametes (sperm and ova).

  • Two Stages of Meiosis:

    • Meiotic Division 1: Chromosome reduction occurs; homologous chromosomes separate.

    • Meiotic Division 2: Sister chromatids separate, leading to four genetically unique gametes.

  • Genetic Uniqueness: This process contributes to genetic variability.

Twins

Types of Twins

  • Monozygotic (Identical) Twins:

    • Develop from a single fertilized egg that splits into two embryos.

    • Share identical genetic material.

    • Occur in approximately 1 in 250 births globally.

  • Dizygotic (Fraternal) Twins:

    • Occur when two separate eggs are fertilized by two different sperm cells.

    • Share the same genetic similarities as regular siblings.

    • Occur in approximately 1 in 125 births globally.

Behavior Genetics

Measuring Nature/Nurture

  • Twin Studies: Investigate the influence of genetics versus environment by comparing identical twins (monozygotic) and fraternal twins (dizygotic) reared together or apart.

  • Adoption Studies: Compare children raised in adoptive families with biological and adoptive parents to determine influences of heredity and environment.

Shared vs. Nonshared Environments

  • Heredity-Environment Correlations:

    • Passive Correlations: Environment provided by parents reflects their genetic traits.

    • Evocative Correlations: An individual's genetic traits evoke certain responses from the environment.

    • Active (Niche-Picking) Correlations: Individuals select environments suited to their genetic predispositions.

Nature-Nurture Interaction

Range of Reaction

  • Concept that any given genotype sets limits on the range of phenotypes possible in different environments.

  • Example:

    • Adult Height: Genotype can determine potential height, but environmental factors (nutrition, health) can affect the final outcome.

Canalization

  • Idea that certain genetic traits are relatively impervious to environmental influences, hence limiting variability in characteristics despite different environmental conditions.

Determining Sex

  • Female: XX chromosome combination; always contributes an 'X' chromosome.

  • Male: XY chromosome combination; contributes either 'X' or 'Y' chromosome.

Principles of Hereditary Transmission: Gene Expression

  • Dominant (B) vs. Recessive (b) traits.

  • Homozygous (e.g., BB): Two identical alleles.

  • Heterozygous (e.g., Bb): One dominant and one recessive allele.

  • Examples:

    • Brown eyes (B) are dominant over blue (b).

Examples of Dominant and Recessive Traits

  • Dominant Traits:

    • Dark hair

    • Dimples

    • Curly hair

    • Normal blood clotting

    • Tongue curling

    • Ear wiggling

    • Hand clasping (left over right)

    • PTC-taster

  • Recessive Traits:

    • Blond hair

    • No dimples

    • Straight hair

    • Hemophilia

    • No tongue curling

    • No ear wiggling

    • Hand clasping (right over left)

    • PTC-nontaster

Complex Inheritance Patterns

Incomplete Dominance

  • Example: Sickle Cell Anemia

    • Homozygous (AA or aa): Severe form.

    • Heterozygous (Aa): Mild form.

Co-Dominance

  • Example: Type AB blood, where both A and B alleles are expressed equally.

Polygenic Inheritance

  • Traits determined by multiple genes.

X-Linked Inheritance

  • Traits determined by genes found on the X chromosome.

Genetic and Chromosomal Abnormalities

Genetic Abnormalities

  • Generally rare; can increase the risk of fetal mortality but are not always lethal.

Genetic Abnormalities: Mutation

  • Defined as a spontaneous change in genetic material due to errors during gene replication.

  • Types of Mutations:

    • Beneficial: E.g., heterozygous sickle-cell anemia (offers malaria resistance).

    • Neutral: E.g., blind cave fish (no vision serves no disadvantage in their environment).

    • Harmful: E.g., hemophilia (a blood clotting disorder).

Examples of Genetic Abnormalities

  • X-linked Defective Genes:

    • Color blindness

    • Hemophilia

  • Recessive Defective Genes:

    • Cystic Fibrosis

    • Tay Sachs

    • Phenylketonuria (PKU)

  • Dominant Defective Genes:

    • Huntington’s Disease

Chromosomal Abnormalities

  • Characteristics include uneven numbers or abnormal consistency of chromosomes.

  • Result from the deletion, duplication, or relocation of whole sections of chromosomes.

Chromosomal Abnormalities: Down Syndrome (Trisomy 21)

  • Occurs when there is an extra copy of Chromosome 21, leading to developmental and intellectual disabilities.

Sex Chromosomal Abnormalities: Fragile X Syndrome

  • A genetic condition occurring due to a defect on the X chromosome, leading to developmental issues, particularly in males.

Female Chromosomal Abnormalities

  • Turner Syndrome (45, XO): A condition where one of the X chromosomes is completely or partially missing, affecting females.

  • Super-Female Syndrome (XXX, XXXX): Presence of one or more extra X chromosomes producing certain developmental and functional traits.

Male Chromosomal Abnormalities

  • Klinefelter Syndrome (XXY, XXXY): A condition resulting from an extra X chromosome in males, affecting physical and cognitive development.

  • Super Male Syndrome (XYY): A condition where males have an extra Y chromosome, which may influence physical development and behavior.

Causes of Chromosomal Abnormalities

  • Environmental Hazards: Exposure to certain environmental factors can lead to chromosomal abnormalities.

  • "Aging Ova" Hypothesis: Suggests that older maternal age may increase the risk of chromosomal abnormalities in offspring due to factors associated with aging ova.

Additional Context

Visit the links and articles referenced for further details, including case studies on twin studies and genetic abnormalities.