Chromosomal Basis of Inheritance

Chromosome Theory of Inheritance

The chromosome theory of inheritance states that genes are located on chromosomes and that the behavior of chromosomes during meiosis accounts for Mendel's laws of segregation and independent assortment.

Key Principles

• Genes have specific loci (positions) on chromosomes
• Chromosomes undergo segregation during meiosis I → explains Mendel's Law of Segregation
• Chromosomes on different pairs assort independently → explains Mendel's Law of Independent Assortment
• Each chromosome carries hundreds to thousands of genes

Historical Development

The Sutton-Boveri hypothesis proposed that chromosomes are the physical carriers of genes — a revolutionary idea that unified cytology and genetics.

Concept Check 🎯

Linkage Groups

A linkage group is a set of genes located on the same chromosome that tend to be inherited together.

Why Linkage Matters

• Mendel's Law of Independent Assortment applies only to genes on different chromosomes
• Genes on the same chromosome do NOT assort independently — they are linked
• The number of linkage groups equals the haploid chromosome number (n)

Linked vs. Unlinked Genes

• Unlinked genes (on different chromosomes): produce a 1:1:1:1 ratio in test cross offspring
• Completely linked genes: produce only parental types (no recombinants)
• Partially linked genes: produce mostly parental types with some recombinant types

💡 Crossing over during prophase I of meiosis can separate linked genes, producing recombinant chromosomes.

Fill in the Blanks 🔍

Morgan's Drosophila Experiments

Thomas Hunt Morgan used the fruit fly Drosophila melanogaster as a model organism because of its advantages:

• Short generation time (~2 weeks)
• Prolific reproduction (hundreds of offspring)
• Only 4 pairs of chromosomes (easy to study)
• Many visible mutations (eye color, wing shape, body color)

The White-Eye Discovery (1910)

Morgan crossed a white-eyed male fly with a red-eyed (wild-type) female:

P cross: ♀ red-eyed × ♂ white-eyed F₁: All red-eyed (red is dominant)

F₁ × F₁ cross:

• F₂ females: all red-eyed
• F₂ males: ½ red-eyed, ½ white-eyed

Morgan's Conclusion

The white-eye gene must be on the X chromosome:

• Males have only one X → a single recessive allele is expressed
• Females have two X chromosomes → need two copies of the recessive allele

This was the first gene mapped to a specific chromosome, directly supporting the chromosome theory of inheritance.

🔬 Morgan won the Nobel Prize in Physiology or Medicine in 1933 for his discoveries concerning the role of chromosomes in heredity.

Concept Check 🎯

Sex-Linked Traits

Sex-linked traits are controlled by genes located on the sex chromosomes (X or Y). Because males and females have different combinations of sex chromosomes, these traits show distinctive inheritance patterns.

Sex Determination

In the XX-XY system:

• The X chromosome is large, carrying ~800 genes
• The Y chromosome is small, carrying ~50 genes (mostly involved in male development)
• The SRY gene on the Y chromosome triggers male development

X-Linked Recessive Traits

X-linked recessive traits are the most commonly tested sex-linked pattern on the AP exam.

Linked Genes & Recombination

When genes are on the same chromosome, they tend to be inherited together — this is genetic linkage. However, crossing over during meiosis can shuffle linked alleles, producing recombinant offspring.

Parental vs. Recombinant Types

Consider two linked genes (A and B) on the same chromosome:

Example

If a parent has alleles AB on one chromosome and ab on the homolog:

• Parental gametes: AB, ab
• Recombinant gametes: Ab, aB (from crossing over)

💡 Key rule: If two genes are linked, the recombinant classes will always be less frequent than the parental classes. If they're unlinked, all four classes appear in roughly equal proportions (~25% each).

Concept Check 🎯

Recombination Frequency & Map Distance

measures how often crossing over separates two linked genes.

Chromosomal Mutations — Changes in Chromosome Number

Chromosomal mutations involve changes in the number or structure of chromosomes. These large-scale alterations can have dramatic effects on phenotype and are a significant source of genetic disorders.

Nondisjunction

Nondisjunction is the failure of chromosomes (or chromatids) to separate properly during cell division.

When It Occurs

Consequences

After fertilization with a normal gamete:

• Gamete with extra chromosome → trisomy (2n + 1)
• Gamete with missing chromosome → monosomy (2n − 1)

⚠️ Most aneuploid embryos are spontaneously aborted. It is estimated that ~25% of all conceptions involve chromosomal abnormalities.

Structural Chromosomal Changes

Structural chromosomal abnormalities involve rearrangements of chromosome segments rather than changes in chromosome number. These can arise from errors in DNA repair, recombination, or breakage.

Four Major Types

💡 These changes affect gene dosage, gene regulation, and can disrupt genes at breakpoints.

Deletions

A deletion occurs when a chromosomal segment is lost.

Effects

• Loss of genes → usually detrimental
• Pseudodominance: A recessive allele on the normal homolog is expressed because the dominant allele is deleted

Genomic Imprinting & Extranuclear Inheritance

Not all inheritance follows standard Mendelian patterns. Genomic imprinting and extranuclear inheritance demonstrate that the source of an allele (which parent) and the location of genes (nucleus vs. organelles) can both influence phenotype.

Genomic Imprinting

Genomic imprinting is an epigenetic phenomenon where certain genes are expressed differently depending on whether they were inherited from the mother or the father.

How It Works

• Specific genes are silenced (imprinted) by DNA methylation during gamete formation
• The imprint is parent-of-origin specific: one parental allele is always silenced
• Result: only one allele is expressed (monoallelic expression)
• Imprints are erased and reset each generation during gametogenesis

Key Points

• ~100–200 imprinted genes have been identified in mammals
• Imprinting does NOT change the DNA sequence — it's epigenetic (methylation, histone modification)
• Both the maternal AND paternal copies are needed for normal development

Imprinting Disorders

Prader-Willi Syndrome vs. Angelman Syndrome

These two syndromes beautifully illustrate genomic imprinting — both involve the same region of chromosome 15 (15q11-13) but result from losing the contribution of different parents.

AP Exam Practice — Chromosomal Inheritance Problems

This section contains multi-step genetics problems similar to those on the AP Biology exam. Work through each problem carefully, applying concepts from the entire unit.

Problem-Solving Strategy

1. Identify the inheritance pattern (autosomal vs. sex-linked, dominant vs. recessive, linked vs. unlinked)
2. Assign genotypes to all known individuals
3. Set up appropriate crosses (Punnett squares, branch diagrams)
4. Calculate expected ratios and compare to observed data
5. Check your work — do the ratios make biological sense?

Practice Problem Set 1: Sex-Linked Crosses 🎯

Gene Mapping Practice

Problem: Two-Point Test Cross

A test cross involving two linked genes produces the following offspring:

Step 1: Identify parental and recombinant classes

• Parental: AB (354) and ab (346) → total = 700
• Recombinant: Ab (52) and aB (48) → total = 100

Step 2: Calculate recombination frequency