Delayed Chromosomal & Extra Chromosomal Inheritance

• Delayed Chromosomal Inheritance

  • Characteristics showing delayed inheritance still conform with the principles of chromosomal genetics but are sidetracked by the ties of the parent.
• DELAYED CHROMOSOMAL INHERITANCE

  The ties are usually between the maternal parent and the offspring.
• 2 important features of the egg but not the sperm

  • The orientation of the mitotic spindle axis
  • The high cytoplasmic continuity between the egg and the oocyte with very little or no contribution from the sperm.
• Caspari (1948)

  maternal influence in the flour moth Esphestia kuhnielle.
• The colors of the larval skin and eye of this moth are controlled by the gene A, where A is pigmented and a is not pigmented.
• The allele A controls the production of Kynurenin – a diffusible, hormone – like substance, involved in pigment synthesis while a does not elaborate kynurenin.
• DELAYED CHROMOSOMAL INHERITANCE

  In some cases, maternal influence does not diminish during the development but lasts throughout adult life.
• DELAYED CHROMOSOMAL INHERITANCE

  This occurs when development is started in a specific direction that cannot be reversed.
• DELAYED CHROMOSOMAL INHERITANCE

  An example is the inheritance of the direction of coiling of the snail Limnea peregra’s shell.
• coiling of the shell may be dextral (right) or sinistral (left).
• a.When reciprocal crosses of dextral and sinistral lines were made, the F1 's showed the same direction of coiling as the maternal parent.

  
  

  b. The F2 's produced by selfing the F1 were all dextrally coiled.

  
  

  c. TheF3's segregated 3 dextral: 1 sinistral.
• dextral – sinistral alternative

  depends on a gene pair in which the allele for dextral dominant.
• The F1 shells in the reciprocal crosses are determined by the genotypes of the maternal parents.

  The F2 shells are dextral since their maternal parents are all heterozygotes.

  The F3 progeny reflects 3:1 phenotypic segregation of the maternal parent of the F2 .
• The developmental consequences of delayed gene action may also be detrimental e.g., grandchildless mutation in Drosphila.
• Homozygous females are fertile, but the sex organs of their offspring fail to mature.
• The offsprings are, therefore, all fully sterile, irrespective of the genotype of the male parent.
• The effect of the grandchildless gene is transmitted through the egg cytoplasm where it acts to prevent normal reproductive development.
• EXTRACHROMOSOMAL INHERITANCE

  There are cytoplasmic factors which are capable of self – perpetuation and independent transmission and are therefore considered genetic units fully equal to those in the chromosome. Because of their location outside the chromosome, these genetic factors have been called plasmagenes, or plasmons, or cytogens or plasmids.
• EXTRACHROMOSOMAL INHERITANCE

  Extranuclear inheritance or inheritance through plasmids tends to be maternal because most of the zygote’s cytoplasm is derived from the egg. Therefore, reciprocal crosses give different results, a situation like delayed chromosomal inheritance.
• EXTRACHROMOSOMAL INHERITANCE

  The unusual phenotypic ratios do not disappear after one generation.
• EXTRACHROMOSOMAL INHERITANCE

  Plasmid inheritance implies perpetuation through DNA replication; hence, it is a second system for the transmission of traits.
• Cytoplasmic Inheritance

  In Chlamydomonas – a single celled green alga, Sager and her co – workers found in 1960 a number of hereditary variables that failed to show segregation in a chromosomal pattern.

  For example, streptomycin resistance (sr) or sensitivity (ss) appears to be inherited in a regular Mendelian fashion so that sr x ss produces 1⁄2 sr and 1⁄2 ss offspring.
• Cytoplasmic Particles

  • Sonneborn in 1943 studied the inheritance of the killer vs. sensitive trait in Paramecium aurelia.
  • He found that to be a killer, a Paramecium must have the gene K (which segregates in the typical chromosomal fashion), and a complement of cytoplasmic particulate material called kappa.
  • Sensitive animals are those that lack of kappa.
• Cytoplasmic Particles

  • Killer animals could maintain as many as 1600 kappa particles, each measuring about 0.2 micron in diameter. Each kappa particle contains DNA, indicating some hereditary independence.
  • The cytochrome pigments that kappa utilizes in oxygen respiration are different from those of the host. Therefore, since Paramecium exists quite well without kappa, its presence seems to be of an accessory organism or symbiont.
• Chloroplast

  • Chloroplasts are responsible for photosynthesis.
  • The structure of these organelles, the pigments contained, and their enzyme systems can all be affected by mutations indicating that chloroplasts are not free from chromosomal genetic apparatus control.
  • Mature plastids arise from pro-plastids, which can divide and structures, they are capable of self-replication.
  • Certain plastid differences are not transmitted along chromosomal lines.
• Chloroplast

  Plastids contains DNA, it may then be concluded that plastids have a genetic machinery of their own, machinery that lies outside the chromosome e.g., variegated four o’clock (Mirabilis jalapa).
• Mitchondria

  -In baker’s yeast, Saccharomyces cerevisae, Ephrussi and his co – workers were able to identify in 1951 three petite varieties.
• Segregational (nuclear) petites

  • when crossed with the wild type, produce ascospores which segregate in the ratio 1 petite: 1 normal. This petite characteristic is chromosomally determined trait.
• Neutral petites

  • when mated with normal strains, will produce only normal or wild type ascorpores and colonies. In further generations, the petite characteristic never reappears and seems to have been physically lost. This behavior indicates an extrachromosomal inheritance. A heterokaryon test further proved this.
• Suppressive petites

  suppress normal respiratory behavior in crosses with the normal strains so that most of diploid cells derived from a zygote are petites. Most of them, when induced to sporulate in a special environment, give rise to asci with four petite spores. The suppressive petite factor, therefore, acts as a dominant trait.
• mitochondria

  also have their own DNA and they have been known to divide or reproduce by themselves. This continuity of the mitochondria and the mitochondrial DNA explains the cytoplasmic continuity of the neutral and suppressive petites.
• The human mitochondrial genome includes only 37 genes.
• Thirteen encode proteins. The gene are important in cellular respiration.
• In 1988, the first human genetic disorder associated with mitochondrial mutation was identified.
• Hereditary mitochondrial diseases are transmitted only through the maternal line, since spermatozoa contain hardly any mitochondria.
• Progressive external opthalmoplegia

  gradual loss of the ability to control eye movements
• Kearn-Sayre Syndrome

  progressive external opthalmoplegia, pigmentary disorders in the eyes, heart disease, cerebellar dysfunction, high cerebrospinal fluid protein, muscle weaknesses, hearing loss, diabetes.
• Pearson Syndrome

  occurs during childhood characterized by a combination of symptoms like anemia, reduction in the numbers of all blood cells, dysfunction of the pancreas, liver and kidneys.
• Leber Hereditary/Optic Neuropathy


  degeneration of the optic nerve that causes rapid onset of blindness (usually in men in their 20s).
• Cytoplasmic-Nuclear Male Sterility

  • Cytoplasmic male sterility (CMS) has been observed in many plant species. Male sterility is due to some cytoplasmic factors (e.g., genes in the mitochondrial genome).