Pollination and types of pollination

 Pollination

   The transferral of pollen grains from the opened anthers to the receptive stigma of the carpel is called pollination. It is of two types-    SELF POLLINATION and CROSS POLLINATION.

SELF POLLINATION

     Self pollination involves the transfer of pollen grains from the anthers of a flower to a stigma of the same flower or genetically similar flower. It occurs by two types s- 

a) Autogamy- ( autos=self, gamos=marriage)

        It is a kind of pollination in which the pollen  from the anthers of a flower are transferred to the stigma of the same flower. It occurs by three methods -

i) Cleistogamy (kleisto=closed,gamos= marriage) 

 Some plants never open to ensure complete self pollination. This condition is called Cleistogamy, e.g, oxalis, viola etc. These cleistogamous flowers are bisexual small, inconspicuous, colourless and do not secrete nectar.

ii) Homogamy 

 Anthers and stigma of bisexual flowers of some plants mature at the same time. They are bought close to each other by growth, bending or folding to ensure self pollination. This condition is called Homogamy.

E.g Mirabilis( Four 'o, clock), potato, sunflower plants etc.

iii) Bud pollination 

 Anthers and stigma of the bisexual flowers of some plants mature before the opening of buds to ensure self pollination, e.g , Wheat, Rice, peas etc.

B) Geitonogamy ( geiton- neighbour, gamos- marriage)

It is a kind of pollination in which pollen from anthers of one flower are transferred to the stigma of another flower borne on same plant. It usually occurs in plants which shows monoecious conditions ( unisexual, male and female flowers are borne on the same plants).

ADVANTAGES of SELF pollination -

1. Chances of pollination are more.

2.self pollination maintains purity of the race and avoids mixing.

3.It needs not to produce a large number of pollen grains.

4. Flowers need not  to possess devices such as large and showy petals , presence of scent and nectar,etc to attract pollinators.

DISADVANTAGES of Self pollination -

 1 . Progeny continuously gets weaker after every generation.

2. Less chances of the production of new species and varieties.

CROSS POLLINATION

It involves the transfer of pollen grains from the flower of one plant to the flower of another plants. It is called xenogamy or allogamy . 

The main floral characteristics which facilitate cross pollination are- 

i) Herkogamy - Flowers possess some mechanical barriers on their stigmatic surface to avoid self pollination e.g, presence of gynostegium and pollinia in calotropis.

ii)Dichogamy - pollen and stigma of the flower mature at different times to avoid self pollination. It is of two types - protogyny and protandry.

iii) Self incompatibility - In some plants, the mature pollen fall on the receptive stigma of the same flower but fails to bring about self pollination. It is called self incompatibility.

iv) Male sterility- the pollen grains of some plants are not functional.Such plants set seeds only after cross pollination.

v) Dioecism- Cross pollination always occurs when plants are unisexual and dioecious i.e , male and female flower occurs on separate plants e.g, papaya, cucurbits etc

vi) Heterostyly - the flower of some plants have different lengths of stamens and styles so that self pollination is not possible e.g, primula, Linum etc.

Agents for cross pollination

 a) Abiotic agents - such as wind current, gravity, water, etc.

b) Biotic agents -  such as animal pollinators , insects, birds.

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Important terms used in genetics

 1.Gene. -  

A gene may be defined as the unit of inheritance which is carried from the parent by a gamete in a chromosome and expression of a character in the young one in cooperation with its allele, other gene and environment. Chemically, a gene is a linear segment of DNA. 

2.Allelomorphs( Allele) -

 A pair of genes that control the two alternative expressions of the same character and have the same loci(sites) in the homologous chromosomes are called allelomorphs or Allele. The term allele used for any two genes of a single character.

Example - The gene T and t for tallness and dwarfness for height of pea plant. Now TT, Tt, tt are considered as allele. 

3.Gene locus -

 A specific region of a chromosome representing a single gene or allele is known as gene locus. The allelic genes occupy the corresponding loci in a pair of homologous chromosomes.

4.Dominant and Recessive Allele -

When two alleles representing the alternative of a character come together in an individual, usually one allele is expressed fully and the other is not at all. The overpowering of one allele by another is called law of Dominance. 

  The allele which always expresses itself even in the presence of its contrasting allele is known as the dominant Allele.

   On the another hand,  the allele which fails to express itself in the presence of its contrasting dominant Allele is called recessive allele.  

  Example - a guinea pig with allele Bb for coat colour is black. The B allele for black colour coat being dominant express itself and the b for white coat colour being recessive, does not express itself.

Wild and Mutant alleles-

An original allele, dominant in expression and wild spread in the population, is called wild allele.

   An allele formed by a mutation in the wild allele, recessive in expression and less common in the population is termed as mutant alleles.

Homozygous organism -

An organism in which both the alleles of a character at the corresponding loci in homologous chromosomes are identical is said to be homozygous or genetically pure  organism. Homozygous individuals breed true for the specific trait i.e produce offspring with similar genotype and phenotype on inbreeding. They are called pure individual.

Example - The pea plant with TT and tt allele pair are homozygous for height. The combination Of TT allele is homozygous dominant; and the combination of tt allele is homozygous recessive.

 5.Heterozygous organism - 

An organism in which the two allele of a character at the corresponding loci in the homologous chromosomes are unlike is said to be heterozygous for their character. Heterozygous individual do not breed true and are said to be hybrid for their character.

Example - The pea plant with Tt allele pair is heterozygous for height ia heterozygote, one allele codes for dominant trait and the other is recessive trait, and usually the dominant allele is expressed and the recessive allele remains latent.

  So, the homozygous and heterozygous individual are called homozygote and heterozygote respectively.

6.F1 generation - 

It is the first filial generation of hybrid offspring produced in a genetic cross between two genetically different but pure parents.

Example - 

F1 generation produced by crossing TT and tt parent Pea Plants consists of Tt individuals.

7.F2 generation - 

It is the second filial generation of young ones produced in a genetic cross between two F1 organisms.

8.Pure Lines - 

A race of genetically pure,true breeding organisms resulting from continued selfing or inbreeding is called a pure Lines. In nature, pure Lines do not produce vigorous offspring because defective alleles may come to exist in homozygous state.

9.Genotype - 

 It is the sum total of genes inherited from both parents irrespective of whether they are expressed or not.

Example - TT, Tt and tt are genotype for the size of stem in pure tall, dwarf and hybrid tall garden pea.

10.Phenotype -

It refers to the expressed or observable structural and functional trait produced by the interaction of genes and environment.

Example - Tallness in pea plant, colour blindness in man are phenotype.both are noticed by eyes.

         Individual of the same genotype have the same phenotype and breed alike. Individuals of different genotype may also have the same phenotype e.g the genotype TT and Tt give rise to tall pea plant.

11.Genome -

A complete set of gene (DNA instructions) contained in the haploid dose of chromosomes and inherited as a unit from one parent is known as genome. A haploid cell contains two genomes, one parental, other maternal. A cell may have more than two genomes also. Such cells are called polyploids.

12.Gene pool- 

 All the genotype of all the individuals in an inbreeding population make up a gene pool.

  

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Mendel's Second Law- The Law of Independent Assortment

Law of independent assortment 

Definition - This law states that the alleles of two different characters located in different pairs of homologous chromosomes are independent of one another during gamete formation and coming together into the offspring by fertilization, both processes occurring randomly.

OR

The principle of Independent Assortment may also stated that during the  formation of gametes in each sex, either one of a pair of alleles may enter the same gamete with either one of another pair.

Explanation with examples

i) Cross between Pea Plants

Cross a pure pea plant having round seeds and yellow cotyledons with a pure pea plant having wrinkled seeds and green cotyledons.

Here , the round form of seeds and yellow colour of cotyledons are dominant over the wrinkled seeds and green colour of cotyledons. The parent plants may beCr represented by RRYY and rryy. During the gametes formation , meiosis separate the two alleles of each trait so that gamete get only one allele of each trait.

The F1 generation formed by the fusion of gametes with alleles RY and ry i.e they have RrYy. They all are hybrid with round seeds and yellow cotyledons.

In F2 generation,  

After crossing the F1 generation four types of gametes are formed and produce four types of plants in F2 generation. They are in ratio of 9:3:3:1 where 9 with round seeds and yellow cotyledons,3 with round seeds and green cotyledons, 3 with wrinkled seeds and yellow cotyledons and 1 with wrinkled seeds and yellow cotyledons . 

ii) Cross between Guinea pig

Mate a black, short haired male guinea pig with a pure white female Guinea pig . Here the black colour is dominant over the white colour and short hair are dominant over the long hair. Each character is represented by two alleles such as BBSS and bbss respectively. Here B stands for the allele of dominant black colour,b for white colour,S for the allele of short hair and s for the allele of recessive long hair. During the formation gamete , two character of each allele segregate so a gamete has only one allele of each character. i.e BS and bs. Now fertilization occurs. The zygote comes to possess two unlike allele for the size the hair it has Bb  Ss . The offspring developed from such zygote will be all hybrid with black colour and short hair.

    Now cross the hybrid black, short haired male and female of F1 generation among themselves. The allele for short and long hair again segregate during gamete formation. B can go with S or s i.e BS or Bs. Similarly b can go with S or s i.e bS or bs. Therefore the gamete show four combinations of allele which are 

a) black short BS

b) black long Bs

C) white short bS

d) white long bs

These gametes on fertilization produce four types of guinea pig in F2 generation. They're in the ratio of 9:3:3:1 i.e

9 black short

3 black long

3 white short

1 white long

The black and white guinea pigs are in ratio of 3:1 and pure black, hybrid black and pure white guinea pigs in ratio of 1:2:1 . Similarly short haired and long haired guinea pigs show ratio of 3:1 and pure short hybrid, hybrid short haired and pure long haired a ratio of 1:2:1 .

   The 9:3:3:1 Dihybrid phenotypic ratio is the product of the two separate 3:1  monohybrid ratios, one for each allele pair.ie 

3:1 X 3:1 = 9:3:3:1

Advantage :-

 This cross shows that the two alleles of different characters do not interfere with one another's behaviour, but sorted out independently of one another

Limitations

The law of independent assortment of genes holds good only if the different pairs of alleles lie in the different chromosomes pair and not individual allele which are segregated during reduction division. 

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