Mendelian Genetics and the Gene Idea: Patterns and 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 (hybridization), producing the parental (P) generation, the first filial (F1) generation, and the second filial (F2) generation.

• P Generation: True-breeding parents.

• F1 Generation: Hybrid offspring of the P generation.

• F2 Generation: Offspring from 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.

• Dominant traits appeared in a 3:1 ratio over recessive traits in the F2 generation.

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

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.

• Homozygous: Two identical alleles for a gene.

• Heterozygous: Two different alleles for a gene.

• Dominant allele: Determines the organism's appearance.

• Recessive allele: Has no noticeable effect on appearance.

Punnett Square and Genetic Ratios

The Punnett square is a tool used to predict the possible combinations of alleles in offspring.

• Capital letters represent dominant alleles; lowercase letters represent recessive alleles.

• F2 generation shows a 3:1 ratio of dominant to recessive phenotypes.

Genetic Vocabulary

Phenotype and Genotype

• Phenotype: The visible or measurable physical or biochemical characteristics of an organism.

• Genotype: The particular pair of alleles present for any given gene.

The Testcross

A testcross is used to determine the genotype of an individual with a dominant phenotype by breeding it with a homozygous recessive individual.

• If any offspring display the recessive phenotype, the mystery parent must be heterozygous.

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.

• 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: Two dominant 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 ABO blood groups.

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.

Polygenic Inheritance

Quantitative characters vary in the population along a continuum, indicating polygenic inheritance—an additive effect of two or more genes on a single phenotype.

• Height is a classic example, influenced by over 180 genes.

Mendelian Patterns in Human Genetics

Recessively Inherited Disorders

Recessive disorders appear only in individuals homozygous for the allele. Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal.

• Albinism is a recessive condition characterized by lack of pigmentation.

Dominantly Inherited Disorders

Some human disorders are caused by dominant alleles, which are rare and often arise by mutation.

• Achondroplasia is a form of dwarfism caused by a dominant allele.

• Huntington's disease is a degenerative nervous system disorder with late onset.

Summary Tables: Relationships Among Genes

Single Gene Relationships

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

Multiple Gene Relationships

Relationship	Description	Example
Epistasis	Phenotypic expression of one gene affects another	BbEe
Polygenic inheritance	Single phenotypic character affected by two or more genes	AaBbCc

Additional info:

• Mendel's principles are foundational for understanding inheritance, but many traits are influenced by more complex genetic interactions.

• Modern genetics incorporates molecular biology to explain gene function and regulation.