The Chromosomal Basis of Inheritance

The Chromosomal Basis of Inheritance

Introduction

The chromosomal theory of inheritance connects Mendel’s principles of heredity with the behavior of chromosomes during meiosis. This chapter explores how genes are located on chromosomes, how chromosomes segregate and assort independently, and how these processes explain inheritance patterns, including those involving sex chromosomes, linked genes, and chromosomal alterations.

Chromosome Theory of Inheritance

Historical Foundations

• Mendel’s Gene Idea (1860s): Mendel proposed that genes are discrete units of inheritance, but lacked cellular evidence.

• Mitosis (1875) and Meiosis (1890): Discovery of cell division processes revealed that chromosomes exist in pairs and segregate during gamete formation, paralleling Mendel’s laws.

• Chromosome Theory (1902): Genes are located on chromosomes at specific loci. Chromosomes undergo segregation (Anaphase I) and independent assortment (Metaphase I) during meiosis.

Thomas Hunt Morgan and Drosophila Genetics

Morgan’s Experiments

• Model Organism: Morgan used Drosophila melanogaster (fruit fly) to study inheritance.

• Key Discovery: Identified a white-eyed mutation (w) and wild-type red eyes (w+), providing the first scientific support for the chromosomal theory of heredity.

Sex-Linked Inheritance

• When Morgan crossed red-eyed females with white-eyed males, he observed that the white-eye trait appeared only in males in the F2 generation, suggesting the gene was located on the X chromosome.

Sex Chromosomes and Sex-Linked Genes

Structure and Function

• X Chromosome: Much larger, contains many genes (about 1,100), not all related to sex determination.

• Y Chromosome: Much smaller, contains few genes, mainly involved in male development.

Inheritance Patterns of Sex-Linked Genes

• Sex-Linked Gene: A gene located on a sex chromosome (usually X-linked).

• Hemizygous: Males have only one X chromosome, so a single recessive allele will be expressed.

• Examples: Color blindness and hemophilia are X-linked recessive traits.

X Inactivation in Females

• In female mammals, one X chromosome per cell is randomly inactivated during early embryogenesis, forming a Barr body.

• This leads to mosaic expression of X-linked genes, as seen in tortoiseshell cats.

Linked Genes and Crossing Over

Linked Genes

• Genes located close together on the same chromosome tend to be inherited together and are called linked genes.

• Genes far apart on the same chromosome can assort independently due to crossing over.

Crossing Over and Genetic Recombination

• During meiosis, homologous chromosomes exchange segments, producing recombinant chromosomes and new allele combinations.

• The frequency of recombination between two genes is proportional to their distance apart on the chromosome.

Linkage Maps

• Linkage maps use recombination frequencies to estimate the relative positions of genes on a chromosome.

• 1% recombination frequency = 1 map unit (centimorgan).

Testcross Example: Cosmos Plants

• Testcross of AaBb × aabb with unlinked genes predicts a 1:1:1:1 phenotypic ratio in offspring.

Offspring from testcross of AaBb × aabb	Purple stem/short petals (A–B–)	Green stem/short petals (aaB–)	Purple stem/long petals (A–bb)	Green stem/long petals (aabb)
Expected ratio if the genes are unlinked	1	1	1	1

Chromosomal Alterations

Types of Chromosomal Alterations

• Duplication: A segment of a chromosome is repeated. Can be beneficial or harmful (e.g., Huntington’s disease).

• Deletion: Loss of a chromosome segment. Often severe (e.g., Cri du chat syndrome, Williams syndrome).

• Inversion: A chromosome segment is reversed. May cause infertility if pairing is disrupted.

• Translocation: A segment from one chromosome is transferred to another non-homologous chromosome. Can lead to disorders such as Burkitt’s lymphoma and chronic myelogenous leukemia.

Abnormal Chromosome Number

Nondisjunction and Aneuploidy

• Nondisjunction: Failure of homologous chromosomes (Anaphase I) or sister chromatids (Anaphase II) to separate properly during meiosis, resulting in gametes with abnormal chromosome numbers.

• Aneuploidy: Presence of an abnormal number of chromosomes (extra or missing).

• Monosomy (2n-1): Missing a chromosome (e.g., Turner syndrome, XO).

• Trisomy (2n+1): Extra chromosome (e.g., Down syndrome, Trisomy 21).

Human Disorders Due to Chromosomal Abnormalities

• Down Syndrome (Trisomy 21): Characterized by developmental delays, heart defects, and increased risk with maternal age.

• Sex Chromosome Aneuploidy: Includes Klinefelter syndrome (XXY), XYY syndrome, Turner syndrome (XO), and XXX syndrome. Symptoms range from mild to severe, and some individuals may be unaware of their condition.

Summary Table: Types of Chromosomal Alterations

Alteration	Description	Example
Duplication	Repeated segment	Huntington’s disease (excess CAG repeats)
Deletion	Missing segment	Cri du chat (chromosome 5), Williams syndrome (chromosome 7)
Inversion	Reversed segment	Acute myeloid leukemia (chromosome 16 inversion)
Translocation	Segment moved to another chromosome	Burkitt’s lymphoma (8;14), Chronic myelogenous leukemia (9;22)

Key Equations

• Recombination Frequency:

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

• Chi-Square Test for Linkage:

$\chi^2 = \sum \frac{(O - E)^2}{E}$

Where $O$ is the observed number and $E$ is the expected number for each phenotype class.

Conclusion

The chromosomal basis of inheritance provides a unifying framework for understanding how genes are transmitted from one generation to the next. The study of chromosomal behavior during meiosis, the discovery of sex-linked and linked genes, and the analysis of chromosomal alterations are fundamental to modern genetics and the diagnosis of genetic disorders.