Heterosis

It is a proven way  of increasing productivity of many crop plants . It is also called as hybrid vigour or outbreeding enhancement.  It leadsto  increase in such characteristics as size , growth rate , fertiity and yield of a hybrid organism . It actually results from crosses between two genetically different , highly inbreed lines. It is always associated with increased heterozygocity. Heterosis is often discussed as the opposite of inbreeding depression .

Types of Heterosis:

Heterosis is of two types-

1. True heterosis:

It is inherited.

It can be further divided into two types:

(a) Mutational true heterosis:

It is the sheltering or shadowing of the deleterious, un-favourable, often lethal, recessive mutant genes by their adaptively superior dominant alleles.

(b) Balanced true heterosis:

It arises out of balanced gene combinations with better adaptive value and agricultural usefulness.

2. Pseudo-heterosis:

Crossing of the two parental forms brings in an accidental, excessive and un-adaptable expression of temporary vigour and vegetative overgrowth. It is also called luxuriance.

For example: Gossypium Hirsutum (Cotton) has two varieties: one is a pure line variety (Variety-A) which is disease-resistant but with low ginning-percentage of cotton, while the other pure-line variety (Variety-B) has high ginning percentage, but is very susceptible to diseases like wilting or boll-worm. The farmer after sowing variety-B expects high commercial returns, but has to spend huge amount in spraying to protect it from diseases.

Cotton breeders cross these two varieties to produce a F1 hybrid which has high disease resistance as well as high ginning percentage of cotton. This hybrid gives the benefit of both the parent varieties and even more than the parents.

1.Heterosis lost due to continuous inbreeding known as
A)Hybrid vigour
B)Inbreeding depression
C)Outbreeding depression
D)none of these

Ans. b

2.When two unrelated individuals or lines are corssed, the performance of F1 hybrid is often superior to both its parents. This phenomenon is called

1. Metamorphosis
2. Heterosis
3. Transformation

4. Spheing

Ans. b

3.Heterosis is commercially exploited in:

A. Cross pollinated species B. Often cross-pollinated species
C. Self-pollinated species D. Vegetatively propagated species

Ans. a

4.Heterosis is -

a. superiority of hybrids over their parents

b. mixture of two or more traits

c. induction of mutation 

d. None of the above 

Ans. a

5. A mating among the following can not lead to heterosis

a.AAbb x aaBB
b.AABBccdd x AABBccdd
c.AABBccdd x aabbCCDD
d.aaBBcc x AABBCC

Ans. b

6.Maximum heterosis is found in -

a.single cross               c. triple cross

b . double cross            d. none of the above 

Ans. a

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