Plant Breeding and Improvement STT 2073

Variation Environmental variation Heritable variation

Environmental variation Abiotic Soil – fertility, pH, texture Water – availability, quality Temperature – cold, hot Light – intensity, photoperiod Biotic Pests and diseases

Environmental variation Environmental stress causes changes: size, shape, colour, Composition development Environmental variation can be observed by growing plants of the same genetics in different environment

Environmental variation Environmental variation is NOT heritable Heritable variation Origin of heritable variation in nature 1.Gene recombination 2.Variation in chromosome number 3.Gene mutation

Important terms Genetics Genome Gene Allele Homologous chromosomes Homozygous Heterozygous Dominant Recessive

Important terms Genetics Study of: gene structure action pattern of inheritance

Important terms Genome Complete set of DNA, including all of its genes

Chromosome

DNA

DNA: Deoxyribonucleic acid The material that makes up the genes found in all cells Controls the function of all the cells in the body. Consists of two thread- like strands that are linked together in the shape of a double helix.

DNA Replication

Important terms Gene  A unit of inheritance for a trait. Occurring at specific locus.  Determines a particular characteristic in an organism  It consists of a sequence that is transcribed to a functional RNA product and regulatory sequences that enable translation to occur

Important terms Gene and allele  Gene – The unit of inheritance for a particular trait  Allele – the alternative forms of a gene. Located at the same locus of homologous chromosomes

Important terms Locus The specific position or location where a gene occupied Homologous chromosome Chromosomes that pair with each other at meiosis

Important terms Homozygous When the two alleles are the same Heterozygous When the two alleles are different. The dominant allele is expressed.

Important terms Dominant Trait that is expressed regardless the second allele Recessive Trait that is only expressed when the second allele is the same

Important terms Gene and allele

Important terms Gene and allele

Important terms Genotype The allelic composition of an organism Phenotype The physical expression of the allelic composition for the trait under study

Important terms GenotypePhenotype

Important terms Letter notation for gene Genes are commonly represented by letters. A and a symbolize alleles AA, Aa or aa symbolize genotypes. AA = homozygous dominant aa = homozygous recessive Aa = heterozygous

Heritable variation Origin of heritable variation in nature 1.Gene recombination 2.Variation in chromosome number 3.Gene mutation

Heritable variation Gene recombination Naturally Human intervention  Artificial pollination  Genetic engineering

Artificial pollination

Monohybrid cross – F 1 and F 2 generations A cross involving segregation of only a single pair of alleles at a given locus (one trait) Gene = seed coat appearance Alleles = smooth (S) and Wrinkled (s)

Monohybrid cross – F 1 and F 2 generations

The principle of segregation

Monohybrid cross – F 1 and F 2 generations

Dihybrid cross A cross that involved two pairs of alleles RR yy rr YY Round, green wrinkled, yellow Genes : Seed coat appearance Seed coat colour Alleles : Smooth vs. wrinkled Yellow vs. green Dominant : Round, Yellow Recessive : Wrinkled, Green

Dihybrid cross – F 1 and F 2 generations

The Chi-Square Test To decide if our data fits any of the Mendelian ratios we have discussed. χ 2 = Σ ( Observed value - Expected value) (Expected value) 2

The Chi-Square Test ( ) 2 ( ) 2 ( ) 2 (32-35) 2 = χ 2 = = n = number of phenotype class = 4 df = degree of freedom = n-1 = 3 With df = 3, χ 2 at 5 % probability = 7.81 (Ref. Table) χ 2 = (< 7.81) means the two traits are segregating at 9:3:3:1 and not by chance.

The Chi-Square Test

Predicting Genetic Ratio Number of phenotypes = 2 n n = no. of segregating gene pairs and assuming complete dominant Monohybrid cross = 2 phenotypes Dihybrid cross = 4 phenotypes Trihybrid cross = 8 phenotypes

Predicting Genetic Ratio Number of genotypes = 3 n where n = no. of segregating gene pairs and assuming complete dominant Monohybrid cross = 3 genotype Dihybrid cross = 9 genotypes Trihybrid cross = 27 genotypes

Modification of Mendelian Ratio The trait may be single gene but not segregate according to Mendelian ratio - Incomplete or partial dominance - Co-dominance - Multiple alleles

Incomplete/ Partial dominance The offspring expresses an intermediate phenotype different to that of both parents. 1:2:1 genotypic phenotypic ratios One gene 2 alleles

Co-dominance When two alleles are responsible for the production of two distinct or detectable gene products In co-dominant situation, both alleles express their gene product in the heterozygote Example : Flower colour. Flower of two distinct colours are often the results of a co-dominant situation

Co-dominance When two alleles are responsible for the production of two distinct or detectable gene products In co-dominant situation, both alleles express their gene product in the heterozygote Example : Flower colour. Flower of two distinct colours are often the results of a co-dominant situation One gene 2 alleles

Multiple alleles Diploid organisms - 2 alleles per gene, one deriving from each parent Polyploid organisms - > 2 alleles per gene Some cases, however, more than two types of allele can code for a particular characteristic (Eg. blood type). When genes are having multiple alleles, numbers such as X1, X2, X3, X4 and so on are used rather than capital and lower-case letters.

Multiple alleles Within a population of plants, as many types of gametes can be produced as there are different types of alleles A heterozygous X1X2 plant can be crossed with a heterozygous X3X4 one resulting in four distinct types of F1 plants with none have the same genotype of either parents X 1 X 2 X3 X4 X3 X4

Test cross A technique used to help determine the genotype of a particular plant Use a ‘tester’ that is homozygous recessive TT : Tall TT Tt : Tall Tttt (Tester) Tt All tall x tt X TallDwarf P F1F1 tt : Dwarf

Test cross

Progeny test A test to determine the genotype of plants ‘Tester ’ is not used but simply allow the F 2 individual to self, then examine the F 3 generation for segregation

Progeny test Tall TT TT F2F2 Gametes F 3 s Selfed

Progeny test T Tall Tt T Tall TT F2F2 Gametes F 3 s tt Tall Tt Dwarf tt

Gene interaction - epistasis Epistasis is the situation in which the alleles at one gene cover up or alter the expression of alleles of another gene (2 genes interaction) It is a form of gene interaction between non-allelic genes affecting the same phenotypic traits

Gene interaction Complementary action Modifying action Inhibiting action Masking action Duplicate action Additive effect Pleiotropic effect

Complementary action The situation in which two non-allelic genes may be required to produce a single effect Eg. In oats, two dominant genes (AB) are required for resistance to crown rust. Ab, aB and ab are susceptible to the disease

Modifying action The situation in which one gene produces an effect only in the presence of a second gene at another locus In corn, a dominant gene P produces purple aleurone color in the presence of dominant R, but expresses no effect in the absence of R. PrR = Purple aleurone prR = Red aleurone Prr, prr = colourless aleurone

Inhibiting effects The situation in which one gene may act as an inhibitor of the expression of another gene In corn, a dominant gene R for red colour aleurone does not produce an effect in the presence of a dominant inhibitor gene I. Ri = red aleurone RI, rI, ri = white aleurone

Masking action The situation in which one gene may hide the effect of a second gene when both are present In oats, a dominant gene Y produces yellow seed coat colour and a dominant gene B produces black seed coat colour. BY, By = black bY = yellow by = white (Dominant gene B masks the effect of gene Y )

Duplicate action The situation in which two genes may produce a similar effect; or the same effect is produced by both of them together In shepherd’s purse, triangular-shaped seed capsule is produced by either the dominant genes C or D or by both together. Cd, cD, CD = triangular capsule cd = ovoid-shaped seed capsule

Additive effect The situation in which two genes may produce the same effect are additive if both genes are present In barley, either A or B will produce medium-length awns while the two dominant genes together produce long awns. AB = Long awns Ab, aB = Medium-length awns ab = Awnless (No awn)

Pleiotropic effect The situation in which a single gene or mutation affects two or more characteristics or traits; eg. Simultaneously influencing size, shape, color or function of several organs The ‘uzu’ gene in barley. Uz is dominant and normal in appearance, but uz is recessive. The plant is semi-dwarf, dense spike, short awns, small seeds, short erect flag leaf.

Genetic linkage Mendel’s Law of independent assortment only applied if the genes located: o on different chromosomes or o very far apart on the same chromosome Crossing over is the process by which segment of chromatids of homologous chromosomes are exchanged as they synapse during meiosis – Recombination occurred If the genes located closely on the same chromosome - tend to be inherited together = genetic linkage (link).

Crossing over

Crossing over

Linkage

Backcross Technique A cross between a hybrid and either one of its parents The backcross method is used in succession to add a gene for a desire character to an otherwise superior parent The backcross method is also used to concentrate genes for a quantitative character

Backcross Technique Original cross Donor parent (RR) A disease resistant cultivar Recurrent parent (rr) An adapted cultivar A 1 st Backcross 3 rd Backcross 2 nd Backcross 4 th Backcross cv A rr X F 1 Rr 50% genes from cv.A BC 1 Rr : rr 75% genes from cv.A BC 2 Rr : rr 87.5% genes from cv.A BC 3 Rr : rr 93.75%genes from cv.A BC 4 Rr : rr 96.87% genes from cv.A cv. A rr 1 RR : 2 Rr : 1 rr Self Rr plant from BC 4 to obtain homozygous RR X X X X

Backcross Technique