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Extra-chromosomal Inheritance

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
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
 Structure, pigments contained & enzymes are affected by mutations
 Chloroplasts are not free from chromosomal genetic apparatus control
 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
Type of plastid
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, green, variegated
Pale, green, variegated
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.
 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
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.