PRINCIPLES OF INHERITANCE AND VARIATION
INHERITANCE OF TWO GENE: Mendel's 2nd law or Law of independent assortment: It states that, factors for different pairs of contrasting characters in a hybrid assorted (distributed) independently during gamete formation. Mendel's 2nd law can be explained by dihybrid cross. Dihybrid cross: The cross between two parents, which differs in two pairs of contrasting characters
DIHYBRID CROSS: Parents Phenotype Genotype Gametes Round Yellow RRYY RY Wrinkled Green rryy ry F1 generation RrYy Round Yellow
Dihybrid Cross Round, yellow seeds (RRYY Wrinkled, green seeds (rryy F1 generation All round, yellow seeds (RrYy) Eggs ry F2 generation RRYY RRYy RrYY RrYy 9/16 are round, yellow 3/16 are round, green Sperm RRYY RRVY RrYy Fry 3/16 are wrinkled, yellow 1/16 are wrinkled, green Phenotypic ratio : 9:3:3
DIHYBRID TEST CROSS. F1 hybrid is crossed with recessive green wrinkled pea plant. Recessive green wrinkled - rryy, Gamete ry . F1 hybrid round yellow- RrYy, Gametes: RY, Ry, rY, ry Gametes RY RyrY ry ry RrYy Rryy rry rryy Phenotypic ratio -1:1:1:1
. Mendel work published On 1865 but remain unrecognized till 1. 2. 3. 4. 1900 Reasons for that: Lack of communication Concept of genes/ factors- clear Mathematical approach for biology was not acceted No proof for existence of factors
CHROMOSOMAL THEORY OF INHERITANCE: It was proposed by Walter Sutton and Theodore Boveri. .They work out the chromosome movement during meiosis. .The movement behavior of chromosomes was parallel to the behavior of genes. The chromosome movement is used to explain Mendel's laws. The knowledge of chromosomal segregation with Mendelian principles is called chromosomal theory of inheritance. . . According to this, Chromosome and genes are present in pairs in diploid cells. Homologous chromosomes separate during gamete formation (meiosis) Fertilization restores the chromosome number to diploid condition.
Thomas Hunt Morgan and his colleagues conducted experimental verification of chromosomal theory of inheritance 'Morgan worked with tiny fruit flies, Drosophila melanogaster. Drosgps melogastey
He selected Drosophila because, . It is suitable for genetic studies. : Grown on simple synthetic medium in the laboratory They complete their life cycle in about two weeks. A single mating could produce a large number of progeny flies. . Clear differentiation of male and female flies Many types of hereditary variations can be seen with low power microscopes.
SEX DETERMINATION Henking (1891) traced specific nuclear structure during spermatogenesis of some insects. 50 % of the sperm received these specific structures, whereas 50% sperm did not receive it. He gave a name to this structure as the X-body. This was later on named as X-chromosome. .
XX-XY TYPE Sex determination in insects and mammals In this type both male and female has same number of chromosomes. Female has autosomes and a pair of X chromosomes. (AA+ Male has autosomes and one large 'X' chromosome and one very small 'Y-chromosomes. (AA+XY) In this type male is heterogamety and female homogamety. .
LINKAGE & RECOMBINTION Morgan carried dihybrid cross in Drosophila to study genes that are sex linked .Crossed- yellow bodied, white eyed females with brown bodied, red eyed males & intercourse F1 progeny Two genes did not segregate independently of each other & F2 ratio deviated from 9:3:3:1 The genes present on X-chromosome & two genes in a dihybrid cross- situated on same chromosome- parental gene combination is much higher than non parental- this is due to physical association/ linkage of two genes on chromosome- Linkage .Generation of non parental combination- Recombination