delayed chromosomal and extra chromosomal inheritance

DELAYED CHROMOSOMAL AND EXTRACHROMOSOMAL

INHERITANCE

Topics:

Delayed Chromosomal Inheritance Extrachromosomal InheritanceCriteria for Extrachromosomal

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

The ties are usually between the maternal parent and the offspring

Delayed Chromosomal Inheritance Such maternal inheritance results from 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

Delayed Chromosomal Inheritance Caspari found in 1948 an example of

Maternal influence in the flour moth 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, involved in pigment synthesis while a,does not elaborate kynurenin

Delayed Chromosomal Inheritance

aa female mated with Aa male

a a

A Aa Aa

a aa aa

Pigmented

Non

Pigmented


Aa female mated with aa male

A a

a Aa aa

a Aa aa

Pigmented Pigmented

Delayed Chromosomal Inheritance The aa individuals cannot elaborate

kynurenin since they lack the allele A. However these aa individuals developed pigments as larvae in their skin and eyes.

The pigment fades and disappear as they grow older

These results are explained by the fact the Aa mother includes in her eggs some of the A hormones elaborated in her own body

Delayed Chromosomal Inheritance These substance present by maternal

influence on the a, as well as in the A eggs, enables the aa offspring to develop some pigment.

But the aa individuals, being unable to elaborate a continuing supply of the hormone themselves, dilute and use up supply that was transmitted by the mother, the effect therefore is transient


A. E. Boycott (1920s) First to study an example of maternal effect Involved morphological features of water

snail Limnea peregra Shell and internal organs can be either right- or

left-handed Dextral or sinistral, respectively Determined by cleavage pattern

of egg after fertilization Dextral orientation is more

common and dominant

Delayed Chromosomal InheritanceA. E. Boycott (1920s) Began with two different true-

breeding strains One dextral, one sinistral

Dextral ♀ x sinistral ♂ dextral offspring

Reciprocal cross sinistral offspring Contradict a Mendelian pattern of

inheritance

A. E. Boycott (1920s); Alfred Sturtevant (1923)

Sturtevant proposed that Boycott’s results could be explained by a maternal effect gene Conclusions drawn from F2

and F3 generations Dextral (D) is dominant to

sinistral (d) Phenotype of offspring is

determined by genotype of mother


EXTRACHROMOSOMAL

INHERITANCE

Extrachromosomal Inheritance Some maternal inheritance indicates

cytoplasmic influence which is independent of the nucleus.

But this is not self perpetuating and disappears in the subsequent generations

Plasmids / Plasmagenes / cytogens or plasmons however are capable of self perpetuation and independent transmission and may therefore be considered as genetic units fully equal to those in the chromosomes

Extrachromosomal Inheritance Extrachromosomal inheritance or

inheritance through plasmids tends to be maternal because most of the zygotes cytoplasm is derived from the egg.

Therefore reciprocal crosses give different results, a situation similar to delayed chromosomal inheritance

1. Cytoplasmic Inheritance2. Cytoplasmic Particles

3. Chloroplast

4. Mitochondria

Cytoplasmic Inheritance

In Chlamydomonas ( a single celled green alga) for example, streptomycin resistance (sr) or sensitivity (ss) appears to be inherited in a regular Mendelian fashion so that srxss produces ½ sr and ½ ss offspring.


One resistant strain, sr-500 acts diferently,

If sr-500 is mt+ and I ss is mt- all offspring are sr

However on a reciprocal cross ss mt- x ss mt+ all offspring are ss.


Since such result cannot be explained on the basis of chromosomal segregation, in which 1 sr: 1 ss ratio is expected, they are ascribed to an extrachromosomal factor transmitted only through the plus mating type

Cytoplasmic Particles

Sonneborn in 1943 studied the inheritance of the killer vs sensitive trait in paramecium aurelia.

To be a killer, paramecium must have the gene K and a complement of cytoplasmic particulate material called kappa.

Sensitive animals are those that lacks kappa.

Genotype kk cannot produce kappa, only the genotype KK and Kk can

Cytoplasmic Particles

For example: conjugation of killer (KK) and sensitive (kk) strains produces exconjugants, with very little or no exchange of cytoplasm.

Separate killer and sensitive clones are produced depending on the parent from whom they were derived

Induces autogamy or self fertilization of the killer exconjugant will produce the homozygotes KK and kk which will give rise to killer and sensitive clones respectively


Autogamy of the sensitive exconjugant gives rise to sensitive clones only, in spite of the segregation of KK and kk.

Chloroplast

Chloroplast The structure of this organelles, the pigment

contained and their enzymes 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 are capable of dividing and thereby increase in number

Since plastids arise from pre-existing structures they are capable of self replication

Moreover plastids are not transmitted along chromosomal lines

Extrachromosomal Inheritance Example in four o’ clock plant

Male parent Female Parent Progeny

Pale PaleGreen

Variegated

PaleGreen

Pale, Green & Variegated

Green PaleGreen

Variegated

PaleGreen


Varieted PaleGreen

Variegated

PaleGreen


Extrachromosomal Inheritance(Chloroplast) Seeds from the pale plant would have

only the pale plastid type: those from green, only green; those from variegated, either pale, green or mixture of the two types.

Neither the genotype of the male gametophyte nor the nuclear genetic constitution of the fertilized egg would be involved in the control of this variation

Mitochondria

In baker’s yeast, Saccharomyces cerevisiae, Ephrussi and his co-workers wre able to identify in 1951 three petite varieties.

1. Segregational (nuclear) petites2. Neutral Petites3. Suppressive petites

Mitochondria

1. 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

Mitochondria

Neutral petites when mated with normal strains, will

produce only normal or wild type ascospores and colonies.

In further generations, the petite characteristic never reappears and seems to have been lost

This behavior indicates an extrachromosomal inheritance

Mitochondria

Suppressive petites suppress normal respiratory behavior

in crosses with the normal strains so that most of the diploid cells derived from a zygote petites

Suppressive factor therefore acts as a dominant trait

Mitochondria

The petite characteristic of yeast has been attributed to the deficiencies of cytochromes b,c and cytochrome oxidase a, a3 that are normally found in the inner membrane of mitochondria

The 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

Criteria for Extrachromosomal

Inheritance Differences in reciprocal cross results When one allows the transmission of

characteristics based on chromosomal heredity, the reciprocal crosses are ordinarily identical, except in the case of sex-linked genes

Non Mappability If the chromosomes of an organism are well

mapped, a characteristic based on chromosomal heredity should show linkages and should be mapped in reference to the other gene controlled characteristics


Inheritance Maternal Inheritance A characteristic form of difference in the

results of reciprocal crosses is maternal inheritance, where the progenies show the characteristics of their female parent. If chromosomal differences can be ruled out, maternal inheritance can be usually implies transmission through the cytoplasm. This is because the female gamete ordinarily provides more cytoplasm to the zygote than the male gamete does.


Inheritance Non Segregation Failure to show segregation under

appropriate circumstances may indicate extrachromosomal heredity

Non Mendelian Segregation When segregation occurs but in the

fashion inconsistent with chromosome segregation, non-chromosomal factors might have accounted for the phenotypic variations


Inheritance Indifference to nuclear substitution When a heritable characteristics persists in

the presence of nuclei known to have been associated with alternative characteristics, the control of the nuclear genetic material over the characteristic may be ruled out

Infection-like transmission When a heritable phenotype is

transmitted without nuclear transmission, it seems unlikely that chromosomes control that phenotype