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Laws of Genetics

Monohybrid Crosses

Unit 11: Genetics & Genetic Engineering (RQF)

Lesson Objectives

  • **Learn the work of Gregor Mendel:
    • Understand how Mendel's research allows the prediction of offspring phenotypes.
    • Learn how to construct and use Punnett squares to predict phenotypes.
    • Understand the meaning and significance of "monohybrid" crosses.**

Gregor Mendel

Introduction

  • Father of Genetics (1822 – 1884)
    • Austrian monk and scientist.
    • Conducted experiments with common pea plants (Pisum sativum) in the 1850s.
    • Discovered that certain traits appear in offspring without blending characteristics from either parent.
Contributions
  • Paved the way for understanding how traits are inherited across generations.
  • Applied statistical analysis to his experiments, allowing him to predict genetic cross outcomes.
  • His findings established the foundational principles of modern genetics.

The Three Principles of Genetics

  1. Dominance
    • Certain characteristics are expressed over others.
  2. Segregation
    • Each parent contributes one allele (part of a gene pair) to the offspring.
  3. Independent Assortment
    • Allele pairs for different traits are passed on independently to offspring.

Key Terms in Genetics

Chromosomes
  • Diploid: Organism with two copies of each chromosome.
  • Genotype: Composed of two alleles from each chromosome.
  • Gene locus: Specific position (“address”) occupied by a gene on a chromosome.
  • Allele: Different versions of a gene at a locus.
  • Homologous chromosomes: Pairs of the same chromosome containing the same genes—one inherited from each parent.
Alleles
  • Mendel recognized two alternate forms of a trait (characteristic).
  • An individual possesses two alleles for each trait, which can be:
    • Homozygous: Same alleles (e.g., BB or bb).
    • Heterozygous: Different alleles (e.g., Bb).

Example - Eye Colour Trait

  • Two alleles:
    • Brown (B): Dominant allele.
    • Blue (b): Recessive allele.
Distribution of Alleles
  • Each gene has two copies, one from each parent.
    • Examples:
    • BB (homozygous brown)
    • bb (homozygous blue)
    • Bb (heterozygous brown with hidden blue allele)
Dominant & Recessive Alleles
  • Dominant Allele: Always expressed if present; represented by capital letters.
  • Recessive Allele: Only expressed if both alleles are recessive; represented by lowercase letters.
  • Example of representation:
    • Brown (B), Blue (b)

Genetic Representation

Expressing Genetic Traits
  • Use consistent letter choices for alleles to avoid confusion.
Examples of Natural Selection through Eye Colour
  1. Example 1:
    • Parent genotypes: BB (brown) × bb (blue)
    • Offspring genotype: Bb (brown eyes).
  2. Example 2:
    • Parent genotypes: BB × BB
    • Offspring genotype: BB (brown eyes).
  3. Example 3:
    • Parent genotypes: bb × bb
    • Offspring genotype: bb (blue eyes).
Definitions of Heterozygous and Homozygous
  • Heterozygous: Different alleles of a gene (e.g., Bb).
  • Homozygous: Same alleles of a gene (e.g., BB or bb).

Homologous Chromosomes

  • Homologous chromosomes have two copies (one from Mum and one from Dad), containing the same genes but possibly different alleles.

Monohybrid Crosses

  • A Punnett square visually represents the genetic combinations possible from a cross between parents.
  • Considers one trait, such as eye colour.
  • Each parent's genotype is separated to depict meiosis in reproducing gametes.

Punnett Squares

Construction of a Punnett Square
  • Structure:
    • The grid consists of four boxes representing possible offspring outcomes.
  • Filling the square:
    • Father's alleles are entered at the top; mother's alleles are entered along the left side.
Example Cross
  • Heterozygous Brown-eyed Father: Alleles = Bb
  • Homozygous Blue-eyed Mother: Alleles = bb
  • Resulting combinations include:
    • Bb (brown eyes)
    • bb (blue eyes)
Interpretation of Outcomes
  • Results outline the likelihood of phenotypes in offspring.
  • The F1 generation is the first filial generation from the cross.

Outcomes of the Cross

  • Results out of four children:
    • 2 will have heterozygous brown eyes (Bb)
    • 2 will have homozygous blue eyes (bb)
  • Phenotype: Observable traits of offspring.
  • Genotype: Genetic makeup of the offspring concerning the trait.
Mendel's Law of Segregation
  • Five key points:
    1. Each trait has two alternative states called alleles.
    2. Offspring receive one allele from each parent; thus, each has two alleles.
    3. Alleles have an equal chance of being passed to offspring.
    4. Alleles remain distinct and do not blend.
    5. There exists a dominant/recessive relationship.

Example Problem for Interpretation

  • What is the outcome of the offspring (genotype & phenotype)?

  • Phenotypic Ratio:

    • Brown eyes: 3
    • Blue eyes: 1

  • Genotypic Ratio:

    • Homozygous dominant (BB): 1
    • Heterozygous (Bb): 2
    • Homozygous recessive (bb): 1
Genetic Terminologies
  • Heterozygous: Different alleles of a gene.
  • Homozygous: Same alleles of a gene.
  • Recessive: Allele expressed only when both are the same.
  • Dominant: Always expressed allele.
  • Locus: Specific location of a gene on a chromosome.
  • Alleles: Different versions of a gene.
  • Phenotype: Physical appearance of an organism.
  • Genotype: Genetic composition of an organism.