Crop water productivity: briefing on concepts, definitions and goals, Andrew Noble
11. Definitions for crop botany
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Transcript of 11. Definitions for crop botany
Gene • Genes act as units of heredity, transferring the same instructions from
parent to offspring.
• Genes are functional units of DNA that contain the instructions for making
proteins or RNA.
• Each somatic cell has two copies of a gene, while gametes have only one
copy
• The name "gene" was coined
by Johannsen in 1909
Allele
• Different forms of the same gene
• Occur at loci on chromosomes
• Alleles govern the contrasting forms of a character
• The genes are particulate. the two alleles of a gene do not modify each
other when they stay together in the same cell
• They separate and pass into different gametes of the hybrid
Locus – fixed position of a gene on a chromosome
Note in the diagram that the genetic sequence of the two alleles is different.
Note that the gene will likely function the same, even though the genetic sequences are slightly different
Character/ trait = a feature that can be inherited
Quantitative traits
Qualitative traits
S.No. Quantitative traits Qualitative traits
1 Governed by polygenes By oligogenes
2 Effect of each gene is not detectable
Detectable
3 Variation is continuous Discontinuous
4 Separation into different classes is possible
Separation into different classes is not possible
5 Highly influenced by environmental factors
Little influenced by environmental factors
Homozygous and Heterozygous
A certain terminology is used to describe how each individual has inherited the alleles of specific genes.
Homozygous - having an identical pair of alleles for a specific character or feature
Heterozygous - having different alleles for a specific character or feature
Hemizygous
A condition in which only one of the alternate allele of a gene pair is present as in sex linkage or as a result of deletion
Dominant • One allele express itself at the expense of the other in the heterozygote
• Only one allele at a locus is expressed. The other (recessive gene) is suppressed
Recessive
• An allele, which is incapable expressing itself phenotypically
• An allele that is suppressed in the presence of a dominant gene. If the two recessive alleles occur at a locus, then the trait is expressed.
Codominant
• Both alleles at a locus are expressed with equal force
Genotype
The genetic constitution of an organism
Not very easily influenced by envt.
Predicted based on phenotypic ratios
Phenotype
The observable characteristics of an organism
Can be altered by envt.
Can be measured or grouped in to distinct classes
Generations
• P generation - represents parents
• F1 generation (first filial) - offspring of these (P) parents
• F2 generation (second filial) - offspring of these (F1)
parents
Monohybrid cross
Hybridization between two individuals or strains, which differ for a single character
Dihybrid cross
Hybridization between two individuals or strains, which differ for two characters
Polyhybrid
The offspring of parents differing from each other in more than two characters.
BackcrossThe cross of an F1 hybrid to one of its parents
TestcrossThe cross of an F1 hybrid with an individual or strain having the recessive phenotype
Purpose: to verify the ratio in which the different gametes of the F1 hybrid are produced
It crosses a dominant phenotype (P_) with a recessive phenotype (pp).
If the cross creates:
PP x pp -> Pp 100% purple offspring, or
Pp x pp -> Pp 50% purple offspring and
pp 50% white offspring
so it proves the existence of a recessive allele in the dominant parent.
Punnett Squares - These are the tools that we use to determine the genetic possibilities of various combinations of parents
BB Bb Bb bb¼ 2/4 ¼
3 : 1
MENDEL’S EXPERIMENTS
• Born in 1822 in Brunn (Austria), now Brno in Czechoslovakia
• Joined Monastery in 1843
• In 1847, became priest
• In 1854, appointed as substitute science teacher
• In 1857, began his famous experiments on pea
• In 1865, presented his paper before Natural Science Society of Brunn
• In 1866, his paper ‘Experiments on plant hybridization” published in the proceedings of the society
• Died in 1884 at an age of 62 years
Gregor Johann Mendel
Mendel's Monastery Garden
Selection of pea as an experimental material
• Self pollinated
• Large flowers
• Seeds are large; Easy to grow
• Needs small space
• Reproduces in one season
• Varieties had contrasting forms of several characters
Seven pairs of contrasting traits
TraitDominant
ExpressionRecessive Expression
Form of ripe seed (R) Smooth (Round) Wrinkled
Color of seed coat (Y) Yellow Green
Color of flower (P) Purple White
Form of ripe pods (I) Inflated Constricted
Color of unripe pods (G) Green Yellow
Position of flowers (A) Axial Terminal
Length of stem (T) Tall Dwarf
Several characters had two contrasting forms
Purple x White White x Purple
Crossing or hybridization
S.No. Structure Character Dominant Recessive Ratio in F2
1 Seed Form 5,474 (R) 1,850 (W) 2.96 : 1
2 Cotyledon Colour 6,022 (Y) 2,001 (G) 3.01 : 1
3 Seed coat Form 882 (I) 299 (C) 2.95 : 1
4 Seed coat Colour 705 (G) 224 (W) 3.15 : 1
5 Unripe pods Colour 428 (G) 152 (Y) 2.82 : 1
6 Flowers Position 651 (A) 207 (T) 3.14 : 1
7 Stem Length 787 (L) 277 (S) 2.84 : 1
Results of Mendel’s original crosses for seven pairs of characters
Hybrid
Aa They do not mix or blend
Hybrid
Aa
Hybrid
Aa
Hybrid
Aa
Remain together in a pure form without affecting each other (Law of purity of gametes)
Separate into different gametes in equal numbers Separation occurs during Anaphase I
When there is complete dominance, segregation leads to phenotypic ratio 3:1
Mendel's First Law The law of segregation
Round Yellow seeds x Wrinkled Green seeds
Mendel's Second Law The law of Independent Assortment/
Law of free recombination
During gamete formation the segregation of the alleles of one gene is independent of the segregation
of the alleles of another gene
Main features of SECOND LAW
• Explains simultaneous inheritance of two plant characters
• In F1 two gene controlling two different characters, come together, each gene exhibits independent dominant behaviour without affecting or modifying the effect of other gene
• Segregation occurs during gamete formation – independently
• Allele of one gene can freely combine with the allele of another gene – equal chance
The two laws of Mendel, especially the law of segregation, are the basis of the science of genetics, and Mendel is aptly known as the
‘Father of Genetics’
Rediscovery of Mendel's work
• Mendel presented his paper in 1865 but the scientific world failed to
recognize its importance for nearly thirty- five years.
• Hugo de Vries in Holland working on Oenothera; Carl Correns in
Germany working on peas and maize and Eric Tschermak in Austria
obtained results similar to those obtained by Mendel, but none of them
had seen Mendel's paper before beginning their own experiments.
• The results were published by
Hugo de Vires in March 1900
Carl Correns in April 1900
Eric Tschermak in June 1900
• The rediscovery initiated a spurt of research activity in the field of genetics
• Thus the ‘Science of Genetics’ was truly borne