Patterns of Inheritance: Mendelian Genetics and Beyond

Patterns of Inheritance

Introduction to Genetics

Genetics is the scientific study of heredity, focusing on how traits are transmitted from one generation to the next. The foundational principles of genetics were established by Gregor Mendel through his experiments with pea plants, leading to the formulation of Mendel's Laws.

Mendel’s Laws

Ancient Theories and Mendel’s Experiments

• Pangenesis Theory: Proposed by Hippocrates, this theory suggested that particles from all parts of the body are collected in the gametes. This idea is now rejected because changes in somatic cells do not affect gametes.

• Blending Hypothesis: Suggested that offspring are a blend of parental traits. This was disproven as traits can reappear after skipping generations.

• Heredity: The transmission of traits from parents to offspring.

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

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

Mendel’s Experimental Approach

• True-breeding: Plants that produce offspring of the same variety when self-pollinated. These are the P generation (parental generation).

• Hybrids: Offspring of two different true-breeding varieties, called the F1 generation.

• When F1 individuals self-fertilize, their offspring are the F2 generation.

Mendel’s Law of Segregation

Mendel’s law of segregation describes how pairs of gene variants (alleles) are separated into reproductive cells.

• Each organism inherits two alleles for each gene, one from each parent.

• If the alleles differ, the dominant allele determines the organism’s appearance, while the recessive allele has no noticeable effect.

• Allele pairs segregate during gamete formation, so each gamete carries only one allele for each gene.

Key Genetic Terms

• Homozygous: Two identical alleles for a gene (e.g., PP or pp).

• Heterozygous: Two different alleles for a gene (e.g., Pp).

• Genotype: The genetic makeup of an organism.

• Phenotype: The observable traits of an organism.

Homologous Chromosomes and Gene Loci

Homologous chromosomes carry alleles for the same genes at corresponding loci. Each diploid cell contains pairs of homologous chromosomes.

Mendel’s Law of Independent Assortment

This law states that allele pairs separate independently during the formation of gametes. It is revealed by tracking two characters at once (dihybrid cross).

• Monohybrid cross: Cross between individuals heterozygous for one character.

• Dihybrid cross: Cross between individuals heterozygous for two characters.

Testcrosses

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

Probability in Genetics

• Rule of multiplication: Probability of two independent events both occurring is the product of their individual probabilities.

• Rule of addition: Probability of an event that can occur in multiple ways is the sum of the probabilities of each way.

Human Genetics and Pedigrees

Pedigree Analysis

Pedigrees are family trees that track the inheritance of traits across generations, helping to determine genotypes and inheritance patterns.

Single-Gene Disorders

Many inherited disorders are controlled by a single gene and can be dominant or recessive. Most individuals with recessive disorders are born to carrier parents.

Disorder	Major Symptoms	Inheritance
Albinism	Lack of pigment in skin, hair, eyes	Recessive
Cystic fibrosis	Excess mucus, infections, early death	Recessive
Phenylketonuria (PKU)	Phenylalanine accumulation, developmental disabilities	Recessive
Sickle-cell disease	Sickled red blood cells, tissue damage	Recessive
Tay-Sachs disease	Lipid accumulation in brain, mental deficiency	Recessive
Achondroplasia	Dwarfism	Dominant
Huntington’s disease	Uncontrollable movements, cognitive decline	Dominant
Hypercholesterolemia	High cholesterol, heart disease	Dominant

Variations on Mendel’s Laws

Incomplete Dominance

In incomplete dominance, the phenotype of heterozygotes is intermediate between the two parental varieties. For example, crossing red and white snapdragons produces pink flowers.

Multiple Alleles and Codominance

Some genes have more than two alleles. The ABO blood group in humans is controlled by three alleles, with IA and IB being codominant (both expressed in type AB individuals).

Pleiotropy

Pleiotropy occurs when one gene influences multiple phenotypic traits. Sickle-cell disease is an example, affecting hemoglobin, red blood cell shape, and resistance to malaria.

Polygenic Inheritance

Polygenic inheritance occurs when a single trait is controlled by two or more genes, resulting in a range of phenotypes. Human height is a classic example.

Environmental Effects

Many traits are influenced by both genetic and environmental factors, leading to variation in phenotypes.

The Chromosomal Basis of Inheritance

Chromosome Theory of Inheritance

Genes occupy specific loci on chromosomes, which segregate and assort independently during meiosis, providing the physical basis for Mendel’s laws.

Linked Genes and Crossing Over

Genes located close together on the same chromosome (linked genes) tend to be inherited together. Crossing over during meiosis can separate linked genes, producing recombinant offspring. The frequency of recombination can be used to map gene loci on chromosomes (linkage maps).

Sex Chromosomes and Sex-Linked Genes

Sex Determination

In mammals, sex is determined by the presence of XX (female) or XY (male) chromosomes. The Y chromosome carries genes for male development. In some species, environmental factors such as temperature can determine sex.

Sex-Linked Inheritance

Genes located on sex chromosomes are called sex-linked genes. X-linked recessive disorders are more common in males, as they have only one X chromosome. Examples include color blindness and hemophilia.

Y Chromosome and Human Evolution

The Y chromosome is passed from father to son and can be used to trace paternal ancestry and study human evolution.

Summary Table: Key Genetic Concepts

Term	Definition
Allele	Alternative version of a gene
Genotype	Genetic makeup of an organism
Phenotype	Observable traits of an organism
Homozygous	Two identical alleles for a gene
Heterozygous	Two different alleles for a gene
Dominant	Allele that determines phenotype in heterozygotes
Recessive	Allele masked in heterozygotes
Pleiotropy	One gene affects multiple traits
Polygenic inheritance	Multiple genes affect one trait
Linked genes	Genes located close together on a chromosome
Sex-linked gene	Gene located on a sex chromosome

Additional info: This guide covers the core concepts of Mendelian genetics, extensions to Mendel’s laws, and the chromosomal basis of inheritance, as well as human genetic disorders and the use of pedigrees in genetic analysis.