Understanding Mendelian Inheritance Patterns
What Mendelian Inheritance Actually Is
Mendelian inheritance describes how traits pass from parents to offspring through genes. Gregor Mendel figured this out in the 1860s by experimenting with pea plants. Scientists ignored his work for decades because it was too ahead of its time.
The core idea is simple: genes come in pairs, offspring get one copy from each parent, and some versions of genes dominate others.
Mendel's Three Laws (The Foundation)
Law of Dominance
If you have one dominant allele and one recessive allele, you show the dominant trait. The recessive allele doesn't disappear—it just stays hidden in your DNA.
Example: Brown eyes (B) dominate blue eyes (b). A person with genotype Bb has brown eyes, not a blend.
Law of Segregation
During reproduction, paired alleles separate. Each gamete (sperm or egg) gets only one allele from each parent pair. This is why you look different from both parents in some ways.
Your children don't inherit your exact genetic makeup. They get a random half.
Law of Independent Assortment
Genes on different chromosomes sort independently during meiosis. What this means: the inheritance of one trait doesn't affect another trait's inheritance.
This law has limits—genes on the same chromosome often travel together. But for genes far apart on different chromosomes, independent assortment holds.
Genotype vs Phenotype—The Difference Matters
Genotype is the genetic code: BB, Bb, or bb.
Phenotype is what you actually see: brown eyes or blue eyes.
Two different genotypes (BB and Bb) can produce the same phenotype (brown eyes). This matters when predicting offspring—you can't just look at parents and know their exact genetic makeup.
The Main Inheritance Patterns
Autosomal Dominant
You need only one copy of the dominant allele to show the trait. If a parent has it, each child has a 50% chance of inheriting it.
Real examples:
- Huntington's disease
- Marfan syndrome
- Neurofibromatosis type 1
The problem with dominant disorders: they don't skip generations. If you have it, at least one parent does too.
Autosomal Recessive
You need two copies of the recessive allele to show the trait. Carriers (one copy) show no symptoms but can pass it to children.
Real examples:
- Cystic fibrosis
- Sickle cell anemia
- Phenylketonuria (PKU)
Two carriers have a 25% chance of having an affected child, 50% chance of a carrier, and 25% chance of a child with no allele at all.
X-Linked Recessive
These genes sit on the X chromosome. Men (XY) only have one X, so one bad copy affects them. Women (XX) need two bad copies.
Real examples:
- Hemophilia A and B
- Duchenne muscular dystrophy
- Red-green color blindness
Men are affected more often than women. A carrier mother has a 50% chance of passing the allele to each son.
Punnett Squares—How to Actually Use Them
Punnett squares predict offspring genotype ratios. Here's the basic setup for two carriers of an autosomal recessive condition (both are Bb):
| B | b | |
|---|---|---|
| b | BB | Bb |
| b | Bb | bb |
Results: 25% BB (not a carrier), 50% Bb (carrier), 25% bb (affected).
Another Example: Autosomal Dominant
| B | b | |
|---|---|---|
| b | Bb | bb |
| b | Bb | bb |
One parent is affected (Bb), one is unaffected (bb). Results: 50% Bb (affected), 50% bb (unaffected). Simple 50/50 split.
Common Misconceptions That Need to Die
Myth: Traits always skip generations.
Reality: Dominant traits don't skip generations. Recessive traits can appear in every generation if both parents carry the allele.
Myth: Blended traits prove Mendelian inheritance wrong.
Reality: Most traits involve multiple genes. What looks like blending is actually incomplete dominance or polygenic inheritance—different mechanisms, not evidence against Mendel.
Myth: Dominant means more common.
Reality: Allele frequency has nothing to do with dominance. Huntington's disease is dominant but rare. Cystic fibrosis is recessive but more common in certain populations.
Getting Started: Predicting Inheritance in Your Cases
Step 1: Identify the inheritance pattern
- Does it affect men and women equally? Likely autosomal
- Are affected men born to carrier mothers? Likely X-linked recessive
- Does every affected person have an affected parent? Likely dominant
Step 2: Determine parent genotypes
- Affected with recessive condition = aa
- Unaffected but had affected child = Aa (carrier)
- Affected with dominant condition = AA or Aa (often Aa if one parent is unaffected)
Step 3: Build your Punnett square
- Put one parent's alleles across the top
- Put the other parent's alleles down the side
- Fill in each box with combined alleles
Step 4: Calculate probabilities
Count each genotype outcome, divide by total boxes. That's your probability for each child.
When Mendelian Rules Break Down
Some situations don't follow simple Mendelian patterns:
- Incomplete dominance: Heterozygotes show an intermediate phenotype (red Ă— white = pink)
- Codominance: Both alleles show fully (AB blood type)
- Polygenic traits: Multiple genes affect one trait (skin color, height)
- Epistasis: One gene masks another gene's expression
Mendel got lucky—he picked pea plant traits that happened to follow clean dominant/recessive patterns. Most human traits are messier.
Why This Matters
Understanding Mendelian inheritance lets you predict genetic disease risk, interpret family health history, and make informed decisions about genetic testing.
If you're counseling patients, breeding animals, or just trying to figure out why your kid has blue eyes when both parents have brown—you need this foundation. It won't answer everything, but it answers most of the basic questions.