AP Biology Unit 5 – Heredity: Ultimate Study Notes

Mendelian Genetics

Key Ideas

  • Traits are passed via genes

  • Genes have different forms called alleles

  • You inherit one allele from each parent

  • Homozygous vs Heterozygous

    • Homozygous: two identical alleles (AA or aa)

    • Heterozygous: two different alleles (Aa)

  • Allele terminology

    • Dominant: masks expression of another allele

    • Recessive: masked by dominant allele

  • Ploidy reminder

    • Haploid: one copy of each chromosome (gametes)

    • Diploid: two copies (somatic cells)

Gregor Mendel

  • Used pea plants

  • Discovered inheritance patterns

Laws of Inheritance

Law of Segregation

  • Two alleles separate during gamete formation

  • Each gamete gets ONE allele

Law of Independent Assortment

  • Genes for different traits separate independently

    • (only true if genes are on different chromosomes or far apart)

  • Testcross

    • Cross unknown genotype with homozygous recessive to determine genotype

    • Example: Aa × aa → 50% offspring show recessive → unknown parent is heterozygous

  • Clarify Independent Assortment

    • Independent assortment applies only if genes are on different chromosomes or far apart on same chromosome

    • Example: Gene A on Chromosome 1, Gene B on Chromosome 2 → assort independently

Punnett Squares & Probability

Pedigree basics

  • Squares = males, circles = females

  • Shaded = affected, unshaded = unaffected

  • Helps determine autosomal vs sex-linked, dominant vs recessive

Monohybrid Cross

  • One trait (e.g., Aa × Aa)

  • Genotype ratio: 1 AA : 2 Aa : 1 aa

  • Phenotype ratio: 3:1 (dominant:recessive)

Dihybrid Cross

  • Two traits (AaBb × AaBb)

  • Phenotype ratio: 9:3:3:1

Probability Rules

  • Multiplication Rule: AND → multiply

  • Addition Rule: OR → add

Non-Mendelian Inheritance

Polygenic Inheritance

  • Trait controlled by multiple genes → continuous variation

  • Example: human height or skin color

Environmental Effects

  • Phenotype may be influenced by environment

  • Example: temperature-sensitive alleles, nutrition affecting height

Incomplete Dominance

  • Heterozygote = blended phenotype

  • Example: red + white → pink

Codominance

  • Both alleles expressed fully

  • Example: AB blood type

Multiple Alleles

  • More than 2 allele options

  • Example: ABO blood system

Chromosomal Basis of Inheritance

Chromosomal Theory of Inheritance

  • Genes are located on chromosomes

  • Explains Mendel’s laws physically

Thomas Hunt Morgan

  • Used fruit flies

  • Discovered sex-linked traits

Sex-Linked Inheritance

X-Linked Traits

  • Found on X chromosome

  • Males (XY) more affected (only one X)

  • Females can be carries, not affected, affected

  • Carrier females

    • Heterozygous females may carry trait without expressing it

    • Can pass X-linked recessive trait to son

  • Example Punnett square for X-linked cross

    • Mother: XᴬXᵃ (carrier) × Father: XᴬY → 25% daughters affected, 25% sons affected, etc.

Examples

  • Color blindness

  • Hemophilia

Key Tip

  • Males inherit X-linked traits from mother

Linked Genes & Recombination

  • Y-linked Traits

    • Only on Y chromosome → passed father → son

  • Sex-influenced & sex-limited traits

    • Sex-influenced: trait more easily expressed in one sex (e.g., male pattern baldness)

    • Sex-limited: trait occurs only in one sex (e.g., milk production in mammals)

  • Epistasis

    • One gene masks expression of another gene

    • Example: coat color in Labrador retrievers

  • Crossing over details

    • Occurs during prophase I of meiosis

    • Increases genetic variation

Linked Genes

  • Genes close together on same chromosome

  • Tend to be inherited together

Crossing Over

  • Exchange of DNA during meiosis I

  • Creates genetic variation

Recombination Frequency

  • Used to map gene distance

  • 1% recombination = 1 map unit

  • Map distance formula

    • Map distance (cM) = (# recombinant offspring ÷ total offspring) × 100

  • Double crossovers

    • Can occur → affect mapping if not detected

Big Picture Connections

  • Link to Meiosis

    • Segregation = separation of homologous chromosomes in anaphase I

    • Independent assortment = random alignment of chromosomes during metaphase I

  • Genetic Variation

    • Crossing over + independent assortment + random fertilization → variation

  • Genotype → Phenotype Mapping

    • Multiple mechanisms affect phenotype: dominance, codominance, incomplete dominance, polygenic inheritance, environmental effects