Topic 5.4 Notes – Non-Mendelian Genetics

1. When Inheritance Does Not Follow Mendel’s Ratios

Mendel’s predictions rely on two big ideas:

• Law of Segregation → two alleles separate during gamete formation.
• Law of Independent Assortment → alleles of different genes separate independently (if unlinked).

In a simple monohybrid cross, you expect a 3:1 phenotypic ratio. In a dihybrid cross, 9:3:3:1.

When your observed offspring numbers don’t match those ratios, you use a chi-square test to compare:

$$\chi^2 = \sum \frac{(O - E)^2}{E}$$

If the difference between observed and expected is statistically significant, the trait likely does not follow simple Mendelian inheritance.

That usually means:

• Alleles interact differently (incomplete dominance or codominance)
• Genes are linked
• The trait is sex-linked
• One gene affects multiple traits (pleiotropy)
• Inheritance is non-nuclear (mitochondria or chloroplasts)

On exams, they often give you offspring counts and expect you to notice that 3:1 doesn’t fit before they ever say “non-Mendelian.”

2. Allele Interactions That Change the Phenotype

These patterns still follow segregation, but the phenotype ratios shift.

Incomplete Dominance

• Heterozygote shows a blended phenotype.
• Neither allele fully masks the other.
• Phenotypic ratio = 1:2:1 (same as genotypic).

Example:

• RR = red
• rr = white
• Rr = pink

The key is that the heterozygote looks intermediate.

Codominance

• Both alleles are fully expressed.
• Heterozygote has a distinct phenotype, not blended.
• Often gives a 1:2:1 phenotypic ratio.

Example:

• RR = red
• WW = white
• RW = red and white spotted

You physically see both traits at the same time.

Multiple Alleles

More than two allele forms exist in the population, even though each individual still has only two.

This increases genotype combinations and may involve dominance or codominance.

Common mistake: thinking “multiple alleles” means one person has three alleles. They don’t. It refers to the gene pool.

Quick Comparison

If you see 1:2:1 in phenotype counts, your brain should immediately think incomplete dominance or codominance.

3. Linked Genes and Gene Mapping

What Linked Genes Are

Genetically linked genes are located on the same chromosome and tend to be inherited together.

This violates Mendel’s Law of Independent Assortment.

Independent assortment only works when:

• Genes are on different chromosomes
• Or far apart on the same chromosome

Crossing Over and Recombination

During prophase I of meiosis, homologous chromosomes exchange segments. The stages of prophase I are shown below, with crossing over occurs during pachytene, when homologous chromosomes are fully synapsed. Chiasmata become visible later during diplotene, as homologs begin to separate.

Stages of prophase I and crossing over

Crossing over produces recombinant gametes.

The farther apart two genes are:

• The more likely crossing over occurs between them.
• The higher the recombination frequency.

Gene Mapping

Recombination frequency tells us distance:

$$\frac{\text{recombinant offspring}}{\text{total offspring}} \times 100$$

• 1% recombination = 1 map unit (centimorgan)
• >50% recombination → genes behave as unlinked

Gene mapping uses this to determine relative gene positions. On tests, they may give recombination percentages and ask you to order genes on a chromosome.

4. Sex-Linked Inheritance

What Sex-Linked Traits Are

Genes located on sex chromosomes:

• X-linked (most common)
• Y-linked

In XY systems:

• XX = female
• XY = male

Males express all X-linked alleles because they only have one X. That’s why X-linked recessive traits appear more often in males.

Recognizing X-Linked Patterns

In pedigrees, look for:

• More affected males than females
• No father-to-son transmission (for X-linked)
• Affected males often have carrier mothers
• Trait can skip generations
• For X-linked recessive traits, all daughters of affected fathers are carriers (if mother unaffected)

That “no father to son” rule is one of the fastest ways to identify X-linkage.

Alternative Sex Determination Systems

Not all species use XY.

• ZW system (birds) → ZZ male, ZW female
• Haplodiploidy (bees) → fertilized (diploid) = female, unfertilized (haploid) = male

Sex linkage depends on the chromosomal system of that organism.

5. When One Gene or One Organelle Breaks the Rules

Pleiotropy

One gene affects multiple traits.

One mutation → multiple phenotypic effects.

Because the same gene influences multiple characteristics, those traits don’t assort independently.

Non-Nuclear Inheritance

Some traits come from DNA in:

• Mitochondria
• Chloroplasts (plants)

These organelles are randomly distributed into gametes and daughter cells.

In animals:

• Mitochondria come from the egg, not sperm.
• Traits show maternal inheritance.

In plants:

• Mitochondria and chloroplasts come from the ovule, not pollen.

Pattern you’ll see:

• Affected mother → all offspring affected
• Affected father → no offspring affected

That’s a huge clue that it’s mitochondrial or chloroplast DNA.