Mendelian Inheritance and Chromosomal Theory

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, through his experiments with pea plants, established the basic laws of heredity, which were later integrated with the chromosomal theory of inheritance.

• Key Terms: Gene (hereditary unit), allele (variant form of a gene), locus (gene location on a chromosome).

• Mendel's Laws: Law of Segregation and Law of Independent Assortment.

• Chromosomal Theory: Genes are located on chromosomes, and their behavior during meiosis explains inheritance patterns.

Mendel’s Experiments and Heredity

Mendel used pea plants to study inheritance, focusing on traits such as flower color, pea color, and pea shape. He performed controlled crosses and analyzed the resulting offspring to deduce patterns of inheritance.

• Parental (P) Generation: True-breeding plants with distinct traits.

• F1 Generation: Offspring of P generation, all showing the dominant trait.

• F2 Generation: Offspring of F1 self-pollination, showing a 3:1 ratio of dominant to recessive traits.

Molecular Basis of Mendelian Inheritance

Genes are segments of DNA located on chromosomes. Different alleles can result in different phenotypes due to variations in the DNA sequence, which may affect the function of the encoded protein.

• Example: The allele for purple flowers encodes an enzyme that synthesizes purple pigment, while the allele for white flowers does not.

Chromosomal Localization of Genes

Modern techniques allow scientists to trace genes to specific locations (loci) on chromosomes using fluorescent tags. This supports the chromosomal theory of inheritance, which links Mendel’s laws to the physical behavior of chromosomes during meiosis.

• Locus: The specific physical location of a gene on a chromosome.

• Alleles: Alternative forms of a gene found at the same locus.

Inheritance and Meiosis

Law of Segregation and Independent Assortment

During meiosis, homologous chromosomes separate, ensuring that each gamete receives only one allele of each gene. The independent assortment of chromosomes leads to genetic variation in offspring.

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation.

• Law of Independent Assortment: Genes located on different chromosomes assort independently during meiosis.

Chromosomal Basis of Sex Determination

Sex Chromosomes and Inheritance

Sex chromosomes (X and Y) determine the biological sex of an organism. In many species, including humans and fruit flies, females have two X chromosomes (XX) and males have one X and one Y chromosome (XY).

• SRY Gene: Located on the Y chromosome, triggers male development.

• Sex-linked Genes: Genes located on sex chromosomes, especially the X chromosome, exhibit unique inheritance patterns.

X-Linked Inheritance

X-linked genes are more likely to affect males, as they have only one X chromosome. Females must inherit two copies of a recessive allele to express an X-linked trait, while males need only one.

• Examples: Color blindness, Duchenne muscular dystrophy, hemophilia.

• Hemizygous: Males are hemizygous for X-linked genes, having only one allele.

Linked Genes and Genetic Recombination

Linked Genes

Genes located close together on the same chromosome tend to be inherited together and are called linked genes. However, crossing over during meiosis can separate linked genes, resulting in recombinant offspring.

• Parental Types: Offspring with phenotypes matching the parents.

• Recombinant Types: Offspring with new combinations of traits due to crossing over.

Genetic Mapping

The frequency of recombination between genes can be used to map their relative positions on a chromosome. The further apart two genes are, the higher the probability of a crossover event between them.

• Map Unit: 1% recombination frequency equals 1 map unit (centimorgan).

• Linkage Map: A genetic map based on recombination frequencies.

Alterations of Chromosome Number and Structure

Aneuploidy and Polyploidy

Errors during meiosis can lead to abnormal numbers of chromosomes (aneuploidy) or extra sets of chromosomes (polyploidy).

• Aneuploidy: Presence of an abnormal number of chromosomes (e.g., trisomy 21 causes Down syndrome).

• Polyploidy: More than two complete sets of chromosomes, common in plants.

Alterations of Chromosome Structure

Chromosome breakage can result in deletions, duplications, inversions, or translocations, sometimes leading to genetic disorders.

• Example: Deletion on chromosome 5 causes cri du chat syndrome; translocation between chromosomes 9 and 22 causes chronic myelogenous leukemia (CML).

Exceptions to Mendelian Inheritance

Genomic Imprinting and Epigenetics

Some genes are expressed in a parent-of-origin-specific manner due to genomic imprinting, often regulated by DNA methylation. Epigenetic changes can affect gene expression without altering the DNA sequence.

• Imprinting: Silencing of certain genes depending on the parent of origin.

• Epigenetics: Heritable changes in gene expression not caused by changes in DNA sequence.

Inheritance of Organelle Genes

Some genes are located in organelles such as mitochondria and chloroplasts. These genes are typically inherited maternally, as the zygote’s cytoplasm comes from the egg.

• Mitochondrial Disorders: Mutations in mitochondrial DNA can cause diseases affecting energy production.

• Chloroplast Inheritance: In plants, plastid genes are also inherited maternally.