Basic Principles of Inheritance

## (a) Genotype and Phenotype

- Genotype: the genetic constitution or gene combination of an organism.
- Phenotype: the observable or expressed appearance/character of an organism.

Example: In tall pea plants, TT and Tt are different genotypes, but both show the same tall phenotype.

- Dominant character: the character that expresses itself in a heterozygous condition and masks the other.
- Recessive character: the character that does not express itself in a heterozygous condition and appears only in homozygous condition.

Example: In pea plant height, tall (T) is dominant over dwarf (t).

- Hybrid individual: an organism having two different alleles for a trait, i.e., heterozygous.
- Pure individual: an organism having two same alleles for a trait, i.e., homozygous.

Example: Tt is a hybrid, while TT or tt is pure.

- Heterozygous progeny: offspring having different alleles for a character, such as Tt.
- Homozygous progeny: offspring having similar alleles for a character, such as TT or tt.

So, heterozygous progeny carry two unlike alleles, whereas homozygous progeny carry two identical alleles.

- Monohybrid cross: a cross involving one pair of contrasting characters.
- Dihybrid cross: a cross involving two pairs of contrasting characters.

Example:
- TT × tt is a monohybrid cross.
- RRYY × rryy is a dihybrid cross.

- Gene: a unit of heredity; a segment of DNA controlling a character.
- Allele: an alternate form of the same gene.

Example: For plant height, T and t are alleles of the same gene.

- Incomplete dominance: neither allele is completely dominant, so the heterozygote shows an intermediate phenotype.
- Codominance: both alleles are equally expressed in the heterozygote.

Example:
- In Mirabilis jalapa, red × white gives pink in F₁, showing incomplete dominance.
- In MN blood group, both M and N are expressed together, showing codominance.

### (a) F₁ progeny with pure dominant parent
From the chapter, F₁ for a monohybrid cross is heterozygous, e.g. Tt. Crossing it with pure dominant TT:

$$Tt \times TT$$

Gametes:
- from Tt: T, t
- from TT: T

Offspring: TT, TT, Tt, Tt

- Genotypic ratio: 1 TT : 1 Tt
- Phenotypic ratio: all dominant = 1 : 0

### (b) F₁ progeny with pure recessive parent
Cross Tt × tt

Gametes:
- from Tt: T, t
- from tt: t

Offspring: Tt, tt

- Genotypic ratio: 1 Tt : 1 tt
- Phenotypic ratio: 1 dominant : 1 recessive

### (c) F₁ progeny with F₁ progeny
Cross Tt × Tt

Offspring: TT, Tt, Tt, tt

- Genotypic ratio: 1 : 2 : 1
- Phenotypic ratio: 3 dominant : 1 recessive

### Test cross
A test cross is a cross between an individual showing a dominant phenotype and a homozygous recessive individual, to find out whether the dominant individual is homozygous or heterozygous.

### Diagrammatic representation
If the tall plant is TT:

$$TT \times tt$$

All offspring: Tt → all tall

If the tall plant is Tt:

$$Tt \times tt$$

Gametes: T, t and t

Offspring:
- Tt → tall
- tt → dwarf

So the appearance of both tall and dwarf offspring confirms that the dominant plant is heterozygous.

Law of dominance states that when two contrasting alleles are present together in an individual, only one of them expresses itself in the F₁ generation, while the other remains hidden.

The expressing allele is called dominant and the hidden one is called recessive.

Example: In the cross TT × tt, all F₁ plants are Tt and appear tall, because T is dominant over t.

Law of segregation states that the two alleles of a gene separate during gamete formation and are passed on separately to different gametes.

Example: In Tt × Tt, each heterozygous parent produces T and t gametes. On selfing, the offspring show TT, Tt, tt in the ratio 1:2:1. This proves that alleles segregate during inheritance.