Genetics – Study Notes for B.Sc Nursing Entrance
Audience: B.Sc Nursing candidates
Purpose: Quick, clear, and exam‑ready notes on the fundamentals of genetics.
1. Simple Explanation
Genetics is the science that deals with the inheritance of traits from one generation to the next.
- Gene: A unit of heredity made of DNA that codes for a specific protein or function.
- Genome: The complete set of genes in an organism.
- DNA (Deoxyribonucleic Acid): Double‑helix structure that stores genetic information.
- Allele: Different versions of the same gene (e.g., A for brown eye, a for blue eye).
Inheritance follows patterns described by Mendel’s laws, but real‑world genetics can be more complex due to linkage, mutations, and chromosomal abnormalities.
2. Key Points
| Concept | What It Means | Nursing Relevance |
|---|---|---|
| Mendelian Inheritance | Traits governed by one gene with dominant (A) & recessive (a) alleles. | Predicting risk of inherited diseases (e.g., cystic fibrosis). |
| Monohybrid Cross | One gene, two alleles (AA, Aa, aa). | Determining probability of disease carriers. |
| Dihybrid Cross | Two genes, four alleles (Aa × Bb). | Understanding compound traits. |
| Punnett Square | Visual tool to calculate genotype/phenotype ratios. | Quick assessment of genetic risk. |
| Sex‑Linked Traits | Genes on X or Y chromosome (e.g., red‑green color blindness). | Explain why some disorders affect males more. |
| Incomplete Dominance | Heterozygote shows intermediate phenotype (e.g., red + white flowers → pink). | Recognize atypical inheritance patterns. |
| Codominance | Both alleles expressed (AB blood type). | Blood group compatibility. |
| Polygenic Traits | Controlled by many genes (e.g., height, skin color). | Acknowledge multifactorial diseases. |
| Linkage & Recombination | Genes close together on a chromosome tend to be inherited together. | Understand why some traits co‑occur. |
| Mutation | Change in DNA sequence → new allele. | Causes genetic disorders (e.g., sickle cell anemia). |
| DNA Replication, Transcription, Translation | Steps from DNA → RNA → Protein. | Basis for understanding genetic expression. |
| Genetic Testing | PCR, karyotyping, sequencing. | Diagnostic tools in clinical genetics. |
3. Worked Example
3.1 Monohybrid Cross – Eye Colour
- Gene: OCA2 (pale eye – a; dark eye – A)
- Parents: Aa (heterozygous) × Aa
| A (dominant) | a (recessive) | |
|---|---|---|
| A | AA | Aa |
| a | Aa | aa |
- Genotype Ratio: 1 : 2 : 1 (AA : Aa : aa)
- Phenotype Ratio: 3 : 1 (dark : pale)
Interpretation: 75 % of children will have dark eyes; 25 % will have pale eyes.
3.2 Dihybrid Cross – Flower Colour & Height
- Genes:
- C – flower colour (C = purple, c = white)
- T – plant height (T = tall, t = short)
- Parents: CcTt × CcTt
| CT | Ct | cT | ct | |
|---|---|---|---|---|
| CT | CCTT | CCTt | CcTT | CcTt |
| Ct | CCTt | CCTt | CcTt | Cc tt |
| cT | CcTT | CcTt | ccTT | ccTt |
| ct | CcTt | Cc tt | ccTt | cc tt |
- Phenotype Ratio: 9 : 3 : 3 : 1
- 9 purple‑tall
- 3 purple‑short
- 3 white‑tall
- 1 white‑short
3.3 Sex‑Linked Trait – Red‑Green Color Blindness
- X‑linked recessive
- Parents: Mother (XᴴXᴴ) – carrier; Father (XᴴY) – normal
| Xᴴ (father) | Y (father) | |
|---|---|---|
| Xᴴ (mother) | XᴴXᴴ | XᴴY |
| Xᴴ (mother) | XᴴXᴴ | XᴴY |
- Offspring:
- 50 % normal males (XᴴY)
- 50 % normal females (XᴴXᴴ)
- No affected males (no recessive allele on Y)
- No affected females (requires two copies)
4. Quick Revision Summary
- DNA → Gene → Allele: Base pairs encode proteins that determine traits.
- Mendel's Laws:
- First Law: Law of Segregation – alleles separate during gamete formation.
- Second Law: Law of Independent Assortment – genes on different chromosomes assort independently.
- Punnett Squares give genotype/phenotype ratios.
- Dominant vs. Recessive: Dominant allele masks recessive in heterozygotes.
- Sex‑Linked: X‑linked recessives affect males more often.
- **Link