Genetics – AIIMS Nursing Study Notes
(Prepared for the Biology section of the AIIMS Nursing entrance exam)
1. Simple Explanation
Genetics is the branch of biology that deals with heredity, genes, and variation in organisms.
- Genes: Units of heredity made of DNA.
- Chromosomes: Thread‑like structures carrying genes; humans have 23 pairs.
- Alleles: Different versions of a gene; one from each parent.
- Phenotype: Observable trait (e.g., eye colour).
- Genotype: Genetic constitution (e.g., AA, Aa).
Genetic information flows in one direction:
DNA → RNA (transcription) → Protein (translation).
2. Key Points
| Category | Highlights |
|---|---|
| DNA Structure | Double helix, 4 bases (A, T, C, G). Complementary base pairing. |
| Replication | Semi‑conservative, leading/lagging strands, DNA polymerase, proofreading. |
| Transcription | RNA polymerase synthesizes mRNA; includes promoter, enhancer, terminator. |
| Translation | Ribosome reads mRNA codons, tRNA brings amino acids, forms polypeptide. |
| Gene Expression | Controlled by transcription factors, epigenetics (DNA methylation, histone modification). |
| Mendelian Genetics | Law of Segregation, Law of Independent Assortment, dominant/recessive alleles. |
| Punnett Square | Tool to predict genotype/phenotype ratios in crosses. |
| Non‑Mendelian Patterns | Codominance, incomplete dominance, polygenic traits, multiple alleles. |
| Chromosomal Disorders | Down syndrome (trisomy 21), Turner syndrome (XO), Klinefelter (XXY). |
| Genetic Testing | PCR, Southern blot, karyotyping, genomic sequencing. |
| Inheritance of Traits | Autosomal dominant/recessive, X‑linked, mitochondrial. |
| Genetic Counseling | Risk assessment, carrier screening, prenatal diagnosis. |
3. Worked Example
Problem
A couple has a 1:1 sex ratio among their children. The father is an obligate carrier of a recessive X‑linked disorder (e.g., red‑green colour blindness). What is the probability that a daughter will be affected?
Solution
-
Identify the genes
- X‑linked recessive trait → allele c (affected) vs C (normal).
- Father: cY (since he has the trait).
- Mother: CC (carrier or normal? We need to deduce).
-
Determine mother’s genotype
- Sex ratio 1:1 (male:female) → normal Mendelian ratio → mother is CC (not a carrier).
- If she were a carrier (Cc), the ratio would be 1:3 or 3:1 depending on pattern.
-
Cross
X (C) X (C) Y (c) XcY XcY -
Outcome
- Females: XcX (carrier, unaffected).
- Males: XCY (unaffected).
-
Probability that a daughter is affected
- Daughters inherit the father’s Y? No, they inherit the father’s X.
- Mother passes C → daughters are XCX → unaffected.
- Probability = 0%.
Key Takeaway: X‑linked recessive disorders affect males more often; females can be carriers but rarely affected if mother is normal.
4. Quick Revision Summary
| Concept | Quick Fact |
|---|---|
| DNA | 4 bases, complementary, double‑helix. |
| Replication | Semi‑conservative, enzyme DNA polymerase. |
| Transcription | mRNA made from DNA template. |
| Translation | Ribosome + tRNA → protein. |
| Alleles | Dominant vs recessive; codominance; incomplete dominance. |
| Punnett Square | Predicts genotype/phenotype ratios. |
| Autosomal Dominant | 1:1 ratio; heterozygote shows trait. |
| Autosomal Recessive | 1:4 ratio; two carriers needed. |
| X‑Linked Recessive | Affects males; females usually carriers. |
| Chromosomal Disorder | Down (trisomy 21); Turner (XO); Klinefelter (XXY). |
| Genetic Testing | PCR, karyotype, sequencing. |
Tip for Nursing Exams
Focus on clinically relevant disorders (e.g., Down, Turner, Klinefelter, sickle‑cell disease) and understand their inheritance patterns. Also, remember key nursing implications such as prenatal screening and patient counseling.
Good luck with your studies!