UPCAT Biology — Genetics & HeredityCheat Sheet
Cheat sheet for UPCAT Biology — Genetics & Heredity. Compact, printable, and organised around the concepts University of the Philippines tests most frequently in the UPCAT 2026. Perfect for the week before exam day.
Exam context
On the UPCAT 2026, the Biology subtest carries a "Core" weight in University of the Philippines's pattern. Genetics & Heredity lands at position 6th out of 7 in the standard review order. Target score is UPG ≤ 2.2 typical, and roughly 20 items come from Biology on a typical UPCAT paper.
Genetics & Heredity - Cheat sheet
Your last-minute revision companion for mastering genetics and heredity concepts for UPCAT and other college entrance exams.
Sections
Formulas
Formula
Dominant × Recessive = 100% Heterozygous Dominant Phenotype
Meaning
AA × aa = Aa (all show dominant trait)
Watch Out
Don't confuse genotype (Aa) with phenotype (dominant trait expressed)
When To Use
When crossing pure breeding dominant with recessive
Formula
Heterozygous × Heterozygous = 3:1 Phenotype Ratio
Meaning
Aa × Aa = 3 dominant : 1 recessive phenotype
Watch Out
Remember it's phenotype ratio, not genotype (which is 1:2:1)
When To Use
F2 generation of monohybrid cross
Section Title
Basic Genetics Terminology
Important Facts
- Genes are located on chromosomes
- Each parent contributes one allele for each trait
- Capital letters represent dominant alleles
- Lowercase letters represent recessive alleles
- Humans have 23 pairs of chromosomes (46 total)
Key Definitions
Term
Gene
Example
Gene for eye color
Definition
A section of DNA that carries information for constructing a protein or trait
Term
Allele
Example
Brown eyes (B) and blue eyes (b)
Definition
Alternative forms or variants of the same gene
Term
Genotype
Example
BB, Bb, or bb
Definition
The genetic composition or allelic makeup of an organism
Term
Phenotype
Example
Brown eyes or blue eyes
Definition
The observable physical expression of the genotype
Term
Homozygous
Example
AA or aa
Definition
Having two identical alleles for a trait
Term
Heterozygous
Example
Aa
Definition
Having two different alleles for a trait
Term
Dominant
Example
B for brown eyes
Definition
Allele that is expressed when present (masks recessive)
Term
Recessive
Example
b for blue eyes
Definition
Allele that is only expressed when homozygous
Diagrams To Know
- Punnett square setup
- DNA structure (double helix)
- Chromosome structure
- Gene location on chromosome
Formulas
Formula
Monohybrid Cross = 3:1 Phenotype Ratio in F2
Meaning
3 dominant phenotype : 1 recessive phenotype
Watch Out
Only applies to simple dominant-recessive inheritance
When To Use
When crossing F1 heterozygotes for one trait
Formula
Dihybrid Cross = 9:3:3:1 Phenotype Ratio in F2
Meaning
9 both dominant : 3 first dominant : 3 second dominant : 1 both recessive
Watch Out
Only works when genes are on different chromosomes (independent assortment)
When To Use
When crossing F1 heterozygotes for two independent traits
Section Title
Mendel's Laws
Important Facts
- Mendel used pea plants for his experiments
- Law of Segregation explains 3:1 ratio in F2 generation
- Independent assortment only works for unlinked genes
- Each gamete receives only one allele per trait
- Fertilization restores diploid condition
Key Definitions
Term
Law of Segregation
Example
Aa parent produces A and a gametes
Definition
Each parent has two alleles for each trait; these separate during gamete formation
Term
Law of Independent Assortment
Example
Eye color doesn't affect hair color inheritance
Definition
Genes for different traits are inherited independently of each other
Term
Law of Dominance
Example
Aa shows dominant phenotype
Definition
In heterozygotes, the dominant allele masks the recessive allele
Diagrams To Know
- Monohybrid cross Punnett square
- Dihybrid cross Punnett square
- Gamete formation diagram
- F1 and F2 generation crosses
Formulas
Formula
Incomplete Dominance = Blended Phenotype
Meaning
RR (red) × WW (white) = RW (pink)
Watch Out
Genotype is still heterozygous, but phenotype is intermediate
When To Use
When neither allele is completely dominant
Formula
Codominance = Both Phenotypes Expressed
Meaning
Both alleles are fully expressed simultaneously
Watch Out
Different from incomplete dominance - no blending occurs
When To Use
When both alleles are equally dominant
Section Title
Non-Mendelian Genetics
Important Facts
- Incomplete dominance shows 1:2:1 phenotype ratio in F2
- Codominance shows both parental phenotypes in offspring
- Sex-linked traits are more common in males
- Multiple alleles can create complex inheritance patterns
- Blood type is an example of multiple alleles and codominance
Key Definitions
Term
Incomplete Dominance
Example
Red and white flowers produce pink offspring
Definition
Neither allele is completely dominant; phenotype is a blend of both
Term
Codominance
Example
AB blood type shows both A and B antigens
Definition
Both alleles are fully expressed simultaneously in heterozygotes
Term
Multiple Alleles
Example
ABO blood system has IA, IB, and i alleles
Definition
More than two allelic forms exist for a single gene
Term
Sex-linked Traits
Example
Color blindness, hemophilia
Definition
Traits controlled by genes located on sex chromosomes
Diagrams To Know
- Incomplete dominance cross (snapdragons)
- Codominance cross (blood types)
- Sex-linked inheritance pattern
- Multiple allele crosses
Formulas
Formula
Base Pairing Rule: A = T, G ≡ C
Meaning
Adenine pairs with Thymine (2 bonds), Guanine pairs with Cytosine (3 bonds)
Watch Out
In RNA, Thymine is replaced by Uracil (A = U)
When To Use
When determining complementary DNA strands
Common Values
Value
23 pairs (46 total)
Symbol
2n = 46
Quantity
Human chromosome pairs
Value
21,000 genes
Symbol
~21K
Quantity
Human genes (approximate)
Section Title
DNA Structure and Function
Important Facts
- DNA has double helix structure discovered by Watson and Crick
- Purines: Adenine (A) and Guanine (G) - two rings
- Pyrimidines: Cytosine (C), Thymine (T), Uracil (U) - one ring
- DNA replication is semiconservative
- RNA contains ribose sugar; DNA contains deoxyribose
Key Definitions
Term
DNA
Example
Contains instructions for protein synthesis
Definition
Double-stranded nucleic acid that stores genetic information
Term
RNA
Example
mRNA, tRNA, rRNA
Definition
Single-stranded nucleic acid involved in protein synthesis
Term
Nucleotide
Example
ATP, dATP, GTP
Definition
Basic unit of nucleic acids; contains sugar, phosphate, and nitrogenous base
Term
Chromosome
Example
Humans have 46 chromosomes in 23 pairs
Definition
DNA wrapped around proteins (histones) forming a compact structure
Diagrams To Know
- DNA double helix structure
- Nucleotide structure
- Base pairing diagram
- Chromosome structure
Formulas
Formula
Central Dogma: DNA → RNA → Protein
Meaning
Replication → Transcription → Translation
Watch Out
Some viruses use reverse transcription (RNA → DNA)
When To Use
Explaining flow of genetic information
Common Values
Value
AUG
Symbol
Met
Quantity
Start codon
Value
UAA, UAG, UGA
Symbol
STOP
Quantity
Stop codons
Section Title
Central Dogma and Protein Synthesis
Important Facts
- Transcription occurs in nucleus; translation in cytoplasm
- mRNA carries genetic code from DNA to ribosomes
- tRNA brings amino acids to ribosomes
- rRNA is structural component of ribosomes
- Genetic code is universal and redundant
Key Definitions
Term
Transcription
Example
DNA template strand creates mRNA
Definition
DNA is copied to make RNA in the nucleus
Term
Translation
Example
mRNA codons specify amino acid sequence
Definition
mRNA is decoded to synthesize proteins at ribosomes
Term
Codon
Example
AUG codes for methionine (start codon)
Definition
Three-nucleotide sequence that codes for a specific amino acid
Term
Anticodon
Example
UAC anticodon pairs with AUG codon
Definition
Three-nucleotide sequence on tRNA complementary to mRNA codon
Diagrams To Know
- Transcription process diagram
- Translation at ribosome
- tRNA structure with anticodon
- Central dogma flow chart
Section Title
Mutations
Important Facts
- Substitution: one base replaced by another
- Insertion: extra bases added
- Deletion: bases removed
- Frameshift mutations often have severe effects
- Some mutations can be beneficial or neutral
Key Definitions
Term
Mutation
Example
Sickle cell anemia from single base change
Definition
Changes in DNA sequence that can alter protein structure or function
Term
Point Mutation
Example
A changed to T
Definition
Change in a single nucleotide base
Term
Frameshift Mutation
Example
Addition of one base shifts all downstream codons
Definition
Insertion or deletion that changes reading frame of codons
Term
Silent Mutation
Example
Due to genetic code redundancy
Definition
Mutation that doesn't change the amino acid sequence
Diagrams To Know
- Types of point mutations
- Frameshift mutation effects
- Silent vs. nonsense mutations
- Mutation consequences flowchart
Must Remember
- Base pairing rules: A-T, G-C (DNA); A-U, G-C (RNA)
- Monohybrid cross F2 ratio: 3:1 phenotype, 1:2:1 genotype
- Dihybrid cross F2 ratio: 9:3:3:1 phenotype
- Central Dogma: DNA → RNA → Protein
- Sex-linked traits are more common in males
- Frameshift mutations change reading frame and are usually severe
- Incomplete dominance = blended phenotype; Codominance = both expressed
- Mendel's laws: Segregation, Independent Assortment, Dominance
- Multiple alleles can exist for one gene (like ABO blood types)
- Homozygous = same alleles (AA, aa); Heterozygous = different alleles (Aa)
Last Minute Tips
- For Punnett squares: Always write parent genotypes first, then list possible gametes
- Remember that phenotype ratios only work with large sample sizes
- Sex-linked problems: Males need only one recessive allele to show trait
- In genetics problems, 'true-breeding' means homozygous
- When calculating dihybrid crosses, use FOIL method for gamete combinations
Comparison Tables
Rows
Values
- Double-stranded
- Single-stranded
Property
Structure
Values
- Deoxyribose
- Ribose
Property
Sugar
Values
- A, T, G, C
- A, U, G, C
Property
Bases
Values
- Nucleus
- Nucleus and cytoplasm
Property
Location
Values
- Store genetic info
- Protein synthesis
Property
Function
Columns
- Feature
- DNA
- RNA
Table Title
DNA vs RNA
Rows
Values
- Capital letter (A)
- Lowercase letter (a)
Property
Symbol
Values
- Expressed in heterozygotes
- Only expressed in homozygotes
Property
Expression
Values
- Can be rare
- Can be common
Property
Frequency
Values
- Masks recessive
- Masked by dominant
Property
Masking
Columns
- Characteristic
- Dominant
- Recessive
Table Title
Dominant vs Recessive Alleles
Rows
Values
- Blended/intermediate
- Both traits fully expressed
Property
Phenotype
Values
- Pink flowers from red × white
- AB blood type
Property
Example
Values
- Reduced protein function
- Both proteins functional
Property
Molecular level
Values
- 1:2:1 phenotype
- 1:2:1 phenotype
Property
F2 ratio
Columns
- Feature
- Incomplete Dominance
- Codominance
Table Title
Incomplete Dominance vs Codominance
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