Question
In a cross between a heterozygous tall plant (Tt) and a short plant (tt), predict the phenotypic and genotypic ratios of offspring. Then explain the systematic approach to solving any genetics problem, including pedigree analysis.
(NEET + CBSE 12 pattern)
Solution — Step by Step
Parents: Tt (tall) tt (short)
| t | t | |
|---|---|---|
| T | Tt | Tt |
| t | tt | tt |
Offspring: 2 Tt : 2 tt
Genotypic ratio: Tt : tt = 1 : 1 (50% heterozygous tall, 50% homozygous short)
Phenotypic ratio: Tall : Short = 1 : 1
This is a test cross — crossing an organism with a homozygous recessive. It reveals the genotype of the unknown parent.
Follow this decision-making process for every genetics question:
- Determine dominance — which trait is dominant?
- Assign allele symbols — capital for dominant, lowercase for recessive
- Write parental genotypes — use phenotype + pedigree clues
- Draw Punnett square — for simple crosses (1 or 2 genes)
- Read ratios — count phenotypes and genotypes from the square
- For pedigree: trace the pattern — autosomal/sex-linked, dominant/recessive
flowchart TD
A["Genetics Problem"] --> B{"What type?"}
B -->|"Simple cross"| C["Identify parental genotypes"]
B -->|"Pedigree analysis"| D["Check if trait skips generations"]
B -->|"Dihybrid cross"| E["Use 4x4 Punnett square"]
C --> F["Draw Punnett square"]
F --> G["Read phenotypic + genotypic ratios"]
D --> H{"Trait in every generation?"}
H -->|"Yes"| I["Likely dominant"]
H -->|"No, skips"| J["Likely recessive"]
J --> K{"Affected females?"}
K -->|"Yes, equal to males"| L["Autosomal recessive"]
K -->|"Rare in females"| M["X-linked recessive"]Why This Works
Mendelian genetics works because alleles segregate independently during gamete formation (Law of Segregation). Each parent contributes one allele to each offspring, and the combination determines the phenotype.
The test cross (crossing with homozygous recessive) is a powerful diagnostic tool. If the tall parent were TT, all offspring would be tall. Since we get a 1:1 ratio, we confirm the parent is Tt. The recessive parent can only contribute recessive alleles, so it acts like a “transparent” background that lets us read the other parent’s genotype directly.
For pedigree analysis, the key patterns are: autosomal recessive traits skip generations and appear equally in both sexes; X-linked recessive traits appear more in males and never pass father-to-son; dominant traits appear in every generation.
Alternative Method — Probability Approach
Instead of drawing the full Punnett square, use probability:
Tt parent produces T gametes (probability 1/2) and t gametes (probability 1/2).
tt parent produces only t gametes (probability 1).
Probability of Tt offspring
Probability of tt offspring
For NEET dihybrid cross questions, remember the standard ratios: 9:3:3:1 (both heterozygous), 1:1:1:1 (test cross with dihybrid), 3:1 (monohybrid F2). If you see a modified ratio like 9:3:4 or 9:7, it indicates epistasis — one gene masking another. Know the common modified ratios and which type of epistasis each represents.
Common Mistake
In pedigree analysis, students often assume that a trait appearing mostly in males must be X-linked. While X-linked recessive traits DO appear more in males, an autosomal recessive trait can also appear predominantly in males by coincidence in a small family. The definitive test for X-linked inheritance is: an affected father NEVER passes the trait to his sons (he gives them his Y chromosome, not X). If an affected father has affected sons, the trait is autosomal, not X-linked.