Lecture on Mendelian Genetics and Chromosomal Disorders
Mendel's Monohybrid Cross Experiments
Gregor Johann Mendel: Discovered the laws of heredity through hybridization experiments with true-breeding pea plants.
Monohybrid Cross: Crossed tall and dwarf pea plants.
P Generation: Tall (dominant) and dwarf (recessive) plants.
F1 Generation: All plants were tall; dwarf trait disappeared.
F2 Generation: Self-pollinated F1, resulting in a 3:1 ratio of tall to dwarf plants.
Conclusion: Inferred that traits are passed from parents to progeny, introducing the concepts of dominant and recessive traits.
Laws Derived from Experiments
Law of Dominance: In a monohybrid cross, the trait that appears in the F1 generation is dominant while the recessive trait reappears in the F2 in a 3:1 ratio.
Law of Segregation: Alleles segregate from each other during gamete formation, leading to a 50% probability of each allele in gametes.
Punnett Square: Predicts genotypes of progeny in genetic crosses.
Chromosomal Disorders
Down's Syndrome: Trisomy 21, causes facial and developmental abnormalities.
Klinefelter's Syndrome: Males with an extra X chromosome (XXY), leading to physical and fertility issues.
Turner's Syndrome: Females missing one X chromosome (XO), leading to sterility and developmental issues.
Aneuploidy: Gain or loss of chromosomes leading to disorders like trisomy (2n+1) or monosomy (2n-1).
Incomplete Dominance and Codominance
Incomplete Dominance: Neither allele is completely dominant, resulting in a blended phenotype (e.g., pink Snapdragon flowers).
Codominance: Both alleles are equally expressed in the phenotype (e.g., AB blood type).
Multiple Alleles: More than two alleles for a gene (e.g., blood type alleles Iá´¬, Iá´®, and i).
DiHybrid Cross Experiments
Example: Crossed pea plants with two different traits (e.g., seed shape and color).
Results: F2 generation showed a phenotypic ratio of 9:3:3:1.
Law of Independent Assortment: Genes for different traits segregate independently during gamete formation.
Chromosomal Theory of Inheritance
Rediscovery: Mendel's work gained recognition in 1900 through De Vries, Correns, and Tschermak.
Sutton and Boveri: Proposed that chromosomes carry hereditary units (genes) and segregate independently during meiosis.
Principles Combined: Called the chromosomal theory of inheritance, integrating Mendelian principles with chromosomal behavior.