Ch 15. 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

Morgan’s Experiments with Drosophila

Thomas Hunt Morgan used the fruit fly Drosophila melanogaster to study inheritance patterns, providing evidence that genes are located on chromosomes.

• Model Organism: Fruit flies are ideal for genetic studies due to their short generation time, high offspring numbers, and only four pairs of chromosomes.

• Wild Type vs. Mutant: The normal phenotype is called wild type; alternative forms are mutant phenotypes.

• Key Discovery: Morgan discovered a white-eyed mutant, linking a specific gene to a chromosome.

Chromosome Theory of Inheritance

Morgan’s findings supported the chromosome theory, which states that genes are located on chromosomes and their behavior during meiosis explains inheritance patterns.

• Law of Segregation: The two alleles for each gene separate during gamete formation.

• Law of Independent Assortment: Alleles of genes on nonhomologous chromosomes assort independently.

Concept 15.2: Sex-Linked Genes and Inheritance Patterns

Sex Chromosomes and Sex Determination

Sex chromosomes (X and Y) determine the sex of individuals in many species. The presence or absence of the Y chromosome, and specifically the SRY gene, determines male development in humans.

• XX: Female

• XY: Male

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

Systems of Sex Determination

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

Sex-Linked Genes

Genes located on sex chromosomes exhibit unique inheritance patterns. X-linked genes are more common than Y-linked genes in humans.

• X-linked Genes: About 1,100 genes on the human X chromosome.

• Y-linked Genes: Only about 78 genes on the Y chromosome.

Inheritance of X-Linked Genes

Recessive X-linked disorders are more common in males, as they have only one X chromosome. Examples include color blindness, Duchenne muscular dystrophy, and hemophilia.

X Inactivation in Female Mammals

In females, one X chromosome per cell is randomly inactivated during embryonic development, forming a Barr body. Heterozygous females are mosaics for X-linked traits.

Concept 15.3: 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.

• Linkage: Linked genes do not assort independently.

• Testcrosses: Used to study linkage and recombination frequencies.

Genetic Recombination

Recombination produces offspring with combinations of traits different from either parent. Crossing over during meiosis creates recombinant chromosomes.

• Parental Types: Offspring with parental phenotypes.

• Recombinant Types: Offspring with new trait combinations.

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

Concept 15.4: Alterations of Chromosome Number or Structure

Nondisjunction and Aneuploidy

Nondisjunction occurs when chromosomes fail to separate properly during meiosis, leading to abnormal chromosome numbers (aneuploidy).

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

• Trisomy: One extra chromosome (2n + 1).

Human Disorders Due to Aneuploidy

• Down Syndrome (Trisomy 21): Three copies of chromosome 21; risk increases with maternal age.

• Klinefelter Syndrome (XXY): Males with an extra X chromosome.

• Turner Syndrome (X0): Females with only one X chromosome.

Polyploidy

Polyploidy is the presence of more than two complete sets of chromosomes. Common in plants, rare in animals.

• Triploidy (3n): Three sets of chromosomes.

• Tetraploidy (4n): Four sets of chromosomes.

Alterations of Chromosome Structure

Chromosome breakage can lead to four 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.

Disorders Caused by Chromosomal Alterations

• Cri du chat Syndrome: Deletion on chromosome 5, causing intellectual disability and a distinctive cry.

• Chronic Myelogenous Leukemia (CML): Caused by a reciprocal translocation between chromosomes 9 and 22 (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	Some developmental disorders
Inversion	Reversal of a segment	Usually no obvious disorder
Translocation	Segment moves to another chromosome	Chronic myelogenous leukemia

Key Terms

• Aneuploidy: Abnormal number of chromosomes.

• Polyploidy: More than two sets of chromosomes.

• Linked Genes: Genes located close together on the same chromosome.

• Sex-linked Gene: Gene located on a sex chromosome.

• Barr Body: Inactive X chromosome in female cells.