Inheritance of Two Genes (Dihybrid Cross) 🌱

1. The classic pea experiment

  • Parents
    • Yellow-round seeds (RRYY) ✨
    • Green-wrinkled seeds (rryy) 🍃
  • Gametes formed: RY and ry. Fertilisation made an F1 hybrid RrYy, and every F1 seed looked yellow + round. :contentReference[oaicite:0]{index=0}
  • When Mendel self-pollinated F1 plants, four kinds of F2 seeds appeared in the famous 9 : 3 : 3 : 1 ratio. :contentReference[oaicite:1]{index=1}

How the 9 : 3 : 3 : 1 ratio arises 🎲

\[ (3\text{ Round} : 1\text{ Wrinkled})\; (3\text{ Yellow} : 1\text{ Green}) \;=\; 9\text{ Round Yellow} : 3\text{ Wrinkled Yellow} : 3\text{ Round Green} : 1\text{ Wrinkled Green} \] :contentReference[oaicite:2]{index=2}

A Punnett square shows four equally common gametes—RY, Ry, rY, ry—coming from every F1 plant. 💡 Each appears with a probability of \( \frac{1}{4} \). :contentReference[oaicite:3]{index=3}

2. Law of Independent Assortment 📊

Statement: “When two pairs of traits combine in a hybrid, segregation of one pair occurs independently of the other pair.” :contentReference[oaicite:4]{index=4} In other words, whether a gamete gets R or r has no effect on whether it gets Y or y.

3. Chromosomes explain the law 🧬

  • Sutton & Boveri (1902) noticed that chromosomes behave just like Mendel’s “factors” (genes): they occur in pairs, separate during gamete formation, and different pairs act independently. :contentReference[oaicite:5]{index=5}
  • During anaphase I of meiosis, two chromosome pairs can line up in either orientation:
    Possibility I 🟠🍃 | Possibility II 🟠🔴. This random orientation gives independent assortment at the chromosomal level. :contentReference[oaicite:6]{index=6}

Quick comparison: genes vs. chromosomes 🤝

  • Both occur in pairs.
  • Both segregate so each gamete gets only one from every pair.
  • Each pair segregates independently of other pairs. :contentReference[oaicite:7]{index=7}

4. Linkage and Recombination 🚦🔀

  • Thomas Hunt Morgan studied dihybrid crosses in fruit fly Drosophila melanogaster. He saw huge numbers of parental-type offspring when both genes lay on the same chromosome. :contentReference[oaicite:8]{index=8}
  • Linkage = physical closeness of genes on one chromosome, keeping parental combinations together.
  • Recombination = appearance of non-parental gene combos caused by crossing-over.
  • Strength of linkage varies:
    • white & yellow eyes: only 1.3 % recombination (very tight) 😮
    • white & miniature wing: 37.2 % recombination (loose). :contentReference[oaicite:9]{index=9}
  • Alfred Sturtevant used recombination frequency to create the first genetic maps—longer map distance ⇒ higher % recombinants. :contentReference[oaicite:10]{index=10}

5. High-yield NEET nuggets ✨

  1. 9 : 3 : 3 : 1 dihybrid phenotypic ratio and how to derive it quickly.
  2. Law of Independent Assortment: genes on different chromosome pairs segregate independently.
  3. Chromosomal Theory of Inheritance: Sutton & Boveri matched chromosome movements to Mendel’s rules.
  4. Linkage vs. Recombination: tight linkage suppresses crossovers; recombination frequency measures gene distance.
  5. Genetic mapping: Sturtevant’s insight that 1 % recombination ≈ 1 map unit helps locate genes quickly.

Keep practising Punnett squares, and you’ll ace these questions! 🚀