Transmission Genetics and Modifications to Mendelian Inheritance

Gregor Mendel and the Foundations of Genetics

Mendel’s Experiments and Laws

Gregor Mendel’s work with pea plants established the basic principles of heredity, now known as Mendelian genetics. He studied seven traits, each controlled by a single gene with two alleles, and observed predictable inheritance patterns.

• Law of Segregation: Each individual has two factors (alleles) for each trait, which segregate during gamete formation so each gamete receives one allele.

• Law of Independent Assortment: Alleles of different genes assort independently during gamete formation, leading to genetic variation.

• Dominant and Recessive Traits: In F1 hybrids, one trait may mask the other (dominant vs. recessive). In the F2 generation, the recessive trait reappears in a 3:1 ratio of phenotypes.

• Genotype and Phenotype Ratios: F2 generation shows a 1:2:1 genotype ratio (AA, Aa, aa) and a 3:1 phenotype ratio (dominant:recessive).

• Dihybrid Crosses: Crossing parents differing in two traits yields a 9:3:3:1 phenotype ratio, supporting independent assortment.

Example: Mendel’s cross between yellow and green seed peas demonstrated dominant (yellow) and recessive (green) inheritance.

Chromosomes and the Chromosome Theory of Inheritance

From Mendel’s Factors to Genes on Chromosomes

After Mendel, scientists discovered that genes are located on chromosomes. The behavior of chromosomes during meiosis explained Mendel’s laws. The Boveri-Sutton chromosome theory proposed that genes reside on chromosomes, which segregate and assort independently during meiosis.

• Chromosome Structure: Chromosomes consist of DNA and proteins, with regions called centromeres and telomeres.

• Homologous Chromosomes: Pairs of chromosomes (one maternal, one paternal) that carry the same genes but may have different alleles.

• Sister Chromatids: Identical copies of a chromosome formed during DNA replication.

Cell Division: Mitosis and Meiosis

The Cell Cycle

The cell cycle consists of interphase (G1, S, G2) and M phase (mitosis or meiosis). DNA replication occurs in S phase, while cell division occurs in M phase.

Mitosis

Mitosis produces two genetically identical diploid daughter cells, maintaining chromosome number. It is essential for growth and repair in multicellular organisms.

• Stages: Prophase, metaphase, anaphase, telophase, and cytokinesis.

• Outcome: Each daughter cell receives a complete set of chromosomes.

Meiosis

Meiosis produces four genetically distinct haploid gametes through two successive divisions (meiosis I and II) after one round of DNA replication. This process explains Mendel’s laws at the chromosomal level.

• Meiosis I: Homologous chromosomes separate, reducing chromosome number by half (reduction division).

• Meiosis II: Sister chromatids separate, similar to mitosis (equational division).

• Genetic Variation: Crossing over during prophase I and independent assortment during metaphase I increase genetic diversity.

Modifications to Mendelian Genetics

Overview

Most traits do not follow simple Mendelian inheritance. Modifications include sex linkage, incomplete dominance, codominance, multiple alleles, lethal alleles, conditional alleles, sex modification, penetrance, expressivity, gene interaction, and genetic linkage.

Sex Linkage

Sex-linked traits are associated with genes located on sex chromosomes (e.g., X-linked traits). Thomas Hunt Morgan’s work with Drosophila melanogaster demonstrated X-linked inheritance of the white-eye mutation, providing evidence for the chromosome theory of inheritance.

• X-linked Traits: Males (XY) express X-linked recessive traits if they inherit the mutant allele, while females (XX) require two copies.

• Reciprocal Crosses: Crossing mutant males with wild-type females and vice versa yields different results, revealing sex linkage.

Incomplete Dominance

In incomplete dominance, the heterozygote phenotype is intermediate between the two homozygotes. The classic example is flower color in snapdragons or fruit color in eggplant, where crossing two true-breeding parents yields an intermediate F1 phenotype and a 1:2:1 ratio in the F2 generation.

• Molecular Basis: One functional allele does not produce enough protein for the full phenotype.

Multiple Alleles

Many genes have more than two alleles in the population. Rabbit coat color and human blood type are classic examples.

• Rabbit Coat Color: Four alleles (C, cch, ch, c) with a dominance hierarchy.

• Human Blood Type: Three alleles (IA, IB, i) produce four phenotypes (A, B, AB, O).

Codominance

In codominance, both alleles in a heterozygote are fully expressed. The human ABO blood group system is a classic example, where IA and IB are codominant, and i is recessive.

• Molecular Basis: Different alleles encode enzymes that add different sugars to red blood cell surfaces.

Lethal Alleles

Lethal alleles cause death when present in certain genotypes, altering expected Mendelian ratios. For example, the yellow coat color mutation in mice is dominant for color but recessive lethal (homozygotes die).

Pleiotropy

Pleiotropy occurs when one gene influences multiple phenotypic traits. For example, the agouti gene in mice affects both coat color and viability.

Conditional Alleles

Conditional alleles express mutant phenotypes only under specific environmental conditions, such as temperature-sensitive mutations in Siamese cats and Himalayan rabbits.

Sex-Influenced and Sex-Limited Traits

• Sex-Limited Traits: Phenotype is expressed in only one sex (e.g., feathering in chickens).

• Sex-Influenced Traits: Allele expression differs between sexes (e.g., bearded phenotype in goats).

Penetrance and Expressivity

• Penetrance: The proportion of individuals with a genotype who display the expected phenotype. Complete penetrance means all individuals show the phenotype; incomplete penetrance means some do not.

• Expressivity: The degree to which a phenotype is expressed among individuals with the same genotype. Variable expressivity means the phenotype varies in severity.

Example: Polydactyly in humans shows incomplete penetrance and variable expressivity.

Summary Table: Modifications to Mendelian Inheritance

Modification	Definition	Example
Sex linkage	Gene located on a sex chromosome	White eyes in Drosophila
Incomplete dominance	Heterozygote phenotype intermediate	Snapdragon flower color
Codominance	Both alleles fully expressed	ABO blood group
Multiple alleles	More than two alleles in population	Rabbit coat color, blood type
Lethal alleles	Allele causes death in certain genotypes	Yellow coat in mice
Pleiotropy	One gene affects multiple traits	Agouti gene in mice
Conditional alleles	Phenotype depends on environment	Siamese cat coloration
Sex-limited/influenced	Trait expression depends on sex	Feathering in chickens
Penetrance/Expressivity	Variation in phenotype among same genotype	Polydactyly in humans