AP Biology Unit 5 – Heredity: Ultimate Study Notes (CED aligned)

Meiosis and Genetic Variation

Meiosis Overview

Purpose of Meiosis

• Produces haploid gametes (sperm and egg)

• Reduces chromosome number by half

• Creates genetic variation

• Cause → Effect

  • Meiosis → haploid cells → maintains chromosome number across generations

Meiosis I vs Meiosis II

Key Differences

• Meiosis I:

  • Homologous chromosomes separate

  • Crossing over occurs

• Meiosis II:

  • Sister chromatids separate

  • Similar to mitosis

• Cause → Effect

  • Separation of homologous chromosomes → genetic diversity

• Tips

  • Meiosis I = most important for variation

Sources of Genetic Variation

Crossing Over

• Exchange of DNA between homologous chromosomes

• Occurs in prophase I

Independent Assortment

• Random orientation of chromosomes in metaphase I

Random Fertilization

• Any sperm can fertilize any egg

• Cause → Effect

  • These processes → increased genetic diversity in offspring

• Tips

  • Always connect variation → survival and evolution

Mendelian Genetics

Basic Genetic Principles

Alleles and Traits

• Genes have different versions (alleles)

• Organisms inherit two alleles (one from each parent)

Dominant and Recessive

• Dominant allele masks recessive

• Recessive expressed only if homozygous

Predicting Inheritance

Punnett Squares

• Used to predict genotype and phenotype ratios

• Show probability of traits

Law of Segregation

• Alleles separate during gamete formation

Law of Independent Assortment

• Genes for different traits assort independently

• Cause → Effect

  • Allele separation → variation in offspring

• Tips

  • Always show work for crosses

  • Use ratios in answers

Non-Mendelian Genetics

Inheritance Patterns

Incomplete Dominance

• Heterozygote shows intermediate phenotype

Codominance

• Both alleles expressed equally

Multiple Alleles

• More than two alleles exist in population

Polygenic Traits

Multiple Gene Influence

• Many genes contribute to one trait

• Results in continuous variation

Sex-Linked Traits

X-Linked Inheritance

• Genes located on X chromosome

• Males more likely to express recessive traits

• Cause → Effect

  • Location of gene → affects inheritance pattern

• Tips

  • If more males affected → likely X-linked

Chromosomal Inheritance

Chromosomal Basis of Inheritance

Genes on Chromosomes

• Genes are located on chromosomes

• Behavior of chromosomes explains inheritance

Errors in Meiosis

Nondisjunction

• Failure of chromosomes to separate

• Results in abnormal chromosome number

• Cause → Effect

  • Nondisjunction → aneuploidy → genetic disorders

Linked Genes

Gene Linkage

• Genes close together on same chromosome

• Tend to be inherited together

• Cause → Effect

  • Linkage → alters expected ratios

• Tips

  • Crossing over can break linkage

Environmental Effects on Phenotype

Gene and Environment Interaction

Phenotypic Expression

• Environment can influence gene expression

• Same genotype → different phenotype under different conditions

• Cause → Effect

  • Environmental factors → changes in phenotype

• Tips

  • Always mention:

    • “phenotype is influenced by both genes and environment”