⚠️ Top Mistakes to Avoid on This Topic
- Confusing genotype and phenotype: Genotype = the letters (Tt). Phenotype = what you SEE (tall). Both Tt and TT look tall!
- Setting up Punnett squares wrong: Put one parent’s alleles on top, the other on the side. Each box gets one allele from each parent.
- Forgetting meiosis produces HAPLOID cells: Meiosis produces 4 cells with HALF the chromosomes (23 in humans). Mitosis produces 2 cells with the FULL set (46).
- Mixing up transcription and translation: Transcription = DNA → mRNA (in the nucleus). Translation = mRNA → protein (at the ribosome).
Question 1 Easy
Define: gene, allele, genotype, phenotype, dominant, recessive, homozygous, heterozygous.
1
Gene: A segment of DNA that codes for a specific protein or trait. Located on a chromosome.
2
Allele: Different versions of the same gene (e.g., T for tall, t for short). You inherit one allele from each parent.
3
Genotype: The combination of alleles an organism has (e.g., TT, Tt, or tt).
4
Phenotype: The observable physical trait (e.g., tall or short).
5
Dominant: An allele that is expressed whenever it is present (represented by a capital letter, e.g., T).
6
Recessive: An allele that is only expressed when TWO copies are present (lowercase, e.g., tt). Masked by the dominant allele when heterozygous.
7
Homozygous: Having two identical alleles for a gene (TT or tt).
8
Heterozygous: Having two different alleles for a gene (Tt). Also called a “carrier” if the recessive allele causes a condition.
Gene = DNA segment for a trait. Allele = version of a gene. Genotype = allele combo. Phenotype = visible trait. Dominant = always expressed. Recessive = only when homozygous.
Question 2 Easy
Cross Rr × Rr. Punnett square, genotypic and phenotypic ratios.
1
Punnett square:
| | R | r |
| R | RR | Rr |
| r | Rr | rr |
| | R | r |
| R | RR | Rr |
| r | Rr | rr |
2
Genotypic ratio: 1 RR : 2 Rr : 1 rr (25% homozygous dominant, 50% heterozygous, 25% homozygous recessive).
3
Phenotypic ratio: 3 round : 1 wrinkled (75% round seeds, 25% wrinkled seeds). RR and Rr both show the dominant phenotype (round).
Genotypic: 1 RR : 2 Rr : 1 rr. Phenotypic: 3 round : 1 wrinkled.
Question 3 Easy
Mitosis vs meiosis. Which produces gametes? Chromosomes in a human egg.
1
Mitosis: Produces 2 genetically IDENTICAL daughter cells (diploid, 2n = 46 chromosomes in humans). Used for growth, repair, and asexual reproduction. ONE division.
2
Meiosis: Produces 4 genetically UNIQUE daughter cells (haploid, n = 23 chromosomes in humans). Used to produce gametes (egg and sperm). TWO divisions (meiosis I and meiosis II). Includes crossing over for genetic variation.
3
Which produces gametes? Meiosis produces gametes (sex cells).
4
Human egg chromosomes: A human egg cell has 23 chromosomes (haploid). When it fuses with a sperm (also 23), the resulting zygote has 46 chromosomes (diploid).
Mitosis: 2 identical cells, 46 chromosomes, growth/repair. Meiosis: 4 unique cells, 23 chromosomes, gametes. Human egg = 23 chromosomes.
Question 4 Medium
DNA structure: double helix, four bases, base pairing rules.
1
What is DNA? Deoxyribonucleic acid — the molecule that carries the genetic instructions for building and maintaining an organism. Found in the nucleus.
2
Double helix: DNA has two strands twisted into a spiral (like a twisted ladder). The sides of the ladder are made of alternating sugar (deoxyribose) and phosphate groups. The rungs are made of paired nitrogenous bases.
3
Four bases: Adenine (A), Thymine (T), Guanine (G), Cytosine (C).
4
Base pairing rules: A always pairs with T (held by 2 hydrogen bonds). G always pairs with C (held by 3 hydrogen bonds). This is called complementary base pairing.
DNA = double helix of sugar-phosphate backbone + base pairs. Bases: A-T (2 H-bonds), G-C (3 H-bonds). Complementary base pairing.
Question 5 Medium
Protein synthesis: transcription and translation.
1
Transcription (DNA → mRNA): Occurs in the nucleus. The DNA double helix unzips. RNA polymerase reads one strand and builds a complementary mRNA strand (A→U, T→A, G→C, C→G). Note: RNA uses Uracil (U) instead of Thymine (T). The mRNA then leaves the nucleus.
2
Translation (mRNA → protein): Occurs at the ribosome (in the cytoplasm or on rough ER). The ribosome reads the mRNA three bases at a time (each group of 3 = a codon). Each codon specifies one amino acid.
3
Roles:
• mRNA: Carries the genetic message from DNA to the ribosome.
• tRNA: Carries amino acids to the ribosome; has an anticodon that matches the mRNA codon.
• Ribosome: The site where amino acids are assembled into a polypeptide chain (protein).
• mRNA: Carries the genetic message from DNA to the ribosome.
• tRNA: Carries amino acids to the ribosome; has an anticodon that matches the mRNA codon.
• Ribosome: The site where amino acids are assembled into a polypeptide chain (protein).
Transcription: DNA→mRNA in nucleus. Translation: mRNA→protein at ribosome. mRNA carries message, tRNA carries amino acids, ribosome assembles the protein.
Question 6 Medium
Mutations: point mutation vs frameshift mutation.
1
Mutation: A permanent change in the DNA sequence. Can be beneficial, harmful, or neutral.
2
Point mutation (substitution): ONE base is replaced with a different base. This changes one codon and may change one amino acid. Example: Sickle cell disease — a single base change (GAG → GTG) causes one amino acid change in haemoglobin, making red blood cells sickle-shaped.
3
Frameshift mutation (insertion/deletion): One or more bases are ADDED (insertion) or REMOVED (deletion). This shifts the entire reading frame, changing EVERY codon after the mutation — producing a completely different (usually nonfunctional) protein. Example: Tay-Sachs disease — caused by a frameshift mutation (insertion of 4 bases) in the HEXA gene.
Point mutation: 1 base changed, 1 amino acid affected (e.g., sickle cell). Frameshift: base added/removed, shifts ALL codons (e.g., Tay-Sachs). Frameshift is usually more damaging.
Question 7 Medium
Incomplete dominance and codominance.
1
Incomplete dominance: The heterozygous phenotype is a BLEND of both homozygous phenotypes. Neither allele is fully dominant. Example: Red flower (RR) × White flower (WW) → Pink flower (RW). The 1:2:1 ratio gives 1 red : 2 pink : 1 white.
2
Codominance: BOTH alleles are fully expressed simultaneously in the heterozygote (not blended). Example: A cow with red hair allele (R) and white hair allele (W) has BOTH red and white hairs (roan pattern — red AND white patches visible). Another example: blood type AB (both A and B antigens expressed).
3
Difference from simple dominance: In simple dominant/recessive, the heterozygote looks the same as the homozygous dominant (Tt looks tall, just like TT). In incomplete dominance, the heterozygote is a blend. In codominance, both phenotypes appear together.
Incomplete dominance: heterozygote = blend (red+white=pink). Codominance: heterozygote = both shown (red+white=red AND white patches). Simple dominance: one allele masks the other.
Question 8 Hard
Sex-linked colour blindness cross. Punnett square and probabilities.
1
a) Punnett square:
Mother: XBXb (carrier) × Father: XBY (normal vision)
| | XB | Y |
| XB | XBXB | XBY |
| Xb | XBXb | XbY |
Mother: XBXb (carrier) × Father: XBY (normal vision)
| | XB | Y |
| XB | XBXB | XBY |
| Xb | XBXb | XbY |
2
b) Probability of a colour-blind son: XbY = colour-blind male. This appears in 1 out of 4 boxes = 25% (or 50% of sons).
3
c) Probability of a colour-blind daughter: A daughter would need XbXb (two recessive alleles). The father only passes XB to daughters, so NO daughter can be colour-blind in this cross = 0%.
4
d) Why more common in males: Males have only ONE X chromosome (XY). If that one X carries the recessive colour-blind allele (Xb), there is no second X to mask it, so the male IS colour-blind. Females need TWO copies (XbXb) to be colour-blind, which is much less likely.
b) 25% chance of colour-blind son. c) 0% colour-blind daughter. d) Males only have 1 X — one recessive allele = affected. Females need 2 copies.
Question 9 Hard
Genetic engineering: inserting human insulin gene into bacteria.
1
Step 1 — Identify and cut the gene: Use restriction enzymes to cut the human insulin gene out of human DNA. Restriction enzymes cut DNA at specific sequences, creating “sticky ends.”
2
Step 2 — Cut the plasmid: Use the SAME restriction enzyme to cut open a bacterial plasmid (a small circular DNA molecule). This creates matching sticky ends.
3
Step 3 — Insert the gene: Mix the human insulin gene with the cut plasmid. The matching sticky ends join together. DNA ligase enzyme seals the bonds, creating a recombinant plasmid.
4
Step 4 — Transform the bacteria: Insert the recombinant plasmid into a bacterial cell (e.g., E. coli). The bacteria now contains the human insulin gene.
5
Step 5 — Grow and harvest: The bacteria reproduce rapidly, each copy carrying the insulin gene. They produce human insulin, which is harvested and purified for medical use.
6
Medical importance: Before genetic engineering, insulin came from pig or cow pancreases, which could cause allergic reactions. Genetically engineered human insulin is identical to natural human insulin, safer, and can be produced in unlimited quantities for diabetic patients.
Steps: cut gene with restriction enzyme → cut plasmid with same enzyme → insert gene (ligase seals) → transform bacteria → bacteria produce insulin. Medically vital for diabetes treatment.
Question 10 Hard
Chromosomal abnormalities: Down, Turner, Klinefelter syndromes.
1
Down Syndrome (Trisomy 21):
a) Cause: Nondisjunction during meiosis — chromosome pair 21 fails to separate, so one gamete gets an extra copy.
b) Chromosome number: 47 (three copies of chromosome 21).
c) Characteristics: Intellectual disability, characteristic facial features, shorter stature, increased risk of heart defects.
a) Cause: Nondisjunction during meiosis — chromosome pair 21 fails to separate, so one gamete gets an extra copy.
b) Chromosome number: 47 (three copies of chromosome 21).
c) Characteristics: Intellectual disability, characteristic facial features, shorter stature, increased risk of heart defects.
2
Turner Syndrome (45, X0):
a) Cause: Nondisjunction — one sex chromosome is missing. The individual receives only one X and no second sex chromosome.
b) Chromosome number: 45 (missing one X).
c) Characteristics: Female phenotype, short stature, webbed neck, usually infertile, may have heart/kidney issues.
a) Cause: Nondisjunction — one sex chromosome is missing. The individual receives only one X and no second sex chromosome.
b) Chromosome number: 45 (missing one X).
c) Characteristics: Female phenotype, short stature, webbed neck, usually infertile, may have heart/kidney issues.
3
Klinefelter Syndrome (47, XXY):
a) Cause: Nondisjunction — an extra X chromosome is passed to a male offspring.
b) Chromosome number: 47 (XXY).
c) Characteristics: Male phenotype, tall stature, reduced muscle mass, may have learning difficulties, usually infertile.
a) Cause: Nondisjunction — an extra X chromosome is passed to a male offspring.
b) Chromosome number: 47 (XXY).
c) Characteristics: Male phenotype, tall stature, reduced muscle mass, may have learning difficulties, usually infertile.
All caused by nondisjunction in meiosis. Down: 47 chromosomes (trisomy 21). Turner: 45 (X0, female). Klinefelter: 47 (XXY, male).
🤖 Stuck? Ask AI for Help!
Prompt 1 — Punnett Squares
I am a 9th grader learning genetics. Can you walk me through how to set up and solve a Punnett square step by step? Use an example with eye colour where brown (B) is dominant over blue (b). Show me a cross between two heterozygous parents (Bb x Bb).
Prompt 2 — Protein Synthesis
I am confused about transcription and translation. Can you explain the full process of how DNA makes a protein, step by step, using simple language? Please explain what mRNA, tRNA, codons, and anticodons do. Use an analogy if possible.
Prompt 3 — Sex-Linked Traits
Can you explain why colour blindness is more common in males than females? I need to understand X-linked inheritance. Please show me a Punnett square for a carrier mother and a normal father, and explain the probabilities for sons and daughters.
📚 Want to Learn More?
- Khan Academy: Search for “Punnett squares genetics khan academy”
- Khan Academy: Search for “DNA transcription and translation khan academy”
- YouTube: Search for “genetics Punnett square practice problems biology”
- YouTube: Search for “sex-linked traits X-linked inheritance explained”