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Study Notes on Monohybrid Cross and Principles of Segregation

Monohybrid Cross & the Principle of Segregation

Overview

  • Presenter: James Cook University, Australia
  • Course: BS1001: Introduction to Biological Processes

Mendel's Laws of Heredity

  1. Principle of Segregation
  2. Principle of Independent Assortment

Monohybrid Cross

Definition

  • A monohybrid cross is performed between two parents who are heterozygous for a single trait, denoted as (F1 x F1).
  • This cross helps determine the relationship of dominance between two alleles for a single gene.

P Generation

  • Requires starting with a pure breeding parent (P generation) to obtain hybrids.

Punnett Square

  • A 2 x 2 Punnett square can be utilized to predict possible genotypes and phenotypes of the offspring.
  • Individual squares show potential combinations of gametes.
  • The principle illustrated is that only one allele for each gene is transmitted from each parent, demonstrating the law of segregation.

Law of Segregation

Statement

  • Mendel’s Law of Segregation states that a parent passes one randomly selected copy of any given gene to its offspring.
  • Example: A Yy individual can pass on either allele Y or y.

Mechanism

  • This halving of genetic information occurs during meiosis I when homologous chromosomes separate.
  • For instance, a Yy parent produces two types of gametes: Y or y.
    Additionally, a yy parent produces identical gametes, both containing y.

Summary

  • Alleles segregate randomly into gametes during reproduction.

Intended Learning Outcomes

  • Understand Mendel's principles of segregation and independent assortment.
  • Solve genetics problems involving monohybrid, dihybrid, and test crosses.
  • Apply the product rule and sum rule to predict outcomes of genetic crosses.

Introduction to Drosophila melanogaster

  • Commonly known as the fruit fly or vinegar fly.
  • Scientific name: Drosophila melanogaster, meaning ‘black-bellied dew lover’.
  • Established by Thomas Hunt Morgan as a model organism for genetic studies.
  • Notable characteristics:
    • Only 4 pairs of chromosomes.
    • Easily recognizable mutations.
    • Simple to care for and cost-effective.
    • Quick reproduction cycle.
  • Confirmed the chromosomal theory of inheritance: genes are carried on chromosomes, with some genes linked together.
  • Genome sequenced in 2000, confirming 4 chromosomes.

Worked Example: Monohybrid Cross (D. melanogaster)

Scenario

  • P (Parent) Generation: Wild type (++) x Ebony (ee)
  • Phenotypes: Wild type and Ebony
  • Genotypes: Wild type (++) and Ebony (ee)

F1 Generation

  • All F1 offspring are wild type with genotype +e.
  • Since + denotes dominance in this context, we can represent it as E for easier discussion.

F2 Generation Predictions

  • The anticipated F2 phenotypic ratio is 3:1 (3 wild type: 1 ebony), while the genotypic ratio is 1:2:1 (1 ++ : 2 +e : 1 ee).

Test Cross

Purpose

  • A test cross helps determine the genotype of an individual exhibiting a dominant phenotype (unknown genotype).

How to Perform a Test Cross

  1. Cross the unknown genotype with a homozygous recessive individual.
  2. For example, if the unknown purple parent is homozygous dominant (PP), all offspring will show the purple phenotype.
  3. If the purple parent is heterozygous (Pp), offspring will display a 1:1 phenotypic ratio.

Practical Application

  • To ascertain the genotype of an individual with a known phenotype, a test cross can clarify whether it is homozygous or heterozygous.

Dihybrid Cross & the Principle of Independent Assortment

Definition

  • A dihybrid cross is conducted between two double heterozygotes, carrying two different alleles for two genes (F1 x F1).
  • Requires pure-breeding parents to achieve hybrids.

Punnett Square

  • A 4 x 4 Punnett square is used to predict the offspring's genotypes and phenotypes.

Independent Assortment

  • The alleles for different genes assort independently, meaning the segregation of alleles at one gene does not influence the segregation of alleles at another (e.g., yellow/green does not influence round/wrinkled).
  • Each double heterozygote (RrYy) produces 4 types of gametes: RY, Ry, rY, ry in equal proportions.

Chromosome Assortment in Dihybrid Crosses

Metaphase I Impact

  • At metaphase I, chromosome arrangement can result in different combinations at the daughter cells during meiosis.
  • This arrangement leads to diverse gametes in the F1 generation.

Practical Enumeration

  • The F2 generation outcomes reflect the dihybrid cross with a phenotypic ratio of 9:3:3:1.

Rules of Probability in Genetics

Probability Concepts

  • All genetic ratios can be expressed in the form of probabilities.
    • Monohybrid cross phenotypic ratio: 3:1 (Genotypic ratio 1:2:1).
    • Dihybrid cross phenotypic ratio: 9:3:3:1.
  • Probability calculations:
    • Certainty: Probability = 1.
    • Impossibility: Probability = 0.
  • The Punnett square allows combining various probabilities.

Applying the Product and Sum Rules

Product Rule

  • Utilized for predicting the combined probabilities of independent events (A and B occurring).
    • For instance, if a coin is tossed, the probability of heads on the first toss is $ rac{1}{2}$, and on the second toss it is $ rac{1}{2}$. Therefore, the outcome of heads twice is given by:
      rac{1}{2} imes rac{1}{2} = rac{1}{4}.

Genetic Application of Product Rule

  • Example in genetics: If both parents are Bb, the probability their child being bb is:
    rac{1}{2} imes rac{1}{2} = rac{1}{4}.

Sum Rule

  • Predicts the combined probabilities of mutually exclusive events (A or B occurring).
  • Example: if two parents are Bb, what is the probability their child will have the Bb genotype?
  • Combine outcomes using addition:
    rac{1}{4} + rac{1}{4} = rac{1}{2}.