Principles of Inheritance and Variation

9:3:3:1

Step 1: In a dihybrid cross, two traits are considered simultaneously. Here R = round (dominant), r = wrinkled (recessive), Y = yellow (dominant), y = green (recessive). Step 2: The F1 generation is RrYy (round yellow). Step 3: When F1 (RrYy × RrYy) is self-pollinated, each trait segregates independently as per the Law of Independent Assortment. Step 4: Round : Wrinkled = 3:1 and Yellow : Green = 3:1. Combining: (3 Round : 1 Wrinkled) × (3 Yellow : 1 Green) = 9 Round Yellow : 3 Wrinkled Yellow : 3 Round Green : 1 Wrinkled Green. Step 5: This gives the classic 9:3:3:1 ratio. The 3:1 ratio is fo

Sutton and Boveri

Step 1: After Mendel's work was rediscovered in 1900, scientists began linking gene behaviour to chromosome behaviour. Step 2: Walter Sutton (a graduate student) and Theodore Boveri (a German biologist) independently noticed that chromosomes behave similarly to Mendel's 'factors' (genes). Step 3: Both chromosomes and genes: occur in pairs, segregate during gamete formation, and assort independently. Step 4: Sutton united this knowledge into what is called the Chromosomal Theory of Inheritance. Step 5: Morgan and Sturtevant worked on linkage maps; de Vries and Correns rediscovered Mendel's work

It has a large number of chromosomes (46 pairs) making analysis easier

Step 1: Drosophila melanogaster is ideal for genetics due to several features. Step 2: It has a VERY SHORT life cycle (~2 weeks), allowing multiple generations to be studied quickly — this is a genuine advantage. Step 3: A single mating produces hundreds of offspring, giving large sample sizes for statistical analysis — this is a genuine advantage. Step 4: Males and females are easily distinguished externally — another genuine advantage. Step 5: However, Drosophila actually has ONLY 4 PAIRS (8) of chromosomes, NOT 46 pairs. 46 chromosomes is the number in HUMANS. Fewer chromosomes actually mak

The physical association of two or more genes on the same chromosome

Step 1: During Morgan's dihybrid crosses in Drosophila, he observed that some gene pairs did NOT segregate independently as Mendel's Law would predict. Step 2: This happened when two genes were located on the SAME chromosome — they tended to be inherited together. Step 3: Morgan coined the term LINKAGE to describe this physical association of genes located on the same chromosome. Step 4: Closely linked genes (physically near each other) show very low recombination, while loosely linked genes show higher recombination. Step 5: Crossing over (recombination) can separate linked genes — this is a