Genetics: Mendelism, Inheritance Patterns & Linkage

What is Genetics?

Genetics is the study of heredity—how traits are passed from parents to offspring. Gregor Mendel, a monk, discovered the basic rules of inheritance using pea plants in the 1800s.

Why Pea Plants?

Mendel chose pea plants because:

• They grow quickly.
• They have clear traits (e.g., tall vs. short, yellow vs. green seeds).
• They can self-pollinate or cross-pollinate easily.

Mendel’s Laws of Inheritance

1. Law of Dominance

• Some traits are dominant (always show up if present), while others are recessive (only show if no dominant gene is present).
• Example: In pea plants, Tall (T) is dominant over short (t).

classDiagram
    class Parent1 {
        +Genotype: TT (Tall)
    }
    class Parent2 {
        +Genotype: tt (Short)
    }
    class Offspring {
        +Genotype: Tt (All Tall)
    }
    Parent1 --> Offspring : Crossed with
    Parent2 --> Offspring : Crossed with

2. Law of Segregation

• Each parent passes one allele (gene variant) for a trait to their offspring.
• Example: A Tt pea plant can produce T or t gametes (sperm/pollen or egg).

pie
    title Gamete Formation in Tt
    "T (50%)" : 50
    "t (50%)" : 50

3. Law of Independent Assortment

• Genes for different traits are inherited independently (unless they are linked).
• Example: Seed shape (round/wrinkled) and seed color (yellow/green) are inherited separately.

Punnett Squares: Predicting Offspring Traits

A Punnett square helps predict possible genotypes of offspring.

Example 1: Monohybrid Cross (One Trait)

• Cross a heterozygous tall (Tt) pea plant with a short (tt) pea plant.

Result:

• 50% Tt (Tall)
• 50% tt (Short)

Example 2: Dihybrid Cross (Two Traits)

• Cross TtYy (Tall, Yellow) with TtYy (Tall, Yellow).

Result:

• 9/16 Tall & Yellow (TY, Ty, TY, Ty)
• 3/16 Tall & Green (Ty, Ty)
• 3/16 Short & Yellow (tY, tY)
• 1/16 Short & Green (ty)

Special Inheritance Patterns

1. Incomplete Dominance

• Neither allele is fully dominant.
• Example: Red (RR) + White (WW) flowers → Pink (RW) flowers.

classDiagram
    class Parent1 {
        +Genotype: RR (Red)
    }
    class Parent2 {
        +Genotype: WW (White)
    }
    class Offspring {
        +Genotype: RW (Pink)
    }
    Parent1 --> Offspring : Crossed with
    Parent2 --> Offspring : Crossed with

2. Codominance

• Both alleles show up equally.
• Example: AB blood type (IA and IB alleles both express).

3. Multiple Alleles

• More than two alleles control a trait.
• Example: Blood types (IA, IB, i).

4. Sex-Linked Traits (X and Y Chromosomes)

• Some genes are on the X chromosome (e.g., color blindness, hemophilia).
• Males (XY) only need one recessive allele to show the trait, while females (XX) need two.

classDiagram
    class X {
        +Color Blindness (Xc)
    }
    class Y {
        +No X-linked genes
    }
    class Female {
        +Genotype: XX
    }
    class Male {
        +Genotype: XY
    }
    Female --> X : Inherits from mother
    Male --> X : Inherits from mother
    Male --> Y : Inherits from father

Example: Color Blindness

• A carrier mother (XCXc) and a normal father (XCY) can have:
  • 25% Normal daughters (XCXC)
  • 25% Carrier daughters (XCXc)
  • 25% Normal sons (XCY)
  • 25% Color-blind sons (XcY)

Linkage and Crossing Over

• Linked genes are located close together on the same chromosome and do not assort independently.
• Crossing over during meiosis can separate linked genes.

Example: Linked Genes in Fruit Flies

• Gray body (B) and Normal wings (V) are linked.
• A BVBV fly crossed with bvbv produces mostly BVBv and bvbv offspring, not a 9:3:3:1 ratio.

Solved NEB-Style Problems

Problem 1: Monohybrid Cross

A tall pea plant (Tt) is crossed with a short pea plant (tt). What is the probability of getting a short plant? Solution:

• Tt × tt → 50% Tt (Tall), 50% tt (Short)
• Answer: 50% chance of a short plant.

Problem 2: Dihybrid Cross

A plant with genotype TtYy is self-pollinated. What is the probability of getting a plant with tall stems and green seeds? Solution:

• TtYy × TtYy → 9:3:3:1 ratio
• Tall & Green = Ty or Ty → 3/16
• Answer: 3/16 (18.75%) chance.

Problem 3: Sex-Linked Trait

A color-blind mother (XcXc) and a normal father (XCY) have a son. What is the probability that the son is color-blind? Solution:

• XcXc × XCY → All sons get XcY (color-blind).
• Answer: 100% chance.

Exam Tip

1. Memorize Punnett squares for monohybrid and dihybrid crosses.
2. Understand dominance vs. recessiveness—know which traits are dominant in pea plants.
3. Practice sex-linked problems—males are more likely to show recessive X-linked traits.
4. Watch for linkage—if genes are linked, they won’t follow independent assortment.
5. NEB often tests blood types and pedigree charts—practice drawing them!

Summary Table: Inheritance Patterns

Final Note:

• Mendel’s laws explain how traits pass from parents to offspring.
• Punnett squares help predict offspring genotypes.
• Sex-linked traits are more common in males.
• Linkage affects inheritance when genes are close on a chromosome.

Practice: Draw a Punnett square for a dihybrid cross (RrYy × RrYy) and find the probability of round and yellow seeds (assuming round and yellow are dominant).