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.

• 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	3:1
Flower position	Axial	Terminal	3:1
Stem length	Tall	Dwarf	2.8:1

Concept 14.1: 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 at a locus.

• Heterozygous: Two different alleles at a locus.

• 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 diagram used to predict the outcome of a genetic cross.

• Capital letter represents dominant allele (e.g., P for purple).

• Lowercase letter represents recessive allele (e.g., p for white).

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

Concept 14.2: The Law of Independent Assortment

Dihybrid Crosses and Independent Assortment

Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation. Mendel's law of independent assortment states that 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.

• Genes located near each other on the same chromosome tend to be inherited together.

Concept 14.3: 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	Different carbohydrate
i	None

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

Pleiotropy

• Pleiotropy: Most genes have multiple phenotypic effects.

• Pleiotropic alleles are responsible for multiple symptoms of hereditary diseases, such as cystic fibrosis and sickle-cell disease.

Epistasis

• Epistasis: Expression of a gene at one locus alters the phenotypic expression of a gene at a second locus.

Polygenic Inheritance

• Polygenic inheritance: Additive effect of two or more genes on a single phenotype.

• Quantitative characters vary along a continuum (e.g., height, skin color).

Concept 14.4: Human Mendelian Genetics

Recessively Inherited Disorders

• Disorders appear only in individuals homozygous for the allele.

• Carriers are heterozygous and phenotypically normal.

• Example: Albinism, characterized by lack of pigmentation.

Dominantly Inherited Disorders

• Caused by dominant alleles; rare and often arise by mutation.

• Example: Achondroplasia, a form of dwarfism.

Timing of Disease Onset

• Some disorders, such as Huntington's disease, have late onset, affecting inheritance patterns.

• Huntington's disease is a degenerative nervous system disorder with onset at 35-40 years of age.

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 modern genetics, including molecular genetics and genomics.

• Complex inheritance patterns such as incomplete dominance, codominance, pleiotropy, epistasis, and polygenic inheritance are common in nature and human disease.