Chromosomal Basis of Inheritance Important points

Chromosomal Basis of Inheritance Important points

Bernstein (1924, 1925) Discovered multiple alleles, codominance and dominant-recessive relationships in determination of human blood groups.

Mendel (1822-1884) simply described his results and drew certain conclusions. Father of genetics. W. Johannsen coined the term pureline (1900), gene (1909), genotype and phenotype (1911). Nilsson-Ehle (1908) first scientist to experimentally prove polygenic/quantitative inheritance.

• Shull (1914) : Coined the term heterosis.
• Galton (1883) : Coined the term eugenics.
• Castello and Steini (1902) : Discovered AB group.

   

• Punnet (1927) : Gave the checker board method for simultaneously obtaining phenotype and genotype. The checker board is now called Punnet square. He along with Bateson worked on gene interaction using sweet pea.
• Rh. factor : Reported by Landsteiner and Wiener in blood of Indian brown Rhesus monkey (Macacus rhesus). It is an autigen protein on RBC surface. 93% Indians have this antigen and called as Rh positive (Rh+) It is also controlled by multiple alleles. Either the mother should be Rh+, or both husband and wife should be Rh–.
• Complete Penetrance : 100% ability of an allelic combination to produce expected phenotype.
• Incomplete penetrance : It is failure of an allelic combination to provide cent percent phenotypic expression. e.g., polydactyly, diabetes mellitus.
• Self Sterility : Found in plants having multiple alleles for compatibility-incompatibility S1, S2, S3, S4, S5.... etc., A plant carries two such alleles e.g., S1, S2, S2, S3, S1, S3, S2, S4, A pollen grain carries only one allele. If it happens to be one of the alleles of pistil the pollen grain fails to form pollentube. Marphan’s syndrome caused in human beings by a pleiotropic gene which is characterised by slender body, limb elongation, hypermobility in joints, lens dislocation and tendency to develop heart diseases.
• Hemizygous condition : The condition where only one gene or allele of allelic pair is present e.g. Xth X Seven traits selected by Mendel in Pea plant are now known to be located on only 4 chromosomes, i.e., on 1, 4, 5, 7 chromosomes. Cotyledon and Seed coat colour on chromosome 1, pod form, flower position and stem length on chromosome 4, Pod colour on chromosome 5, seed form on chromosome 7.

Wild species are disease resistant, so disease resistance can be introduced in cultivated forms from wild relatives by hybridization. Interrelationship between different metabolic pathways contributing to different phenotypes is the basis of pleiotropy.

Mendel’s law of independent assortment is not universally accepted and is applied only to genes present on different chromosomes and not to genes present on the same chromosome.

Mendel’s Postulates/Principle Laws : Some geneticists include only two laws in Mendelism - Law of segregation and Law of independent assortment.

Others consider three principle or laws - law of dominance, law of segregation and law of independent assortment.

Presently we have four postulates each called principle or law

• Postulate 1 : Principle of paired unit factors
• Postulate 2 : Principle/Law of dominance
• Postulate 3 : Principle/Law of segregation
• Postulate 4 : Principle/Law of independent assortment

• Nonalleles : Unit factors or alleles present on different loci.
• Isoalleles : Alleles producing similar phenotypes but distinguishable amongst them through changed optima. Like change in biochemicals, temperature, pH, Humidity etc. eg., IA1, IA2, IA3.
• Pseudoalleles : Genes lying side by side, producing related phenotypic effect and distinguishable through a rare crossing over eg., star (dominant) and asteroid (recessive) traits in Drosophila.
• Gene Pool : Its is the aggregate of all the genes and their alleles present in an interbreeding population.
• Cistron : It is a functional segment of DNA which codes for polypeptide, tRNA or rRNA.
• Muton : It is a smallest segment of gene which undergoes mutation.
• Recon : It is a smallest segment of gene which undergoes recombination.
• Ramets : The individuals which are carbon copies of one another and/or the parent are called ramets.
• Replication : It is the process of formation of daughter DNA from the parent DNA.
• Transcription : It is the formation of RNA from a DNA template.
• Translation : It is the synthesis of polypeptide chain from mRNA.
• Split genes : The genes which contain extra or non essential regions interspersed with essential or coding parts. The non essential parts are called introns spacer DNA or IVS (Intervening sequences). Essential or coding parts are called Exons.
• Overlapping genes : They are those genes which overlap with one another.
• Jumping genes (Transposons): They are segments of DNA that can jump or move from one place in the genome to another.

Gene is a piece of DNA having about 500-2000 base pairs. About 100000 genes are present on 46 chromosome in Man.

• Actinobiology : Study of affects of radiations on living organisms.
• Altenberg : Used UV rays and gamma rays as mutagen.
• Stadier : Used X-rays to induce mutations in Barley and Maize. Zech (1970) discovered y-spot. Males are identified by number of Y spots. This is equal to number of y-chromosomes. Auerbach and Robson used mustard gas as first chemical mutagen. Winkler (1916) discovered aneuploidy.

Artificial or man-made Allopolyploids

• Raphanobrassica between Raphanus sativum and Brassica oleracea by Karpenchenko (1927).
• Triticale (=Tritisecale) is a octaploid between Triticum aestivum and Secale cereale by Muntzing (1966).

• Sturtevant (1913) : First chromosome map was given by him.
• Stebbins (1950) : Distinguished three types of polyploids-autopolyploids, allopolyploids and auto allopolyploids.
• Daltonism or Dichromatism : Discovered by Dalton. Congental colour blindness is called Daltonism. Alternate name of red-green colourblindness.
• Protanopia : Red colour blindness.
• Deuteranopia : Green colour blindness.
• Tritanopia : Blue colour blindness.
• Ishihara Cards : Cards used for checking colour blindness.
• Pseudodominance : Phenotypic expression of a recessive allele when the dominant allele has been lost due to deletion.
• Bar-eye : Narrow and smaller eye is called Bar-eye. Normal eye has 780 facets but a bar eye has 200-358 facets. Presence of an extra B-gene on x-chromosome of fruitfly Drosophila results in formation of Bar-eye. The abnormality is due to duplication of gene.

Barr body : Barr and Bertram (1949) found a small dark stained chromatin body adhered to the nuclear membrane of nerve cells of female cats but not in male cats. Number of barr bodies is always one less than the total number of X chromosomes in female. Normal male has no Barr body.

Drum stick : It is related to Blood Corpuscles. The Heteropycnotic X-chromosome occurs as a round pedunculated body attached to the nucleus in called Drum stick. A normal female has 1 drum stick per nucleus of neutrophil. In neutrophils of males drumstick is absent. Sudden change in pH and temperature causes breaking of sugar PO4 bonds. Radiations break chromosomes and bring aberrations. Gamma rays generated by cobalt 60 in gamma garden are used in plants to induce mutations.

Prochromosome : The hereditary material of virus may similarly be considered to be a single chromosome or prochromosome.

Intraspecific hybrids are sterite.e.g.

• Tigon : Male tiger X lioness
• Hinny (Pony) : Male horse X female donkey
• Mule : Mare X Male donkey
• Liger : Male lion X female tiger
• Fairchild Mule : Sweet william (Centaurial) X Carnation (Dianthus plant) by Thomas fairchild.
• Pomato : Potato X Tomato
• Raphanobrassica : Radish X Cabbage
• Kappa particles : Believed to be Symbiont bacterium Caedobacter taeniospiralis which is active in presence of dominant gene K.
• Planosome : Supernumerary chromosome formed due to nondisjunction at the time of meiosis.
• Polysomy : Occurrence of one or more individual chromosomes in excess of normal complement.