Genetics Fundamentals: Mendel to Modern Concepts
Mendel’s Pea Plant Experiments
Mendel chose pea plants because they were:
- Fast-growing plants
- Found in a large number of varieties
- Flowers can be easily pollinated artificially
- Study features are found in pairs, not intermediates (e.g., yellow or green peas, no blending)
Key Genetic Terms
Dominant Character:
- Appears in the first generation when crossing two pure species. Notated with capital letters.
- Determined by a dominant gene, which is always expressed when present.
Recessive Character:
- Traits that “skip” a generation, observed only when the dominant character is absent. Notated with lowercase letters.
- Less expressed traits (e.g., blue eyes, light hair) that can reappear, though less frequently.
Genotype:
- Not directly observable, passed from one generation to the next.
- Biological characteristics resulting from the union of egg and sperm.
Phenotype:
- The appearance of an organism, all that is visible.
- Features presented by an individual (their appearance).
Chromosomes:
- Structures in the cell nucleus that transmit inheritance, made of DNA.
- Thread-like structures consisting of chromatin, visible during cell division. Humans have 46 chromosomes in pairs.
Homozygous:
- Genetically pure plants.
- Having two identical alleles for a gene (e.g., AA or aa).
Heterozygous:
- Resulting from crossing two homozygous plants.
- Having a combination of alleles (e.g., Aa).
Codominance:
Process where an individual expresses two dominant genetic characteristics.
Contributions to Genetics
Mendel (Pisum Sativum):
Noted that characteristics are transmitted from generation to generation. Proposed that particles (genes) are inherited.
Walter Sutton (Grasshopper Chromosomes):
Observed that chromosomes come in pairs. Proposed that genes are located on chromosomes.
Thomas Morgan (Drosophila Melanogaster):
Studied sex-linked characteristics. Proposed that sex-linked characteristics are determined by genes on the X chromosome.
Drosophila Characteristics
- Giant chromosomes in salivary glands
- Females are larger than males
- Female abdomen is pointed, male abdomen is oval
- Small size, rapid life cycle, large number of offspring
Cell Division
Chromosome Number:
Species-specific number of chromosomes.
Somatic Cells:
Form tissues and organs, originating from stem cells during embryonic development.
Sex Cells (Gametes):
Haploid cells (eggs and sperm).
Morgan’s Cross:
Mutations linked to sex are recessive and appear mostly in males.
Sex Determination:
Determined by chromosomes. XX for females, XY for males.
Meiosis
Division of a diploid cell (2n) into four haploid cells.
First Meiotic Division:
Prophase I: Chromosomes condense, homologous chromosomes synapse, crossing-over occurs.
Metaphase I: Tetrads align at the metaphase plate.
Anaphase I: Homologous chromosomes separate.
Telophase I: Nuclear membrane reforms, cells separate.
Second Meiotic Division:
Similar to mitosis.
Crossing-Over:
Exchange of genetic material between chromosomes, increasing genetic variation.
Gametogenesis
Spermatogenesis (in testes):
Proliferation: Spermatogonia (2n) form.
Growth: Spermatogonia form primary spermatocytes (2n).
Maturation: Meiosis produces spermatids.
Oogenesis (in ovaries):
Proliferation: Oogonia form during embryonic development.
Growth: Oocytes grow at puberty.
Maturation: Oocyte undergoes meiosis.
Mitosis
Process by which a somatic cell (2n) divides into two somatic cells (2n).
Phases:
Prophase: Chromosomes condense, centrioles form asters.
Metaphase: Chromosomes align at the metaphase plate.
Anaphase: Sister chromatids separate and move to opposite poles.
Telophase: Nuclear membrane reforms, chromosomes decondense.
Karyokinesis:
Maintenance of genetic heritage.
Cytokinesis:
Physical separation of cytoplasm.
Cell Types
Mitosis:
Diploid cell division.
Meiosis:
Diploid cell division resulting in four haploid cells.
Reproductive Cells (Gametes):
Have half the genetic material (23 chromosomes in humans).
Somatic Cells:
Make up the body, contain the full genetic load (46 chromosomes in humans).
Cell Cycle:
Time from one cell division to the next.
Synapse:
Junctions for nerve cell signaling.