Introduction to Mendelian Genetics

Basics of Human Genetics

• Human cells contain 46 chromosomes.
  • 23 chromosomes from the father.
  • 23 chromosomes from the mother.
• Chromosomes contain DNA, making each cell unique.
• Humans have 23 pairs of chromosomes, each having a maternal and paternal copy.

Key Terms

Allele

• A small section on a chromosome that codes for a specific gene.
• Humans typically have at least two alleles for every specific gene (one from each parent).

Blood Type Example

• Blood types (A, B, O) are coded by specific alleles.
• Example:
  • Male with alleles AA = Blood type A (homozygous)
  • Female with alleles AO = Blood type A (heterozygous)

Dominant and Recessive Alleles

• Dominant alleles (e.g., A) mask recessive alleles (e.g., O).
• Heterozygous individuals (different alleles) express the dominant trait.

Genotype vs. Phenotype

• Genotype: The genetic makeup (e.g., AA, AO).
• Phenotype: The physical expression (e.g., Blood type A).
• Different genotypes can result in the same phenotype due to dominant alleles.

Gene Inheritance

• Punnett Square:
  • A tool to predict the genotypes of offspring from parental alleles.
  • Each parent contributes one allele per gene.
  • Example cross:
    • Father (AA) x Mother (AO): 50% offspring AA, 50% AO ➔ phenotype: Blood type A
    • Father (AO) x Mother (AO): 25% AA, 50% AO, 25% OO ➔ 75% phenotype A, 25% phenotype O

Conclusion

• Definitions of allele, homozygous, heterozygous.
• Understanding dominant vs. recessive traits.
• Distinguishing genotype from phenotype.
• Application of Punnett Squares in predicting inheritance patterns.