B. Tech. (Biotechnology) 1

st year- II semester

Basics of Plant Breeding (PB - 101)

Course instructor:

Dr. Nilanjaya

Dr. M. K. Singh

Mass selection

In mass selection, a large number of plants of similar phenotype are selected and their seeds are mixed

together to constitute the new variety.*Generally, the selected plants are not subjected to progeny test.

Since several plants are selected and their seeds are mixed, the selected population is a mixture of several

similar looking Purelines. Thus, a variety developed through mass selection would have considerable

genetic variation and, consequently, further mass selection or Pureline selection may be done in such a

variety at a later time.

Procedure of mass selection:

1. First year – a large number of phenotypically similar plants are selected for vigour, plant type, disease

resistance and other characters. Seeds from the selected plants are composited to raise the next generation.

2. Second year – the composited seed is planted in a preliminary yield trial along with standard varieties

as check. The variety from which the selection was made should also be included as check to

determine if there has been an improvement due to selection.

3. 3rd

- 5th year- co-ordinated yield trial.

4. 6th year – the promising variety in co-ordinated trial is recommended by the competent variety release

committee for release.

Applications of mass selection:

1. Purification of existing Pureline varieties – Pureline tend to become variable with time due to

mechanical mixture, natural hybridization and mutation. Therefore, purity of Pureline variety has to be

maintained through regular mass selection. On the same principle, nucleus seed of Pureline is

produced through mass selection.

2. Improvement of desi/local variety – Mass selection is useful in improvement of desi/local variety of

SPC. The local varieties are mixture of several Purelines which may differ for several characters. Many of

the component Purelines would be inferior in performance that lowers the performance of local variety.

Elimination of such poor plant types improves the performance and uniformity of the original variety.

Additionally, local varieties are under cultivation for generations and are adapted to local environment

and are stale in performance. Mass selection improves the local variety without adversely affecting its

adaptability and stability as the new variety is still composite of several superior purelines present in the

original population.

Merits and demerits

Merits

1. Since a large number of plants are selected, the adaptation of the original variety is not changed.

2. Mass selection retains considerable genetic variability in the new variety. Therefore, another round

of mass selection after few years would be effective in improving the variety in future.

3. It is conducted on homozygous and heterogeneous population and no crossing is done to generate

new variability. So, extensive and prolonged yield trials are not necessary. This reduces time and cost in

developing new variety.

4. It is less demanding and hence breeders can devote more time to other breeding programmes.

Demerits

1. Variety developed by mass selection show variation and is not as uniform as Pureline varieties.

2. The improvement through mass selection is generally less than that could be achieved through Pureline

selection

3. In absence of progeny test, it is not possible to determine if the selected plants are homozygous.

Even in SPC, some degree of cross pollination does occur and some of the selected plants may be

heterozygous. It is also not known if the phenotypic superiority of selected plant is due to environment or

genotype.

4. Varieties developed by mass selection are more difficult to identify than Pureline varieties in seed

certification programme.

5. Mass selection utilizes the variability already present in a variety/population and does not generate

new variability.

Hybridization: Aims and objectives, Types of hybridization

Hybridization: The mating/crossing of two plants/lines of dissimilar genotype is known as hybridization.

Requirements for a successful hybridization:

1. Prevention of selfing and chance cross pollination in female flower – Emasculation and aging of female

flower.

2. Pollination by the selected male parent – Hand pollination and bagging of male flower.

Objectives of hybridization:

The chief objective of hybridization is to create genetic variation. The degree of genetic variation

produced in the segregating generation depends on the number of genes that differ in parents of the F1.

The other objectives for hybridization may be –

1. Combination breeding: Transfer of one/more oligogenic and/or polygenic traits into a single variety

from another variet(y)es.

The increase in the yield of new variety is obtained by correcting the weakness in yield contributing traits,

e.g. tiller number, disease resistance, grains/spike etc.

Backcross method and pedigree method can be used for combination breeding.

For combination breeding genetic divergence between the parents is not the major consideration rather

one of the parents must have the trait to be transferred in high intensity.

The intensity of traits in question in the new variety may be comparable or low than the source variety

from where it is transferred.

2. Transgressive breeding: Refers to improvement in the yield or its contributing traits through

transgressive segregation. Transgressive segregation is appearance of individuals in F2 or subsequent

generation that are superior to both the parents. The cause for transgressive segregation is the

accumulation of favourable genes from both the parents as a result of recombination.

Pedigree method and its modification and population approach are suited for high recovery of

transgressive segregants and hence transgressive breeding.

Unlike combination breeding genetic divergence of parents each contributing different favourable genes

is prime consideration viz–a-viz they should combine well with each other.

As a result, the intensity of trait in transgressive segregants is greater than that in either of the parents.

3. Hybrid variety: It has been well established that F1 between two pure/inbred lines is more vigorous

than the parents. Wherever, commercially feasible, F1generation may be used as hybrid variety.

Genetic divergence and combining ability are two major considerations for a successful hybrid variety.

Types of Hybridization:

Based on taxonomic relationship of the parents involved, hybridization may be classified into:

1. Intervarietal hybridization: Hybridization between two strains/varieties/races of the same species.

This is also known as intraspecific hybridization. Such hybridization is most commonly used in plant

breeding. Intervarietal hybridization may be simple or complex depending upon the number of parents

involved.

a. Simple cross/ single cross: Two parents are crossed to produce F1.

E.g. A×B F1 (A×B).

b. Complex cross/ convergent cross: more than two parents are crossed to produce the F1hybrid. Such

cross ring together genes from several parents into a single variety.

Three-way cross- when F1 is crossed to the third parent.

E.g. F1 (A×B) × C F1 [(A×B) × C]

Double cross- when two F1s are crossed together.

E.g. F1 (A×B) × F1 (A×B) F1 [(A×B) × (C×D)]

2. Distant hybridization: refers to the hybridization between different species of the same genus or

different genera.

a. Interspecific / Intrageneric- when two species of the same genus are hybridized.

b. Intergeneric- when two species belonging to the different genera are hybridized.

Objective of the distant hybridization is to transfer one or few simply inherited traits like resistance for

diseases or biotic or abiotic stresses. Distance hybridization is likely to become increasingly more

frequent in correction of specific defects in crop plants in light of current desire of consumers for

uniformity and resulting genetic erosion.

Handling of Segregating Generations

The process of hybridisation is fairly simple and easy. But the chief difficulty in using hybridization for

crop improvement lies in the handling of segregating generations. F2 and subsequent generation

generations consists of several thousand plants. Raising of large segregating generations from several

crosses at the same time require money, labour, land and other facilities which are often limited. Further,

selection for desirable plant types has to e done in the segregating generations. If a breeder desires to

practice selection based on scientific considerations, he will be able to handle only a limited number of

cross at a time. Under such situation it is suggested that increasing the number of crosses is more

desirable than increasing the population size of each cross. Further the number of cross a reeder can

handle depends on the available resources and method used for handling of segregating generation.

Various methods for handling of segregating generations are:

1. Pedigree method

2. Bulk method

3. Backcross method

Pedigree method

In pedigree method, individual plants are selected from F2 and subsequent generation, their progenies are

grown and a record of parent-progeny relationship (pedigree) is maintained. Pedigree method was first

outlined by Love in 1927.

Procedure:

1. Hybridization - the selected parents are crossed to produce single or complex cross.

2. F1 generation – F1 seeds are space planted to produce maximum number of seeds.

3 F2 generation – F2 seeds are space-planted to facilitate selection. Usually, 1-10 % selection intensity is

practised. When closely related varieties are crossed, the number of F2 individuals selected would be

considerably smaller then when the parents are unrelated by descent. When the objective is to breed for

quantitative traits, a relatively larger number of F2 plants would be selected. Selection in F2 is based on

the traits that are simply inherited.

4. F3 – F5 generation – Individual plant progenies are space planted in F3 and F4 generation. Selection is

practiced from within and between the progeny row. If two or more progenies are coming from the

same progeny row are similar, only one of them may be retained. In the F5 generation, variation within

the progeny row vanishes and the focus for section should be between progeny row.

5. F6 generation – Individual selected progenies in F5 are planted in multi-row for visual comparison

among progeny rows. Superior progenies are bulk harvested as they have become homozygous. Progenies

showing segregation are discarded unless the segregants are outstanding. Under such situation, individual

plant is selected.

6. F7 generation – PYT with 3 replications are conducted along with standard check. Progenies are

evaluated for plant height, lodging and disease resistance flowering and maturity date, yield and quality

traits with respect to the check. 2-5 outstanding lines superior to the check would be advanced to the co-

ordinated yield trials.

7. F8 – F10 generation – The superior lines are tested in replicated yield trials at several locations.

These are evaluated for plant height, lodging and disease resistance flowering and maturity date, yield and

quality traits. A line that is superior to the best commercial variety included in the trial as check in yield

and other traits is identified for release as new variety.

8. F11 generation – when the strain is likely to be released as variety, the breeder usually multiplies its

seed during the last year in trial. Breeder has the responsibility to supply the breeder seed for production

of foundation seed. Thus, in F11 to F12 the seed of the new variety will be multiplied for the distribution

to the farmers.

Pedigree Record

Maintenance of a detailed record of the relationship between the selected plants and their progenies is

called as pedigree record or pedigree.

Advantage-

1. Each progeny in the every generation can be traced back to the F2 plant from which it has

originated.

2. It helps to find out that two individuals may share some allele in common if they are related by

decent.

3. Allelic composition of the final population could be predicted if there is such history in its ancestry.

Maintenance of pedigree record:

There are two systems followed for maintaining the pedigree record – 1. Pedigree based on the location of

progeny row in the field and 2. Pedigree based on serial number of selected plants.

Pedigree based on the location of progeny row in the field:

1. Each cross is given a number. The first two digits of the number refer to the year in which the cross is

made and the remaining two digits denote the serial number of the cross. E.g. 7911 --- Cross number 11

made in the year 1979.

2. Individual plant progeny row in F3 and latter generation are assigned row number based on its location

in the plot.

3. Progeny in F4 and latter generation is identified by its row number in the current generation preceded y

the row number of the progeny row in previous generation from where it was selected.

Thus, using this method each progeny can be traced back to the F2 plant from which it originated

provided record of all the previous year is available. E.g. –

Generation Progeny number Description

F3 7911-7 Progeny in the 7th row in F3 plot.

F4 7911-7-4 Progeny in the 4th row in F4 plot, selected from the progeny in the7th row of F3 plot.

F5 7911-4-14 Progeny in the 14th row in F5 plot, selected from the

progeny in 4th row of F4 plot.

F6 7911-14-3 ?

Pedigree based on serial number of selected plants:

1. Each cross is given a number. The first two digits of the number refer to the year in which the cross is

made and the remaining two digits denote the serial number of the cross. E.g. 7911 --- cross number 11

made in the year 1979.

2. Each selected individuals if F3 generation is given serial number and the individuals in subsequent

generation is given serial number in that generation preceded y serial no of plants in previous generation

from which it is derived.

Thus, using this method each progeny can be tracked back to its originating F2 individual without

consulting previous year record. E.g. –

Generation Progeny number Description

F3 7911-7 Progeny obtained from plat no. 7 selected in F2

F4 7911-7-4 Progeny from plant no. 4, selected from the F3 progeny

derived from plant no. 7 selected in F2

F5 7911-7-4-2 Progeny from plant no. 2, selected from the F4 progeny

derived from plant no. 4 selected from the F3 progeny

obtained from plant no. 7 selected in F2.

F6

In both the systems, these progenies are assigned a different serial number when they become

homozygous and are included in preliminary yield trials.

Applications of pedigree method

1. Most commonly used method for selection from segregating generation of cross in SPC.

2. Combination breeding

3. Transgressive breeding

Merits and demerits of pedigree method

Merits

1 Provides maximum opportunity for the reeder to use his skills for selection of desirable plants from

segregating generation.

2. Transgressive segregants for yield and other quantitative traits may be recovered in addition to the

improvement in specific trait

3. The breeder may often be able to obtain information about inheritance of qualitative trait from

pedigree record.

4. Plants progenies with visible defects and weaknesses are eliminated at an early stage.

Demerits

1. Maintenance of pedigree record is tedious and time consuming job.

2. Selection among and within a large number of progenies in every generation is laborious that limit

the number of crosses a breeder can handle.

3. Success of this method depends on skill of the reeder.

4. No opportunity for the natural selection to influence the population.

5. Selection for yield in F2/F3 is ineffective. If care is not taken care to retain a sufficient number of

progenies, valuable genotypes may be lost in the early segregating generation.

Bulk Method

In bulk method, F2 and subsequent generation are harvested in bulk to raise the next generation until the

genotypes attain Homozygosity or the favourable environment for selection is encountered, following

which individual plants are selected and evaluated.

Procedure –

1. Hybridization - the selected parents are crossed to produce single or complex cross.

2. F1 generation - F1 generation is space planted and harvested in bulk.

3. F2 – F6 generation - F2 – F6 generation are planted at commercial seed rate and spacings and

harvested in bulk. During bulking period, natural selection alters the genotypic frequencies in the

population. Artificial selection is generally not done.

4. F7 generation – plants are space planted, seeds from phenotypically superior plants are harvested

separately.

5. F8 generation – individual plant progenies are grown in single/multiple row. Superior plant progenies

are harvested in bulk.

6. F9 generation – PYT is conducted with standard commercial check. Progenies are evaluated for

plant height, lodging and disease resistance flowering and maturity date, yield and quality traits with

respect to the check. 2-5 outstanding lines superior to the check would be advanced to the co-ordinated

yield trials.

7. F10 – F12 generation - The superior lines are tested in replicated yield trials at several locations.

These are evaluated for plant height, lodging and disease resistance flowering and maturity date, yield and

quality traits. A line that is superior to the best commercial variety included in the trial as check in yield

and other traits is identified for release as new variety.

8. F13 generation - when the strain is likely to be released as variety, the breeder usually multiplies its

seed during the last year in trial. Breeder has the responsibility to supply the breeder seed for production

of foundation seed.

Applications of Bulk Method:

Bulk method of handling segregating generations has three prominent applications:

1. Isolation of homozygous lines

2. Waiting for the opportunity for selection and

3. To allow natural selection to alter genetic composition of population.

Isolation of homozygous lines: Using this method homozygous lines are isolated with minimum effort

and expense. Individual plant selection starts with end of bulking period and a PYT can be conducted in

second year after the end of bulking period.

Waiting for the opportunity for selection: Environment favourable for selection usually do not occur

every year, and this method allow segregating generations to be bulked until its commencement without

loss of any potential genotype as artificial selection is usually not practiced during bulking period. Many a

times, segregating generations are bulked for want of favourable environment and subsequently the

population is handled according to pedigree method. This method is called as mass-pedigree method of

Harlan.

To allow natural selection to alter genetic composition of population: Long bulking generation

provides an opportunity for natural selection to act on their genetic composition. It is assumed that

natural selection would favour higher yielding and eliminate the poorer yielding ones. Therefore,

from a population maintained as a bulk for long period, one may expect to isolate superior lines at a much

higher frequencies than that from F2 of the same cross. Bulk method, particularly when bulking period is

quite long, was termed as evolutionary method of breeding by Suneson as it allows natural selection to

act on the population and change its genotypic composition.

Merits and demerits of bulk method

Merits

1. Natural selection increases the frequencies of superior genotypes.

2. Since large population is grown, there is greater chance of isolation of transgressive segregants than

in the pedigree method.

3. Individual plant selection is done after population attains homozygosity, therefore, selection for

quantitative traits are more effective.

4. It is particularly suited for small grain crops, since, they are planted at high crop densities.

5. It is suitable for studies on the survival of genes and genotypes in the population.

6. Simple, convenient and inexpensive method allows breeder to focus on other breeding projects.

Demerits

1. Natural selection becomes more important only after F10 and bulking may have to be done up to F20 or

more which is considerably longer than the time taken in pedigree method.

2. Short term bulks are useful for the isolation of homozygous lines but natural selection has little effect

on such bulk population

3. Provides little opportunity for breeder to exercise his skill.

4. Information on inheritance of character cannot be obtained.

5. In some cases natural selection may act against agronomically desirable types.

6. Off-season crops and green house cannot be used to advance the generation since such environment

is entirely different from that in the target location.

Single seed descent method

SSD is modifications of bulk method. In SSD, a single seed from each of the plant from F2 and

subsequent generations are bulked to raise the next generation. In F5/F6 individual plant selection is done

and individual plant progenies are grown in next generation. Further selection is done mainly among the

progenies. Superior progeny rows are bulk harvested to conduct PYT in F7/F8 and co-ordinated trials are

initiated in F8/F9.

Objective - Rapid advancement of generation of crosses.

Application – To obtain RILs from selected cross in minimum time.

Advantage-

1. Advances the generation with maximum possible speed in conventional breeding.

2. Require very little space, effort and labour

3. Makes best use of greenhouse and off-season nursery facilities.

4. End population represent random sample of homozygous genotypes derived from F2 population.

Disadvantage-

1. Does not permit any form of selection during the segregating generation.

2. In each successive generation, the population size becomes progressively smaller due to poor

germination, death of plant and other factors.

Backcross Method

A cross between a hybrid and one of its parents is known as backcross. In backcross method of breeding,

the hybrid and the progenies in the subsequent generation are repeatedly backcrossed to one of the parents

of the F1. As a result, the genotype of backcross progeny becomes increasingly similar to that of the

parent to which the backcrosses are made.

Requirements of a backcross programme-

1. A suitable recurrent parent- otherwise popular variety lacking one/two characters.

2. A suitable donor parent- parent having the trait lacking in the recurrent parent in high intensity.

3. High heritability of trait.

4. Sufficient number of backcross- to recover RPG.

Procedure of backcross method:

Transfer of a dominant gene

Assume two parents A and B.

A- High yielding and widely adapted wheat variety but susceptibility to stem rust.

B- Wheat variety resistant to stem rust.

1. Hybridization: A is crossed to B.

2. F1 generation: F1 plants are backcrossed to variety A.

3. BC1: Resistant and Susceptible genotypes in BC1 would be 1:1. Rust resistant plants are selected and

backcrossed to variety A.

4. BC2-BC5: In each backcross generation resistant plants are selected and backcrossed to the recurrent

parent A.

5. BC6 generation: On an average, BC6 plants will have 99% genes from variety A. Rust resistant

plants are selected and selfed, their seeds are harvested separately.

6. BC6F2: Individual plant progenies from the selfed seeds are grown. Rust resistant plants similar to the

plant type of variety A are selected and their selfed seeds are harvested separately.

7. BC6F3: Individual plant progenies are grown. Progenies homozygous for rust resistance and similar to

the plant type of variety A are harvested in bulk.

8. Yield test- The new variety is tested in PYT along with the variety A as check. Ordinarily, the new

variety would be identical to variety A, so, detailed yield tests are generally not required.

Merits and Demerits of backcross method

Merits

1. The genotype of new variety is nearly identical with that of the RP, except for the gene transferred.

Thus, outcome of backcross programme is known in advance.

2. It is not necessary to test the variety developed by backcross method in extensive yield tests.

3. Much smaller population is needed in this method than in pedigree.

4. Defects could be rectified without affecting its performance and adaptability.

5. This is the only method for interspecific gene transfer and for the transfer of cytoplasm.

Demerits

1. The new variety cannot be superior to the recurrent parent except for the character transferred.

2. Undesirable genes closely linked with the gene being transferred may also be transferred to the new

variety producing linkage drag.

3. Repeated hybridization is difficult and time taking.

4. By the time backcross programme improves it, the RP may have been replaced by other varieties

superior in the yielding ability.

Hardy-Weinberg law

Two scientists, G. H. Hardy (1908) (British mathematician) & W. Weinberg (1909) (German physician)

working independently proposed a method to estimate genotypic frequency from gene frequency and

vice-versa in a natural population provided that the population mates randomly and the genotypes are

created from the random union of gametes in absence of any evolutionary forces. This method is referred

to as Hardy-Weinberg law. The law states that in a random mating population the gene and genotypic

frequencies remains constant generation after generation if there is no selection, mutation

migration and random genetic drift. Alternatively, Hardy-Weinberg law advocates constancy of gene

and genotypic frequency over the generations in a random mating population in absence of

evolutionary forces. Any population of diploid individuals may have utmost three genotypic classes, AA,

Aa and aa (single locus-biallelic system). Assume the frequency of allele ‘A’ in that population is ‘p’ and

that of ‘a’ is ‘q’. Then, the population is said to be in Hardy-Weinberg equilibrium when frequencies of

the three genotypes, AA, Aa and aa are p2, 2pq and q

2, respectively. Equivalence of the genotypic

frequencies could be tested using chi square test.

Derivation:

A random mating base population (G0) of diploid individuals

Assume a single locus-biallelic system. There would be three genotypes- Genotypes AA Aa aa Total

Number D H R (D+H+R) = N

Number of alleles 2D 2H 2R (2D+2H+2R) = 2N

(A) = p= (2D+H)/2N

(a) = q= (2R+H)/2N

P+q = (2D+H)/2N + (2R+H)/2N = (2D+2H+2R) 2N = 1 Therefore, p+q = 1 or p=1-q or q=1-p.

p and q are called gene frequencies which are proportion of allele ‘A’ and ‘a’ in the population.

First generation (G1) of random mating

Random union of ‘A’ and ‘a’ in G0 would produce three kind of genotypes with following frequencies:

A (p) a (q)

A (p) AA (p2) Aa (pq)

a (q) Aa (pq) Aa (q2)

Here, (AA), (Aa) and (aa) is p2,

2pq and q2. p

2, 2pq and q

2 are called as genotypic frequencies of

genotype AA, Aa and aa which are proportion of respective genotypes in the population.

Gene frequencies in G1:

(A) = p= (2p2 + 2pq)/ 2(p

2 +2pq+ q

2) = 2p (p+q)/2 = p

(a) = q= (2q2 + 2pq)/ 2(p

2 +2pq+ q

2) = 2q (q+p)/2 = q

Second generation (G2) of random mating

The genotypes in the G1 generation would randomly mate to generate genotypes in G2 generation in the

following manner.

Mating Mating frequency Frequency of resulting progenies/genotypes in G2

AA Aa aa

AA×AA p4 [p2×p2] p4

Aa×Aa 4p2q2 [2pq×2pq] p2q2 2p2q2 p2q2

aa×aa q4

[q2×q

2] - - q

4

AA×Aa 4p3q [2×p2 ×2pq] 2p3q 2p3q -

AA×aa 2p2 q2 [2×p2 × q2] - 2p2 q2 -

Aa×aa 4pq3 [2×2pq×q2] 2pq3 2pq3

(AA) = p2(p2+2pq+q2)

= p2

(Aa) = 2pq(p2+2pq+q2) = 2pq

(aa) = q2(p2 +2pq+q2) = q2

Gene frequencies in G2:

(A) = p= (2p2 + 2pq)/ 2(p

2 +2pq+ q

2) = 2p (p+q)/2 = p.

(a) = q= (2q2 + 2pq)/ 2(p

2 +2pq+ q

2) = 2q (q+p)/2 = q.

Thus, it can be seen that under random mating and in absence of evolutionary forces, the gene and the

genotypic frequencies remain constant in G0 and G1 generations.

When genotypic frequencies are disturbed, the population restores equilibrium after one

generation of random mating.

When the number of genes increases, the equilibrium is not attained in one generation,

additionally, multiple genes with linkages further slows the rate of attainment of

equilibrium.

Breeding population a breeder maintains is finite that restrict the strict random mating even if the

crop is highly cross pollinated. Therefore, Hardy-Weinberg law has limited in plant breeding. The

chief application this law finds is in the study of evolution.

Factors that disturbs Hardy-Weinberg equilibrium:

Constancy of gene and genotypic frequencies over the generations are changed by selection, mutation

migration and random genetic drift. These factors disturbs the Hardy-Weinberg equilibrium are driving

forces of evolution. Evolution is defined as change in gene and genotypic frequency over long period of

time.

Selection: differential rate of reproduction is called selection. Strength of Selection is measured by

selection coefficient (s) and fitness (w). Selection differential is proportion of reduction in gametic

contribution of a genotype with respect to the most fit one. The fitness of a genotype is proportion of

gametic contribution of that genotype towards the next generation in relation to that of the most fit

individual (w=1). Selection differential and fitness are complementary to each other such that s+w =1.

Mutation: mutation is the ultimate source of all the variation present in biological systems. It acts on

existing alleles and creates entirely new alleles that alter the gene frequency. But, since the mutation rate

is generally very low (10-6

), the effects of mutation on gene frequency would be detectable only after

large number of generations. Therefore, in breeding populations such effects are often ignored.

Migration: migration is the movement of individuals from a population into a different population, and

participation in the reproduction of this population. Migration introduces new allele into the population

and alters the frequency of existing alleles. In plant breeding, a suitable example of migration is

represented by wide hybridization.

Random genetic drift: random genetic drift is a random change in gene frequency due to sampling

error. This is more pronounced in small population. The ultimate result of random genetic drift is fixation

of alleles (p=1 or 0). In small population all the genes are expected to be fixed in course of time. Genetic

drift in breeding population can be prevented either by growing infinite population which is impractical or

by resorting to phenotypic disassortative mating which is not economically feasible as well.

Inbreeding: mating between individuals related by descent is known as inbreeding. Inbreeding is

common feature of small population. Inbreeding is deviation from random mating. Under such situation

the allele frequency remain the same but the genotypic frequencies change. Inbreeding increases the

frequency of homozygotes and decreases the frequency of heterozygote. The rate of reduction in

heterozygosity per generation is 1/2N in monoecious or hermaphrodite species while it is 1/(2N+1) in

dioceious species, where N is the number of plants in the population.

Applications of Hardy-Weinberg law:

1. Calculation of frequencies of recessive and dominant genes in a population.

2. Calculate the frequency of carriers/heterozygotes in a population.

3. To test if a population is in agreement of Hardy-Weinberg equilibrium.