Principles of Inheritance and Variation
Mendel's laws, sex determination, genetic disorders.
Mendel's laws
Law of dominance, segregation, independent assortment.
Mendel's pea-plant experiments (1866) gave us three foundational principles of inheritance:
1. Law of Dominance. When two contrasting alleles for a trait are present, only one (the dominant) is expressed; the other (recessive) is masked. Example: tall (T) is dominant over dwarf (t). Tt → tall.
2. Law of Segregation. During gamete formation, the two alleles for a trait separate so each gamete carries only one. Example: a Tt parent produces 50% T gametes and 50% t gametes. This explains 3:1 phenotypic ratio in F2 of a monohybrid cross.
3. Law of Independent Assortment. Alleles for different traits sort independently of each other during gamete formation. Example: in a dihybrid cross TtYy × TtYy, F2 ratio is 9:3:3:1 (tall yellow : tall green : dwarf yellow : dwarf green).
Where Mendel's laws break down (modern exceptions):
| Phenomenon | Example |
|---|---|
| Incomplete dominance | RR × rr → Rr is pink (not red or white) — snapdragons |
| Codominance | AB blood group expresses both A and B antigens |
| Multiple alleles | ABO blood group has I^A, I^B, i alleles |
| Pleiotropy | Sickle cell allele affects multiple traits |
| Linkage | Genes on the same chromosome don't assort independently |
| Polygenic inheritance | Skin colour, height — controlled by many genes |
| Epistasis | One gene masks another (e.g. coat colour in mice) |
Test cross. To find if an organism showing dominant phenotype is homozygous (TT) or heterozygous (Tt), cross with the homozygous recessive (tt). If all offspring are dominant, parent is TT. If 1:1 split, parent is Tt.
Sex determination in humans
XX/XY, sex-linked inheritance, hemophilia.
Genetic disorders
Mendelian (sickle cell, PKU), chromosomal (Down, Klinefelter).
Three categories of inherited disease. Memorize the example for each — NEET asks every year.
1. Mendelian disorders (single gene).
Autosomal recessive (need two copies):
- Sickle cell anemia — β-globin gene mutation; HbS instead of HbA. Heterozygotes have malaria resistance (balanced polymorphism).
- Phenylketonuria (PKU) — phenylalanine hydroxylase deficiency. Untreated → mental retardation. Detected by Guthrie test at birth.
- Cystic fibrosis — CFTR chloride channel defect.
- Thalassemia — α or β globin chain underproduction.
Autosomal dominant (one copy enough):
- Huntington's disease — CAG repeat expansion in HTT gene. Late-onset.
- Marfan syndrome — fibrillin defect.
X-linked recessive:
- Hemophilia — factor VIII (A) or IX (B) deficiency. Famous in European royal families. Females usually carriers; males express.
- Color blindness — red-green most common; about 8% of males.
- Duchenne muscular dystrophy — dystrophin gene.
2. Chromosomal disorders (whole-chromosome aberrations).
Aneuploidy (wrong number of chromosomes):
| Disorder | Karyotype | Key features |
|---|---|---|
| Down syndrome | Trisomy 21 (47, +21) | Mongolian fold, intellectual disability, congenital heart defects |
| Edward syndrome | Trisomy 18 | Severe; usually die in infancy |
| Patau syndrome | Trisomy 13 | Cleft palate, polydactyly |
| Klinefelter syndrome | 47, XXY (male) | Feminine features, sterility |
| Turner syndrome | 45, X (female) | Short stature, sterility, no menstruation |
Polyploidy (3n, 4n) is common in plants but lethal in humans.
3. Sex determination basics.
In humans: XY = male, XX = female. Y chromosome has the SRY gene that triggers male development.
Sex-linked traits are usually X-linked because X is much larger. Y is gene-poor (mostly SRY and male-fertility genes).
X-linked inheritance pattern:
- Affected father → all daughters carriers, no sons affected (sons get Y, not X from father).
- Carrier mother → 50% sons affected, 50% daughters carriers.
Pedigree analysis tips:
- Skipping a generation → recessive.
- Affects every generation → dominant.
- More males affected, no father-to-son → X-linked recessive.
- All daughters of an affected father affected → X-linked dominant.