Fundamentals of Genetics, Heredity, and Human Ecology

Posted on Mar 26, 2025 in Biology

Nucleotide Base Pairing in DNA

In DNA, the nucleotide bases pair specifically: Adenine (A) pairs with Thymine (T), and Guanine (G) pairs with Cytosine (C).

What Makes Individuals Unique?

Individuals are unique primarily due to variations in their alleles, which are different versions of the same gene.

Autosomes vs. Sex Chromosomes

Humans typically have 22 pairs of autosomes (homologous chromosomes that are not sex chromosomes) and one pair of sex chromosomes (XX for females, XY for males), totaling 46 chromosomes.

Total Human Genes

Humans have approximately 20,000 to 25,000 genes.

Gregor Mendel’s Discoveries

Gregor Mendel, often called the “father of genetics,” discovered the fundamental principles of heredity by studying how traits (phenotypes) were passed down. He conducted his foundational studies using pea plants.

Mendel’s Laws: Segregation & Assortment

  • Law of Segregation: During gamete (sperm or egg) formation, the two alleles for a heritable character separate (segregate) from each other, so that each gamete ends up carrying only one allele for that gene.
  • Law of Independent Assortment: When two or more characteristics are inherited, the alleles for different genes assort independently of one another during gamete production. This means the inheritance of one trait does not influence the inheritance of another, provided the genes are on different chromosomes or far apart on the same chromosome.

Persistence of Harmful Alleles

Harmful alleles can persist in a population through several mechanisms:

  • Some harmful alleles are recessive, meaning they only cause disease in individuals who inherit two copies (homozygous). Individuals with one copy (heterozygotes) are carriers but often show no symptoms.
  • These alleles can persist because heterozygotes survive and reproduce, passing the allele to their offspring.
  • In some cases, heterozygotes may even have a survival advantage in certain environments (heterozygote advantage), which helps maintain the harmful allele in the population (e.g., sickle cell trait providing malaria resistance).
  • The frequency of individuals homozygous for severely harmful recessive alleles may be kept low due to reduced survival or reproduction.

Sources of Genetic Variability in Siblings

Siblings from the same parents (except identical twins) exhibit genetic differences due to several key sources of genetic variability arising during sexual reproduction:

  • Independent Assortment: Alleles for genes located on different chromosomes are sorted into gametes independently of one another during meiosis.
  • Crossing Over (Recombination): During meiosis, homologous chromosomes exchange segments, creating new combinations of linked alleles on a single chromosome.
  • Random Fertilization: The combination of which specific sperm fertilizes which specific egg is random, leading to a vast number of potential genetic combinations in the offspring.

Incomplete Dominance vs. Codominance

Incomplete Dominance

Some alleles do not follow a simple dominant/recessive pattern. In incomplete dominance, the heterozygous genotype results in a phenotype that is an intermediate blend between the two homozygous phenotypes. For example, in snapdragons, crossing a red-flowered plant (RR) with a white-flowered plant (rr) results in pink-flowered offspring (Rr).

Codominance

In codominance, both alleles in a heterozygote are fully and equally expressed, resulting in a phenotype that displays both traits simultaneously, not a blend. A classic example is the ABO blood group system in humans, where alleles IA and IB are codominant, resulting in the AB blood type where both A and B antigens are present.

Genes and Cancer Development

Genes play a critical role in cancer development. Cancer often arises from an accumulation of mutations (changes in DNA) in genes that regulate cell growth, division, and death (such as proto-oncogenes, tumor suppressor genes, and DNA repair genes). When these control mechanisms fail due to mutations, cells can grow uncontrollably, forming tumors. Cancer develops when the body’s systems for repairing DNA damage or eliminating abnormal cells are overwhelmed or bypassed.

Environmental Effects on Genes (Epigenetics)

The environment can significantly influence how genes are expressed without altering the underlying DNA sequence itself—a field known as epigenetics. Environmental factors can lead to modifications (like methylation) that switch genes on or off, affecting traits and disease susceptibility. Examples include:

  • Diet: Nutrients can influence the expression of genes related to metabolism.
  • Toxins: Exposure to pollutants or chemicals can alter gene expression patterns, potentially increasing disease risk.
  • Lifestyle: Factors like physical activity, stress, and sleep can impact gene expression.
  • Environmental Pressures: External conditions can trigger adaptive changes in gene activity.

Primary Sources of Genetic Variation

The primary sources introducing or rearranging genetic variation within a population are:

  • Mutations: These are changes in the DNA sequence and represent the ultimate source of new alleles and genetic novelty.
  • Gene Flow (Migration): The movement of genes between populations through the migration of individuals or the dispersal of gametes introduces new alleles or alters existing allele frequencies.
  • Sexual Reproduction: This process shuffles existing alleles into new combinations through:
    • Crossing Over: Exchange of genetic material between homologous chromosomes during meiosis.
    • Independent Assortment: Random orientation and separation of chromosome pairs during meiosis.
    • Random Fertilization: The chance union of specific sperm and egg cells.

Chromosomal Conditions and Syndromes

Errors during meiosis, such as nondisjunction (failure of chromosomes to separate properly), can lead to gametes with an abnormal number of chromosomes, resulting in genetic conditions:

  • Trisomy 21 (Down Syndrome): Caused by having three copies of chromosome 21 instead of the usual two.
  • XYY Syndrome (Jacob Syndrome): Males with an extra Y chromosome (XYY). Often associated with tall stature; significant cognitive or behavioral issues are less common than once thought.
  • XXY Syndrome (Klinefelter Syndrome): Males with an extra X chromosome (XXY). Characteristics can include tall stature, infertility, and sometimes mild cognitive impairment or development of some female secondary sex characteristics.
  • XO Syndrome (Turner Syndrome): Females with only one X chromosome (XO). Characteristics typically include short stature, specific physical features, infertility, and potential heart or kidney problems.

Human Ecology vs. General Ecology

General Ecology studies the interactions between various organisms (plants, animals, microbes) and their physical and biological environment.

Human Ecology focuses specifically on the interactions between human populations and their environments (natural, social, built). It is distinct due to unique human factors:

  • Culture and Technology: Humans utilize complex tools, technology, language, and social systems that dramatically shape their environmental interactions.
  • Global Impact: Human activities, driven by technology and large populations, have significant and often global-scale impacts on ecosystems and resource use.
  • Waste Accumulation: The scale and type of waste generated by human societies pose unique environmental challenges.

Natural Resources: Types and Examples

Natural Resources are materials and energy sources found in the environment that humans use.

They are broadly categorized into two types:

  • Renewable Resources: Resources that can be replenished naturally at a rate comparable to human consumption. Examples include:
    • Solar energy
    • Wind energy
    • Water (though finite in specific locations/times)
    • Timber (if harvested sustainably)
    • Wildlife populations (if managed properly)
  • Nonrenewable Resources: Resources that exist in finite quantities and are consumed much faster than they are formed by geological processes. Examples include:
    • Fossil fuels (coal, oil, natural gas)
    • Minerals (iron, copper, aluminum)
    • Nuclear fuels (uranium)