12.1

The Chromosomal Basis of Inheritance

Introduction

The chromosomal theory of inheritance connects Mendel’s laws of heredity with the behavior of chromosomes during meiosis. Early geneticists, including Gregor Mendel and Thomas Hunt Morgan, made key discoveries that established the foundation for modern genetics.

Mendel’s Laws and Chromosome Behavior

Mendel’s Laws

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation so that each gamete receives only one allele.

• Law of Independent Assortment: Genes for different traits assort independently of one another in gamete formation, provided they are on different chromosomes or far apart on the same chromosome.

These laws were deduced from Mendel’s experiments with pea plants, but the physical basis was not understood until chromosomes were observed during meiosis.

Chromosome Theory of Inheritance

• Chromosomes carry genes, and their behavior during meiosis explains Mendel’s laws.

• Homologous chromosomes separate during meiosis I, accounting for segregation of alleles.

• Nonhomologous chromosomes assort independently, explaining independent assortment.

Thomas Hunt Morgan and Drosophila Genetics

Morgan’s Experiments

• Morgan used Drosophila melanogaster (fruit flies) to study inheritance patterns.

• He discovered the first mutant phenotype: white-eyed flies, compared to the wild-type red-eyed flies.

• Crosses between red-eyed and white-eyed flies revealed that the white-eye trait was sex-linked, appearing only in males in the F2 generation.

Sex-Linked Inheritance

• The gene for eye color in Drosophila is located on the X chromosome.

• Males (XY) inherit their X chromosome from their mother and Y from their father; females (XX) inherit one X from each parent.

• Recessive X-linked traits are more likely to be expressed in males, who have only one X chromosome.

Example: If a male inherits the X chromosome carrying the white-eye allele, he will express the trait because he lacks a second X chromosome with a dominant allele.

Linkage and Independent Assortment

Linked Genes

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

• Linked genes do not assort independently unless crossing over occurs between them during meiosis.

Crossing Over

• During meiosis, homologous chromosomes can exchange segments in a process called crossing over.

• This recombination can separate linked genes, producing new combinations of traits.

Meiosis and Mendel’s Laws

Segregation and Independent Assortment in Meiosis

• During meiosis I, homologous chromosomes (and thus alleles) separate, explaining segregation.

• During metaphase I, the random orientation of chromosome pairs explains independent assortment.

Diagram Description: Diagrams in the original material likely illustrated the separation of homologous chromosomes and the assortment of different chromosome pairs during meiosis.

Key Terms and Definitions

• Gene: A unit of heredity that encodes information for a specific trait.

• Allele: Different forms of a gene.

• Homozygous: Having two identical alleles for a gene.

• Heterozygous: Having two different alleles for a gene.

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

• Mutant phenotype: A phenotype resulting from a change (mutation) in the wild-type gene.

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

Summary Table: Mendel’s Laws and Chromosome Behavior

Mendel’s Law	Chromosomal Basis	Example
Law of Segregation	Separation of homologous chromosomes during meiosis I	Alleles for flower color segregate into different gametes
Law of Independent Assortment	Random orientation of chromosome pairs during metaphase I	Seed color and seed shape genes assort independently

Equations and Genetic Ratios

• Monohybrid Cross Ratio:

$3:1$ (dominant:recessive phenotype in F2 generation)

• Dihybrid Cross Ratio:

$9:3:3:1$ (phenotypic ratio for two independently assorting genes in F2 generation)

Conclusion

The chromosomal theory of inheritance provides a physical explanation for Mendel’s laws. Morgan’s work with fruit flies demonstrated the connection between genes and chromosomes, especially for sex-linked traits. Understanding these principles is fundamental to modern genetics and explains how traits are inherited across generations.