7.2 Genetics, Heredity, and Molecular Biology

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1 Introduction & Exam Overview

1.1 AdUCET Overview, Format, and Admission Process 1.2 Test-Taking and Time-Management Strategies

2 Chapter 2: Mathematics I — Numbers, Algebra, and Functions

2.1 Number Sense, Operations, and Fraction–Decimal–Percent Conversions 2.2 Algebra, Polynomials, and Linear/Quadratic Equations 2.3 Inequalities, Absolute Value, and Systems of Equations 2.4 Radicals, Rational Expressions, and Scientific Notation 2.5 Coordinate Geometry, Lines, and Functions

3 Chapter 3: Mathematics II — Geometry, Measurement, and Trigonometry

3.1 Plane Geometry: Angles, Triangles, Polygons, and Circles 3.2 Solid Mensuration, Measurement, and Unit Conversion 3.3 Right-Triangle Trigonometry and Applications

4 Chapter 4: Mathematics III — Sequences, Proportion, and Data

4.1 Arithmetic and Geometric Sequences and Series 4.2 Ratio, Proportion, Percentage, Work, and Mixture Problems 4.3 Descriptive Statistics and Data Interpretation 4.4 Probability: Rules, Compound Events, and Conditional Probability 4.5 Counting Principles: Permutations and Combinations

5 Chapter 5: English I — Reading Comprehension and Vocabulary

5.1 Reading Comprehension: Main Idea, Purpose, and Passage Structure 5.2 Inference, Supporting Detail, and Author's Tone 5.3 Word Roots, Prefixes, and Suffixes 5.4 Vocabulary in Context, Synonyms, Antonyms, and Confusables

6 Chapter 6: English II — Grammar, Usage, and Written Expression

6.1 Subject-Verb Agreement and Verb Tenses 6.2 Pronouns, Modifiers, and Parallel Structure 6.3 Sentence Boundaries, Clauses, and Punctuation 6.4 Sentence Correction, Redundancy, and Idiomatic Usage 6.5 Verbal Analogies and Relationship Bridges 6.6 Paragraph Organization, Coherence, and Transitions

7 Chapter 7: General Science I — Biology and Chemistry

7.1 Cell Biology, Transport, and Cellular Energetics 7.2 Genetics, Heredity, and Molecular Biology 7.3 Human Body Systems, Ecology, and Evolution 7.4 Matter, Phase Changes, and Atomic Structure 7.5 Periodic Trends, Chemical Bonding, Reactions, Acids and Bases

8 Chapter 8: General Science II — Physics and Earth Science

8.1 Mechanics: Motion, Newton's Laws, Work, and Energy 8.2 Waves, Sound, Light, Heat, Electricity, and Magnetism 8.3 Earth's Structure, Plate Tectonics, and the Rock Cycle 8.4 The Atmosphere, Weather, the Water Cycle, and Astronomy

9 Chapter 9: Aptitude and Abstract Reasoning

9.1 Figure Series and Pattern Progression 9.2 Matrix Reasoning and Figural Analogies 9.3 Spatial Reasoning: Rotation, Reflection, and Chirality 9.4 Paper Folding, Cube Nets, and Orthographic Views 9.5 Deductive Reasoning, Syllogisms, and Venn Diagrams 9.6 Number, Letter, and Alphanumeric Series 9.7 Odd-One-Out, Classification, and Logic Puzzles

7.2 Genetics, Heredity, and Molecular Biology

Key Takeaways Explain This Topic AI

A cross between two heterozygotes gives a 1:2:1 genotypic ratio but a 3:1 phenotypic ratio, while a test cross against a homozygous recessive gives 1:1 only if the unknown parent is heterozygous.

Two double heterozygotes crossed for two independent genes produce the 9:3:3:1 phenotypic ratio, obtained by multiplying three-quarters and one-quarter for each gene separately.

Blood type AB is codominant and type O is the only ABO phenotype with a single possible genotype, two recessive alleles.

Chargaff's rule states that %A equals %T and %G equals %C in double-stranded DNA, so 22% cytosine forces 22% guanine and 28% each of adenine and thymine.

X-linked recessive traits such as red-green colour blindness and haemophilia appear far more often in males because a male's single X chromosome has no second allele to mask the recessive one.

Last updated: August 2026

7.2 Genetics, Heredity, and Molecular Biology

Genetics items at senior-high level are almost always solvable on paper inside a minute, provided you can translate a word problem into symbols and then draw the right grid. This section assumes you already know where the nucleus and ribosomes sit, which is covered in 7.1, and concentrates instead on what the DNA inside them does across generations.

1. Mendel's two laws and the vocabulary that carries them

Gregor Mendel's pea experiments produced two rules that still frame every cross:

Law of segregation — the two alleles of a gene separate during gamete formation, so each sperm or egg carries only one of them.

Law of independent assortment — alleles of different genes are handed to gametes independently, provided the genes sit on different chromosomes.

Term Meaning Example
Gene A stretch of DNA coding for one trait The gene for pea-plant height
Allele One version of that gene T for tall or t for short
Genotype The pair of alleles an organism carries Tt
Phenotype The trait you can actually observe Tall
Homozygous Two identical alleles TT or tt
Heterozygous Two different alleles Tt
Dominant Masks its partner; written as a capital T
Recessive Shows only when doubled t
Carrier A heterozygote showing the dominant trait while able to pass on the recessive allele Tt

A dominant allele is not the common one. Polydactyly is dominant and rare, while the recessive allele behind blood type O is extremely common. Frequency and dominance are unrelated ideas.

Monohybrid crosses and the test cross

Cross two heterozygous tall plants, Tt × Tt :

T t
T TT tall Tt tall
t Tt tall tt short

Genotypic ratio 1 TT : 2 Tt : 1 tt ; phenotypic ratio 3 tall : 1 short . Items very often ask for one and supply the other as bait.

A test cross answers a different question. A tall plant may be TT or Tt and you cannot tell by looking, so you cross it with the homozygous recessive tt :

If the unknown parent is TT , every offspring is Tt and therefore 100% tall .

If the unknown parent is Tt , half the offspring are Tt and half are tt , giving 1 tall : 1 short .

A single short offspring is enough to prove the unknown parent was heterozygous.

Worked example 1 — a dihybrid cross without drawing sixteen boxes

In peas, round seed R is dominant to wrinkled r , and yellow Y is dominant to green y . Cross RrYy × RrYy and predict 320 seeds.

Treat each gene on its own. Rr × Rr gives three-quarters round and one-quarter wrinkled; Yy × Yy gives three-quarters yellow and one-quarter green.

Multiply the separate probabilities (the product rule ):

round and yellow = 3/4 × 3/4 = 9/16
round and green = 3/4 × 1/4 = 3/16
wrinkled and yellow = 1/4 × 3/4 = 3/16
wrinkled and green = 1/4 × 1/4 = 1/16

That is the classic 9 : 3 : 3 : 1 phenotypic ratio.

With 320 seeds, one-sixteenth is 320 ÷ 16 = 20. Expect 180 round-yellow, 60 round-green, 60 wrinkled-yellow and 20 wrinkled-green.

2. When Mendel's ratios do not appear

Pattern What happens Signature result

Incomplete dominance The heterozygote is an intermediate blend Red × white snapdragon gives all pink; pink × pink gives 1 red : 2 pink : 1 white, so genotype and phenotype ratios match

Codominance Both alleles are fully and separately expressed Roan cattle show red hairs and white hairs; blood type AB displays both antigens

Multiple alleles More than two alleles exist in the population, though each person carries only two The ABO system with its A, B and O alleles

Sex linkage The gene lies on the X chromosome Colour blindness and haemophilia, far commoner in males

Polygenic inheritance Many genes add their small effects together Human height and skin colour form a continuous range

ABO blood typing

The A and B alleles are codominant with each other and both are dominant over the O allele, written as a lowercase i .

Blood type Possible genotypes Antigen on the red cell Antibody in plasma
A I A I A I^A I^A I A I A or I A i I^A i I A i A anti-B
B I B I B I^B I^B I B I B or I B i I^B i I B i B anti-A
AB I A I B I^A I^B I A I B only A and B none, the universal recipient
O i i ii ii only none anti-A and anti-B, the universal donor

Worked example 2 — can these parents have that child?

A man with type A blood and a woman with type B blood present a child with type O . Is that genetically possible?

Type O is i i ii ii , so the child received an O allele from each parent.

The father must therefore be I A i I^A i I A i , not I A I A I^A I^A I A I A .

The mother must likewise be I B i I^B i I B i .

Crossing I A i I^A i I A i × I B i I^B i I B i gives I A I B I^A I^B I A I B (AB), I A i I^A i I A i (A), I B i I^B i I B i (B) and i i ii ii (O), each with probability one-quarter.

So yes — and more strikingly, two parents of types A and B can produce children of all four blood types.

Sex-linked traits

Males are XY . A gene on the X has no partner on the much smaller Y, so a male is hemizygous and one recessive allele is enough to express the trait. Females are XX and need two copies. Writing X C X^C X C for normal vision and X c X^c X c for colour blindness, take a carrier mother X C X c X^C X^c X C X c and a father with normal vision X C Y X^C Y X C Y :

X C X^C X C from father Y from father
X C X^C X C from mother X C X C X^C X^C X C X C normal daughter X C Y X^C Y X C Y normal son
X c X^c X c from mother X C X c X^C X^c X C X c carrier daughter X c Y X^c Y X c Y colour-blind son

Half the sons are affected, no daughter is affected, half the daughters are carriers, and one quarter of all the children are colour blind — all of them boys. Note too that a father never passes an X-linked allele to a son, because what he gives a son is the Y.

3. Reading a pedigree

Squares are males and circles are females; a shaded symbol means affected; a horizontal line joins mates and a vertical line drops to their children; generations carry Roman numerals. Three diagnostic patterns cover almost every item:

Autosomal recessive — two unaffected parents produce an affected child, the trait appears to skip generations, and both sexes are affected roughly equally.

Autosomal dominant — every affected child has at least one affected parent, so the trait shows in every generation.

X-linked recessive — affected individuals are overwhelmingly male, and the trait travels from an affected grandfather through an unaffected carrier daughter to a grandson.

4. DNA, RNA and the central dogma

A nucleotide is a phosphate group plus a five-carbon sugar plus a nitrogenous base. In DNA (deoxyribonucleic acid) the sugar is deoxyribose , and two antiparallel strands twist into a double helix held together by hydrogen bonds between complementary bases: A pairs with T using two bonds, G pairs with C using three.

Chargaff's rule follows directly — %A = %T and %G = %C. If a sample is 30% adenine, thymine is also 30%, and the remaining 40% splits evenly into 20% guanine and 20% cytosine.

DNA RNA
Sugar Deoxyribose Ribose
Bases A, T, G, C A, U , G, C
Strands Double Single
Where it works Nucleus (also mitochondria and chloroplasts) Built in the nucleus, works in the cytoplasm
Role Permanent archive Working copy: mRNA, tRNA, rRNA

Replication is semiconservative : helicase unzips the helix, DNA polymerase lays complementary nucleotides along each exposed strand, and each new double helix keeps one old strand paired with one new one.

Transcription copies one DNA strand into messenger RNA (mRNA) using RNA polymerase, substituting uracil wherever thymine would have gone. Translation then happens on a ribosome: mRNA is read three bases at a time as a codon , and each codon is matched by the anticodon of a transfer RNA (tRNA) carrying one amino acid. AUG is the start codon and codes for methionine; UAA, UAG and UGA are stop codons and code for no amino acid at all. There are 64 codons for only 20 amino acids, so the code is degenerate — several codons can specify the same amino acid.

A short worked reading: the template DNA 3'-TAC GGA TTT ACG ATC-5' transcribes to mRNA 5'-AUG CCU AAA UGC UAG-3', which translates to methionine, proline, lysine, cysteine and then stops — a peptide of four amino acids.

5. Mutations, chromosomes and karyotypes

A mutation is any change in the base sequence.

A point or substitution mutation swaps a single base. It may be silent because the code is degenerate, missense when one amino acid changes — sickle-cell anaemia comes from one GAG to GTG change that puts valine where glutamic acid belongs — or nonsense when a stop codon appears early and truncates the protein.

A frameshift mutation inserts or deletes bases in a number not divisible by three. Every codon after the change is read in the wrong grouping, so frameshifts are usually far more destructive than substitutions.

Mutations in body cells affect only that individual. Only mutations arising in gametes are inherited.

A human body cell carries 46 chromosomes in 23 pairs: 22 pairs of autosomes and one pair of sex chromosomes , XX in females and XY in males. A gamete carries 23. A karyotype is a photograph of those chromosomes sorted by size, and it is how doctors detect nondisjunction , the failure of chromosomes to separate during meiosis.

Condition Chromosome formula Total chromosomes
Down syndrome Trisomy 21 47
Edwards syndrome Trisomy 18 47
Patau syndrome Trisomy 13 47
Klinefelter syndrome XXY 47
Turner syndrome Single X (monosomy X) 45
Common traps

Giving the genotypic ratio 1:2:1 when the item asked for the phenotypic ratio 3:1.

Treating a test cross as a cross between two heterozygotes. A test cross is always against the homozygous recessive .

Confusing incomplete dominance with codominance. Pink snapdragons are a blend; an AB red cell shows both antigens intact and unblended.

Assuming an affected father passes an X-linked allele to his sons. He passes it to every daughter and to no son.

Writing that Down syndrome involves 45 chromosomes. A trisomy adds one, giving 47.

Applying Chargaff's rule to RNA. It holds only for double-stranded DNA.