Revised Ch 8 Punnett Sq

Chapter 8: Path of Inheritance

Learning Objectives

  • Understand how traits are inherited from parent organisms.

  • Distinguish between genotype and phenotype.

  • Explain the concepts of dominance, recessiveness, and heterozygosity.

  • Apply Mendel’s laws of inheritance using Punnett squares.

  • Differentiate between monohybrid and dihybrid crosses.

  • Describe Mendel’s Laws: Law of Segregation and Law of Independent Assortment.

1. Introduction to Inheritance

  • Inheritance refers to the passing of genetic traits from parents to offspring.

  • All organisms receive their genetic material (DNA) from their parents.

  • Although offspring may resemble their parents, they possess unique DNA combinations.

Asexual vs. Sexual Reproduction

  • Asexual reproduction produces offspring genetically identical to the parent (clones).
    → Results in continuous variation — traits remain constant through generations.

  • Sexual reproduction mixes genes from two parents.
    → Results in discontinuous variation — offspring display unique combinations of traits.

Example:

Two siblings from the same parents can look very different due to the unique combination of alleles they inherit.

2. Mendel and the Foundation of Genetics

  • Gregor Mendel (1822–1884), an Austrian monk, is known as the Father of Genetics.

  • He studied pea plants (Pisum sativum) because they showed clear, contrasting traits and reproduced quickly.

Traits Mendel Studied in Peas

  • Seed color: Yellow or Green

  • Seed texture: Smooth (Round) or Wrinkled

  • Flower color: Purple or White

  • Plant height: Tall or Short

Each trait existed in two distinct forms — ideal for studying inheritance patterns.

3. Key Genetic Terms

Term

Definition

Example

Trait

A characteristic that can vary (e.g., flower color).

Purple vs. white flowers

Gene

A segment of DNA that codes for a trait.

Gene for flower color

Allele

Different versions of a gene.

Purple (P) or white (p)

Dominant allele

Expressed when present (uppercase letter).

P (purple)

Recessive allele

Masked by the dominant allele (lowercase letter).

p (white)

Genotype

The genetic makeup (allele combination).

PP, Pp, or pp

Phenotype

The physical appearance or expression.

Purple or white flowers

Homozygous

Two identical alleles.

PP (dominant) or pp (recessive)

Heterozygous

Two different alleles.

Pp

⸻

4. Mendel’s Experiments

Generations

1. P (Parental) Generation – purebred parents (homozygous).

• Example: Purple (PP) × White (pp)

2. F₁ (First Filial) Generation – offspring of P.

• All plants: Pp → Purple (dominant trait expressed).

3. F₂ (Second Filial) Generation – offspring from F₁ × F₁.

• Genotypic ratio: 1 PP : 2 Pp : 1 pp

• Phenotypic ratio: 3 Purple : 1 White

⸻

5. Punnett Squares and Monohybrid Crosses

Example: Pea Color

Parental Cross:

• Yellow (YY) × Green (yy)

F₁ Generation:

• All offspring: Yy (heterozygous)

• Phenotype: All yellow

• Genotype: All Yy

F₂ Generation:

Cross F₁ × F₁ (Yy × Yy)

Y

y

Y

YY

Yy

y

Yy

yy

• Genotypic ratio: 1 YY : 2 Yy : 1 yy

• Phenotypic ratio: 3 Yellow : 1 Green

Interpretation: Even though two yellow parents (Yy × Yy) can produce a green offspring, the probability is 25%.

6. Human Example: Eye Color (Simplified)

B = Brown (dominant)

b = Blue (recessive)

  • P Generation: BB (brown) × bb (blue)

  • F₁ Generation: All Bb (brown eyes)

  • F₂ Generation: Bb × Bb → 1 BB : 2 Bb : 1 bb

    • Phenotypes: 3 brown : 1 blue

Even if both parents have brown eyes (Bb), there’s a 25% chance of having a blue-eyed child.

7. Mendel’s Laws

Law of Segregation

  • Each organism has two alleles for each trait.

  • During meiosis, these alleles separate so that each gamete carries only one allele.

  • Fertilization restores the pair (one from each parent).

  • Example: A heterozygous (Aa) individual produces gametes with 50% A and 50% a.

Law of Independent Assortment

  • Genes for different traits segregate independently during gamete formation.

  • Inheritance of one trait (like color) does not affect inheritance of another (like shape).

  • Example: Eye color genes assort independently from hair color genes.

8. Dihybrid Crosses

A dihybrid cross studies two traits simultaneously.

Example: Pea Color and Texture

  • Y = Yellow (dominant)

  • y = Green (recessive)

  • R = Round (dominant)

  • r = Wrinkled (recessive)

P Generation:

YYRR (yellow, round) × yyrr (green, wrinkled)

F₁ Generation:

All YyRr (yellow, round)

F₂ Cross: YyRr × YyRr

→ Produces 16 combinations with the classic 9:3:3:1 phenotypic ratio:

Trait Combination

Genotype Example

Probability

Yellow, Round

Y_R_

9/16

Yellow, Wrinkled

Y_rr

3/16

Green, Round

yyR_

3/16

Green, Wrinkled

yyrr

1/16

Meaning:

  • 9 show both dominant traits.

  • 3 show dominant color, recessive texture.

  • 3 show recessive color, dominant texture.

  • 1 shows both recessive traits.

9. Summary of Key Ratios

Cross Type

Genotypic Ratio

Phenotypic Ratio

Monohybrid (heterozygous × heterozygous)

1 : 2 : 1

3 : 1

Dihybrid (heterozygous × heterozygous)

—

9 : 3 : 3 : 1

10. Study Tips

  • Always identify dominant and recessive alleles before setting up a Punnett square.

  • Label P, F₁, and F₂ generations clearly.

  • Distinguish between genotype (letters) and phenotype (appearance).

  • Practice setting up both monohybrid and dihybrid crosses.

  • Review Mendel’s two laws until you can explain them without notes.