🧬 Chromosomal Basis of Inheritance

Chromosome Theory of Inheritance

Key principles:

1. Genes located on chromosomes
2. Chromosomes segregate during meiosis
3. Explains Mendel's laws at cellular level

Linked Genes

Genes on same chromosome tend to be inherited together

• Linkage: genes close together on chromosome
• Violates independent assortment
• Linked genes don't assort independently

Parental types: Original allele combinations Recombinant types: New allele combinations (from crossing over)

Recombination and Gene Mapping

Crossing over can separate linked genes

• Occurs during Prophase I of meiosis
• Exchanges DNA between homologous chromosomes

Recombination frequency:

• % of offspring showing recombinant phenotypes
• Depends on distance between genes
• 1% recombination = 1 map unit (m.u.) or centimorgan (cM)

Closer genes:

• Less likely to be separated by crossing over
• Lower recombination frequency
• More tightly linked

Farther genes:

• More likely to be separated
• Higher recombination frequency
• Less tightly linked

Maximum recombination: 50% (genes on different chromosomes or very far apart)

Gene Mapping Example

If genes A and B show 20% recombination → 20 map units apart

If three genes:

• A-B: 20% recombination
• B-C: 10% recombination
• A-C: 30% recombination

Gene order: A----B--C (B is between A and C)

Sex Determination

Humans:

• XX = female (homogametic)
• XY = male (heterogametic)
• Y chromosome has SRY gene → male development

Sex ratio: ~1:1 (50% male, 50% female)

Other systems:

• Birds: ZW female, ZZ male
• Bees: diploid females, haploid males
• Some reptiles: temperature determines sex

X-Inactivation (Dosage Compensation)

In female mammals:

• One X chromosome randomly inactivated in each cell
• Forms Barr body (condensed, inactive X)
• Equalizes gene expression between XX and XY

Example: Calico cats

• Heterozygous for coat color gene on X
• Random X-inactivation creates patches
• Orange and black patches
• Only females can be calico (need two X chromosomes)

Chromosomal Alterations

Changes in Chromosome Number

Polyploidy: Extra complete sets of chromosomes

• Triploidy (3n): usually lethal in animals
• Tetraploidy (4n): common in plants (larger, hardier)

Aneuploidy: Missing or extra individual chromosomes

• Monosomy (2n-1): one chromosome missing
• Trisomy (2n+1): one extra chromosome

Human aneuploidies:

• Down syndrome: Trisomy 21 (3 copies chromosome 21)
• Edwards syndrome: Trisomy 18
• Patau syndrome: Trisomy 13
• Turner syndrome: 45, X (monosomy X)
• Klinefelter syndrome: 47, XXY

Changes in Chromosome Structure

Deletion: Segment of chromosome lost Duplication: Segment repeated Inversion: Segment reversed Translocation: Segment moved to another chromosome

Effects:

• Often harmful
• Can cause genetic disorders
• Can lead to evolution (duplications)

Key Concepts

1. Linked genes on same chromosome don't assort independently
2. Recombination frequency indicates distance between genes
3. 1% recombination = 1 map unit
4. Sex determination: XY system in humans
5. X-inactivation: one X randomly inactivated in female cells
6. Aneuploidy: abnormal chromosome number (monosomy, trisomy)
7. Structural changes: deletion, duplication, inversion, translocation