The Chromosomal Basis of Inheritance

The Chromosomal Basis of Inheritance

Introduction

The chromosomal theory of inheritance connects Mendelian genetics with the behavior of chromosomes during meiosis. This chapter explores how genes are carried on chromosomes, how their behavior explains inheritance patterns, and how alterations in chromosome number or structure can lead to genetic disorders.

Concept 15.1: Mendelian Inheritance and Chromosome Behavior

The Chromosome Theory of Inheritance

The chromosome theory of inheritance states that genes are located on chromosomes, and the behavior of chromosomes during meiosis accounts for inheritance patterns. Chromosomes carry alleles, which segregate and assort independently during gamete formation.

• Genes are segments of DNA located on chromosomes.

• Alleles are alternative forms of a gene found at the same locus on homologous chromosomes.

• During meiosis, homologous chromosomes (and thus alleles) segregate and assort independently, explaining Mendel's laws.

Morgan’s Experiments with Drosophila

Thomas Hunt Morgan used the fruit fly Drosophila melanogaster to provide evidence that genes are located on chromosomes. He observed wild-type (normal) and mutant phenotypes, such as red and white eyes, and linked these traits to specific chromosomes.

• Fruit flies are ideal for genetic studies due to their short generation time and few chromosomes.

• Wild type refers to the most common phenotype in a population; mutants have alternative traits.

Correlating Gene Behavior with Chromosomes

Morgan’s findings supported the chromosome theory by showing that the inheritance of certain traits correlated with the inheritance of specific chromosomes.

Concept 15.2: Sex-Linked Genes and Inheritance Patterns

Sex Chromosomes and Sex Determination

Sex chromosomes determine the biological sex of an organism. In humans, females have two X chromosomes (XX), and males have one X and one Y chromosome (XY). The SRY gene on the Y chromosome triggers male development.

• Short homologous regions on X and Y allow pairing during meiosis.

• The SRY gene (Sex-determining Region Y) is crucial for testis development.

Systems of Sex Determination

Different organisms use various systems for sex determination, including the X-Y, X-0, Z-W, and haplo-diploid systems.

• X-Y system: Humans and most mammals (XX = female, XY = male).

• X-0 system: Some insects (XX = female, X0 = male).

• Z-W system: Birds, some fish, and insects (ZW = female, ZZ = male).

• Haplo-diploid system: Bees and ants (females are diploid, males are haploid).

Sex-Linked Genes

Genes located on sex chromosomes are called sex-linked genes. X-linked genes are found on the X chromosome, while Y-linked genes are on the Y chromosome. X-linked recessive disorders are more common in males due to their single X chromosome.

• Examples of X-linked disorders: color blindness, Duchenne muscular dystrophy, hemophilia.

X Inactivation in Female Mammals

In female mammals, one X chromosome in each cell is randomly inactivated during embryonic development, forming a Barr body. This leads to mosaic expression of X-linked genes in heterozygous females.

• Example: Calico cats display mosaic fur color due to X inactivation.

Concept 15.3: Linked Genes and Genetic Recombination

Linked Genes

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

• Parental types: Offspring with phenotypes matching the parents.

• Recombinant types: Offspring with new combinations of traits.

• Recombination frequency can be used to map gene loci on chromosomes.

Concept 15.4: Alterations of Chromosome Number or Structure

Nondisjunction and Aneuploidy

Nondisjunction occurs when chromosomes fail to separate properly during meiosis, resulting in gametes with abnormal chromosome numbers. Fertilization involving these gametes leads to aneuploidy.

• Monosomy: One chromosome is missing (2n-1).

• Trisomy: An extra chromosome is present (2n+1).

• Example: Down syndrome (trisomy 21).

Polyploidy

Polyploidy is the presence of more than two complete sets of chromosomes. It is common in plants and can result in triploidy (3n) or tetraploidy (4n).

• Polyploids are often more normal in appearance than aneuploids.

Alterations of Chromosome Structure

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

• Deletion: Loss of a chromosome segment.

• Duplication: Repetition of a segment.

• Inversion: Reversal of a segment within a chromosome.

• Translocation: Movement of a segment to a nonhomologous chromosome.

Human Disorders Due to Chromosomal Alterations

Some genetic disorders are caused by changes in chromosome number or structure. For example, cri du chat syndrome results from a deletion in chromosome 5, and chronic myelogenous leukemia (CML) is associated with a specific translocation (the Philadelphia chromosome).

Summary Table: Types of Chromosomal Alterations

Type	Description	Example Disorder
Deletion	Loss of a chromosome segment	Cri du chat syndrome
Duplication	Repetition of a segment	Charcot-Marie-Tooth disease (in some cases)
Inversion	Reversal of a segment within a chromosome	Usually no specific disorder, but can disrupt gene function
Translocation	Segment moves to a nonhomologous chromosome	Chronic myelogenous leukemia (CML)

Key Equations

• Recombination Frequency:

$\text{Recombination frequency} = \frac{\text{Number of recombinant offspring}}{\text{Total number of offspring}} \times 100$

Conclusion

The chromosomal basis of inheritance explains how genes are transmitted from one generation to the next and how chromosomal abnormalities can lead to genetic disorders. Understanding these principles is fundamental to genetics and modern biology.