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Choice of Experimental Organism
• Pea Plants (Pisum sativum): Mendel chose pea plants because they have
distinct, heritable traits, can be easily manipulated in breeding
experiments, and have a relatively short generation time.
Mendel's Techniques
• Controlled Cross-Pollination: Mendel manually cross-pollinated plants to
control parentage and track traits through generations.
• True-Breeding Lines: Mendel used plants that, when self-pollinated,
consistently produced offspring with the same traits.
Key Principles from Mendel's Work
Monohybrid Crosses
• Single Trait Crosses: Mendel studied one trait at a time (e.g., flower color)
by crossing true-breeding plants with different traits.
• F1 and F2 Generations: Crossed plants with contrasting traits (e.g., purple
and white flowers), observed the F1 generation (all purple), and then self-
pollinated F1 to produce the F2 generation, showing a 3:1 ratio of traits.
Law of Segregation
• Alleles and Gametes: Each individual has two alleles for each gene, which
segregate during the formation of gametes. Thus, each gamete carries only
one allele.
• Dominant and Recessive Alleles: Dominant alleles mask the expression of
recessive alleles in heterozygotes.
, Dihybrid Crosses and Independent Assortment
Two-Trait Crosses
• Studying Two Traits Simultaneously: Mendel crossed plants differing in two
traits (e.g., seed shape and seed color) to see if traits are inherited
independently.
• 9:3:3:1 Ratio: Observed in the F2 generation, indicating that alleles of
different genes assort independently during gamete formation.
Law of Independent Assortment
• Independent Assortment: Alleles of different genes segregate
independently of each other during gamete formation, leading to genetic
variation.
Extensions of Mendelian Genetics
Incomplete Dominance
• Intermediate Phenotype: Heterozygotes show a phenotype that is
intermediate between the two homozygous phenotypes (e.g., red and
white snapdragon flowers producing pink offspring).
Codominance
• Both Alleles Expressed: In heterozygotes, both alleles are fully expressed
(e.g., AB blood type where both A and B alleles are expressed).
Multiple Alleles
• More than Two Alleles: For some genes, more than two alleles exist within
a population (e.g., ABO blood group system with three alleles: I^A, I^B, i).
Pleiotropy
• One Gene, Multiple Effects: A single gene influences multiple phenotypic
traits (e.g., sickle cell disease affecting red blood cells, resistance to malaria,
etc.).
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