what's the difference between homozygous and heterozygous
Homozygous means “same alleles,” heterozygous means “different alleles” for a given gene. In symbols, homozygous is like AA or aa, while heterozygous is like Aa, which usually shows the dominant trait.
What these words actually mean
- Homozygous : Both copies (alleles) of a gene are identical, either both dominant (AA, BB) or both recessive (aa, bb).
- Heterozygous : The two alleles for that gene are different, typically written as one capital and one lowercase letter (Aa, Bb).
In humans and many other organisms, each gene comes in pairs, one from each parent. If the pair matches, it’s homozygous; if it doesn’t, it’s heterozygous.
How this affects traits (phenotype)
- In a simple dominant–recessive situation:
- Homozygous dominant (AA): the dominant trait is expressed.
- Homozygous recessive (aa): the recessive trait is expressed.
- Heterozygous (Aa): the dominant trait is expressed, and the recessive allele is “hidden” but can still be passed on to offspring.
- Example: Suppose brown eyes (B) are dominant over blue eyes (b).
- BB → homozygous dominant → brown eyes.
- bb → homozygous recessive → blue eyes.
- Bb → heterozygous → brown eyes, but carries the blue allele.
Simple comparison
| Feature | Homozygous | Heterozygous |
|---|---|---|
| Allele pattern | Two same alleles (AA or aa) | [5][1]Two different alleles (Aa) | [1][5]
| Types | Homozygous dominant, homozygous recessive | [3][1]One main type, mixed pair with one dominant and one recessive | [7][3]
| Trait expression (simple dominance) | Either clearly dominant or clearly recessive | [3]Usually shows dominant trait, recessive can be carried silently | [10][3]
| Genotype notation | AA, aa, BB, bb | [1][3]Aa, Bb | [1][3]
Quick story to lock it in
Imagine a “gene locker” with two slots:
- If both slots hold the same jersey (both red or both blue), that locker is homozygous.
- If one slot has a red jersey and the other has a blue jersey, that locker is heterozygous.
For most traits, as long as there is at least one “red” (dominant) jersey in the locker, that’s the color you’ll see on the field. TL;DR:
- Homozygous = same alleles (AA or aa), trait is purely dominant or purely recessive.
- Heterozygous = different alleles (Aa), usually shows the dominant trait but carries the recessive one.
Information gathered from public forums or data available on the internet and portrayed here.