Mendel · crosses · linkage · sex determination · genetic disorders
Genetics is the branch of biology that studies the principles of inheritance and its practices.
Humans knew inheritance from ~8000–1000 BC (domestication, breeding) but the scientific basis was given by Gregor Johann Mendel (1856–1863) through pea-plant experiments.
Mendel worked on the garden pea (Pisum sativum) and analysed results mathematically/statistically — a first in biology. He studied 7 pairs of contrasting traits.
| Character | Dominant | Recessive |
|---|---|---|
| Stem height | Tall | Dwarf |
| Flower colour | Violet | White |
| Flower position | Axial | Terminal |
| Pod shape | Inflated (full) | Constricted |
| Pod colour | Green | Yellow |
| Seed shape | Round | Wrinkled |
| Seed colour | Yellow | Green |
| Term | Meaning |
|---|---|
| Factor / Gene | Unit of inheritance controlling a character (Mendel's "factor"). |
| Allele | Slightly different forms of the same gene (e.g. T and t). |
| Genotype | Genetic constitution (e.g. Tt). |
| Phenotype | Observable trait (e.g. Tall). |
| Homozygous | Identical alleles (TT or tt). |
| Heterozygous | Different alleles (Tt). |
| Dominant | Allele expressed in heterozygote (capital, T). |
| Recessive | Allele masked in heterozygote (small, t). |
Convention: capital letter = dominant allele, same small letter = recessive allele.
A cross tracking one pair of contrasting traits.
Test cross: cross the unknown dominant phenotype with a homozygous recessive to reveal its genotype (offspring ratio 1:1 → heterozygous; all dominant → homozygous). Back cross: cross F₁ with either parent.
| Feature | Incomplete Dominance | Co-dominance |
|---|---|---|
| F₁ phenotype | Intermediate (blend) | Both alleles expressed together |
| Example | Snapdragon / Mirabilis jalapa: Red × White → Pink | Human ABO blood group (Iᴬ & Iᴮ) |
| F₂ ratio | 1 : 2 : 1 (Red:Pink:White) — phenotype = genotype ratio | Depends on cross; both antigens appear (AB) |
Gene I has three alleles: Iᴬ, Iᴮ, i (a case of multiple allelism — more than two alleles for one gene). Iᴬ and Iᴮ each make a surface sugar (antigen A / B); i makes none. Iᴬ and Iᴮ are dominant over i but co-dominant to each other.
| Blood group | Genotype(s) |
|---|---|
| A | IᴬIᴬ or Iᴬi |
| B | IᴮIᴮ or Iᴮi |
| AB | IᴬIᴮ (co-dominance) |
| O | ii |
A cross tracking two pairs of traits together (seed shape + seed colour).
Mendel's work was rediscovered in 1900 (de Vries, Correns, von Tschermak).
Walter Sutton & Theodore Boveri (1902) noticed that chromosome behaviour during meiosis parallels the behaviour of Mendel's factors, and united the two ideas as the Chromosomal Theory of Inheritance.
| Chromosome behaviour | Gene / factor behaviour |
|---|---|
| Occur in pairs (homologues) | Alleles occur in pairs |
| Segregate at gamete formation | Alleles segregate (Law of Segregation) |
| Independent pairs assort independently | Independent Assortment |
Thomas Hunt Morgan worked on the fruit fly Drosophila melanogaster and gave experimental proof of the chromosomal theory.
| Linkage | Recombination |
|---|---|
| Physical association of genes on the same chromosome; tend to be inherited together. | Generation of non-parental gene combinations (crossing over). |
| Genes close together → tightly linked, low recombination. | Genes far apart → loosely linked, high recombination. |
Morgan's Drosophila dihybrid data: genes for white eye & yellow body were tightly linked (~1.3% recombination); white eye & miniature wing showed ~37.2% recombination.
| Polygenic inheritance | Pleiotropy |
|---|---|
| Many genes → one trait | One gene → many traits |
| Phenotype = additive effect of alleles + environment; shows a range/gradient. | A single gene affects several, seemingly unrelated, characters. |
| Examples: human skin colour, height. | Examples: phenylketonuria, sickle-cell anaemia, starch synthesis in pea (round/wrinkled). |
In skin colour, the more dominant alleles present (e.g. of genes A,B,C), the darker the skin — a quantitative, continuous trait.
| System | Female | Male | Example |
|---|---|---|---|
| XX–XO | XX | XO (one X, no Y) | Grasshopper / many insects |
| XX–XY | XX (homogametic) | XY (heterogametic) | Humans, Drosophila |
| ZW–ZZ | ZW (heterogametic) | ZZ (homogametic) | Birds |
Humans: 22 pairs of autosomes + 1 pair of sex chromosomes. Female = XX, Male = XY. Because the father is heterogametic (X- or Y-bearing sperm), the sex of the child is decided by the father.
| Disorder | Type | Key facts |
|---|---|---|
| Haemophilia | X-linked recessive | A clotting-factor protein not made → non-stop bleeding; mostly affects males; classic royal pedigree (Queen Victoria). |
| Sickle-cell anaemia | Autosomal recessive | Glutamic acid → Valine at 6th position of β-globin (GAG→GUG); HbˢHbˢ affected, HbᴬHbˢ carrier; RBCs sickle at low O₂. |
| Phenylketonuria | Autosomal recessive | Enzyme converting phenylalanine → tyrosine is absent; phenylalanine + derivatives accumulate → mental retardation; excreted in urine. |
| Thalassemia | Autosomal recessive | Reduced synthesis of globin chains (α on chr 16 / β on chr 11); a quantitative problem (too little globin). Contrast: sickle-cell is qualitative. |
| Colour blindness | X-linked recessive | Red-green defect; ~8% of males, ~0.4% of females. |
Aneuploidy = gain/loss of a chromosome due to non-disjunction (failure of segregation). Polyploidy = gain of a whole chromosome set (common in plants).
| Disorder | Karyotype | Features |
|---|---|---|
| Down's syndrome | Trisomy 21 (extra 21) | Short stature, small round head, furrowed protruding tongue, partly open mouth, broad palm with a characteristic crease; retarded mental & physical development. (Described by Langdon Down.) |
| Klinefelter's syndrome | 47, XXY (extra X) | Overall masculine but with some feminine development (gynaecomastia = breast development); sterile. |
| Turner's syndrome | 45, X0 (missing X) | Sterile female; rudimentary ovaries; lack of other secondary sexual characters. |
Pedigree analysis studies a trait across generations using standard symbols (□ male, ○ female, filled = affected), used for genetic counselling.