Genetics Symbols Table
Standard Notation for Alleles, Pedigrees, and Chromosomes.
Alleles & Genotypes
Dominant Allele
An allele that expresses its trait even when only one copy is present.
Recessive Allele
An allele that is expressed only when two copies are present (homozygous).
Homozygous Dominant
Genotype with two identical dominant alleles.
Homozygous Recessive
Genotype with two identical recessive alleles.
Heterozygous
Genotype with two different alleles (one dominant, one recessive).
Generations & Pedigree
Parental Generation
The initial generation in a genetic cross.
First Filial Generation
The first generation of offspring from the P generation.
Second Filial Generation
The offspring produced by interbreeding the F1 generation.
Male (Pedigree)
Square symbol represents a male family member.
Female (Pedigree)
Circle symbol represents a female family member.
Chromosomes & DNA
Female Chromosomes
Sex chromosome pair responsible for female biological sex in humans.
Male Chromosomes
Sex chromosome pair responsible for male biological sex in humans.
Affected Female
Note for X-linked recessive traits (like Colorblindness).
Affected Male
Male with X-linked recessive trait (Hemizygous).
Adenine-Thymine
Complementary base pair in DNA double helix.
Guanine-Cytosine
Complementary base pair in DNA and RNA.
Uracil
Nitrogenous base found only in RNA (Replaces Thymine).
Reading the Language of Life
Genetics relies on specific symbols to map the flow of traits from parents to offspring. Understanding these abbreviations is key to solving problems in inheritance and heredity.
The Punnett Square Grid
Visualizing a Hybrid Cross (Tt x Tt)
A classic Mendelian cross between two Tall Hybrid plants. This predicts the probability of offspring genotypes.
- 25% Homozygous Dominant (TT)
- 50% Heterozygous (Tt)
- 25% Homozygous Recessive (tt)
Pedigree Chart Legend
Frequently Asked Questions
What is the difference between Heterozygous and Homozygous?
Homozygous means having two identical alleles (e.g., TT or tt). Heterozygous means having two different alleles (e.g., Tt), where the dominant allele typically masks the recessive one.
How are males and females represented in a pedigree chart?
In pedigree charts, a Square (□) represents a Male, and a Circle (○) represents a Female. A shaded shape usually indicates an individual affected by the trait being tracked.
What does "F1 Generation" stand for?
F1 stands for "First Filial" generation. It refers to the first set of offspring produced by the parental (P) generation cross.
Why are dominant alleles written in capital letters?
Capital letters (e.g., T) are used by convention to show that the allele is Dominant and will express its trait over a recessive allele (lowercase, e.g., t).
What is a "Carrier"?
A Carrier is a heterozygous individual (e.g., XᶜX) who carries a recessive allele for a genetic disorder but does not show symptoms. They can pass the allele to their offspring.
What do the letters A, T, G, and C stand for?
They represent the four nitrogenous bases of DNA: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C). A always pairs with T, and G always pairs with C.
How is X-linked inheritance noticed?
X-linked recessive traits (like colorblindness) are more common in Males (XY) because they only have one X chromosome. If that single X has the recessive gene, they express the trait.
What is a Genotype vs Phenotype?
Genotype is the genetic makeup (e.g., Tt). Phenotype is the physical appearance resulting from that genotype (e.g., Tall Plant).
What does "n" and "2n" mean?
n represents a Haploid cell (one set of chromosomes, e.g., gametes). 2n represents a Diploid cell (two sets of chromosomes, e.g., body cells).
Does "Dominant" mean "Most Common"?
No. Dominant only means the allele masks the recessive one. It does not necessarily mean the trait is more common in a population (e.g., Huntingtons disease is dominant but rare).