🫛 Mendel's Laws — Dominance, Segregation & Crosses
Law of Dominance (First Law)
The Law of Dominance is actually composed of two conclusions that Mendel drew from his experiments:
First Conclusion (Law of Paired Factors)
- According to Mendel, every trait is controlled by paired factors (now called genes/alleles)
- Each organism has two alleles for each trait (one from each parent)
- Factors come in pairs as discrete units — they do not blend or dilute each other
Second Conclusion (Law of Dominance)
- When two contrasting alleles are present together (heterozygous condition), only one allele expresses its phenotype
- The allele that expresses is called the Dominant allele
- The allele that is masked (hidden) is called the Recessive allele
- In the heterozygote (F1), the recessive allele is present but does not express — this is Mendel's principle of dominance
IMPORTANT
Law of Dominance = First Conclusion (Paired Factors) + Second Conclusion (Dominance)
Two exceptions to the Law of Dominance:
- A. Incomplete dominance — neither allele is fully dominant; the heterozygote shows an intermediate phenotype
- B. Codominance — both alleles express simultaneously in the heterozygote
Law of Segregation (Second Law / Third Conclusion) — Law of Purity of Gametes
This law is based on the F2 generation results and is considered Mendel's most fundamental contribution.
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Law of Dominance (First Law)
The Law of Dominance is actually composed of two conclusions that Mendel drew from his experiments:
First Conclusion (Law of Paired Factors)
- According to Mendel, every trait is controlled by paired factors (now called genes/alleles)
- Each organism has two alleles for each trait (one from each parent)
- Factors come in pairs as discrete units — they do not blend or dilute each other
Second Conclusion (Law of Dominance)
- When two contrasting alleles are present together (heterozygous condition), only one allele expresses its phenotype
- The allele that expresses is called the Dominant allele
- The allele that is masked (hidden) is called the Recessive allele
- In the heterozygote (F1), the recessive allele is present but does not express — this is Mendel's principle of dominance
IMPORTANT
Law of Dominance = First Conclusion (Paired Factors) + Second Conclusion (Dominance)
Two exceptions to the Law of Dominance:
- A. Incomplete dominance — neither allele is fully dominant; the heterozygote shows an intermediate phenotype
- B. Codominance — both alleles express simultaneously in the heterozygote
Law of Segregation (Second Law / Third Conclusion) — Law of Purity of Gametes
This law is based on the F2 generation results and is considered Mendel's most fundamental contribution.
- During gamete formation, the two alleles of a gene separate (segregate) from each other so that each gamete carries only one allele
- When gametes fuse during fertilization, the paired condition is restored
- Each gamete receives only one allele of each gene — gametes are therefore "pure" for each trait
- Segregation of factors occurs during Anaphase I of meiosis (when homologous chromosomes separate)
- Segregation begins during gametogenesis (gamete formation)
- The conclusion of segregation is that all sexually reproducing organisms must undergo segregation during cell division
- If segregation does not occur properly (non-disjunction), it can lead to genetic abnormalities such as Down syndrome
- This is Mendel's most fundamental law and is universally valid — it applies to all sexually reproducing organisms
Monohybrid Cross
A cross involving the study of one pair of contrasting characters. This is the simplest type of genetic cross:
Parents: TT (Tall) × tt (Dwarf)
↓
F1: All Tt (Tall) — Impure
↓ Self-pollination
F2: Tall : Dwarf
3 : 1 (Phenotypic ratio or Basic Mendelian ratio)
In this cross, all F1 offspring are tall because the T (tall) allele is dominant over the t (dwarf) allele. When F1 plants are selfed, the recessive trait (dwarf) reappears in F2, proving that the recessive allele was present but hidden in F1.
Checker Board Method (Punnett Square): First used by Reginald C. Punnett (1875-1967) to determine the combinations of gametes from a cross. This visual tool makes it easy to predict offspring ratios.
| T | t | |
|---|---|---|
| T | TT (Tall, Pure) | Tt (Tall, Impure) |
| t | Tt (Tall, Impure) | tt (Dwarf, Pure) |
| Ratio Type | Values |
|---|---|
| Phenotypic ratio | Tall : Dwarf = 3 : 1 |
| Genotypic ratio | TT : Tt : tt = 1 : 2 : 1 |
F3 Generation (from selfing F2)
When F2 plants are selfed, the results depend on their genotype:
F2 plants selfed:
- 1 TT (pure tall) → F3: All Tall
- 2 Tt (impure tall) → F3: 3 Tall : 1 Dwarf
- 1 tt (pure dwarf) → F3: All Dwarf
This confirms that the 3:1 ratio in F2 actually represents 1 pure dominant : 2 heterozygous : 1 pure recessive.
Fork Line Method (Branch Diagram)
For determining gamete types and phenotypic/genotypic categories in complex crosses, use these formulas:
- 2^n = Types of gametes AND Phenotypic categories
- 3^n = Genotypic categories
- 4^n = Total zygote combinations from selfing
Where n = number of heterozygous pairs (hybrid characters)
| n (hybrid pairs) | Gamete types (2^n) | Phenotypes (2^n) | Genotypes (3^n) | Zygotes (4^n) |
|---|---|---|---|---|
| 1 (Monohybrid) | 2 | 2 | 3 | 4 |
| 2 (Dihybrid) | 4 | 4 | 9 | 16 |
| 3 (Trihybrid) | 8 | 8 | 27 | 64 |
TIP
These formulas are extremely useful for quickly calculating the expected outcomes of any cross. Just count the number of heterozygous gene pairs (n) and plug into the formula.
Image Generation Prompt
A Punnett square diagram for a monohybrid cross (Tt x Tt). Show the parents at top and left side with gametes T and t. Fill the 4 boxes with genotypes TT, Tt, Tt, tt. Color-code: TT in dark green (homozygous dominant), Tt in medium green (heterozygous), tt in light/white (homozygous recessive). Label phenotypic ratio 3:1 and genotypic ratio 1:2:1 below. Include small pea plant illustrations showing tall and dwarf phenotypes. Clean educational genetics style.
Test Cross
The test cross is the most important cross in genetics. When an F1 hybrid is crossed with the homozygous recessive parent, it is called a Test Cross.
- The purpose of a test cross is to reveal the genotype of an organism showing the dominant phenotype. Since a tall plant could be either TT or Tt, crossing it with tt tells us which one it is.
- In test cross offspring, both phenotypic and genotypic ratios are the same — this is a unique and important property.
(A) Monohybrid Test Cross
F1 (Hybrid) × Recessive parent
Tt tt
↓
┌────┬────┐
│ │ t │ t │
├────┼────┤
│ T │ Tt │ Tt │ → 50% Tall
│ t │ tt │ tt │ → 50% Dwarf
└────┴────┘
Result: Phenotypic and Genotypic ratio = 1 : 1 (1 Tall : 1 Dwarf)
If the test cross gives a 1:1 ratio, the parent was heterozygous (Tt). If all offspring show the dominant trait, the parent was homozygous dominant (TT).
(B) Dihybrid Test Cross
F1 (Dihybrid) × Recessive parent
TtRr ttrr
↓
| Tr | tR | tr | TR | |
|---|---|---|---|---|
| tr | Ttrr | ttRr | ttrr | TtRr |
Four types of offspring, each at 25% probability.
Result: Phenotypic and Genotypic ratio = 1 : 1 : 1 : 1
IMPORTANT
Test cross is the most important cross in genetics. From it, both phenotypic and genotypic ratios are identical. This makes it the definitive method for determining an organism's genotype.
Back Cross
- Crossing F1 offspring with either parent is called a Back Cross
- Two types:
- Out cross: F1 × Dominant parent → All offspring show dominant trait
- Test cross: F1 × Recessive parent → 1:1 ratio
TIP
Remember: Every test cross is a back cross, but not every back cross is a test cross. A back cross with the recessive parent is specifically called a test cross.
Out Cross
F1 (Hybrid) × Dominant parent
Tt TT
↓
┌────┬────┐
│ │ T │ T │
├────┼────┤
│ T │ TT │ TT │
│ t │ Tt │ Tt │
└────┴────┘
Result: All offspring are Tall (all dominant phenotype). This cross does not help us determine the genotype because all offspring look the same regardless of whether the parent was TT or Tt.
Reciprocal Cross
A pair of crosses in which the sex of the parents is reversed:
Cross (a): TT (Female) × tt (Male) → F1: All Tall
Cross (b): tt (Female) × TT (Male) → F1: All Tall
Reciprocal crosses are a powerful diagnostic tool:
- If reciprocal crosses give the same result, the gene is on an autosome (nuclear gene, biparental inheritance)
- If reciprocal crosses give different results, the gene may be on a sex chromosome or in the cytoplasm
- Reciprocal crosses are used to determine whether a trait shows sex linkage or cytoplasmic inheritance
- Mendel found no difference in reciprocal crosses because the traits he studied were autosomal
NOTE
Karyogene: Genes on autosomes; inherited biparentally (from both parents). This contrasts with cytoplasmic genes (plasmagenes) which show maternal inheritance.
Key Points to Remember
- Law of Dominance: Dominant allele masks recessive in heterozygote (exceptions: incomplete dominance, codominance)
- Law of Segregation: Alleles separate during gamete formation (Anaphase I of meiosis); each gamete carries only one allele
- Monohybrid F2: Phenotypic ratio 3:1 | Genotypic ratio 1:2:1
- Punnett Square invented by Reginald C. Punnett
- Test cross (F1 × recessive parent) → both phenotypic and genotypic ratio = 1:1
- Back cross = F1 × either parent; test cross is a specific back cross
- Reciprocal crosses same result → autosomal gene; different result → sex-linked or cytoplasmic
- Key formula: Gamete types = 2^n; Genotypes = 3^n; Zygote combos = 4^n
Summary Cheat Sheet
| Concept / Topic | Key Details / Explanation |
|---|---|
| Law of Dominance (1st Law) | Composed of two conclusions: 1. Paired Factors — every trait controlled by two alleles 2. Dominance — dominant allele masks recessive in heterozygote |
| Exceptions to Law of Dominance | Incomplete dominance (intermediate phenotype) and Codominance (both alleles express simultaneously) |
| Law of Segregation (2nd Law) | During gamete formation, two alleles separate so each gamete gets only one allele; occurs at Anaphase I of meiosis |
| Most fundamental law | Law of Segregation — universally valid for all sexually reproducing organisms |
| Non-disjunction | Failure of proper segregation → genetic abnormalities (e.g., Down syndrome) |
| Monohybrid cross | Study of one pair of contrasting characters |
| Monohybrid F2 phenotypic ratio | 3 : 1 (dominant : recessive) |
| Monohybrid F2 genotypic ratio | 1 : 2 : 1 (TT : Tt : tt) |
| Punnett Square | Checker board method; invented by Reginald C. Punnett (1875–1967) |
| Fork Line formulas | Gamete types = 2^n Phenotypic classes = 2^n Genotypic classes = 3^n Zygote combinations = 4^n (n = number of heterozygous pairs) |
| Test cross | F1 hybrid × homozygous recessive parent; reveals genotype of dominant phenotype organism |
| Monohybrid test cross ratio | Phenotypic and genotypic ratio both = 1 : 1 |
| Dihybrid test cross ratio | 1 : 1 : 1 : 1 |
| Key property of test cross | Phenotypic ratio = Genotypic ratio (unique property) |
| Back cross | F1 × either parent; test cross is a specific type of back cross |
| Out cross | F1 × dominant parent → all offspring show dominant trait |
| Reciprocal cross | Sex of parents is reversed; same result → autosomal gene; different result → sex-linked or cytoplasmic |
| Karyogene | Genes on autosomes; inherited biparentally |
| F3 from selfing F2 | TT → all Tall; Tt → 3 Tall : 1 Dwarf; tt → all Dwarf |
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