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Linkage, recombination, and chromosomal disorders
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Key principles:
Genes on same chromosome tend to be inherited together
Parental types: Original allele combinations Recombinant types: New allele combinations (from crossing over)
Crossing over can separate linked genes
Recombination frequency:
What is the chromosomal theory of inheritance? Who were the key scientists involved in developing it?
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:
Walter Sutton (1903) • Studied grasshopper chromosomes • Observed parallel behavior of chromosomes and Mendelian factors
Theodor Boveri (1903) • Studied sea urchin embryos • Showed chromosomes carry genetic information
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
| Section | Format | Questions | Time | Weight | Calculator |
|---|---|---|---|---|---|
| Multiple Choice | MCQ | 60 | 90 min | 50% | 🚫 |
| Free Response (Long) | FRQ | 2 | 50 min | 30% | 🚫 |
| Free Response (Short) | FRQ | 4 | 40 min | 20% | 🚫 |
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Closer genes:
Farther genes:
Maximum recombination: 50% (genes on different chromosomes or very far apart)
If genes A and B show 20% recombination → 20 map units apart
If three genes:
Gene order: A----B--C (B is between A and C)
Humans:
Sex ratio: ~1:1 (50% male, 50% female)
Other systems:
In female mammals:
Example: Calico cats
Polyploidy: Extra complete sets of chromosomes
Aneuploidy: Missing or extra individual chromosomes
Human aneuploidies:
Deletion: Segment of chromosome lost Duplication: Segment repeated Inversion: Segment reversed Translocation: Segment moved to another chromosome
Effects:
This unified cytology and genetics, explaining the physical basis of heredity.
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?
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).
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?
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:
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!
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?
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.
Explain genomic imprinting. How does it violate Mendelian genetics, and what is an example in humans?
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.