The Chromosomal Basis of Inheritance

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Chapter 12: The Chromosomal Basis of Inheritance

Introduction to Chromosomal Inheritance

The chromosomal theory of inheritance connects Mendel’s laws of heredity with the behavior of chromosomes during meiosis. This chapter explores how genes are located on chromosomes, how their arrangement and behavior explain inheritance patterns, and how chromosomal changes can lead to genetic disorders.

Sex-Linked Genes and X-Inactivation

Sex-Linked Inheritance Patterns

Sex-linked genes are located on sex chromosomes (X or Y). In humans, most sex-linked traits are X-linked because the X chromosome contains more genes than the Y chromosome. Males (XY) are hemizygous for X-linked genes, so recessive alleles are expressed more frequently in males.

X-linked recessive inheritance: Traits such as color blindness are more common in males. Females can be carriers if they have one normal and one mutant allele.
Carrier females: Heterozygous females (XNXn) do not express the trait but can pass it to offspring.
Inheritance patterns: The combination of parental genotypes determines the probability of affected offspring.

Punnett square for X-linked inheritance: carrier female x affected male

X Chromosome Inactivation and Dosage Compensation

In female mammals, one of the two X chromosomes in each cell is randomly inactivated during early embryonic development. This process, called dosage compensation, ensures that females, like males, have one functional copy of the X chromosome in each body cell. The inactivated X condenses into a Barr body.

Mosaicism: Because X inactivation is random, females are mosaics for X-linked genes, with different cells expressing genes from either the maternal or paternal X chromosome.
Barr body: The condensed, inactive X chromosome visible in the nucleus of interphase cells.
XIST gene: The X-inactive specific transcript (XIST) gene produces RNA that coats the X chromosome to be inactivated.

Mosaicism in a cat due to X inactivation Fluorescent image showing XIST RNA on inactive X chromosome

Linkage and Recombination

Linked Genes and Deviation from Mendel’s Laws

Genes located close together on the same chromosome are called linked genes and tend to be inherited together. This linkage can cause deviations from Mendel’s law of independent assortment.

Linked genes: Do not assort independently unless separated by crossing over during meiosis.
Recombination: Crossing over between homologous chromosomes can break the physical connection between linked genes, producing recombinant chromosomes and phenotypes.

P generation cross: wild type and double mutant Drosophila F1 dihybrid testcross in Drosophila Testcross offspring showing four phenotypes

Chromosomal Basis of Recombination

During meiosis, homologous chromosomes can exchange segments in a process called crossing over. The frequency of recombination between two genes is proportional to their distance apart on the chromosome.

Recombinant frequency: The percentage of recombinant offspring is used to estimate the distance between genes (1% recombination = 1 map unit or centimorgan).
Linkage maps: Diagrams showing the relative positions of genes on a chromosome based on recombination frequencies.

Diagram of crossing over between homologous chromosomes Daughter cells with recombinant chromosomes Linkage map with recombination frequencies between genes b, cn, and vg

Alterations of Chromosome Number and Structure

Aneuploidy and Polyploidy

Errors during meiosis can result in gametes with abnormal numbers of chromosomes, leading to aneuploidy (extra or missing chromosomes) or polyploidy (extra sets of chromosomes).

Aneuploidy: The presence of an abnormal number of chromosomes (e.g., trisomy 21, which causes Down syndrome).
Polyploidy: More than two complete sets of chromosomes, common in plants.
Nondisjunction: Failure of homologous chromosomes or sister chromatids to separate properly during meiosis.

Diagram of nondisjunction in meiosis I and II Polyploidy in plants: wheat, banana, strawberry Small-scale chromosomal gains and losses: trisomy and monosomy

Structural Alterations of Chromosomes

Chromosome structure can be altered by deletions, duplications, inversions, and translocations, often due to DNA breakage and faulty repair. These changes can disrupt gene function and cause genetic disorders.

Deletion: Loss of a chromosome segment.
Duplication: Repetition of a chromosome segment.
Inversion: Reversal of a chromosome segment.
Translocation: Movement of a segment from one chromosome to another.

Chromosomal deletions and inversions Chromosomal translocations

Chromosomal Alterations and Human Disorders

Alterations in chromosome number or structure can lead to genetic disorders. For example, trisomy 21 causes Down syndrome, and certain translocations are associated with cancers such as chronic myelogenous leukemia.

Down syndrome: Caused by an extra copy of chromosome 21 (trisomy 21).
Cancer: Some cancers are linked to specific chromosomal translocations that create fusion genes and disrupt normal cell regulation.

Summary Table: Types of Chromosomal Alterations

Type of Alteration	Description	Example
Aneuploidy	Abnormal number of chromosomes	Down syndrome (trisomy 21)
Polyploidy	Extra sets of chromosomes	Wheat (6n), strawberry (8n)
Deletion	Loss of a chromosome segment	Cri du chat syndrome
Duplication	Repetition of a segment	Charcot-Marie-Tooth disease
Inversion	Reversal of a segment	Some forms of hemophilia
Translocation	Segment moves to another chromosome	Chronic myelogenous leukemia

Key Concepts and Learning Outcomes

Explain how the behavior of chromosomes during meiosis accounts for Mendel’s laws of segregation and independent assortment.
Describe the inheritance patterns of sex-linked genes and the effects of X inactivation.
Understand how genetic linkage and crossing over affect inheritance and how recombination frequencies are used to map genes.
List and explain different types of chromosomal alterations and their consequences for genetic disorders.