The vagaries of non-traditional mendelian recessive inheritance in uniparental disomy: AA x Aa = aa !

The vagaries of non-traditional mendelian recessive inheritance in uniparental disomy: AA x Aa = aa !


Eric Engel M.D.

Department of Medical Genetics
Geneva University Hospital

January 2005



A situation of Non-Traditional Mendelian Inheritance is achieved whenever an exceptional mode of recessive transmission, distinctly alien to the usual pattern, leads paradoxically to a similar phenotype.

Laws and Out-Laws in Mendelian Recessive Inheritance
Two basic traditional tenets of the Mendelian Laws of Inheritance are the segregation (separation) and the independent assortment of alleles.
In the Non-Traditional Mode of Mendelian Inheritance, the non-congruent path of transmission usually resorts to a stratagem, the meiotic mis-segregation of alleles followed by a revised assortment at mitosis. Both steps may end-up in the uniparental transmission of allele pairs while euploidy is maintained, a situation referred to as Uniparental Disomy.

Exemples of Non-Traditional Mendelian Inheritance causing genetic harm through UPD
1) duplication of a same dominant mutant (must often be lethal)
2) duplication of the same recessive mutant
3) overdose of the duplicated active domain of a parentally imprinted area
4) loss of a singly active domain of a parentally imprinted area
5) combination of 3) and 4)
6) combination of 2) with 3) or 4) or 5) - quite rare

What is Uniparental Disomy (UPD)?
UPD is the occult presence of a chromosome pair inherited from only the mother or father in a diploid conceptus (or cell-line).

How does UPD come about?
A two-step twist of chromosomal inheritance (usually one step occurring at meiosis, the other at mitosis) leads to a diploid state with one pair issued from one parent only.

What is the meiotic error leading to UPD?
The usual meiotic process entails the same one that leads to zygotic trisomy or monosomy (i.e. MeI or MEII non-segregation).

What is the mitotic step superseding the meiotic one and resulting in UPD?
An early somatic error, namely a mitotic non-disjunction or a chromosome lag restoring diploidy.

What is the end-result of the two-step process leading to UPD?
- Trisomy rescue by loss of one or the other member of the uniparental pair, (thus restoring the biparental parity) or by the loss of the normaly inherited chromosome (leaving in place the monoparental pair).
- Monosomy duplication (always resulting in a monoparental pair).

What may be the broad genetic make-up of the uniparental pair?
The overall content of the pair be one of isodisomy (i.e. both members are a carbon copy of one over the other) with complete allelic sameness as seen in the process resulting of mitotic monosomy duplication...
...or one of heterodisomy (non-identical allelism between homologous members) as may occur for the complete lack of crossing-over of two homologues failing MeI segregation. Aside from these extreme examples, the UPD content resulting from meiotic non-segregation is often a mix-pot of iso- and heterodisomy.

Definition of isodisomy
The homoallelic duplication of a chromosome or a chromosome segment in one pair of a diploid individual or in a 2n cell-line or a 2n cell is called isodisomy.

NOn-TRAditional Mendelian Inheritance (NOTRAMI)!
Conditions for the occurrence of an isodisomic recessive trait:
1) Inheritance of a UPD pair showing duplication of a same chromosome or duplication of chromosome segments shared by homologues as a result of crossing over.
2) Presence of a locus with a recessive mutant on that chromosome or segment.

What can bring about duplication of a same segment (isodisomy) in a uniparentally inherited pair:
1) Transmission at normal MeII of a pair including the two same crossed-over segments derived from a tetravalent, which failed MeI segregation.
2) Transmission of the non-crossed-over segment of the chromatids of a reduplicated chromosome, which failed MeII segregation.
3) The mitotic non-disjunction of a reduplicated monosomic member.
Note: the centromeric and juxta-centromeric areas of a tetravalent do not cross-over and stay heterodisomic. As a result, the above areas of a MeII reduplicated chromosome remain isodisomic. The typing of these areas with appropriate polymorphisms allows to distinguish between MeI and MeII non-disjunction but not between MeII and mitotic non disjunction.

Reduction to homozygosity by isodisomy formation
Depending on the dual meiotic-mitotic comedy of errors, an F1 individual can become homozygous for a recessive allele, if this allele gets duplicated in a uniparental pair. Thus, the singly heterozygous partner of a couple (Aa)* can sire a homozygous child (aa)** by "reduction to homozygosity" as shown below:

Specific factors potentially favoring complementation of aneuploid gametes *
a) A high frequency of germ cell aneuploidy targeting a same homologous member chromosome in both sexes causing a number of gametes to be either disomic or nullisomic for that chromosome (scarce evidence).
b) Presence in both parents of a balanced translocation involving a same homologous member favoring unbalanced segregation of the translocated partner in the germ cells in a complementary pattern (some very suggestive evidence).
(*Probably the least common pathway to UPD)

Sometime the UPD does not involve the whole of a chromosome and remains confined to a segment of a pair as it arises from a somatic crossing over between two homologous non-sister chromatids. When interstitial, the segmental UPD results from two symmetrical breaks, which are shown below as the result of an "interchromatid kiss"! Mitotic segregation of the duplicated chromosomes, thereafter leads to mosaicism with one native and one reshuffled balanced cell line.

In other instances the segmental UPD is terminal and results from a single symmetrical break in each of two homologous non-sister chromatids, as seen below. Mosaicism involving two somatic cell types also results from this.

On the slide below are presented examples of both types of segmental UPD, terminal or interstitial, as found for various chromosomes, 4, 6, 7, 11, 14 and 20.
Some were discovered because of reduction to homozygosity causing recessive traits, while others involved imprinted domains and disrupted them.

Uniparental disomies for chromosome 1 as a cause of recessive phenotypes

Aneuploid mode and mood of chromosomes:
The results for chromosome 1 UPD's emphasize the major role of maternal meioses errors causing gamete nullisomy and disomy in cases of either maternal or parental UPD1 (8 cases). Paternal meioses errors also account for 4 cases.

The perfect culprit for Non-Traditional Mendelian Inheritance in UPD
-A chromosome from a disomic or nullisonic ovum responsible after fertilization for trisomies or monosomies respectively liable to rescue by mitotic deletion or duplication.
-A large member of the set.
-A member carrying a locus with a common mutation or an imprinted domain.

Factors influencing the chance occurrence of trisomy rescue
a) The relatively large pool size of the trisomy liable to rescue (i.e. 16, 15, 21, 22, X).
b) The factors modulating the size and nature of such a pool:
- the aneuploid mode and mood of specific chromosome members
- maternal age
- abnormal segregation of common centric fusions
- etc...

At meiosis, what affects the occurrence and genetic content of the non-disjoined pair?
- Some ill-understood specific factors of the chromosome member implicated which I have just called the aneuploid mode and mood of that particular chromosome.
- Recombination which may stay normal or be perturbed in the process.

The variable aneuploid mode and mood at the origin of numerical abnormalities of different chromosome numbers of the set
- Great difference in the incidence of trisomy for different chromosomes *
- Variable association with increased parental (maternal) age *
- Absence of all monosomies except for the X chromosome *
- Differential selection and survival at different stages of pregnancy *
- Different frequencies of non-disjunction among different chromosomes *
And, consequently, great differences in the chances of UPD formation and isodisomy for individual numbers.
(*Jacobs PA, Hassold TJ. Adv Genet. 1995;33:101-33.)

Aneuploid mode and mood of some clinically significant chromosomes

Except for the 45,X, most cases are of maternal origin, the meiotic stage of non-disjunction is variable for different numbers (from 100% MeI to 66% MeII) and the lethality rate quite disparate.

Possible end-points of the various factors affecting transition to isodisomy in the formation of uniparental pairs
1) Some mechanisms rule out or lower the chances of a transition from hetero- to isodisomy and minimize the risks of transmission of a recessive allele.
2) Other mechanisms carry-out a higher to much higher risk of isodisomy along with an increased chance of homozygosity for a recessive allele.

What suppresses or lowers the risk of isodisomy as a cause of reduction to homozygosity in UPD?
1) an MeI nulli-chiasmate or pauci-chiasmate synapsis (i.e. an absolute or relative recombination failure of homologues) followed by MeI non-segregation and normal MeII separation.
2) an MeI failure of synapsis (i.e. the lack of homologous pairing) followed by the same pole migration of the two resulting univalent an normal MeII separation.

On the following few slides, we show some specific examples of the role of the aneuploid mechanisms on the production of UPD for chromosomes 21, 1, 7 and 15:


1) Presence of a uniparental pair in a diploid genome (UPD) results from an aberrant mode of transmission separate from Traditional Mendelian Inheritance. Instead of the classic tenets of allele segregation and independent assortment, an abnormal and complex pattern of segregation leads to this unusual unilateral assortment of alleles in the offspring.
2) The phenotype effect of this odd transmission then depends on the character -normal, mutant or imprinted- of the mis-assorted gene(s).
3) In the case of recessive inheritance, a singly heterozygous parent may sire a homozygous affected child. This will occur when the uniparental pair is homoallelic (isodisomy).
4) Isodisomy in uniparental pairs depends on various factors such as the aneuploid ways and means of the chromosome number involved, the recombination process, the meiotic disjunctional pattern and the post-zygotic mitotic adjustment of the chromosomes which all concur in shaping the end-product of the uniparental pair and its genetic impact.
5) To this day, some thirty different recessive phenotypes have been traced once or a few times to the presence of uniparental pairs. The good thing about the uniparental inheritance of recessive traits is that their risk of recurrence is almost nil.

Illustrations by Mr Jean-Claude Malgouyres

Written2005-01Eric Engel
of Medical Genetics, Development, University of Geneva, Geneva, Switzerland


This paper should be referenced as such :
Engel, E
Some vagaries of non-traditional mendelian recessive inheritance in uniparental disomy: AA x Aa = aa !
Atlas Genet Cytogenet Oncol Haematol. 2005;9(1):63-72.
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