Biology Notes – Genetics, Heritability & Pedigrees

1. Sexual Reproduction & Life Cycle

  • Adults (egg producer + sperm producer) → have germ cells (diploid, 2N = 46 chromosomes).

  • Germ cells undergo meiosis → produce gametes (egg & sperm).

    • Gametes are haploid (N = 23) → contain one copy of each chromosome.

  • Fertilization: egg + sperm fuse → diploid zygote (2N = 46).

  • Zygote → grows by mitosis + differentiation → develops into an adult.

2. Chromosomes & DNA

  • Chromosome = condensed form of DNA.

  • Gene = portion of DNA coding for a protein.

  • Allele = different versions of a gene.

  • Sister chromatids = identical copies of a chromosome (after replication).

  • Homologous pair = one chromosome from mom + one from dad (carry same genes, may have different alleles).

  • Karyotype = image of paired homologous chromosomes.

  • Autosomes = first 22 pairs (44 total).

  • Sex chromosomes = X and Y → determine biological sex.

    • Variations exist: XX, XY, XXX, X0, XXY, XYY, etc.

3. Genotype vs. Phenotype

  • Genotype = genetic makeup (alleles present).

  • Phenotype = physical expression of alleles.

  • Allele combinations:

    • Homozygous dominant (AA)

    • Heterozygous (Aa)

    • Homozygous recessive (aa)

  • Dominant allele = expressed with just one copy.

  • Recessive allele = expressed only if both copies are recessive.

  • Dominant ≠ most common (e.g., 5 fingers is recessive but common).

  • Wild type = most common (normal) trait.

  • Mutant = altered version.

4. Punnett Squares

  • Used to predict possible offspring genotypes/phenotypes.

  • Show genotypic ratio (allele combinations) & phenotypic ratio(traits shown).

  • Carrier = heterozygous for a recessive disease allele.

    • Example: cystic fibrosis.

5. Beyond Simple Dominance

Codominance

  • Both alleles are expressed at the same time.

  • Example: blood type (A, B, AB, O).

    • 3 alleles: A, B, O.

    • A and B are dominant over O.

    • AO = type A; BO = type B.

    • AB = both expressed (codominance).

    • OO = type O.

6. Sex-Linked Traits

  • Found on sex chromosomes (usually X).

  • X chromosome has more genes than Y.

  • Example: color blindness (X-linked recessive).

    • Females (XX): 3 genotypes (normal, carrier, affected).

    • Males (XY): 2 genotypes (normal or affected).

  • Males more likely to inherit X-linked recessive traits.

  • Passed mother → son.

7. Pedigrees

  • Used to track inheritance patterns in families.

  • Symbols:

    • ⬜ = male, ⚪ = female.

    • Shaded = affected.

    • Half-shaded = carrier (sometimes shown).

  • Generations marked with Roman numerals.

Patterns of Inheritance

  1. Autosomal Dominant

    • Equal males & females affected.

    • Appears in every generation.

    • Example: Huntington’s disease.

  2. Autosomal Recessive

    • Equal males & females affected.

    • Can skip generations.

    • Carriers pass recessive allele.

  3. X-Linked Recessive

    • More males than females affected.

    • Skips generations.

    • Passed mother → son.

    • Example: hemophilia, color blindness.