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 because of its short generation time, high offspring numbers, and only four pairs of chromosomes. He distinguished between wild type (normal) and mutant phenotypes, discovering sex-linked inheritance with a white-eyed mutant.

• Wild type: The most common phenotype in a population.

• Mutant phenotype: Traits that differ from the wild type due to mutations.

• Chromosome theory of inheritance: Genes are located on chromosomes, and their behavior during meiosis explains inheritance patterns.

Relationship Between Genes and Chromosomes

Genes are located on chromosomes, which duplicate and segregate during meiosis. Each parent contributes one set of chromosomes, and thus one allele for each gene, to their offspring.

• Homologous chromosomes: Chromosome pairs, one from each parent, carrying the same genes at the same loci.

• Alleles: Different versions of a gene.

• 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 during gamete formation.

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. Females are XX, males are XY. The SRY gene on the Y chromosome triggers male development.

• Sex-linked gene: A gene located on a sex chromosome (X or Y).

• SRY gene: Sex-determining region on the Y chromosome, responsible for testis development.

• Inheritance patterns: X-linked traits are more commonly expressed in males, as they have only one X chromosome.

Inheritance of X-Linked Genes

X-linked recessive disorders are more common in males. Examples include color blindness, Duchenne muscular dystrophy, and hemophilia. Females can be carriers if heterozygous for the trait.

• Carrier: A female with one normal and one mutant allele for an X-linked gene.

• Pedigree analysis: Used to track inheritance of X-linked traits in families.

X Inactivation in Female Mammals

In females, one X chromosome per cell is randomly inactivated during embryonic development, forming a Barr body. This leads to mosaic expression of X-linked genes, as seen in calico cats.

• Barr body: The condensed, inactive X chromosome in female cells.

• Mosaicism: Presence of two or more populations of cells with different genotypes in one individual.

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. Morgan’s experiments with Drosophila showed that linked genes do not assort independently.

• Linkage: Tendency of genes located on the same chromosome to be inherited together.

• Testcross: Used to determine the genotype of an individual by crossing with a homozygous recessive.

Genetic Recombination and Crossing Over

Genetic recombination produces offspring with combinations of traits different from either parent. Crossing over during meiosis exchanges genetic material between homologous chromosomes, creating recombinant chromosomes.

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

• Recombinant types: Offspring with new combinations of traits.

• Recombination frequency: Proportion of recombinant offspring, 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 of these gametes leads to aneuploidy, such as monosomy (one chromosome) or trisomy (three chromosomes).

• Nondisjunction: Failure of homologous chromosomes or sister chromatids to separate during meiosis.

• Aneuploidy: Abnormal number of chromosomes in a cell.

• Monosomy: Only one copy of a chromosome.

• Trisomy: Three copies of a chromosome.

Human Disorders Due to Aneuploidy

Down syndrome (trisomy 21) is a common aneuploid condition. Aneuploidy of sex chromosomes can result in syndromes such as Klinefelter (XXY), Turner (X0), and others. Polyploidy, having more than two sets of chromosomes, is common in plants but rare in animals.

• Down syndrome: Caused by three copies of chromosome 21.

• Klinefelter syndrome: XXY males, often with mild symptoms.

• Turner syndrome: X0 females, the only viable monosomy in humans.

• Polyploidy: More than two complete sets of chromosomes.

Alterations of Chromosome Structure

Chromosome breakage can lead to four types of structural changes: deletion, duplication, inversion, and translocation. These changes can cause genetic disorders, such as cri du chat syndrome and chronic myelogenous leukemia (CML).

• Deletion: Loss of a chromosome segment.

• Duplication: Repetition of a chromosome segment.

• Inversion: Reversal of a chromosome segment.

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

Summary Table: Types of Chromosomal Alterations

Type	Description	Example Disorder
Deletion	Removes a chromosomal segment	Cri du chat syndrome
Duplication	Repeats a chromosomal segment	Some forms of Charcot-Marie-Tooth disease
Inversion	Reverses a segment within a chromosome	Usually no specific syndrome, but can disrupt gene function
Translocation	Moves a segment from one chromosome to another	Chronic myelogenous leukemia (CML)