The Chromosomal Basis of Inheritance: Study Notes

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The Chromosomal Basis of Inheritance

Introduction

The chromosomal basis of inheritance explains how genes are transmitted from parents to offspring through chromosomes. This concept integrates Mendelian genetics with cytological observations of chromosomes during cell division, providing a molecular foundation for inheritance patterns.

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 and independent assortment during meiosis, explaining Mendel's observations.

Diagram showing the relationship between genes and chromosomes, meiosis, and fertilization

Key Principles

Segregation: Homologous chromosomes (and thus alleles) separate during Anaphase I of meiosis.
Independent Assortment: Chromosome pairs align randomly during Metaphase I, leading to genetic variation.

Diagram illustrating Mendel's laws of segregation and independent assortment

Thomas Hunt Morgan and Drosophila Experiments

Scientific Support for Chromosomal Theory

Model Organism: Morgan used Drosophila melanogaster (fruit fly) to study inheritance.
Key Discovery: Identified a white-eye mutation, providing the first evidence that genes are located on chromosomes.

Comparison of red-eyed and white-eyed Drosophila

Morgan's White-Eyed Fly Crosses

Tested whether the white-eye gene was autosomal or sex-linked.
Observed inheritance patterns that matched X-linked transmission, not autosomal.

Punnett square for Morgan's white-eyed fly cross showing X-linked inheritance Punnett squares comparing sex-linked and autosomal inheritance

Sex Chromosomes and Sex-Linked Genes

Structure and Function

X Chromosome: Large, contains many genes (about 1,100), not all related to sex determination.
Y Chromosome: Small, few genes, mainly involved in male development.
Small regions of homology at the tips allow pairing during meiosis.

Image of X and Y chromosomes

Sex-Linked Genes

Definition: Genes located on sex chromosomes, especially the X chromosome (X-linked genes).
Inheritance Patterns: X-linked recessive traits are more common in males (hemizygous for X).
Examples: Color blindness, hemophilia.

Pedigree showing inheritance of a sex-linked trait (hemophilia)

Practice Problem: Hemophilia Inheritance

If a carrier female (XHXh) marries a normal male (XHY):
1/4 daughters will be carriers, 0 will have hemophilia; 1/4 sons will have hemophilia, 1/4 will be normal.

Punnett square for X-linked inheritance (hemophilia)

X Inactivation

In females, one X chromosome is randomly inactivated in each cell during early embryogenesis, forming a Barr body.
This leads to mosaic expression of X-linked genes (e.g., tortoiseshell cats).

Microscopic image showing Barr bodies in XX cells Diagram of X inactivation mosaicism in early embryo Photo of a tortoiseshell cat showing mosaic fur color

Linked Genes and Crossing Over

Linked Genes

Genes located close together on the same chromosome tend to be inherited together.
Linkage can be disrupted by crossing over during meiosis, producing recombinant chromosomes.

Diagram of linked genes on a chromosome Diagram showing crossing over and recombinant chromosomes

Linkage Mapping

Recombination frequency between genes is used to construct genetic maps (1% recombination = 1 map unit).
Genes far apart on the same chromosome can appear unlinked (50% recombination frequency).

Diagram of a chromosome with map units and linked/independent genes Diagram showing recombination frequencies between genes

Testcross Example: Cosmos Plants

Cross: AABB × aabb → AaBb (F1), then AaBb × aabb (testcross).
If genes are unlinked, expect a 1:1:1:1 phenotypic ratio in offspring.

Handwritten cross setup for AaBb × aabb Handwritten Punnett square for AaBb × aabb

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

Table showing expected phenotypic ratios for unlinked genes

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 number of offspring (of 900)	225	225	225	225

Table showing expected numbers of offspring for unlinked genes

Heritable Changes in Chromosome Structure

Types of Chromosomal Alterations

Duplication: Repetition of a chromosome segment (e.g., Huntington's disease: excess CAG repeats).
Deletion: Loss of a chromosome segment (e.g., Cri du chat syndrome, Williams syndrome).
Inversion: Reversal of a chromosome segment; can cause infertility or disease (e.g., acute myeloid leukemia).
Translocation: Segment moves to a non-homologous chromosome; can be reciprocal (e.g., Burkitt's lymphoma, 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): Characteristic facial features, heart defects, developmental delays; risk increases with maternal age.
Sex Chromosome Aneuploidy:
- Klinefelter Syndrome (XXY): Male with extra X, some female characteristics, sterile.
- XYY Syndrome: Male with extra Y, usually taller.
- Turner Syndrome (XO): Female with one X, short stature, sterile.
- XXX Syndrome: Female with extra X, usually no significant symptoms.

Summary Table: Types of Chromosomal Alterations

Alteration	Description	Example
Duplication	Repeated segment	Huntington's disease (CAG repeats)
Deletion	Missing segment	Cri du chat, Williams syndrome
Inversion	Reversed segment	Acute myeloid leukemia
Translocation	Segment moves to another chromosome	Burkitt's lymphoma, CML

Additional info: The study of chromosomal inheritance is foundational for understanding genetic disorders, genetic mapping, and the molecular mechanisms underlying heredity.