Mendelian Inheritance
Genotypes and Phenotypes
Your genotype is your genetic makeup for a particular trait, written as a combination of alleles (the different versions of a gene you can carry). Your phenotype is the observable trait that results from that genotype, like flower color or eye shape.
Here's how dominance works:
- A dominant allele (written as an uppercase letter, like A) only needs one copy to show up in the phenotype. It masks the effect of a recessive allele.
- A recessive allele (written as a lowercase letter, like a) only shows up in the phenotype when two copies are present. There's no dominant allele around to mask it.
The three possible genotype combinations and their phenotypes:
| Genotype | Term | Phenotype |
|---|---|---|
| AA | Homozygous dominant | Dominant trait |
| Aa | Heterozygous | Dominant trait (the dominant allele masks the recessive one) |
| aa | Homozygous recessive | Recessive trait |
Notice that both AA and Aa produce the same phenotype. This is exactly why test crosses exist, which you'll see below.
Punnett Squares for Genetic Prediction
A Punnett square is a grid that helps you predict the probability of offspring genotypes and phenotypes. A monohybrid cross tracks just one trait.
To set one up:
- Determine each parent's genotype (e.g., both parents are Aa).
- Draw a 2×2 grid.
- Write one parent's alleles across the top (one per column).
- Write the other parent's alleles along the left side (one per row).
- Fill in each box by combining the column allele with the row allele.
For a cross between two heterozygous parents (Aa × Aa):
| A | a | |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
This gives you a genotypic ratio of 1 AA : 2 Aa : 1 aa, and a phenotypic ratio of 3 dominant : 1 recessive. That 3:1 ratio is one of the most recognizable results in genetics.
Purpose of Test Crosses
If an organism shows the dominant phenotype, you can't tell just by looking whether it's AA or Aa. A test cross solves this by crossing the unknown individual with a homozygous recessive (aa) individual.
The logic is straightforward:
- Cross the organism with the unknown genotype with an aa individual.
- Observe the offspring phenotypes.
- Interpret the results:
- If all offspring show the dominant phenotype, the unknown parent is most likely AA (every offspring gets at least one A).
- If roughly half the offspring show the recessive phenotype, the unknown parent is Aa (some offspring receive a from both parents).
The aa parent can only contribute recessive alleles, so the offspring phenotypes directly reveal what the unknown parent is passing on.
Non-Mendelian Inheritance
Beyond Simple Dominance
Mendelian inheritance follows the classic rules: complete dominance, segregation, and independent assortment. But not every trait plays by those rules.
Incomplete dominance occurs when neither allele is fully dominant. The heterozygous individual shows a blended or intermediate phenotype. A classic example: crossing a red-flowered plant (RR) with a white-flowered plant (WW) produces pink-flowered offspring (RW), not red.
Sex-linked traits involve genes located on the sex chromosomes (X or Y). Females carry two X chromosomes (XX), while males carry one X and one Y (XY). For X-linked recessive traits, males only need one copy of the recessive allele on their single X chromosome to express the trait. Females would need two copies. This is why conditions like color blindness and hemophilia are far more common in males.
Genetic Material and Inheritance
Heredity and Genes
Heredity is the transmission of traits from parents to offspring. The physical units of heredity are genes, which are segments of DNA that code for specific proteins or traits. Genes sit at specific locations on chromosomes, structures found in the cell nucleus that package and organize DNA.
Mutations and Inheritance Patterns
A mutation is a change in the DNA sequence. Mutations can create entirely new alleles, which may alter how a trait is inherited or expressed.
Traits can follow different inheritance patterns depending on where the gene is located and how the alleles interact:
- Autosomal dominant: Only one copy of the mutant allele (on a non-sex chromosome) is needed to express the trait.
- Autosomal recessive: Two copies of the recessive allele are needed; carriers (one copy) don't show the trait.
- X-linked dominant: One copy on the X chromosome is enough to express the trait in both males and females.
- X-linked recessive: Expressed in males with one copy on the X; females need two copies.
Recognizing which pattern a trait follows is key to predicting how it will appear across generations, which is exactly what pedigree analysis builds on.