Chromosomal Basis of Inheritance

Linkage, recombination, and chromosomal disorders

🧬 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

📚 Practice Problems

1Problem 1easy

❓ Question:

What is the chromosomal theory of inheritance? Who were the key scientists involved in developing it?

💡 Show Solution

Chromosomal Theory of Inheritance: The theory states that genes are located on chromosomes, and the behavior of chromosomes during meiosis accounts for Mendel's laws of inheritance.

Key Scientists:

  1. Walter Sutton (1903) • Studied grasshopper chromosomes • Observed parallel behavior of chromosomes and Mendelian factors

  2. Theodor Boveri (1903) • Studied sea urchin embryos • Showed chromosomes carry genetic information

  3. Thomas Hunt Morgan (1910s) • Provided experimental proof using Drosophila • Discovered sex-linkage and genetic recombination • Won Nobel Prize (1933)

Key Evidence: • Chromosomes and genes both come in pairs • Homologous chromosome pairs separate during meiosis (explains Mendel's law of segregation) • Different chromosome pairs assort independently (explains Mendel's law of independent assortment) • Chromosome number is restored during fertilization

This unified cytology and genetics, explaining the physical basis of heredity.

2Problem 2medium

❓ Question:

In Drosophila, white eye color is X-linked recessive (Xw) and red is dominant (X+). What are the expected phenotypes and their ratios from a cross between a white-eyed female and a red-eyed male?

💡 Show Solution

Parental genotypes: Female (white eyes): XwXw Male (red eyes): X+Y

Gametes: Female produces: Xw only Male produces: X+ or Y

Punnett square: X+ Y Xw X+Xw XwY Xw X+Xw XwY

Offspring: Females: 100% X+Xw (red eyes, carriers) Males: 100% XwY (white eyes)

Phenotypic ratio: • All females: red eyes (2/4 = 50%) • All males: white eyes (2/4 = 50%)

This is a classic example of sex-linkage discovered by Morgan. The reciprocal cross (red female × white male) gives different results, proving the gene is on the X chromosome.

Key observation: Males show the recessive trait more frequently because they have only one X chromosome (hemizygous).

3Problem 3hard

❓ Question:

Explain why genes on the same chromosome don't always follow Mendel's law of independent assortment. How did Morgan's work on linked genes modify Mendelian genetics?

💡 Show Solution

Mendel's Law of Independent Assortment: • Assumes genes are on different chromosomes • Each gene pair separates independently during meiosis • Predicts 9:3:3:1 ratio for dihybrid cross

Linked Genes (on same chromosome): • Do NOT assort independently • Tend to be inherited together • Produce parental combinations more frequently than recombinant types

Morgan's Discovery:

  1. Found genes for body color and wing size in Drosophila were linked
  2. Expected 1:1:1:1 from testcross, but got parental types > 50%
  3. Explained by genes being on same chromosome

Modification to Mendelian genetics: • Genes on same chromosome = linked • Closer genes = more tightly linked (less recombination) • Crossing over can separate linked genes • Recombination frequency proportional to distance between genes • Led to chromosome mapping

General principle: • Genes on DIFFERENT chromosomes: independent assortment (50% recombination) • Genes on SAME chromosome: linkage (< 50% recombination) • Distance determines recombination frequency

This explained exceptions to Mendel's laws and showed chromosomal location matters!

4Problem 4hard

❓ Question:

A female Drosophila heterozygous for genes A and B (on the same chromosome) is testcrossed. Of 1000 offspring, 450 show AB phenotype, 450 show ab, 50 show Ab, and 50 show aB. What is the map distance between genes A and B?

💡 Show Solution

Step 1: Identify parental vs recombinant types Testcross: AaBb × aabb

Offspring: AB: 450 } Parental types (most common) ab: 450 } Total = 900

Ab: 50 } Recombinant types (less common) aB: 50 } Total = 100

Step 2: Calculate recombination frequency Recombination frequency = (# recombinants / total offspring) × 100%

RF = (100 / 1000) × 100% = 10%

Step 3: Convert to map units 1% recombination = 1 map unit (m.u.) = 1 centimorgan (cM)

Map distance = 10 map units (or 10 cM)

Interpretation: • Genes A and B are 10 map units apart on the chromosome • 10% of gametes show recombination between these loci • Genes are linked but not tightly (moderate distance) • Crossing over occurs between them in 10% of meioses

Note: Maximum recombination frequency is 50% (for unlinked genes). Values < 50% indicate linkage.

5Problem 5hard

❓ Question:

Explain genomic imprinting. How does it violate Mendelian genetics, and what is an example in humans?

💡 Show Solution

Genomic Imprinting: A phenomenon where gene expression depends on which parent the allele came from. One allele is silenced based on parent of origin.

Violates Mendelian Genetics: • Mendel assumed both alleles contribute equally • Imprinting means maternal and paternal alleles are NOT equivalent • Phenotype depends on parent of origin, not just genotype

Mechanism: • Epigenetic modification (usually DNA methylation) • Occurs during gamete formation • Silences one allele while leaving the other active • Imprinting is "erased" and "reset" each generation

Human Example: Prader-Willi and Angelman Syndromes Both involve deletion on chromosome 15, but different phenotypes:

Prader-Willi Syndrome: • Deletion of paternal chromosome 15q11-13 • Symptoms: obesity, intellectual disability, short stature • Maternal copy is imprinted (silenced), so deletion of paternal copy = no gene expression

Angelman Syndrome: • Deletion of maternal chromosome 15q11-13 • Symptoms: severe intellectual disability, seizures, inappropriate laughter • Paternal copy is imprinted (silenced), so deletion of maternal copy = no gene expression

Same chromosomal region, different diseases depending on parent of origin!

Evolutionary significance: May reflect parent-offspring conflict over resource allocation.

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