Lecture 12 Monohybrid Crosses and the Law of Segregation

Mendelian Genetics

Introduction to Gregor Mendel

• Gregor Mendel is known as "The Father of Genetics".

• Before Mendel's work in 1866, knowledge about genetics was minimal.

• Common opinion held that inheritance followed the concept of "blending" inheritance.

• Mendel's contribution included experimentation and testable theories of inheritance.

• His distinctive approach shifted the understanding of heredity in science.

Why Mendel Chose Pea Plants (Pisum)

• Several key reasons:

  • Availability: Mendel had access to pea plants.

  • Pollination Methods: Pea plants can self-pollinate or cross-pollinate.

  • Prolific Offspring: They produce a high number of offspring.

  • Short Generation Time: Quick turnaround between generations.

  • Manageability: Unlike animals, pea plants remain stationary, facilitating breeding and counting offspring.

Traits Studied by Mendel

• Mendel examined a total of seven traits, each with two forms:

  • Flower Color: Purple vs. White

  • Plant Height: Tall vs. Short

  • Seed Color: Green vs. Yellow

• Example used for the explanation: Flower color (Purple and White).

Experimental Rationale

• Create pure-breeding lines for both purple and white-flowered plants.

  • Pure breeding lines, when selfed, yield offspring with identical appearance.

• Definition:

  • Phenotype: The observable appearance or characteristic of an organism regarding a trait.

• Initial crossings:

  • Purple x Purple = Purple

  • White x White = White

The Key Question

• Inquiry: If true-breeding purple plants are crossed with true-breeding white plants, what are the offspring's phenotypes?

  • Cross: Parental Generation (P) Purple x White

  • Initial Prediction (according to blending hypothesis): Offspring should be pink.

  • Actual Result: All offspring were PURPLE (F1 Generation).

F1 Generation Observations

• Mendel found that the “whiteness” in the white parent seemed to disappear in the F1 generation.

• This necessitated further investigation.

F2 Generation Results

• Inquiry: What happens if the F1 offspring are selfed?

  • Cross: F1 purple x F1 purple

  • Initial Prediction: All offspring would be purple.

  • Actual Outcome: The F2 generation produced offspring with a phenotypic ratio of:

  • 3 Purple: 1 White

  • Expressed as: $rac{3}{4}$ Purple : $rac{1}{4}$ White.

Summary of Monohybrid Cross

• P Generation: Purple x White

• F1 Generation: All Purple

• F1 Self Cross: F1 purple x F1 purple produces F2 Generation: 3 Purple : 1 White

Results Across All Traits

• Mendel consistently observed the same results across all seven traits studied:

  • F1 offspring resembled one of the parents.

  • Selfing the F1 generation led to consistently obtaining a 3:1 ratio for the trait.

Antagonistic Pairs Examined by Mendel

• Traits and their Antagonistic Pairs:

  • Seed Color: Yellow vs. Green

  • Seed Shape: Round vs. Wrinkled

  • Flower Color: Purple vs. White

  • Pod Color: Green vs. Yellow

  • Pod Shape: Round vs. Pinched

  • Stem Length: Tall vs. Short

  • Flower Position: Along stem vs. At tip of stem.

Interpretation of Results

• Mendel's First Law (The Law of Segregation)

  • States that for each trait, there are discrete units of information, which Mendel called “factors” (now known as genes).

  • Each adult plant possesses two copies of a gene for each trait.

  • During gamete formation (eggs and sperm), copies segregate so that each gamete contains one copy.

  • Fertilization combines two gametes, forming a new individual with two copies of the gene for a particular trait.

Important Terminology

• Genes and Alleles:

  • Genes come in two forms, called alleles.

  • Dominant alleles express the trait when present.

  • Recessive alleles require two copies for expression.

  • The alleles present in an organism are referred to as its genotype.

• Genotype Definitions:

  • Homozygous: Organism with two identical alleles for a trait.

  • Heterozygous: Organism with two different alleles for a trait.

  • Gene locus: The physical location of a gene on a chromosome.

Final Understanding of Results

• Summarized Monohybrid Cross:

  • P generation: Purple (PP) x White (pp)

  • F1 generation: All Purple (Pp).

The Punnett Square

• Example for Purple and White Pea Plants:

  • Cross: PP x pp using a Punnett Square yields:

  • P | P

  • p | Pp

  • p | Pp

  • Result: All F1 offspring are Purple (Pp).

F2 Generation Punnett Square Analysis

• F1 Generation (Pp) Self Cross:

  • Cross: Pp x Pp using a Punnett Square yields:

  • P | p

  • P | PP

  • p | Pp

  • p | pp

  • Result: F2 generation shows a ratio of 3 Purple (PP, Pp) to 1 White (pp).

Testcross Concept

• Definition: The genotype of a plant exhibiting the dominant trait can be identified through crossing with a recessive trait.

• Example Crosses:

  • Cross A: PYY x yy

  • Cross B: Yy x yy

• Result Analysis:

  • Offspring from Cross A: All yellow

  • Offspring from Cross B: 1:1 Yellow to Green ratio.

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