fundamentals of genetics and plant breeding
TRANSCRIPT
Genetics
Genetics can be defined as the study of heredity, the process in which a parent passes certain
genes onto their offspring. Heredity describes how some traits are passed from parents to their
children. The traits are expressed by genes, which are small sections of DNA that are coded for
specific traits. Genes are found on chromosomes. Genetic material (genes, chromosomes, DNA)
is found inside the nucleus of a cell.
Gene: Genes are segments of DNA located on chromosomes. Genes exist in alternative forms
called alleles. Alleles determine distinct traits that can be passed on from parents to offspring.
Chromosome: Chromosome is thin, thread-like structure present in the nucleus showing
physic-chemical changes in the morphology during cell division. They become dark coloured
when stained with basic dye, hence the name Chromosome. Chromosome is an organized structure of DNA and protein, found in cells. It is a single piece of
coiled DNA containing many genes, regulatory elements and other nucleotide sequences.
Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control
its functions. Chromosomes vary widely between different organisms.
Crop Center of Origin Chromosome
Wheat Near East and Ethiopian
Highlands
2n=42
Rice Asia 2n=24
Maize North America 2n=20
Millets West Africa 2n=18
Sugarcane New Guinea & North India 2n=80, 126
Cotton Africa 2n=52
Potato South America 2n=42
Deoxyribonucleic acid (DNA): DNA is a nucleic acid containing the genetic instructions
used in the development and functioning of all known living organisms (with the exception of
RNA viruses).
Gregor Mendel • Austrian Monk.
• Experimented with ―pea plants‖.
• Used pea plants because:
• They were available
• They reproduced quickly
• They showed obvious differences in the traits
Understood that there was something that carried traits from one generation to the next- known
as―FACTOR‖.
Mendelian Genetics • Dominant traits- traits that are expressed.
• Recessive traits- traits that are covered up.
• Alleles- alternative form of a gene
• Punnett Squares- show how crosses are made.
• Probability- the chances/ percentages that something will occur.
• Genotype- the types of genes (Alleles) present.
• Phenotype- what it looks like.
• Homozygous- two of the same alleles.
• Heterozygous- two different alleles.
Mendel’s Laws:
1. Law of Segregations: - During the formation of gametes (eggs or sperm), the two alleles (hereditary units)
responsible for a trait separate from each other.
- Alleles for a trait are then "recombined" at fertilization, producing the genotype for the
traits of the offspring.
P generation (parental generation) F1 generation (first filial generation, the word filial from the
Latin word for "son") are the hybrid offspring.Allowing these F1 hybrids to self-pollinate
produces:F2 generation (second filial generation).It is the analysis of this that lead to an
understanding of genetic crosses.
2. Law of Independent Assortment:
- Each factor (gene) is distributed (assorted) randomly and independently of one another in
the formation of gametes.
Phenotypic ratio:- 9:3:3:1
Genotypic ratio:- 1:2:1:2:4:2:1:2:1
Some exception ratio-
Dominant epistasis - 12:3:1
Recessive epistasis- 9:3:4
Complementary gene or Duplicate recessive gene- 9:7
Dominant and recessive interaction- 13:3
Duplicate dominant gene- 15:1
Polymeric Genes - 9:6:1
Phenotype vs. Genotype:
𝑷 = 𝑮+ 𝑬+ (𝑮 × 𝑬) P is called the phenotypic value, i.e., the measurement associated with a particular individual.
G is the genotypic value, the effect of the genotype (averaged across all environments) .
E is the effect of environments (averaged across all genotypes).
Plant breeding: Plant breeding can be defined as the art and science of changing the genetic
combination of plants in order to produce desired characteristics.
Objective of plant breeding: Higher yield Improved quality Abiotic resistance Biotic resistance Change in maturity Duration / Earliness Desirable Agronomic Characteristics Elimination of Toxic Substances Varieties for New Seasons
Requirements for plant breeding:
1. Scissors: It is used for removing of any plant materials such as extra buds, leaves and
awn, etc. sharp-edged scissors of small and medium size are necessary.
2. Needles: Needles are used to open the small buds and also to emasculate the flower buds
and make holes in the butter-paper bags.
3. Forceps: Fine pointed and long forceps are used for the removal of anthers and holdings
anther, during transfer of pollen grains to the stigma.
4. Brushes: Camel-hair brushes of various sizes are used. More commonly these brushes
can be found useful to collect pollen from the anthers, particularly where the anthers
produced in small quantity.
5. Rectified spirit: A tube or vial with alcohol or rectified spirit is used to sterilize the
instruments together with hands which are used during hybridization programme.
6. Magnifying lens: It is used to observe the small flowers (buds) on a large scale, in order
to carry out emasculation successfully and also to ensure that stigma does not carry
foreign pollen grains.
7. Bags: Bags are used according to size of flower buds to protect them after emasculation
and pollination. Different kinds of materials are used to prepare the bags such as butter-
paper, bamboo, khakhi-paper, muslin-cloth and plastic sheets.
8. Tags: Proper size of tags made up of thick paper or ply-board tags which have been
waxed after labeling can be used. Now a days aluminium bags are commonly used. On
bags, information like name of parents, date of emasculation and cross pollination, etc.,
written with a pencil.
9. Meter tape: Meter scale or meter tape is used for measuring various morphological
parameters of plants during recording the experimental data.
10. Weighing balance: Small-size balance is needed by which large number of samples can
be weighed immediately.
11. Field notebook: Field notebook is used to note the following observations at the time of
crossing:
Basis of selection of parents.
Nature of the cross—intervarietal / interspecific / intergeneric.
Purpose of crossing.
Number of crosses made.
Name of the parents to be used for crossing.
Date of emasculation and pollination.
Other points of interest, such as sowing-date, germination %, date of maturity, etc.
Name of the breeder.
Reproduction: Reproduction refers to the process by which living organism give rise to the
offspring of similar kind (species).In other words, reproduction is the process of multiplication of
living being. In crop plants, the mode of reproduction is of two types: viz. 1. Asexual reproduction and
2. Sexual reproduction
Asexual reproduction: A sexual reproduction does not involve fusion of male and female
gametes. New plants may develop from vegetative parts of the plant (vegetative reproduction) or
may arise from embryos that develop without fertilization (apomixis).
Sexual reproduction: Multiplication of plants through embryos which have developed by
fusion of male and female gametes is known as sexual reproduction.
Pollination: pollination is the process by which pollen grains are transferred from anthers to
stigma. Pollination is of two types viz.
Autogamy/self pollination and
Allogamy /cross pollination
Self-Pollination: Pollen is transferred from the anther to the stigma of the same flower or to
the stigma of another flower of same plant.
Cross-Pollination: In this process, the pollen from one flower is transferred to the stigma of
another flower on a different plant of the same kind.
Self-Pollinated crops Cross-Pollinated crops
Oat =2n=42 Millet = 2n=18
Lettuce =2n=18 Onions = 2n=16
Wheat = 2n=42 Corn =2n=20
Rice =2n=24 Carrot =2n=18
Tomato =2n=24 Rye=2n=14
Barley =2n=14 Maize =2n=20
Sorghum =2n=20 Mustard =2n=36
Pea =2n=22 Sunflower =2n=34
Cowpea =2n=24 Cotton =2n=52
Cotton =2n=52
Gram =2n=16
Groundnut =2n=40
Soybean =2n=40
Sesame =2n=26
Jute =2n=14
Tobacco =2n=48
Mechanism of self pollination:
Bisexuality : Male and female organ present in the same flower is called bisexuality.
Homogamy: Anther and stigma of a flower mature at the same time is called homogamy.
Cleistogamy: Pollination and fertilization take place in unopened (flower do not open at
all) flower bud is known as cleistogamy. E.g. wheat.
Chasmogamy: Flowers open only after the completion of pollination is called
chasmogamy. E.g. Rice.
Mechanism followed cross pollination: Dicliny: It refers to unisexual flowers. This is two types:
a. Monoecious plants
– female flowers and males are found in separate flowers on the same plant
– no perfect flowers
– E.g. Mango, maize, cucurbits, grapes, strawberry, rubber etc.
b. Dioecious plants
–female flowers (pistillate) and males flowers (staminate) are found on different
plants
–no perfect flowers
– E.g. Papaya, date palm, spinach, asparagus etc
Dichogamy :
– ♀ and ♂ flowers are both found on the same plant but mature at different times
–pollen shed when pistil is not receptive.
Dicogamy is of two types: viz. i) Protogyny and ii) Protoandry
protogyny: When female phase comes before the male phase
protandry: When male phase comes before the female phase.
Herkogamy: Hindrance to self-pollination due to some physical barriers such as presence
of hyline membrance around the anther is known as herkogamy. The spatial separation of
the anthers and stigmas within a flower. E.g. Alfaalfa.
Heterostyly :When styles and filaments in a flower are of different length is called
heterostyly. E.g. Linseed.Two (distyly) or three (tristyly) style morphs differ in the
reciprocal placement of anthers and stigmas.
Self-incompatibility: The ability of plant to reject its own pollen and that of closely
related individuals.
Male sterility: In some species, the pollen grains are non functional. Such condition is
known as male sterility.
Methods of Breeding Autogamous species: 1. Plant Introduction 2. Pureline selection
3. Mass selection 4. Pedigree method
5. Bulk method 6. Single seed descent method
7. Backcross method 8. Heterosis breeding
9. Mutation breeding 10. Polyploidy breeding
11. Distant hybridization 12.Transgenic breeding.
Methods or Breeding Allogamous species: (1) Plant introduction (2) Mass and progeny selection
(3) Backcross method (4) Heterosis breeding
(5) Synthetic breeding (6) Composite breeding
(7) Polyploidy breeding (8) Distant hybridization
(9) Transgenic breeding
Methods of Breeding Asexually Propagated Species: (1) Plant Introduction (2) Clonal selection
(3) Mass selection (4) Heterosis breeding
(5) Mutation breeding (6) Polyploidy breeding
(7) Distant hybridization (8) Transgenic breeding.
Plant introduction:
• It is introducing a plant into new regions from its growing locality. • It is the easiest and simplest method. • Proper management and acclimatization is very important to prevent losses. • Quarantine has to play an important role in introduction to ensure that the material which
is to be introduced should not carry pest& diseases with it. • It can be used in both self and crossed pollinated plants.
Procedure of plant introduction: Type of plant to be introduced.
Place of plant collection.
Medium of collection.
Packing.
Procurement Quarantine. Cataloguing.
Multiplication and Distribution. Regional yield trail.
Evaluation Variety release.
Purpose of plant introduction:
To Obtain An Entirely New Crop Plant. To Serve as New Varieties. To Be Used in Crop Improvement. To Save the Crop from Diseases And Pests. For Scientific Studies. For Aesthetic Value. Varieties Selected from Introductions. Varieties Developed through Hybridization .
Pure Line: (Recount Johannsen. 1903) usually no hybridization
Initial parents (IPs) selected from a heterogeneous population (i.e. genetically variable)
procedure continues until homogeneity is achieved
last phase is field testing
A pure line consists of progeny descended solely by self-pollination from a single
homozygous plant
Pure line selection is therefore a procedure for isolating pure line(s) from a mixed
population
Pure line cultivars are more uniform than cultivars developed through mass selection (by
definition, a pure line cultivar will be composed of plants with a single genotype)
Progeny testing is an essential component of pure line selection
Improvement using pure line breeding is limited to the isolation of the ‗best‘ genotypes
present in the mixed population
More effective than MS in development of self-pollinated cultivars
However, leads to rapid depletion of genetic variation
Genetic variability can be managed through directed cross hybridizations
Essential to progeny test selections.
Pure-line Selection-Steps: Select desirable plants
• Number depends on variation of original population, space and resources for
following year progeny tests
• Selecting too few plants may risk losing superior genetic variation
• A genotype missed early is lost forever
Seed from each selection is harvested individually.
Single plant progeny rows grown out
• Evaluate for desirable traits and uniformity
• Should use severe selection criteria (rogue out all poor, unpromising and variable
progenies)
Selected progenies are harvested individually
In subsequent years, run replicated yield trials with selection of highest yielding plants
After 4-6 rounds, highest yielding plant is put forward as a new cultivar
Advantages of pure line selection: 1. The pure lines are extremely uniform since all the plants in the variety will have the same
genotype.
2. Attractive and liked by the farmers and consumers.
3. Pure lines are stable and long test for many years.
4. Due to its extreme uniformity the variety can be easily identified in seed certification
programs.
Disadvantages of pure line selection: 1. New genotypes are not created by pure line selection
2. Improvement is limited to the isolation of the best genotype present in population. No
more improvement is possible after isolation of the best available genotype in the
population.
3. Selection of pure lines require great skill and familiarity with the crop.
4. Difficult to detect small differences that exist between cultures
5. The breeder has to devote more time
6. Pure lines have limited adaptability hence can be recommended for cultivation in limited
area only.
Mass selection:
It is the oldest & easiest method of selection and is useful in self- pollinated species and rarely in
cross-pollinated species. It is based on the ability to recognize desirable or undesirable traits in
plants of a population.
Procedure of Mass Selection: First year: Large numbers of phenotypically similar plants having desirable characters are
selected. The range of number may vary from 500-1000 plants. The seeds from the selected
plants are composited to rise the next generation.
Second year: composited seed planted in a preliminary field trial along with standard checks.
The variety from which the selection was made should also be included as check. Phenotypic
characteristics of the variety are critically examined and evaluated.
Third to sixth year: The variety is evaluated in coordinated yield trials at several locations. It is
evaluated in an initial evaluation (IET) trial for one year. If founded superior it is promoted to
main yield trial for 2 or 3 years.
Seventh year: if the variety is proved superior in main yield trials it is multiplied and released
after giving a suitable name.
Modification of mass selection Mass selection is used for improving a local variety. Large number of plants are selected (I year) and
individual plant progenies are raised (II year). Inferior, segregating progenies are reflected. Uniform,
superior rows are selected and the seed is bulked. Preliminary yield trials are conducted in third year.
Fourth to seventh year multilocation tests are conducted and seed is multiplied in eight year and
distributed in ninth year. Many other modifications also are followed depending on the availability of
time and purpose for each it is used.
Objectives of Mass Selection: To increase the frequency of superior genotypes from a genetically variable population.
Purify a mixed population with differing phenotypes.
Develop a new cultivar by improving the average performance of the population.
Disadvantages of Mass selection: 1 To be most effective, the traits of interest should have high heritability.
2 Because selection is based on phenotypic values, optimal selection is achieved if it is
conducted in a uniform environment.
3 Phenotypic uniformity is less than in cultivars produced by pure-line selection.
4 With dominance, heterozygotes are indistinguishable from homozygous dominant genotypes.
Without progeny testing, the selected heterozygotes will segregate in the next generation.
Mass selection vs pure line selection:
Mass selection pure line selection
1. Used both in self and cross pollinated crops. 1. Practiced in self pollinated crops only.
2. Large number of plants are selected. 2. Comparatively less number of plants are
selected.
3. The produce of the selected plants is mixed
and sown as such in next year.
3. Produce of individual plants is kept separate
and progeny rows are raised next year.
4. No control of pollination. 4. Pollination is controlled.
Mass selection pure line selection
5. Variety developed is heterozygous and not
uniform.
5. Variety is homozygous homogeneous and
uniform.
6 Due to heterozygosity the variety deteriorates
Quickly.
6. Due to homozygosity the variety lasts long.
7. No knowledge of science is required. It is
more an art.
7. Knowledge of science and genetics is
required.
8. Selection within a variety is effective. 8. Selection with in a pure line variety is not
effective.
9. The variety is relatively difficult to identify. 9. It is relatively easy to identify in seed
Certification programs.
10. The method has to be repeated once in 2-3
years to purify the variety.
10. No need to repeat
Pedigree method: It is a widely used method of breeding self-pollinated species (and even cross-pollinated species
such as crops produced as hybrids). Detailed records of the origin of the selected lines are
maintained. It produces new cultivars faster than mass selection. In self-pollinated crop, it is used
to release new varieties. In cross-pollinated crops, it is used to develop inbred lines.
Procedure of Pedigree method: Year 1 Identify desirable homozygous parents and make about 20–200 crosses.
Year 2 Grow 50–100 F1 plants including parents for comparison to authenticate its hybridity.
Year 3 Grow about 2,000–5,000 F2 plants. Space plant is to allow individual plants to be
examined and documented. Include check cultivars for comparison. Desirable plants are selected
and harvested separately keeping records of their identities. In some cases, it may be
advantageous not to space plant F2s to encourage competition among plants.
Year 4 Seed from superior plants are progeny-rowed in the F3–F5 generations, making sure to
space plant the rows for easy record keeping. Selection at this stage is both within and between
rows by first identifying superior rows and selecting 3–5 plants from each progeny to plant the
next generation.
Year 5 By the end of the F4 generation, there should be between 25–50 rows with records of the
plant and row. Grow progeny of each selected F3.
Year 6 Family rows are planted in the F6 to produce experimental lines for preliminary yield
trials in the F7. The benchmark or check variety is a locally adapted cultivar. Several checks may
be included in the trial.
Year 7 Advanced yield trials over locations, regions, and years are conducted in the F8–F10
generations, advancing only superior experimental material to the next generation. Ultimately,
the goal is to identify one or two lines that are superior to the check cultivars for release as a new
cultivar.
Figure: Generalized steps in breeding by pedigree selection.
Advantages of pedigree selection: 1. Eliminates unpromising material at early stages;
2. Multi-year records allow good overview of inheritance, and more effective selection
through trials in different environments;
3. Multiple families (from different F2 individuals) are maintained yielding different gene
combinations with common phenotype
4. Allows for comparison to other breeding strategies
Disadvantages of pedigree selection:
1. Most labor, time and resource intensive method.
2. Very dependent on skill of breeder in recognizing promising material.
3. It is not effective for accumulating the number of minor genes needed to provide horizontal resistance.
4. Pedigree selection is a long procedure, requiring about 10–12 years or more to complete 5. Selecting in the F2 (early generation testing) on the basis of quantitative traits such as
yield may not be effective. Bulk Method: This method can handle segregating generations, in which F2 and subsequent generations are harvested in bulk to grow the next generation. At the end of bulking period, individual plant selection and evaluation is carried out in the similar way as in pedigree method. This method is used in self- pollinated plant species.
Procedure of bulk method: Year 1 Identify desirable parents and make a sufficient number of crosses between them
Year 2 Following a cross between appropriate parents, about 50–100 F1 plants are planted and
harvested as a bulk, after rouging out selfs.
Year 3 The seeds from the second year are used to plant a bulk plot of about 2,000–3,000 F2
plants. The F2 is bulk harvested.
Year 4–6 A sample of the F2 seed is planted in bulk plots, repeating the steps for year 2 and year
3 until the F4 is reached or when a desired level of homozygosity has been attained in the
population. Space plant about 3,000–5,000 F5 plants and select about 10% (300–500) superior
plants for planting F6 progeny rows.
Year 7 Select and harvest about 10% (30–50) progeny rows that exhibit genes for the desired
traits for planting preliminary yield trails in the F7.
Year 8 and later Conduct advanced yield trials from F8 through F10 at multiple locations and
regions, including adapted cultivars as checks. After identifying a superior line, it is put through
the customary cultivar release process.
Figure: Generalized steps in breeding by bulk selection.
Advantages of bulk selection: Less record keeping than pedigree Inexpensive Easy to handle more crosses Natural selection is primarily for competitive ability Large numbers of genotypes can be maintained Works well with unadapted germplasm Can be carried on for many years with little effort by the breeder
Disadvantages of bulk selection: Environmental changes from season to season so adaptive advantages shift
Most grow bulk seed lots in area of adaptation
Less efficient than pedigree method on highly heritable traits (because can purge non-
selections in early generations)
Not useful in selecting plant types at a competitive disadvantage (dwarf types)
Final genotypes may be able to withstand environmental stress, but may not be highest
yielding.
If used with a cross pollinated species, inbreeding depression may be a problem.
Comparison between bulk and pedigree method:
Pedigree method Bulk method 1 Most widely used Breeding method. 1 Used only to a limited extent.
2 Individual plants are selected in F2 and
subsequent generations and individual plant
progenies are grown.
2 F2 and subsequent generations are grown in
bulk.
3 Pedigree Records have to be maintained
which is often time consuming and laborious.
3 No pedigree records are maintained.
4 Generally it‘s taken 12-13 years to release a
new variety.
4 Takes more than 15 years.
5 Requires close attention of breeder from F2
onwards.
5 It is quite simple and does not require much
attention.
6 Planting (spacing) the segregating
generations are space planted to permits
effective individual plant selection.
6 The bulk populations are generally planted at
commercial planting rates.
7 Population size is small in comparison to
bulk.
7 The population size is large.
Back cross method: A cross between a hybrid (F 1 or a segregating generation) and one of its parents is known as
backcross. Same form whether self or cross pollinated species In the B.C. method, the hybrid
and the progenies in the subsequent generations are repeatedly back crossed to one of their
parents. To improve or correct one or two specific defects of a high yielding variety, which is
well adapted to the area and has other desirable characteristics.
Recipient parent: Well adapted, high yielding variety, lacking one or two characters and hence
receives these genes from other variety.
Donor parent: The variety which donates one or two useful genes.
Recurrent parent: Since the recipient parent is repeatedly used in the backcross program, it is
also known as the recurrent parent.
Non-recurrent parent: The donor parent, on the other hand, is known as the non-recurrent
parent because it is used only once in the breeding program (for producing the F1 hybrid).
Procedure of backcross:
Transfer of a Dominant Gene
Let us suppose that a high yielding and widely adapted variety A is susceptible to stem rust.
Another variety B is resistant to stem rust, and that resistance to stem rust is dominant to
susceptibility. A generalized scheme of the backcross program for the transfer of rust resistance
from variety B to variety A is given below.
Hybridization: Variety A is crossed to variety B. Generally, variety A should be used as the
female parent. This would facilitate the identification of self pollinated plants, if any.
F1 Generation: F1 plants are backcrossed to variety A. Since all the F1 plants will be
heterozygous for rust resistance, selection for rust resistance is not necessary.
First Backcross Generation (BC1): half of the plants would be resistant and the remaining half
would be susceptible to stem rust. Rust resistant plants are selected and backcrossed to variety A.
BC1 plants resistant to rust may be selected for their resemblance to variety A as well.
BC2-BC5 Generations: In each backcross generation, segregation would occur for rust
resistance. Rust resistant plants are selected and backcrossed to the recurrent parent A. Selection
for the plant type of variety A may be practiced, particularly in BC2 and BC3.
BC6- Generation: On an average, the plants will have 98.4 per cent genes from variety A.
Rust resistant plants are selected and self pollinated; their seeds are harvested separately.
BC6 F2 Generation: Individual plant progenies are grown. Progenies homozygous for rust
resistance and similar to the plant type of variety A are harvested in bulk. Several similar
progenies are mixed to constitute the new variety.
Yield Tests: The new variety is tested in a replicated yield trial along with the variety A as a
check. Plant type, date of flowering, date of maturity, quality etc. are critically evaluated.
Ordinarily, the new variety would be identical to the variety A in performance. Detailed yield
tests are, therefore, generally not required and the variety may directly be released for
cultivation.
Transfer of a Recessive Gene
When rust resistance is due to a recessive gene, all the backcrosses cannot be made one after the
other. After the first backcross, and after every two backcrosses, F2 must be grown to identify
rust resistant plants. The F1 and the backcross progenies are not inoculated with rust because
they would be susceptible to rust. Only the F2 is tested for rust resistance. A generalized scheme
for the transfer of a recessive gene for rust resistance is given below.
Hybridization: The recurrent parent is crossed with the rust resistant donor parent. The recurrent
parent is generally used as the female parent.
F1 Generation: F1 plants are backcrossed to the recurrent parent.
BC1 Generation: Since rust resistance is recessive, all the plants will be rust susceptible.
Therefore, there is no test for rust resistance. All the plants are self-pollinated.
BC1 F2 Generation: Plants are inoculated with rust spores. Rust resistant plants are selected
and backcrossed with the recurrent parent. Selection is done for the plant type and other
characteristics of the variety A.
BC2 Generation: There is no rust resistance test. Plants are selected for their resemblance to the
recurrent parent A, and backcrossed with the recurrent parent.
BC3 Generation: There is no disease test. The plants are self-pollinated to raise F2 selection is
usually done for the plant type of variety A.
BC3F2 Generation: Plants are inoculated with stem rust. Rust resistant plants resembling
variety A are selected and backcrossed to variety A. Selection for plant type of A is generally
effective.
BC4 Generation: There is no rust resistance test. Plants are back-crossed to variety A.
BC5 Generation: There is no rust test. Plants are self -pollinated to rise F2 generation.
BC5F2 Generation: Plants are subjected to rust epidemic. A rigid selection is done for rust
resistance and for the characteristics of variety A. Self pollinated seeds from the selected plants
are harvested separately.
BC5F3 Generation: individual plant progenies are grown and subjected to rust epiphytotic.
A rigid selection is done for resistance to stem rust and for the characteristics of variety A.
Seeds from several similar rust resistant homogeneous progenies are mixed to constitute the new
variety.
Yield Tests: It is the same as in the case of transfer of a dominant gene.
Genetics & Plant Breeding Related Terminology
Karyokinesis: A nuclear division is called karyokinesis.
Cytokinesis: Division of cytoplasam is called cytokinasis.
Mitosis: It is the division of somatic nucleus.
Meiosis: It may be defined as two nuclear divisions where chromosomes divide only once.
Synapsis: It is the pairing of homologous which leads to the formation of bivalents.
Genome: A haploid set of chromosomes of an organism.
Linkage: Described as presence of number of different gene of the same chromosome. They
are inherited together.
Crossing over: Exchange of segments between non-sister chromatids of homologous
chromosomes during synapsis. Crossing over breaks the linkage.
Map-unit: It is the unit which gives distance between two genes measured in terms of crossing
over percentage.
1 map-unit = 1% cross over (centimorgan)
1 morgan unit represents 100% crossing over.
Inversion: When a segment of chromosome breaks and reunites in the inverted order of gene
sequences, it is called inversion.
Allele: A member of the pair of a gene, situated at a particular locus on a pair of homozygous
chromosomes.
Back cross/Test cross: The cross between F1 hybrid and homozygous recessive parent (test
cross) or a homozygous dominant parent (back cross). In test cross, the purpose is to test
heterozygosity of an organism.
Monohybrid cross: A cross between two organisms heterozygous for one pair of alleles.
Dihybrid cross: A cross between organisms heterozygous for two unlike pair of alleles.
Dominance: the complete expression of one allele over another allele in the pair of gene; the
suppressed allele is called recessive and the allele which expresses is called dominant.
Epistasis: The suppression of one gene by another gene on different locus is called epistass.
F1 generation: The first filial generation, usually the hybrid of two homozygous parents.
F2 generation: The second filial generation usually referred after the F1× F1 cross.
Genotype: The genetic constitution of an organism with respect to specific allele.
Phenotype: The appearance of an organism, i.e., its physical and chemical features.
Incomplete dominance: It refers to partial expression of an allele in the phenotype of a
heterozygous individual.
Lethal gene: A gene which causes death of the organism, bearing it, at any stage from the
zygote onwards.
Locus: The site occupied by a member of the gene on a chromosome.
Self-sterility gene: Multiple alleles which prevent pollen tube formation and self fertilization
in some species of flowering plants.
Male sterility: It is characterised by non-functional pollen grains. Male sterility is used in
hybrid seed production.
Plant breeding: plant breeding is an art and science of crop improvement in respect of yield
and quality.
Inbred line: It is a relatively true-breeding strain resulting from at least 3 successive
generations of controlled self-fertilization or back crossing.
Pure line: A strain in which all individuals have descended by self-fertilization from a single
homozygous individual. A pure line is genetically pure.
Hybrid: Product of hybridization. It can‘t store for the next year. It is heterozygous.
HYV: HYV can store for next year. It is homozygous.
References:
http://www.foodfirst.org/media/opeds/2000/4-greenrev.html Lessons from the Green Revolution.
http://www.arches.uga.edu/~wparks/ppt/green/Biotechnology and the Green Revolution.
Interview with Norman Borlaug.
http://cuke.hort.ncsu.edu/cucurbit/wehner/741/hs741hist.html History of plant breeding.
http://agronomy.ucdavis.edu/gepts/pb143/pb143.thmGepts, P. 2002. The evolution of crop
plants.
http://cucurbitsvr.hort.ncsu.edu/breeding/usplantbreeding/uspbmain.html Plant breeding in the
USA. List of land grant institutions and seed companies.
http://pas.byu.edu/AgHrt100/evolutio.htm Synopsis on plant breeding and evolution.
http://www.barc.usda.gov/psi/ngrl/ngrl.html Website of National Germplasm Resources Lab.
http://www.plantstress.com/admin/WRFiles/germplasmwr.htm List of websites for plant
germplasm resource centers worldwide.
http://www.ciesin.org/docs/002-256a/002–256a.htmlPaper on current status of biological
diversity by E. O. Wilson of Harvard University.
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/A/AsexualReproduction.html Asexual
reproduction in plants.
http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookflowers.htmlExcellent
illustrations and discussion of aspects of reproduction in flowering plants.
http://www.ukans.edu/~bio152/17/sld001. htm Excellent slides on plant reproduction.