Botany · Class 12 · Chapter 5
Mendel's ratios through to pedigree analysis, with a working Punnett builder, a recombination model and three readable pedigrees.
This chapter is worth 3–4 NEET questions every year and it is the most reasoning-heavy chapter in Botany. Nothing here can be bluffed: ratios, crosses and pedigrees are either worked correctly or not at all.
Priority 1
Priority 2
| Character | Dominant | Recessive |
|---|---|---|
| Stem height | Tall | Dwarf |
| Flower colour | Violet | White |
| Flower position | Axial | Terminal |
| Pod shape | Inflated | Constricted |
| Pod colour | Green | Yellow |
| Seed shape | Round | Wrinkled |
| Seed colour | Yellow | Green |
Two facts NEET repeats
Mendel worked from 1856 to 1863 and published in 1865 (Proceedings of the Natural History Society of Brünn). His work was ignored and was rediscovered in 1900 independently by de Vries, Correns and von Tschermak. He used statistics and large samples — that was the real novelty.
Tall (TT) × dwarf (tt) → F1 all Tt, tall. Selfing the F1 gives F2 phenotypic ratio 3 : 1 and genotypic ratio 1 : 2 : 1.
Law of dominance (not a universal law)
Characters are controlled by discrete units called factors, which occur in pairs. In a dissimilar pair one member dominates and expresses itself; the other is recessive and is masked in the heterozygote but reappears unchanged in F2.
Law of segregation (universal — no exception)
The two alleles of a pair separate during gamete formation (at anaphase I of meiosis) so that each gamete receives only one of them. Gametes are always pure for a character.
| Test cross | Back cross | |
|---|---|---|
| Crossed with | the homozygous recessive parent only | either parent (dominant or recessive) |
| Purpose | to find whether a dominant-looking individual is TT or Tt | general breeding technique |
| Result if heterozygous | 1 : 1 ratio in the progeny | depends on which parent was used |
| Result if homozygous | all offspring show the dominant trait | — |
Where it went wrong
Every test cross is a back cross, but not every back cross is a test cross. If the question says “crossed with the recessive parent” it is a test cross; if it just says “crossed with a parent”, do not assume. This exact confusion (test-cross vs carrier-cross) has cost marks on more than one paper.
Round yellow (RRYY) × wrinkled green (rryy) → F1 RrYy, all round yellow. F2 gives 9 : 3 : 3 : 1 — 9 round yellow, 3 round green, 3 wrinkled yellow, 1 wrinkled green.
Law of independent assortment
When two pairs of traits are considered together, the segregation of one pair is independent of the other pair. Valid only for genes on different chromosomes (or very far apart on the same chromosome) — linkage is the exception.
| n genes | Gamete types | F2 phenotypes | F2 genotypes | Ratio |
|---|---|---|---|---|
| 1 | 2 | 2 | 3 | 3 : 1 |
| 2 | 4 | 4 | 9 | 9 : 3 : 3 : 1 |
| 3 | 8 | 8 | 27 | 27 : 9 : 9 : 9 : 3 : 3 : 3 : 1 |
Do it without the square
Use the product rule. For RrYy × RrYy, probability of wrinkled green = P(rr) × P(yy) = ¼ × ¼ = 1/16. For a test cross RrYy × rryy, all four classes come out 1 : 1 : 1 : 1. Multiplying single-gene probabilities is far faster and far less error-prone than filling sixteen boxes under time pressure.
Antirrhinum majus (snapdragon) and Mirabilis jalapa (four o'clock plant): red (RR) × white (rr) → pink F1 (Rr). F2 is 1 red : 2 pink : 1 white — the phenotypic ratio now equals the genotypic ratio. The dominant allele produces only enough pigment for an intermediate phenotype.
Both alleles express themselves fully and independently in the heterozygote. Human AB blood group is the standard case: IA and IB both make their own sugar antigen. Also roan cattle (red and white hairs both present, not blended).
Three alleles, IA, IB, i, but any one person carries only two. IA and IB are codominant to each other and both dominant over i.
| Blood group | Genotype(s) | Antigen on RBC | Antibody in plasma |
|---|---|---|---|
| A | IAIA, IAi | A | anti-B |
| B | IBIB, IBi | B | anti-A |
| AB (universal recipient) | IAIB | A and B | none |
| O (universal donor) | ii | none | anti-A and anti-B |
A single gene affecting several apparently unrelated traits. Phenylketonuria: one defective enzyme → mental retardation, reduced hair and skin pigmentation. Also sickle-cell anaemia and starch synthesis in pea (round vs wrinkled seed involves both starch grain size and water content).
Do not mix these up
Incomplete dominance = blended, intermediate phenotype (pink). Codominance = both phenotypes visible separately and fully (AB blood, roan coat). Both give a 1 : 2 : 1 F2 ratio, so the ratio alone will not tell you which is which — the description of the heterozygote will.
Sutton and Boveri (1902) noticed that the behaviour of chromosomes in meiosis parallels the behaviour of Mendel's factors: both occur in pairs, both segregate, both assort independently. Sutton united chromosome movement with Mendelian inheritance.
T. H. Morgan used Drosophila melanogaster — grown on simple synthetic medium, a life cycle of about two weeks, a single mating gives many progeny, males and females are easily told apart, and there are many hereditary variations visible with a low-power microscope.
The 50% ceiling
Recombination frequency can never exceed 50%. At 50% the two genes behave exactly as if they were on different chromosomes — that is independent assortment, and a test cross gives the 1 : 1 : 1 : 1 ratio. Anything below 50% is evidence of linkage.
| System | Female | Male | Heterogametic sex | Examples |
|---|---|---|---|---|
| XX – XY | XX | XY | male | humans, Drosophila, most mammals |
| XX – XO | XX | XO (one X, no Y) | male | grasshopper, roundworm, many insects |
| ZZ – ZW | ZW | ZZ | female | birds, some reptiles, moths, butterflies |
| Haplodiploidy | diploid (fertilised egg) | haploid (unfertilised egg, parthenogenesis) | — | honeybee |
Human specifics
Sex of the child is decided by the father's gamete: 50% of sperm carry X, 50% carry Y; all ova carry X. The Y chromosome carries the SRY gene for maleness. In Drosophila the Y is not male-determining — the ratio of X chromosomes to autosomes decides sex, which is why XO Drosophila is a sterile male but XO human (Turner's) is female.
A sudden, heritable change in the DNA sequence, and hence in the phenotype. Mutation is the ultimate source of new variation.
| Disorder | Inheritance | Key facts |
|---|---|---|
| Haemophilia | X-linked recessive | A protein in the clotting cascade is missing; a simple cut bleeds on and on. Transmitted from an unaffected carrier female to some sons. A female is affected only if both parents carry it — the homozygous condition is usually lethal before birth. Queen Victoria's family is the classic pedigree. |
| Colour blindness | X-linked recessive | Red–green confusion; about 8% of men and 0.4% of women. |
| Sickle-cell anaemia | Autosomal recessive | HbS/HbS affected, HbA/HbS carrier. Glu → Val at position 6 of the β chain; the RBC becomes sickle-shaped under low oxygen tension. |
| Phenylketonuria | Autosomal recessive | Enzyme converting phenylalanine to tyrosine is missing; phenylalanine and its derivatives accumulate, damaging the brain, and are excreted in urine. |
| Thalassaemia | Autosomal recessive | Reduced synthesis of a globin chain (quantitative problem); α-thalassaemia involves HBA1/HBA2 on chromosome 16, β-thalassaemia HBB on chromosome 11. |
| Cystic fibrosis | Autosomal recessive | Thick mucus in lungs and pancreas. |
The comparison NEET loves
Thalassaemia is quantitative — too few normal globin chains are made. Sickle-cell anaemia is qualitative — the right number of chains is made but one is structurally wrong. Same organ, different defect.
| Disorder | Karyotype | Features |
|---|---|---|
| Down's syndrome | trisomy 21, 45 + XX or XY = 47 | Described by Langdon Down (1866). Short stature, small round head, furrowed protruding tongue, partially open mouth, palm crease, retarded physical and mental development. Risk rises sharply with maternal age. |
| Klinefelter's syndrome | 47, XXY | Overall masculine build but feminine development (gynaecomastia); sterile. |
| Turner's syndrome | 45, X0 | Sterile female; ovaries rudimentary, lack of other secondary sexual characters. |
Aneuploidy = failure of chromatids to separate during cell division (non-disjunction) → gain or loss of a chromosome. Polyploidy = failure of cytokinesis after the replication of DNA → an extra whole set; common in plants.
Pedigree rules that settle most questions
| Ratio / number | What it means |
|---|---|
| 3 : 1 | monohybrid F2, complete dominance |
| 1 : 2 : 1 | monohybrid genotypic ratio; also the F2 phenotypic ratio for incomplete dominance and codominance |
| 9 : 3 : 3 : 1 | dihybrid F2 |
| 1 : 1 | test cross of a monohybrid heterozygote |
| 1 : 1 : 1 : 1 | test cross of a dihybrid heterozygote (independent assortment) |
| 2n / 3n / 4n | gamete types / F2 genotypes / Punnett boxes for n heterozygous genes |
| 1.3% and 37.2% | Morgan's recombination values: white–yellow (tight) and white–miniature wing (loose) |
| 47, XXY / 45, X0 / trisomy 21 | Klinefelter / Turner / Down |
Final checklist