AP Biology Crash Course: Unit 5 - Heredity

Introduction

• Topic: Meiosis and Mendel
• Focus on heredity and how genetic information is passed from parents to offspring.

Mitosis Recap

• Mitosis: Process where a cell divides to form two identical daughter cells.
• Used for growth and repair.

Meiosis Overview

• Purpose: To produce gametes (egg and sperm) with half the number of chromosomes (23 each).
• Fertilization: Fusion of egg and sperm to form a zygote with a full set of chromosomes (46).

Meiosis Process

• Two divisions: Each division reduces chromosome number.
• Start: Full set of chromosomes (46).
• End: Four non-identical daughter cells each with 23 chromosomes.
• Key Differences from Mitosis:
  • Involves two divisions.
  • Leads to genetic diversity.

Stages of Meiosis

Meiosis I

• Prophase I: Chromosomes condense; crossing over occurs.
  • Crossing Over: Exchange of genetic material between homologous chromosomes.
  • Generates genetic diversity.
  • Terms: Chiasmata (crossing over sites), Synaptonemal complex.
• Metaphase I: Homologous chromosomes align at cell center.
• Anaphase I: Homologous chromosomes separate.
• Telophase I and Cytokinesis: Cells split, resulting in two cells with half the number of chromosomes.

Meiosis II

• Prophase II: Chromosomes re-condense.
• Metaphase II: Chromosomes align at center.
• Anaphase II: Sister chromatids separate.
• Telophase II and Cytokinesis: Final split, resulting in four haploid cells.

Mendelian Genetics

• Gregor Mendel: Discovered principles of heredity using pea plants.
• Alleles: Different versions of a gene.
• Genotype vs. Phenotype:
  • Genotype: Genetic makeup (e.g., RR, Rr).
  • Phenotype: Physical expression (e.g., hair color).

Dominance

• Dominant Alleles (e.g., R): Masks the effect of a recessive allele.
• Recessive Alleles (e.g., r): Only expressed if both alleles are recessive.
• Homozygous: Two identical alleles (e.g., RR or rr).
• Heterozygous: Two different alleles (e.g., Rr).

Punnett Squares

• Used to predict the probability of offspring inheriting particular traits.
• Example calculations for monohybrid and dihybrid crosses.

Pedigrees

• Symbols: Circle for female, square for male.
• Used to trace inheritance patterns through a family.

Mendel's Laws

• Law of Segregation: Two alleles for a gene separate during gamete formation.
• Law of Independent Assortment: Genes for different traits segregate independently.

Exceptions to Mendel's Laws

• Incomplete Dominance: Heterozygous phenotype is intermediate between the two homozygous types.
• Codominance: Both alleles are fully expressed (e.g., AB blood type).
• Multiple Alleles: More than two forms of a gene (e.g., blood types A, B, O).
• Pleiotropy: One gene influences multiple traits.
• Polygenic Inheritance: Multiple genes influence a single trait.
• Linked Genes: Genes located close together on the same chromosome.
  • Sex-linked Genes: Genes located on sex chromosomes (X or Y).

Recombination Frequency

• Measure of genetic linkage.
• Percentage of offspring with new combinations of traits.
• Used to map gene locations.

Conclusion

• Meiosis and Mendelian genetics form the basis of genetic inheritance.
• Understanding these processes aids in comprehending genetic diversity and inheritance patterns.

Next Steps

• Review the process of meiosis and Mendelian genetics regularly.
• Practice drawing and interpreting Punnett squares and pedigrees.
• Understand the exceptions to Mendel's laws for a comprehensive grasp of genetics.