Mendel and the Gene Idea: Principles of Inheritance

Mendel and the Gene Idea

Introduction to Mendelian Genetics

Mendelian genetics forms the foundation of classical genetics, describing how traits are inherited from one generation to the next. Gregor Mendel's experiments with garden peas led to the discovery of fundamental laws governing heredity.

• Character: A heritable feature that varies among individuals (e.g., flower color).

• Trait: Each variant for a character (e.g., purple or white flowers).

• True-breeding: Plants that produce offspring of the same variety when self-pollinated.

Mendel's Experimental Approach

Mendel tracked characters with two distinct forms and started with true-breeding varieties. He mated two contrasting, true-breeding varieties, a process called hybridization.

• P generation: Parental generation (true-breeding parents).

• F1 generation: Hybrid offspring of the P generation.

• F2 generation: Offspring produced by self- or cross-pollination of F1 individuals.

Results of Mendel's Crosses

Mendel observed consistent ratios in the F2 generation for several characters, demonstrating predictable inheritance patterns.

• For flower color: 705 purple-flowered plants, 224 white-flowered plants (ratio ≈ 3:1).

• Similar ratios were observed for other traits such as seed shape, seed color, pod shape, flower position, and stem length.

Character	Dominant Trait	Recessive Trait	F2 Ratio
Flower color	Purple	White	3:1
Seed shape	Round	Wrinkled	2.8:1
Seed color	Yellow	Green	3:1
Pod shape	Inflated	Constricted	2.9:1
Flower position	Axial	Terminal	3:1
Stem length	Tall	Dwarf	2.8:1

Mendel's Laws of Inheritance

Law of Segregation

Mendel's first law states that two alleles for a heritable character segregate during gamete formation and end up in different gametes.

• Allele: Alternative versions of a gene.

• Locus: Specific location of a gene on a chromosome.

• Each parent contributes one allele for each character.

Genotype and Phenotype

• Genotype: The particular pair of alleles present for any given gene (e.g., PP, Pp, pp).

• Phenotype: The visible or measurable physical or biochemical characteristics of an organism (e.g., purple or white flowers).

Punnett Square and Genetic Ratios

The Punnett square is a diagram used to predict the outcome of a genetic cross.

• Capital letter represents dominant allele; lowercase represents recessive allele.

• F2 generation from F1 (Pp) cross yields a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio.

The Law of Independent Assortment

Dihybrid Crosses

Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation. A dihybrid cross determines whether two characters are transmitted as a package or independently.

• Law of Independent Assortment: Each pair of alleles segregates independently during gamete formation.

• Applies only to genes on different, nonhomologous chromosomes or those far apart on the same chromosome.

Complex Patterns of Inheritance

Degrees of Dominance

• Complete dominance: Phenotypes of heterozygote and dominant homozygote are identical.

• Incomplete dominance: Phenotype of F1 hybrids is intermediate between parental varieties.

• Codominance: Both alleles affect the phenotype in separate, distinguishable ways.

Multiple Alleles and Codominance

Some genes have more than two alleles, and codominance can occur, as seen in the ABO blood group system.

Allele	Carbohydrate
A	Specific carbohydrate
B	Specific carbohydrate
O	None

Genotype	Phenotype (Blood Group)
IAIA or IAi	A
IBIB or IBi	B
IAIB	AB
ii	O

Pleiotropy

Most genes have multiple phenotypic effects, a property called pleiotropy. For example, pleiotropic alleles are responsible for multiple symptoms of hereditary diseases such as cystic fibrosis and sickle-cell disease.

Epistasis

In epistasis, expression of a gene at one locus alters the phenotypic expression of a gene at a second locus.

• Example: Coat color in Labrador retrievers is determined by two genes, one affecting pigment color and the other affecting pigment deposition.

Polygenic Inheritance

• Quantitative characters: Traits that vary along a continuum, usually indicating polygenic inheritance.

• Polygenic inheritance: Additive effect of two or more genes on a single phenotype (e.g., human height, skin color).

Mendelian Patterns in Human Genetics

Recessively Inherited Disorders

• Appear only in individuals homozygous for the allele.

• Carriers are heterozygous and phenotypically normal.

• Example: Albinism, a recessive condition characterized by lack of pigmentation.

Dominantly Inherited Disorders

• Caused by dominant alleles, often rare and arising by mutation.

• Example: Achondroplasia, a form of dwarfism.

Late-Onset Dominant Disorders

• Timing of onset affects inheritance patterns.

• Example: Huntington's disease, a degenerative nervous system disorder with symptoms appearing in mid-adulthood.

Summary Table: Relationships Among Genes and Alleles

Relationship	Description	Example
Complete dominance	Heterozygous phenotype same as homozygous dominant	PP, Pp
Incomplete dominance	Heterozygous phenotype intermediate	CRCR, CRCW, CWCW
Codominance	Both phenotypes expressed in heterozygotes	IAIB
Multiple alleles	More than two alleles in population	ABO blood group
Pleiotropy	One gene affects multiple phenotypic characters	Sickle-cell disease
Epistasis	Phenotypic expression of one gene affects another	Coat color in dogs
Polygenic inheritance	Single phenotypic character affected by two or more genes	Skin color

Key Equations

Probability of Genotype in Monohybrid Cross

• For a cross between two heterozygotes (Pp x Pp):

$\text{Genotypic ratio} = 1 \text{PP} : 2 \text{Pp} : 1 \text{pp}$

Probability of Phenotype in Monohybrid Cross

$\text{Phenotypic ratio} = 3 \text{purple} : 1 \text{white}$

Probability in Dihybrid Cross

$\text{Phenotypic ratio} = 9 : 3 : 3 : 1$

Conclusion

Mendel's principles of inheritance—segregation and independent assortment—form the basis of classical genetics. Complex patterns such as incomplete dominance, codominance, pleiotropy, epistasis, and polygenic inheritance further expand our understanding of genetic variation and inheritance in both plants and humans. Additional info: Some explanations and tables were expanded for clarity and completeness.