Chromosomal Basis of Inheritance

Historical Foundations

The understanding of heredity advanced significantly with the discovery of chromosomes and their behavior during cell division. Early geneticists, including Mendel, proposed the existence of hereditary units, but it was not until the early 20th century that parallels between these units and chromosomes were recognized.

• Mendel's "hereditary units" were theoretical constructs to explain inheritance patterns observed in pea plants.

• In 1902, Sutton and Boveri independently noted similarities between chromosome behavior and Mendel's factors, leading to the chromosome theory of inheritance.

Model Organism: Drosophila melanogaster

Thomas Hunt Morgan used the fruit fly, Drosophila melanogaster, as a model organism for genetic studies due to several advantageous characteristics:

• Short generation time (about two weeks per generation)

• High number of offspring

• Only four pairs of chromosomes

Wild Type and Mutant Phenotypes

• Wild type: The normal phenotype commonly found in natural populations.

• Mutant phenotype: Traits that differ from the wild type, often due to genetic mutations (e.g., white eyes in fruit flies instead of the wild-type red eyes).

Sex-Linked Genes and Chromosome Behavior

Sex-Linked Genes

Genes located on sex chromosomes (X or Y) are called sex-linked genes:

• X-linked genes: Genes found on the X chromosome.

• Y-linked genes: Genes found on the Y chromosome.

The inheritance patterns of sex-linked genes differ from those of autosomal genes:

• An allele on the Y chromosome is passed to all male offspring of a male with the allele, but not to females.

• An allele on the X chromosome is passed to all offspring of a female with the allele, but only to sons from a male with the allele.

X Inactivation in Mammals

• In female mammals, one of the two X chromosomes in each cell is randomly inactivated during embryonic development.

• The inactive X chromosome condenses into a structure called a Barr body.

• Females heterozygous for X-linked genes are mosaics for those traits, as different cells may express different alleles.

Linkage and Genetic Recombination

Linked Genes

Genes located on the same chromosome tend to be inherited together and are called linked genes. However, genetic recombination can separate linked genes, producing new combinations of traits.

• Parental types: Offspring with phenotypes matching one of the parental (P) phenotypes.

• Recombinant types (recombinants): Offspring with new combinations of traits not found in either parent.

Genetic Recombination and Crossing Over

• A 50% recombination frequency is observed for genes on different chromosomes (independent assortment).

• Linked genes can be separated by crossing over during meiosis, which exchanges segments between homologous chromosomes.

Genetic Mapping

Alfred Sturtevant developed the concept of a genetic map, an ordered list of gene loci along a chromosome, based on recombination frequencies.

• The farther apart two genes are, the higher the probability of a crossover and thus a higher recombination frequency.

• The closer two genes are, the lower the recombination frequency.

• Map units (centimorgans, cM): One map unit equals a 1% recombination frequency.

• Linkage maps show the relative order and distance between genes, not their exact physical locations.

Alterations of Chromosome Number and Structure

Nondisjunction and Aneuploidy

Nondisjunction occurs when homologous chromosomes or sister chromatids fail to separate properly during meiosis, resulting in gametes with abnormal chromosome numbers.

• Aneuploidy: Condition where offspring have an abnormal number of a particular chromosome.

• Monosomic (2n - 1): Zygote has only one copy of a chromosome.

• Trisomic (2n + 1): Zygote has three copies of a chromosome.

Polyploidy

• Polyploidy: Organism has more than two complete sets of chromosomes (e.g., triploidy (3n), tetraploidy (4n)).

• Common in plants, rare in animals.

• Polyploids are often more normal in appearance than aneuploids.

Alterations in Chromosome Structure

Chromosome breakage can lead to four main types of structural changes:

• Deletion: Loss of a chromosomal fragment.

• Duplication: Repetition of a chromosomal segment.

• Inversion: Reversal of a segment within a chromosome.

• Translocation: Movement of a segment from one chromosome to another.

Human Disorders Due to Chromosomal Alterations

Down Syndrome (Trisomy 21)

• Caused by three copies of chromosome 21.

• Incidence: About 1 in 830 live births in the United States.

• Risk increases with maternal age.

Sex Chromosome Aneuploidies

• XXX females: Occur in about 1 in 1,000 births; generally healthy but may have learning disabilities.

• Turner syndrome (Monosomy X, X0): Produces sterile females; only known viable monosomy in humans.

Summary Table: Chromosomal Alterations and Effects

Key Equations and Concepts

• Recombination Frequency (RF):

• One map unit (centimorgan, cM) = 1% recombination frequency.

Conclusion

The chromosomal basis of inheritance explains how genes are transmitted through generations and how alterations in chromosome number or structure can lead to genetic disorders. The study of model organisms like Drosophila melanogaster has been crucial in uncovering these principles, which remain foundational to modern genetics.