PRINCIPLES OF INHERITANCE AND VARIATION
· Genetics: deals with the inheritance, as well as the variation of characters from parents to offsprings.
· Inheritance: is the process by which characters are passed on from parent to progeny.
· Variation: is the degree by which progeny differ from their parents.
MENDEL’S LAWS OF INHERITANCE:
· Gregor Jhon Mendel. Conducted hybridization experiments on garden peas for seven years (1856 – 1863) and proposed laws of inheritance.
· Mendel conducted artificial pollination/cross pollination experiments using several true-breeding pea lines.
· A true breeding line is one that, having undergone continuous self-pollination for several generations.
· Mendel selected 14 true-breeding peas’ plant varieties, as pair’s which were similar except for one character with contrasting traits.
o True breed selected by Mendel
o Stem height- Tall / dwarf
o Flower color- Violet/white
o Flower position – Axial / terminal
o Pod shape- Inflated / beaded or constricted
o Pod color- Green / yellow
o Seed color- Yellow/ green
o Seed shape – round / wrinkled
INHERITANCE OF ONE GENE:
· Mendel crossed tall and dwarf pea plants to study the inheritance of one gene.
· He collected the seeds produced as a result of this cross and grew them to generate plants of the first hybrid generation. This generation is called filial progeny or the F1.
· Mendel observed that all the F1 progeny plants ere tall, like one of its parents; none were dwarf.
· He made similar observations for the other pairs of traits – he found that the F1 always resembled either one of the parents, and that the trait of the other parent was not seen in them.
· Mendel then self – pollinated the tall F1 plants and to his surprise found that in the F2 generation some of the offsprings were ‘dwarf; the character that was not seen in the F1 generation was now expressed.
· The proportion of plants that were dwarf was 1/4th of the F2 plants while 3/4th of the F2 plants were tall.
· The tall and dwarf traits were identical to their parental type and did not show any blending, that is all the offsprings were either tall or dwarf, none were of in between height.
· Similar results were obtained with the other traits that he studied: only one of the parental traits was expressed in the F1 generation while at the F2 stage both the traits were expressed in the proportion of 3:1.
· The contrasting traits did not show any blending at either F1 or F2 stage.
· Mendel proposed that something was being stably passed down, unchanged, from parent to offspring through the gametes, over successive generations. He called these things as ‘factors’.
· Now a day we call them as genes.
· Gene is therefore are the units of inheritance.
· Genes which codes of a pair of contrasting traits are known as alleles, i.e. they are slightly different forms of the same gene.
· Capital letters used for the trait expressed at the F1 stage.
· Small alphabet for the other trait.
· ‘T’ is used for Tall and‘t’ is used for dwarf.
· ‘T’ and ‘t’ are alleles of each other.
· Hence in plants the pair of alleles for height would be TT. Tt. or tt.
· In a true breeding tall or dwarf pea variety the allelic pair of genes for height are identical or homozygous, TT and tt respectively.
· TT and tt are called the genotype.
· Tt plant is heterozygous for genes controlling one character (height).
· Descriptive terms tall and dwarf are the phenotype.
· When F1 hybrid is crossed back with the recessive parent, it is known as test cross.
· It is used to know the genotype of the given plant/animal.
Law of Dominance:
· Characters are controlled by discrete units called factors.
· Factors occur in pairs.
· In a dissimilar pair of factors one member of the pair dominates (dominant) the other (recessive).
Law of Segregation:
· The alleles do not show any blending and that both the characters are recovered as such in the F2 generation though one of these is not seen at the F1 stage.
· The parents contain two alleles during gamete formation; the factors or alleles of a pair segregate or separate from each other such that a gamete receives only one of the two factors.
· Homozygous parent produces all gametes that are similar i.e contain same type of allele.
· Heterozygous parents’ produces two kinds of gametes each having one allele with equal proportion.
· When a cross between two pure breed is done for one contrasting character, the F1 hybrid phenotype dose not resemble either of the two parents and was in between the two, called incomplete dominance.
· Inheritance of flower colour in the dog flower (snapdragon or Antirrhinum sp.) is a good example of incomplete dominance.
· F2 generation phenotypic ratio is 1:2:1 in stead of 3:1 as Mendelian monohybrid cross.
· Genotypic ratio of F2 generation is 1:2:1.
Co – dominance:
· F1 resembled either of the two parents (complete dominance).
· F1 offspring was in-between of two parents (incomplete dominance).
· F1 generation resembles both parents side by side is called (co-dominance).
· Best example of co-dominance is the ABO blood grouping in human.
· ABO blood group is controlled by the gene I.
· The plasma membrane of the RBC has sugar polymers (antigen) that protrude from its surface and the kind of sugar is controlled by the gene-I.
· The gene I has three alleles I A,I B and i.
· The alleles I A and I B produce a slightly different form of sugar while allele i doesn’t produce any sugar.
· Each person possesses any two of the three I gene alleles.
· I A and I B are completely dominant over i.
· When I A, and I B present together they both express their own types of sugar; this because of co-dominance. Hence red blood cells have both A and B type sugars.
· Example of ABO blood grouping produces a good example of multiple alleles.
· There are more than two i.e. three allele, governing the same character.
A single gene product may produce more than one effect:
· Starch synthesis in pea seeds is controlled by one gene.
· It has two alleles B and b.
· Starch is synthesized effectively by BB homozygote and therefore, large starch grains are produced.
· The ‘bb’ homozygous has less efficiency hence produce smaller grains.
· After maturation of the seeds, BB seeds are round and the bb seeds are wrinkle.
· Heterozygous (Bb) produce round seed and so B seems to be dominant allele, but the starch grains produced are of intermediate size.
· If starch grain size is considered as the phenotype, then from this angle the alleles show incomplete dominance.
INHERITANCE OF TWO GENES:
Law of independent Assortment:
· When two characters (dihybrid) are combined in a hybrid, segregation of one pair of traits is independent of the other pair of traits.
Genetic disorders is a disease that is caused by an abnormality in an individual's DNA. Genetic disorders are mainly of two types: Chromosomal disorder, cause by an abnormality in the chromosomes of individual. Mendelian disorder; caused by mutation or alteration in the single gene.
Chromosomal theory of inheritance states that genes are located on specific site on chromosomes which are responsible for the inheritance of characters. Linkage and crossing over, polygenic inheritance, pleiotropy are based on this theory. Sex determination and mutation are the phenomenon which which influence the inheritance pattern of organisms.