TEAS Science: Genetics, DNA, and Heredity Essentials for the Science Section

Genetics is one of the most consistently tested topics on the TEAS Science section. Questions range from the molecular level — DNA structure and protein synthesis — to classical heredity problems involving Punnett squares and inheritance patterns. The good news is that the TEAS tests a focused set of concepts, and with the right review, genetics can become one of your highest-scoring areas.

This guide covers every genetics concept you are likely to see on the TEAS, from nucleotide bases to dominant and recessive inheritance. We will work through the material systematically so that each concept builds on the last, giving you a clear mental framework for test day.

DNA: The Blueprint of Life

DNA Structure

DNA (deoxyribonucleic acid) is a double-stranded molecule shaped like a twisted ladder — the famous double helix. Each strand is made of repeating units called nucleotides. Every nucleotide contains three parts: a phosphate group, a deoxyribose sugar, and a nitrogenous base.

There are four nitrogenous bases in DNA: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C). The two strands are held together by hydrogen bonds between complementary base pairs: A always pairs with T (2 hydrogen bonds), and G always pairs with C (3 hydrogen bonds). This base-pairing rule is essential — the TEAS tests it directly.

DNA vs. RNA

RNA (ribonucleic acid) differs from DNA in three key ways that the TEAS frequently tests:

• Sugar: RNA uses ribose; DNA uses deoxyribose (missing one oxygen atom)
• Structure: RNA is typically single-stranded; DNA is double-stranded
• Bases: RNA uses Uracil (U) instead of Thymine (T). So in RNA, Adenine pairs with Uracil (A-U)

There are three main types of RNA: messenger RNA (mRNA) carries the genetic code from DNA to the ribosome, transfer RNA (tRNA) brings amino acids during protein assembly, and ribosomal RNA (rRNA) forms part of the ribosome structure itself.

DNA Replication

Before a cell divides, it must copy its DNA so each daughter cell gets a complete set of genetic instructions. DNA replication is semiconservative — each new double helix contains one original strand and one newly synthesized strand.

• Step 1: The enzyme helicase unwinds and separates the two strands at the replication fork
• Step 2: DNA polymerase reads each template strand and adds complementary nucleotides (A with T, C with G)
• Step 3: The result is two identical DNA molecules, each with one old strand and one new strand

Protein Synthesis: From Gene to Protein

Transcription

Transcription is the first step in protein synthesis and takes place in the nucleus. The enzyme RNA polymerase reads one strand of DNA (the template strand) and builds a complementary mRNA molecule. Remember that RNA uses Uracil instead of Thymine, so a DNA sequence of TAC GGA becomes an mRNA sequence of AUG CCU.

Translation

Translation occurs at the ribosome in the cytoplasm. The ribosome reads the mRNA three bases at a time. Each three-base sequence is called a codon, and each codon specifies one amino acid. Transfer RNA (tRNA) molecules bring the correct amino acids to the ribosome, matching their anticodon to the mRNA codon. The amino acids are linked together to form a polypeptide chain — which folds into a functional protein.

• Start codon: AUG (codes for methionine and signals the beginning of translation)
• Stop codons: UAA, UAG, UGA (signal the ribosome to stop building the protein)
• The genetic code is universal — the same codons code for the same amino acids in nearly all organisms
• The genetic code is degenerate — multiple codons can specify the same amino acid (e.g., GGU, GGC, GGA, and GGG all code for glycine)

Mutations

A mutation is any change in the DNA sequence. Mutations can occur during DNA replication or from environmental factors like UV radiation or chemicals. The TEAS tests these common mutation types:

• Point mutation (substitution): One base is replaced by another. May change one amino acid (missense), create a stop codon (nonsense), or have no effect (silent) due to code degeneracy
• Insertion: One or more bases are added to the sequence. This causes a frameshift — every codon after the insertion is read incorrectly, usually producing a nonfunctional protein
• Deletion: One or more bases are removed. Also causes a frameshift with the same severe consequences as insertion

Cell Division: Mitosis vs. Meiosis

Understanding cell division is crucial because it connects DNA replication to heredity. The TEAS tests the differences between mitosis and meiosis regularly.

Mitosis

Mitosis is cell division for growth, repair, and maintenance. One parent cell produces two genetically identical daughter cells, each with the same number of chromosomes as the parent (diploid, 2n = 46 in humans). The phases are Prophase, Metaphase, Anaphase, and Telophase (remember: PMAT).

Meiosis

Meiosis is cell division for producing gametes (eggs and sperm). One parent cell produces four genetically unique daughter cells, each with half the chromosomes (haploid, n = 23 in humans). Meiosis involves two rounds of division (meiosis I and meiosis II) and includes crossing over, where homologous chromosomes exchange segments of DNA, increasing genetic diversity.

• Mitosis: 1 division → 2 identical diploid cells (for growth/repair)
• Meiosis: 2 divisions → 4 unique haploid cells (for reproduction)
• Crossing over occurs in meiosis I, not in mitosis
• Nondisjunction (failure of chromosomes to separate properly) in meiosis causes conditions like Down syndrome (trisomy 21)

Mendelian Genetics and Heredity

Key Vocabulary

Before tackling inheritance problems, make sure you know these terms — they appear directly in TEAS questions:

• Gene: A segment of DNA that codes for a specific trait
• Allele: A version of a gene. For example, the gene for eye color might have a brown allele and a blue allele
• Dominant allele: Expressed when one or two copies are present (represented by a capital letter, e.g., B)
• Recessive allele: Only expressed when two copies are present (represented by a lowercase letter, e.g., b)
• Genotype: The genetic makeup of an organism (e.g., BB, Bb, or bb)
• Phenotype: The observable trait (e.g., brown eyes or blue eyes)
• Homozygous: Two identical alleles (BB or bb)
• Heterozygous: Two different alleles (Bb) — also called a 'carrier' for recessive traits

Punnett Squares

A Punnett square is a grid used to predict the genotype and phenotype ratios of offspring from a genetic cross. The TEAS includes Punnett square problems regularly, and they are straightforward once you know the method.

Example: Cross two heterozygous parents (Bb × Bb). Write one parent's alleles across the top and the other's down the side. Fill in each box by combining the row and column alleles. Result: 1 BB : 2 Bb : 1 bb. Genotype ratio = 1:2:1. Phenotype ratio = 3 dominant : 1 recessive (75% dominant, 25% recessive).

Mendel's Laws

• Law of Segregation: During meiosis, the two alleles for each gene separate so that each gamete carries only one allele
• Law of Independent Assortment: Genes on different chromosomes are inherited independently of each other. This is the basis for dihybrid crosses
• Law of Dominance: In a heterozygous organism, the dominant allele is expressed and the recessive allele is masked

Beyond Simple Dominance

While most TEAS genetics questions focus on simple dominant/recessive inheritance, you should be familiar with these additional patterns:

• Incomplete dominance: The heterozygous phenotype is a blend of both alleles. Example: red flower (RR) × white flower (WW) → pink flower (RW)
• Codominance: Both alleles are fully expressed in the heterozygous phenotype. Example: ABO blood type — a person with genotype I^A I^B has type AB blood, expressing both A and B antigens
• Sex-linked traits: Genes located on the X chromosome. Males (XY) are more likely to express recessive X-linked traits because they have only one X. Example: color blindness and hemophilia
• Multiple alleles: Some genes have more than two allele forms in the population. ABO blood type has three alleles: I^A, I^B, and i

ABO Blood Type — A TEAS Favorite

Blood type genetics combines multiple alleles and codominance, making it a frequent TEAS topic. The three alleles are I^A (produces A antigen), I^B (produces B antigen), and i (produces no antigen). I^A and I^B are codominant with each other and both are dominant over i.

• Type A: genotype I^A I^A or I^A i
• Type B: genotype I^B I^B or I^B i
• Type AB: genotype I^A I^B (codominance — both antigens present)
• Type O: genotype ii (recessive — no antigens)
• Universal donor: Type O (no antigens to trigger a reaction)
• Universal recipient: Type AB (has both antigens, so no antibodies against A or B)

High-Yield Genetics Facts for the TEAS

• Human cells have 46 chromosomes (23 pairs): 22 pairs of autosomes and 1 pair of sex chromosomes (XX female, XY male)
• Chromosomes are found in the nucleus and are made of tightly coiled DNA wrapped around histone proteins
• A karyotype is an image of an individual's chromosomes arranged by size and shape, used to detect chromosomal abnormalities
• Trisomy means three copies of a chromosome instead of two (e.g., trisomy 21 = Down syndrome)
• DNA is packaged as chromatin during normal cell activity and condenses into visible chromosomes during cell division
• Genetic diversity comes from three sources: crossing over in meiosis, independent assortment of chromosomes, and random fertilization

Study Strategy for Genetics on the TEAS

• Start with DNA structure and base pairing — this foundation makes everything else click
• Practice writing complementary DNA and mRNA strands until it is automatic
• Work through at least 10 Punnett square problems of varying types (monohybrid, test cross, sex-linked)
• Create a comparison chart for mitosis vs. meiosis and review it daily
• Focus on ABO blood types — know the genotypes for each blood type and how to cross them
• Use the Central Dogma as your organizing framework: DNA → RNA → Protein

Genetics may seem like a dense topic, but the TEAS tests it predictably. If you know DNA structure, base pairing, the difference between mitosis and meiosis, how to work a Punnett square, and the basics of inheritance patterns, you have covered the vast majority of what appears on the exam. Practice until these concepts feel natural, and genetics will be a reliable point-earner on test day.