Chromosomal Basis of Inheritance: Key Concepts and Applications

Chromosomal Theory of Inheritance

Overview

The Chromosomal Theory of Inheritance connects Mendel's principles of heredity to the physical behavior of chromosomes during cell division. It explains that genes are located on chromosomes, and their movement during meiosis underlies the segregation and independent assortment of alleles.

• Genes are specific DNA sequences coding for proteins, located at a particular locus on a chromosome.

• Diploid organisms inherit one set of chromosomes from each parent, forming homologous pairs (not identical, but code for the same types of proteins).

• During meiosis, homologous chromosomes segregate and their alleles assort independently, providing a physical explanation for Mendel's laws.

• Humans have 23 pairs of chromosomes; one set from each parent. Each pair contains genes for the same traits, but different alleles may be inherited from each parent.

Thomas Hunt Morgan and Fruit Flies

Experimental Evidence for Chromosomal Inheritance

Thomas Hunt Morgan's work with Drosophila melanogaster (fruit flies) provided the first definitive proof that genes are located on chromosomes.

• Fruit flies are ideal model organisms due to their short generation time, high offspring number, and simple genome (four pairs of chromosomes).

• Wild type: The most common trait in nature (e.g., red eyes in fruit flies).

• Mutant phenotype: A less common variant (e.g., white eyes).

• Morgan's experiments showed that a male fruit fly inheriting a white-eye mutation on his X chromosome will have white eyes, since males are hemizygous for X-linked traits.

Example: If a male fruit fly inherits a white-eye mutation on his X chromosome, his eye color will be white because he has only one X chromosome.

Sex-Linked Inheritance

Patterns of Inheritance for Sex Chromosomes

Some traits are tied to sex chromosomes (X and Y), leading to different inheritance patterns in males and females.

• Morgan's classic cross: Red-eyed females × white-eyed males produced all red-eyed F1 offspring. F1 crosses yielded a 3:1 red:white ratio, but only males had white eyes.

• This demonstrated that the gene for eye color was on the X chromosome.

• Males (XY) are more likely to express X-linked traits because they have only one X chromosome; females (XX) can mask recessive alleles with a second X.

Systems of Sex Determination

Different Mechanisms Across Species

• XY System (mammals, fruit flies): Females are XX, males are XY. The SRY gene on the Y chromosome triggers male development.

• ZW System (birds, fish): Females are ZW (heterogametic), males are ZZ (homogametic).

• Haplo-diploid System (bees, ants): Females develop from fertilized (diploid, 2n) eggs; males from unfertilized (haploid, n) eggs.

Example: A honeybee queen can produce diploid females or haploid male drones depending on whether she fertilizes the egg.

X-Linked Genes and Disorders

Inheritance and Expression of X-Linked Traits

• The X chromosome codes for ~1,100 proteins; the Y chromosome codes for ~78, mostly related to sex determination.

• Males are hemizygous for X-linked genes, so any recessive allele will be expressed.

• Females must inherit two copies of a recessive allele to express the disorder.

Examples of X-linked recessive disorders: Red-green color blindness, hemophilia.

Example: A female carrier for color blindness (XNXn) and a normal male (XNY) have a 50% chance of having a color-blind son.

X-Chromosome Inactivation

Dosage Compensation in Females

• To balance gene dosage, one of a female's two X chromosomes is randomly inactivated in each cell, forming a Barr body.

• Heterozygous females are genetic mosaics, expressing different alleles in different cells.

Example: Calico cats (almost always female) have patches of black and orange fur due to random X inactivation.

Probability of a normal calico cat being male: 0% (since X inactivation only occurs in females).

Linked Genes and Genetic Mapping

Gene Linkage and Recombination

Genes located close together on the same chromosome are often inherited together (gene linkage), but crossing over can separate them, producing recombinant offspring.

• The frequency of recombination is used to create genetic maps; 1% recombination = 1 map unit.

Gene Pair	Recombination Frequency (%)
A and Z	10
B and Y	30
A and Y	45
Z and B	5
Z and Y	35
A and B	15

Interpretation: Genes A and Y are furthest apart (highest recombination frequency), so they are on opposite ends of the chromosome.

Chromosomal Alterations

Numerical and Structural Changes

• Nondisjunction: Failure of chromosomes to separate properly during meiosis, leading to aneuploidy (abnormal chromosome number).

• Monosomy: Missing a chromosome (e.g., Turner syndrome, XO).

• Trisomy: Extra chromosome (e.g., Down syndrome, trisomy 21; Klinefelter syndrome, XXY).

• Structural alterations:

  • Deletion: Segment lost

  • Duplication: Segment repeated

  • Inversion: Segment reversed

  • Translocation: Segment moves to another chromosome (e.g., Philadelphia chromosome in leukemia)

Examples:

• Turner syndrome (XO): Sterile females, short stature, possible heart defects.

• Klinefelter syndrome (XXY): Sterile males, taller than average, possible female secondary sexual characteristics.

Exceptions to Mendelian Inheritance

Non-Chromosomal Patterns

• Genomic Imprinting: Expression of a gene depends on the parent of origin, often due to DNA methylation.

• Extranuclear Inheritance: Genes in mitochondria and chloroplasts are inherited maternally (from the egg cytoplasm).

Example: Mitochondrial diseases are always passed from mother to all her children, regardless of sex.