Extra-chromosomal Inheritance Inheritance patterns that differ from chromosomal type of inheritance Hereditary processes that disregard the principle of segregation and the tendency to look like the mother Delayed chromosomal inheritance Still conforms with the principle of chromosomal genetics but is sidetracked by ties to maternal parent Maternal inheritance results from 2 important features of the egg 1. Orientation of mitotic axis 2. High cytoplasmic continuity between egg and diploid oocyte from which the egg came Eg. Flour moth Color of larval skin and eye is controlled by gene A Allele A controls production of a hormone(kynurenin) involved in pigment synthesis Allele a does not elaborate kynurenin Aa X aa - Aa aa A = pigmented a = not pigmented a individuals can’t elaborate kynurenin since they lack A allele, however, they develop some pigment in larval skin and eyes; pigment fades as they grow older and lost in next generation Aa mother includes in her eggs some of the A hormone elaborated in her body. This substance present by maternal influence in the a and A eggs, enables aa offspring to develop some pigments. But aa individuals can’t elaborate continuous supply of kynurenin for themselves and so use up supply transmitted from the mother. Extra-chromosomal inheritance Some maternal inheritance indicate cytoplasmic inheritance (independent of nucleus) Plasmogenes = plasmons = cytogenes =plasmids (genetic units located outside the chromosome) Inheritance through plasmids is maternal because most of zygote’s cytoplasm is derived from egg Chloroplast Structure, pigments contained & enzymes are affected by mutations Chloroplasts are not free from chromosomal genetic apparatus control Plastids Have DNA Have a genetic machinery that lies outside chromosome Eg. variegated 4 o’clock plants (Mirabilis jalopa) With 2 kinds of plastids transmitted only through females Phenotype of plant Pale Green Variegated Type of plastid Pale Green Pale & green Seed from pale plant with pale plastid Seed from green plant with green plastid Seed from variegated plant with pale & green plastids Progeny of intercrosses of 4 o’clock plants Male parent Female parent Pale Pale Green Variegated Green Pale Green Variegated Variegated Pale Green Variegated Progeny Pale Green Pale, green, variegated Pale Green Pale, green, variegated Pale Green Pale, green, variegated Neither the genotype of the male gametophyte nor the gametic constitution of the fertilized egg would be involved in the control of this variation. Mitochondria With DNA that divide & reproduce by themselves Human mitochondrial genome Has 37 genes These genes are important in cellular respiration Some human disorders are associated with mitochondrial mutation Hereditary mitochondrial diseases are transmitted only through the maternal line (spermatozoa hardly contain any mitochondria) Human disorders caused by deletions in mitochondrial genome 1. Progressive external ophthalmoplegia (PEO) Gradual loss of ability to control eye movements 2. Kearn-Sayre syndrome PEO positive Pigmentary d/o in eye Heart disease Cerebellar dysfunction High CSF protein Muscle weakness Hearing loss Diabetes 3. Pearson syndrome Childhood anemia Low number of all blood cells Dysfunction of pancreas. Liver & kidneys 4. Leber hereditary optic neuropathy Degeneration of optic nerve causing rapid onset of blindness Criteria for extra chromosomal inheritance 1. Difference in reciprocal cross results In transmission of characteristics based on chromosomal heredity, reciprocal crosses are identical except in cases of sex-linked genes. 2. Maternal inheritance A characteristic form of difference in results of reciprocal crosses where progenies show characteristics of a female parent Implies transmission through cytoplasm because female gamete provides more cytoplasm than male gamete 3. Non-mappability If chromosomes of an organism are well mapped, a characteristic based on chromosomal heredity should show linkages & should be mapped in reference to other gene-controlled characteristics 4. Non segregation Failure to show segregation indicates extra-chromosomal heredity 5. Non-Mendelian segregation When segregation is inconsistent with chromosome segregation, non-chromosomal factors might have accounted for phenotypic variations 6. Indifference to nuclear substitution When a heritable characteristic persists in the presence of nuclei known to have been associated with alternative characteristic, control of nuclear genetic material over the characteristic maybe ruled out 7. Infection-like transmission When a heritable phenotype is transmitted without nuclear transmission, it seems unlikely that chromosomes control the phenotype.
