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 through 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.

• Sex linkage: Genes located on sex chromosomes show unique inheritance patterns.

Chromosome Theory of Inheritance

Morgan’s work provided evidence that genes are located on chromosomes, supporting the chromosome theory of inheritance. The behavior of chromosomes during meiosis explains Mendel’s laws of segregation and independent assortment.

• Law of Segregation: Alleles for a 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

Humans and many other organisms have sex chromosomes (X and Y) that determine biological sex. Females are XX, and males are XY. The SRY gene on the Y chromosome triggers male development.

• X-linked genes: Genes on the X chromosome; about 1,100 in humans.

• Y-linked genes: Genes on the Y chromosome; only about 78 in humans.

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

Inheritance of X-Linked Genes

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

• Carrier female: Heterozygous for an X-linked trait; can pass the allele to offspring.

• Affected male: Has the recessive allele on his single X chromosome.

X Inactivation in Female Mammals

In females, 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, as seen in the coat color of calico cats.

• Barr body: Inactive X chromosome in a female cell.

• 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 always assort independently.

• Linked genes: Genes on the same chromosome that tend to be inherited together.

• Genetic recombination: Production of offspring with combinations of traits differing from either parent.

Recombination and Crossing Over

Crossing over during meiosis can break the linkage between genes, producing recombinant chromosomes. The recombination frequency can be used to map the distance between genes on a chromosome.

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

• Recombinant types: Offspring with new combinations of traits.

• 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, resulting in gametes with abnormal chromosome numbers. Fertilization of such gametes leads to aneuploidy, which can cause developmental disorders.

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

• Aneuploidy: Abnormal number of chromosomes (e.g., monosomy, trisomy).

• Monosomic: Zygote with only one copy of a chromosome.

• Trisomic: Zygote with three copies of a chromosome.

Human Disorders Due to Aneuploidy

Down syndrome (trisomy 21) is caused by an extra copy of chromosome 21. Aneuploidy of sex chromosomes can result in conditions such as Klinefelter syndrome (XXY), Turner syndrome (X0), and others.

• Down syndrome: Trisomy 21; associated with developmental delays and characteristic physical features.

• Klinefelter syndrome: XXY males; often sterile with some female characteristics.

• Turner syndrome: X0 females; sterile and often with short stature.

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).

• Polyploidy: More than two sets of chromosomes in an organism.

• 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, 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 segment within a chromosome.

• 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 developmental disorders
Inversion	Reverses a segment within a chromosome	Usually no obvious disorder
Translocation	Moves a segment to a nonhomologous chromosome	Chronic myelogenous leukemia (CML)

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.