Chapter 15: The Chromosomal Basis of Inheritance – Study Notes

Chapter 15: The Chromosomal Basis of Inheritance

Review: Mendelian Traits

Mendelian traits are inherited according to the principles established by Gregor Mendel. These traits are typically determined by single genes located on autosomes (non-sex chromosomes).

• Carried on autosomes: Most Mendelian traits are found on autosomes, not sex chromosomes.

• Two possible alleles per gene: Each gene has two alleles, one from each parent.

• Inheritance independent of sex: The inheritance of these traits does not depend on the sex of the organism.

• Example: For a homologous chromosome pair, each chromosome carries one allele for a specific trait, which can be dominant or recessive.

Relationship Between Genes and Chromosomes

Genes are segments of DNA located on chromosomes. Chromosomes duplicate before cell division, and during meiosis, homologous chromosomes separate, allowing alleles to segregate into gametes.

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

• Inheritance: Offspring inherit one allele from each parent for each gene.

• Homologous chromosomes: Have the same genes at the same loci, but possibly different alleles.

Concept 15.1 – Thomas Hunt Morgan and Drosophila melanogaster

Thomas Hunt Morgan used the fruit fly (Drosophila melanogaster) to study inheritance patterns, providing key evidence for the chromosomal theory of inheritance.

• Model organism: Drosophila melanogaster produces many offspring, has a short generation time, and four pairs of chromosomes (three autosomes, one pair of sex chromosomes).

• Wild-type vs. mutant phenotypes: Wild-type (normal) phenotypes are denoted with a capital letter and a "+" (e.g., W+), while mutant phenotypes use a lowercase letter (e.g., w).

• Example: Red eye color (W+W+ or W+w) is wild-type; white eye color (ww) is mutant.

Morgan's Experiments

• P Generation: Homozygous red-eyed female (W+W+) × white-eyed male (ww).

• F1 Generation: All offspring had red eyes.

• F2 Generation: 3:1 ratio of red to white eyes, but only males had white eyes.

• Conclusion: The gene for eye color is located on the X chromosome (sex-linked), not on autosomes as in Mendelian traits.

Chromosomal Theory of Inheritance

• Support: Morgan's findings supported the idea that specific genes are carried on specific chromosomes.

• Sex chromosomes: Genes located on sex chromosomes (X or Y) exhibit unique inheritance patterns.

Concept 15.2 – Sex-linked Genes

Sex-linked genes are located on sex chromosomes and show distinct inheritance patterns.

• Sex determination in humans: XX (female), XY (male).

• Other systems: ZW system (birds), XO system (insects), haplo-diploid system (bees).

Types of Sex-linked Genes

• Y-linked genes: Mostly related to male sex determination.

• X-linked genes: Many traits unrelated to sex determination are found here.

Inheritance Patterns of X-linked Genes

• Females (XX): Can be homozygous or heterozygous for X-linked traits.

• Males (XY): Are hemizygous (only one X chromosome), so a single recessive allele will be expressed.

• Notation: Use "X" and a letter to designate alleles (e.g., XN = dominant, Xn = recessive).

Examples of X-linked Crosses

Parents	Genotypes	Offspring Phenotypes
Homozygous dominant female × recessive male	XNXN × XnY	All offspring show dominant phenotype
Heterozygous female × dominant male	XNXn × XNY	All females dominant; 50% males dominant, 50% males recessive
Heterozygous female × recessive male	XNXn × XnY	Varied; 50% chance for recessive phenotype in males

X-inactivation in Females

• Random X-inactivation: In female mammals, one X chromosome in each cell is randomly inactivated during early embryonic development, forming a Barr body.

• Mosaicism: Females are genetic mosaics for X-linked genes, as different cells may express different alleles.

• Example: Calico cats display mosaic fur color due to X-inactivation.

Concept 15.4 – Alteration of Chromosome Number

Changes in chromosome number can lead to genetic disorders and are often the result of errors during meiosis.

Nondisjunction

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

• Results: Gametes with abnormal numbers of chromosomes (n+1 or n-1).

Aneuploidy

• Definition: Presence of an abnormal number of a particular chromosome.

• Types:

  • Monosomy: Missing one chromosome (2n-1).

  • Trisomy: Having an extra chromosome (2n+1).

• Examples:

  • Trisomy 21: Down syndrome (extra chromosome 21).

  • Sex chromosome aneuploidy: XXX (female), XXY (Klinefelter syndrome), X0 (Turner syndrome).

Polyploidy

• Definition: More than two complete sets of chromosomes (e.g., triploid 3n, tetraploid 4n).

• Common in plants: Polyploidy is rare in animals but common in plants and can lead to new species.

Alterations of Chromosome Structure

Type	Description
Deletion	A chromosomal segment is removed.
Duplication	A segment is repeated.
Inversion	A segment is reversed within the chromosome.
Translocation	A segment moves from one chromosome to a nonhomologous chromosome.

Summary: The chromosomal basis of inheritance explains how genes are transmitted via chromosomes, how sex-linked traits are inherited, and how alterations in chromosome number or structure can lead to genetic variation and disorders.