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LECTURE 10: DELAYED CHROMOSOMAL AND EXTRANUCLEARINHERITANCECases of inheritance patterns different from the chromosomal type ofinheritance were reported. Some gave entirely different segregation ratios whileothers observed maternal transmission patterns. This total disregard of theprinciples of segregation and the tendency to look like the mother are effects of twodifferent hereditary processes: delayed chromosomal inheritance and extranuclearinheritance.A.Delayed Chromosomal Inheritance-Characteristics showing delayed inheritance still conform with theprinciples of chromosomal genetics but are sidetracked by the ties to theparent. The ties are usually between the maternal parent and theoffspring.-Such maternal inheritance results from two important features of the eggbut not the sperm:a.The orientation of the mitotic axis;b.The high cytoplasmic continuity between egg and the diploid oocytefrom which the egg came.-Caspari found in 1948 an example of maternal influence in the flour mothEsphestia kuhnielle.-Kynurenin – a diffusible, hormone – like substance.-In some cases, maternal influence does not diminish during thedevelopment but lasts throughout adult life.-This occurs when development is started in a specific direction thatcannot be reversed.-An example is the inheritance of the direction of coiling of the snailLimnea peregra’sshell.-coiling of the shell may be dextral (right) or sinistral (left).a.When reciprocal crosses of dextral and sinistral lines were made, theF1' sshowed the same direction of coiling as the maternal parent.b.TheF2' sproduced by selfing theF1were all dextrally coiled.c.TheF3' ssegregated 3 dextral: 1 sinistral.-The dextral – sinistral alternative depends on a gene pair in which theallele for dextral dominant.-TheF1shells in the reciprocal crosses are determined by the genotypesof the maternal parents; theF2shells are dextral since their maternalparents are all heterozygotes. TheF3progeny reflects 3:1 phenotypicsegregation of the maternal parent of theF2.-The developmental consequences of delayed gene action may also bedetrimental e.g., grandchildless mutation inDrosphila.-Homozygous females are fertile, but the sex organs of their offspring failto mature.-The offsprings are, therefore, all fully sterile, irrespective of the genotypeof the male parent. The effect of the grandchildless gene is transmitted
through the egg cytoplasm where it acts to prevent normal reproductivedevelopment.B.Extranuclear Inheritance-There are cytoplasmic factors which are capable of self – perpetuation andindependent transmission and are therefore considered genetic units fullyequal to those in the chromosome. Because of their location outside thechromosome, these genetic factors have been calledplasmagenes,orplasmons, orcytogensorplasmids.

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