Revision sheet: Basics of Genetics and Chromosomes

📋 Course Outline

1. DNA, chromosomes and genes
2. Chromosome number and species
3. Sex chromosomes and genetic disorders
4. Genes and inherited traits
5. Alleles and genetic variation
6. Dominant and recessive inheritance

📖 1. DNA, chromosomes and genes

🔑 Key Concepts & Definitions

• DNA : DNA is the chemical molecule that carries hereditary information inside cells.
• Chromosomes : Chromosomes are rod-like structures in the nucleus made of tightly packed DNA.
• Genes : Genes are specific DNA segments that contain instructions for inherited traits.
• Condensation of DNA : DNA condenses to form chromosomes inside the cell nucleus.

📝 Essential Points

• Chromosomes are formed by condensed DNA inside the cell nucleus.
• Chromosomes contain information such as sex, species, and some diseases.
• Genes are located at precise positions on chromosomes.
• Genes determine inherited characteristics like traits of the individual.

💡 Memory Hook

DNA condenses into chromosomes; chromosomes carry genes; genes “write” traits.

📖 2. Chromosome number and species

🔑 Key Concepts & Definitions

• Human chromosome number : Human cells contain 23 pairs of chromosomes, for a total of 46 chromosomes.
• Chromosome pairs : Chromosomes occur in pairs within each cell for a given species.
• Species-specific chromosome count : Different species have different numbers of chromosome pairs while still sharing genes overall.
• Same genes, different counts : Species are similar in having the same types of genes, but they differ in the chromosome organization/count.

📝 Essential Points

• In humans, each cell has 46 chromosomes (23 pairs).
• The human species has 23 pairs of chromosomes.
• Dogs have 39 chromosome pairs, as stated in the source.

💡 Memory Hook

Think “pairs”: human 23 pairs; dogs 39 pairs.

📖 3. Sex chromosomes and genetic disorders

🔑 Key Concepts & Definitions

• Sex chromosomes : Sex chromosomes are a special chromosome pair that determines biological sex.
• XX : XX is the sex-chromosome combination associated with a female in the source.
• XY : XY is the sex-chromosome combination associated with a male in the source.
• Trisomy 21 : Trisomy 21 is a genetic condition caused by having 3 chromosomes in pair 21.

📝 Essential Points

• The sex pair is XX for a woman and XY for a man.
• Trisomy 21 means the pair 21 has 3 chromosomes instead of 2.
• The source links certain diseases to chromosome information.

💡 Memory Hook

XX vs XY decides sex; trisomy 21 means one extra chromosome in pair 21.

📖 4. Genes and inherited traits

🔑 Key Concepts & Definitions

• Inheritance : Inheritance is the transmission of genetic characteristics from parents to children.
• Gene position : Genes are found at specific locations on chromosomes.
• Trait determination : Genes determine inherited characteristics such as visible traits.
• Mutation : A mutation is a change in DNA that can lead to new characteristics.

📝 Essential Points

• A child receives half of their chromosomes from the mother and half from the father.
• Genes determine hereditary characters such as eye color and hair color, as examples.
• Mutations can produce new characteristics by changing DNA.
• The source connects inheritance with why siblings resemble parents but are not identical.

💡 Memory Hook

Half-from-mom + half-from-dad; mutation can add novelty.

📖 5. Alleles and genetic variation

🔑 Key Concepts & Definitions

• Alleles : Alleles are different versions of the same gene.
• Genetic variation : Genetic variation comes from the presence of different alleles for a trait.
• Eye color alleles : Blue and brown are presented as different alleles for eye color.
• Blood group alleles : The blood-group gene has alleles A, B, and O in the source.

📝 Essential Points

• One gene can carry a trait, while multiple alleles provide different versions of that trait.
• For eye color, the source lists M+M=M, B+B=B, and M+B=M.
• For blood groups, the source lists A+A=A, B+B=B, O+O=O, A+B=AB, A+O=A, and B+O=B.

💡 Memory Hook

Same gene, different alleles: M/B for eyes; A/B/O for blood.

📖 6. Dominant and recessive inheritance

🔑 Key Concepts & Definitions

• Dominant allele : A dominant allele expresses with only one copy when present in the genotype.
• Recessive allele : A recessive allele expresses only when two copies are present.
• Brown allele dominance : The brown eye allele (M) is dominant over the blue eye allele (B) in the source.
• O allele recessivity : The O blood-group allele is recessive relative to A and B in the source.

📝 Essential Points

• Brown is dominant because it needs only one presence to express, while blue is recessive and needs two presences.
• For eyes, the source’s outcomes match dominance: M+B=M and B+B=B.
• For blood groups, A and B are dominant to each other, producing AB when both are present, while O is recessive.
• The source states that some alleles are always expressed (dominant) and others only expressed when identical (recessive).

💡 Memory Hook

Dominant needs one copy; recessive needs two (M+M or B+B, not just one).

⚠️ Common Pitfalls & Confusions

1. Confusing chromosomes (structures made of condensed DNA) with genes (DNA segments that carry instructions).
2. Mixing up chromosome number by species, since humans are given as 23 pairs (46) while dogs are given as 39 pairs.
3. Thinking XY or XX are chromosomes pairs for disease; in the source they are specifically the sex-chromosome pair.
4. Assuming inheritance means you get the whole set from one parent; the source says you get half from each parent.
5. Mistaking recessive expression conditions; blue eyes require two blue alleles to express.
6. For blood groups, mixing up AB with a case where O is present; the source gives specific A+B and A/B+O results.

✅ Exam Checklist

1. Define DNA and describe what happens to it to form chromosomes.
2. State what chromosomes are made of and where they are located in the cell (nucleus).
3. Explain what a gene is and what it determines (inherited traits).
4. Give the human chromosome number in chromosomes and in pairs (46 and 23 pairs).
5. State the chromosome-pair pattern for sex chromosomes: XX and XY.
6. Explain what trisomy 21 means using the “pair 21 has 3 chromosomes” idea.
7. Define inheritance as the transmission of genetic characteristics from parents to children.
8. State the rule for chromosome contribution to offspring: half from mother and half from father.
9. Define mutation and state its potential effect on characteristics.
10. Define alleles as different versions of the same gene.
11. Use the eye-color allele outcomes M+M=M, B+B=B, and M+B=M.
12. Use the blood-group allele outcomes A+A=A, B+B=B, O+O=O, A+B=AB, A+O=A, and B+O=B.
13. State the dominant vs recessive rule using the “one copy vs two copies” description.