Inheritance - Topic 16

Introduction

• Focus on inheritance, the passage of information from parents to offspring.
• Key terms:
  • Haploid (n): One set of chromosomes in cells.
  • Diploid (2n): Two sets of chromosomes.
  • Homologous pairs of chromosomes: Contain the same genes in the same positions.

Meiosis

• Type of Cell Division: Known as reduction division, resulting in gametes with half the number of chromosomes.
• Importance: Ensures zygotes have the correct diploid number post-fertilization.
• Stages:
  • Meiosis I: Introduction of genetic diversity through independent assortment and crossing over.
    • Independent Assortment: Random alignment of maternal/paternal chromosomes.
    • Crossing Over: Exchange of chromosome segments, leading to new allele combinations.
  • Meiosis II: Further division to produce four genetically diverse haploid cells.
  • Stages include prophase, metaphase, anaphase, telophase, and cytokinesis.
  • Genetic Variation: Over 8 million combinations possible due to independent assortment (2^23 for humans).

Genetic Terms

• Genotype: Genetic constitution of an organism (alleles possessed).
• Phenotype: Expression of genes and interaction with the environment.
• Homozygous vs. Heterozygous:
  • Homozygous: Two identical alleles for a gene.
  • Heterozygous: Two different alleles for a gene.
• Dominance in Alleles:
  • Recessive: Only expressed if no dominant allele is present.
  • Dominant: Always expressed if present.
  • Codominant: Both alleles equally dominant, both expressed.
• Multiple Alleles: More than two possible alleles for a gene.
• Sex Linkage: Gene located on a sex chromosome.
• Autosomal Linkage: Genes located on the same non-sex chromosome.
• Epistasis: One gene masks/modifies the expression of another.
• Monohybrid Inheritance: Inheritance of a single gene.
• Dihybrid Inheritance: Inheritance of two genes.

Genetic Crosses

• Monohybrid Cross Example: Cystic fibrosis, a recessive allele.
  • Probability calculations using Punnett Squares.
• Codominance Example: Cattle color; red and white alleles.
• Blood Groups: Example of codominance and multiple alleles.
  • Alleles: A, B (dominant), O (recessive).

Sex Linkage and Epistasis

• Example: Color blindness, a recessive allele on the X chromosome.
• Epistasis Example: Coat color in Labradors.
  • Gene 1 determines pigment production.
  • Gene 2 determines pigment color.

Dihybrid Crosses

• Involves two genes; common in Mendelian studies.
• Expected Ratios: 9:3:3:1 for heterozygous parents unless influenced by crossing over or genetic linkage.

Autosomal Linkage

• Genes on the same chromosome can reduce the variation in gametes.
• Crossing over can still introduce new combinations.
• Example Explanation: Variation from expected ratios indicates crossing over.

Chi-Square Test

• Used to compare expected vs. observed frequencies.
• Null Hypothesis: No significant difference between observed and expected data.

Test Crosses

• Determine genotype of individuals expressing dominant phenotypes.
• Cross with homozygous recessive to reveal heterozygous or homozygous dominant genotype.

Conclusion

• Study inheritance through genetic diagrams and statistical tests.
• Understand variations caused by linkage and crossing over.