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.

• Genes are segments of DNA located on chromosomes.

• Chromosomes duplicate before cell division, ensuring each gamete receives one set.

• During meiosis I, homologous chromosomes separate; during meiosis II, sister chromatids separate.

• Each chromosome carries one version (allele) of a gene.

• Fertilization restores the diploid number, combining alleles from both parents.

Morgan’s Experiments with Drosophila

Thomas Hunt Morgan used Drosophila melanogaster (fruit flies) to demonstrate the chromosomal basis of inheritance.

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

• Wild type refers to the most common phenotype in a population; mutant phenotypes are alternatives.

• Morgan’s discovery of a white-eyed mutant fly provided evidence that genes are located on chromosomes.

Correlating Gene Behavior with Chromosome Behavior

Morgan’s findings supported the chromosome theory by showing that the inheritance of a specific gene correlated with the inheritance of a specific chromosome.

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 and many animals, these are the X and Y chromosomes.

• Females: XX; Males: XY.

• The SRY gene on the Y chromosome triggers male development.

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

Systems of Sex Determination

Different organisms use various systems for sex determination:

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

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

• Z-W system: Birds, some fish, and butterflies (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.

• The human X chromosome contains about 1,100 genes; the Y chromosome has about 78 genes.

• Inheritance patterns differ for X-linked and Y-linked genes due to the difference in chromosome number between sexes.

Inheritance of X-Linked Genes

X-linked recessive disorders are more common in males because they have only one X chromosome.

• Examples: Color blindness, Duchenne muscular dystrophy, hemophilia.

• Females must inherit two copies of the mutant allele to express the disorder; males need only one.

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 heterozygotes.

• Example: Tortoiseshell cats display patches of different fur color due to X inactivation.

Concept 15.3: Linked Genes and Genetic Recombination

Linked Genes

Genes located near each other on the same chromosome tend to be inherited together and are called linked genes.

• Linked genes do not assort independently during meiosis.

• Testcrosses can reveal linkage by showing deviations from expected Mendelian ratios.

Genetic Recombination

Genetic recombination produces offspring with combinations of traits different from either parent. This can occur through independent assortment or crossing over.

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

• Recombinant types: Offspring with new combinations of traits.

• Crossing over during meiosis exchanges segments between homologous chromosomes, creating recombinants.

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

Concept 15.4: Alterations of Chromosome Number or Structure

Nondisjunction and Aneuploidy

Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during meiosis, leading to gametes with abnormal chromosome numbers.

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

• 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): Caused by three copies of chromosome 21; associated with developmental delays and characteristic physical features.

• Klinefelter syndrome (XXY): Males with an extra X chromosome; often sterile with some female characteristics.

• Turner syndrome (X0): Females with only one X chromosome; sterile and may have other physical abnormalities.

• Other aneuploidies include XYY males and XXX females, often with mild or no symptoms.

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 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: Caused by a deletion in chromosome 5; leads to intellectual disability and a distinctive cry.

• Chronic myelogenous leukemia (CML): Often 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 within a chromosome	Usually no obvious disorder
Translocation	Segment moves to a nonhomologous chromosome	Chronic myelogenous leukemia (CML)

Key Equations

• Recombination Frequency: $\text{Recombination Frequency} = \frac{\text{Number of Recombinant Offspring}}{\text{Total Number of Offspring}} \times 100$