Genetic Inheritance and Mendelian Principles: Structured Study Notes

Genetics and Heredity

Introduction to Genetics

Genetics is the study of heredity, the process by which parents pass genes and traits to their offspring. Sexual reproduction introduces genetic diversity through mechanisms such as crossing-over, independent assortment, and mate selection.

• Genotype: The genetic makeup of an organism; the set of genes it possesses.

• Phenotype: The observable traits or characteristics of an organism, determined by its genotype.

Example: In pea plants, flower color (purple or white) is a phenotype determined by the genotype (PP, Pp, or pp).

Genotype Dictates Phenotype

Chromosomal Basis

The genome consists of pairs of homologous chromosomes, each carrying genes at specific locations called loci. Variants of a gene are known as alleles.

• Locus: The specific location of a gene on a chromosome.

• Allele: Different forms of a gene found at the same locus.

Example: The allele for white flowers (locus for flower color) may be present on one chromosome, while the allele for purple flowers is on the homologous chromosome.

Pea Plants as a Model Organism

Why Pea Plants?

Pea plants were chosen for genetic studies due to their short generation times and the ability to produce large numbers of offspring. They can undergo self-pollination or cross-pollination, making them ideal for controlled genetic experiments.

• Traits studied include seed shape, seed color, flower color, pod shape, pod color, and stem height.

Gregor Mendel and His Experiments

Methodology

Gregor Mendel systematically directed pollination in pea plants and recorded the resulting offspring's phenotypes. His work laid the foundation for understanding the chromosomal basis of inheritance.

• Cross-pollination involves transferring pollen from one plant to another after removing the stamen from the recipient plant.

Recording Pea Plant Phenotypes

Generational Analysis

Mendel began with true-breeding plants (P generation) and observed the F1 and F2 generations:

• P Generation: True-breeding parents (e.g., purple or white flowers).

• F1 Generation: All offspring had the same phenotype (e.g., all purple flowers).

• F2 Generation: Offspring showed a mixture of phenotypes, typically in a 3:1 ratio.

Mendel's Four Principles

Key Principles

• 1. Alternative Versions of Genes: Different genotypes account for variations in traits (phenotypes).

• 2. Law of Dominance: If two alleles are present, only one is expressed (dominant), while the other is masked (recessive).

• 3. Principle of Segregation: During meiosis, homologous chromosomes (and thus alleles) are separated into different gametes.

• 4. Independent Assortment: Genes located on different chromosomes are inherited independently of each other.

Alternative Versions of Genes

Genotype and Phenotype Ratios

Genotype is the combination of two alleles for a specific gene. For example, the alleles for flower color can be PP (homozygous dominant), Pp (heterozygous), or pp (homozygous recessive).

Phenotype	Genotype	Ratio
Purple	PP, Pp	3
White	pp	1

Law of Dominance

Dominant and Recessive Alleles

Even if alleles differ, only the dominant allele is expressed in the phenotype. The recessive allele is masked unless both alleles are recessive.

• Example: For cherries, AA and Aa produce red cherries (dominant), while aa produces yellow cherries (recessive).

Law of Segregation

Gamete Formation

Gametes are haploid and contain only one set of chromosomes/genes/alleles due to the separation of homologous chromosomes during meiosis.

• Homozygous: Identical alleles for a trait.

• Heterozygous: Different alleles for a trait.

Self-Pollination of Heterozygous Plants

Genotype and Phenotype Distribution

Self-pollination of heterozygous plants (e.g., Pp) results in offspring with the following genotype ratios:

• 25% homozygous dominant (PP)

• 50% heterozygous (Pp)

• 25% homozygous recessive (pp)

Phenotype ratio: 3 purple : 1 white

Punnett Squares

Visualizing Heredity

Punnett squares are used to predict the genotypes and phenotypes of offspring from genetic crosses.

• Monohybrid Cross: Involves a single gene (e.g., Gg x Gg).

• Dihybrid Cross: Involves two genes (e.g., RrYy x RrYy).

Example:

Cross	Genotype Ratio	Phenotype Ratio
YY x yy	100% Yy	100% yellow
YY x Yy	50% YY, 50% Yy	100% yellow
Yy x yy	50% Yy, 50% yy	50% yellow, 50% green

Independent Assortment

Genetic Diversity

Each pair of homologous chromosomes lines up independently during meiosis, resulting in different combinations of maternal and paternal chromosomes in gametes.

• Dihybrid Crosses: Crosses involving two genes, leading to 16 possible genotype combinations in the offspring.

Genotype	Phenotype
RRYY, RRYy, RrYY, RrYy	Round, yellow
RRyy, Rryy	Round, green
rrYY, rrYy	Wrinkled, yellow
rryy	Wrinkled, green

Assortment of Alleles on the Same Chromosome

Linked Genes

Alleles on the same chromosome are likely to be inherited together, unlike alleles on different chromosomes, which assort independently. This affects genetic diversity and the outcome of dihybrid crosses.

Review of Dihybrid Crosses

Practice Problems

• Set up a dihybrid cross between homozygous tall, yellow-seeded plants (TTYY) and homozygous short, green-seeded plants (ttyy). Complete the Punnett square and determine the phenotype ratio.

• Allow the F1 generation to self-pollinate and complete the Punnett square for the F2 generation. Determine the phenotype ratio.

Deviations from Mendel's Rules

Complex Genetic Behavior

• Incomplete Dominance: Neither allele is completely dominant; heterozygotes show an intermediate phenotype.

• Codominance: Both alleles are expressed equally in the phenotype.

• Multiple Alleles: More than two alleles exist for a gene, increasing possible genotypes and phenotypes.

• Polygenic Inheritance: Multiple genes contribute to a single trait, resulting in continuous variation.

Nature vs. Nurture

Environmental Influence

Gene expression can be influenced by environmental conditions, demonstrating that both genetics and environment contribute to phenotype.

Summary Table: Mendelian Principles

Principle	Description	Example
Alternative Versions of Genes	Genotypes account for trait variation	PP, Pp, pp for flower color
Law of Dominance	Dominant allele masks recessive	Aa produces red cherries
Law of Segregation	Alleles separate during gamete formation	Gametes carry one allele per gene
Independent Assortment	Genes on different chromosomes assort independently	Dihybrid cross results

Additional info: These notes expand on the original slides by providing definitions, examples, and structured tables for clarity and completeness.