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. He studied seven traits, each controlled by a single gene with two alleles, and observed how these traits were inherited across generations.

• Law of Segregation: Each organism carries two 'factors' (now called genes) for each trait, one from each parent. These segregate during gamete formation, so each gamete carries only one factor.

• Law of Independent Assortment: Genes for different traits 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.

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

Example: Crossing true-breeding yellow and green peas yields all yellow F1, but F2 shows a 3:1 yellow:green ratio.

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 linked gene inheritance to chromosome movement.

• Chromosomes: Structures in the cell nucleus that carry genetic information.

• Homologous Chromosomes: Chromosome pairs (one from each parent) 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 and Mitosis

The cell cycle consists of interphase (G1, S, G2) and mitosis (M phase). Mitosis produces two genetically identical diploid daughter cells, maintaining chromosome number.

• Interphase: Cell grows (G1), replicates DNA (S), and prepares for division (G2).

• Mitosis: Division of the nucleus and chromosomes into two identical cells.

Meiosis

Meiosis is the process by which diploid cells produce haploid gametes. It involves one round of DNA replication followed by two cell divisions (meiosis I and II), resulting in four genetically unique haploid cells.

• Meiosis I: Homologous chromosomes separate, reducing chromosome number by half.

• Meiosis II: Sister chromatids separate, similar to mitosis.

• Genetic Variation: Crossing over and independent assortment during meiosis increase genetic diversity.

Modifications to Mendelian Genetics

Overview of Non-Mendelian Inheritance

Most traits do not follow simple Mendelian inheritance. Modifications include:

• Sex linkage

• Incomplete dominance

• Multiple alleles

• Codominance

• Lethal alleles

• Conditional alleles

• Sex modification

• Penetrance and expressivity

• Gene interaction

• Genetic linkage

Sex Linkage

Some genes are located on sex chromosomes (X or Y), leading to sex-linked inheritance patterns. For example, the white-eye mutation in Drosophila melanogaster is X-linked.

• X-linked Traits: Males (XY) express X-linked recessive traits if they inherit one mutant allele; females (XX) require two mutant alleles.

• Example: Human color blindness is X-linked.

Incomplete Dominance

In incomplete dominance, the heterozygote phenotype is intermediate between the two homozygotes. The classic 1:2:1 ratio is observed in both genotype and phenotype.

• Example: Flower color in snapdragons; fruit color in eggplant.

• 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, leading to a variety of phenotypes.

• Example: Rabbit coat color gene (C) has four common alleles: C (full color), cch (chinchilla), ch (Himalayan), c (albino).

• Example: Human ABO blood groups (IA, IB, i).

Codominance

In codominance, both alleles in a heterozygote are fully expressed, resulting in a phenotype that shows both traits.

• Example: Human blood type AB (IAIB).

Lethal Alleles

Lethal alleles cause death when present in certain genotypes, often altering expected Mendelian ratios (e.g., 2:1 instead of 3:1).

• Example: Yellow coat color in mice (AY allele is dominant for color but recessive lethal).

Pleiotropy

Pleiotropy occurs when one gene influences multiple phenotypic traits.

• Example: Agouti gene in mice affects both coat color and viability.

Conditional Alleles

Conditional alleles express mutant phenotypes only under certain environmental conditions (e.g., temperature-sensitive alleles).

• Example: Himalayan rabbit and Siamese cat coat color depend on temperature.

Sex-Influenced and Sex-Limited Traits

• Sex-Limited Traits: Phenotype is only expressed in 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 percentage of individuals with a genotype who express the expected phenotype.

• Expressivity: The degree or severity of the phenotype among individuals with the same genotype.

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

Summary Table: Key Modifications to Mendelian Inheritance

Modification	Definition	Example
Sex linkage	Gene located on a sex chromosome	Color blindness in humans
Incomplete dominance	Heterozygote phenotype intermediate	Snapdragon flower color
Multiple alleles	More than two alleles for a gene	ABO blood groups
Codominance	Both alleles expressed in heterozygote	Blood type AB
Lethal alleles	Certain genotypes are lethal	Yellow mice (AY allele)
Pleiotropy	One gene affects multiple traits	Agouti gene in mice
Conditional alleles	Phenotype depends on environment	Siamese cat color
Sex-limited/influenced	Trait limited or influenced by sex	Feathering in chickens
Penetrance/Expressivity	Variation in phenotype expression	Polydactyly in humans