The human genome is comprised of nuclear DNA sequences, tightly packed into distinct subunits called chromosomes and hundreds to thousands of circular DNA molecules within the mitochondrion. DNA sequences that encode for proteins account for few percent of the total genome, and the rest of the genome is involved in coding for various RNA molecules that do not code for protein (noncoding RNA) as well as regulatory function. DNA packed into chromosomes is coated with histone and nonhistone proteins, and these proteins play an important role in the regulation of gene expression. There are a total of 22 autosomal chromosomes (1–22) and the sex-determining X and Y chromosomes (Figure 2-1). The gametes, eggs and sperm, each contain a haploid set of 23 different chromosomes that upon fertilization gives rise to the diploid set, 46 chromosomes. In a normal diploid human cell, 23 chromosome pairs are present: 22 pairs of autosomes and two sex chromosomes—XX in females and XY in males. In each individual, each member of a pair is derived from either the father or the mother.

Chromosomes must be replicated at high fidelity and separated into their daughter cells with each cell division. Centromeres and telomeres are important specialized DNA structures on chromosomes. DNA replication during the interphase stage of mitosis produces two copies of a chromosome (sister chromatids) that are connected at the centromere (Figures 2-1 and 2-2). The kinetochore, a complex of proteins that interact with the centromere, serves as the attachment point for the spindle fibers during cell division and enable separation of sister chromatids into individual cells. Chromosome fragments that lack a centromere (acentric fragments) do not become attached to the spindle, and fail to be included in the nuclei of the daughter cells. Telomeres are located at the end of each chromosome, are composed of DNA and protein complexes and serve as caps to maintain chromosome integrity and to prevent chromosomes from fusing and from degradation. The telomere consists of a simple 5′-TTAGGG-3′ sequence that is repeated thousands of times. With each round of DNA replication in a somatic cell, telomeres are shortened and finally lost, providing a mechanism for natural aging and death.¹

Meiosis is a specialized cell division that generates haploid gametes ready for fertilization, and consists of meiosis I and meiosis II. Meiosis I, also known as a reductional division, reduces the number of chromosomes from 46 (92 chromatids) to 23 (46 chromatids), The sister chromatids will separate and segregate to daughter cells during meiosis II to produce four haploid cells containing 23 chromosomes. Meiosis I prophase is the most unique aspect of meiosis and is composed of leptotene, zygotene, pachytene, diplotene, and diakinesis stages. During the first four stages homologues pair, and recombine. In a female, meiosis I commences during first trimester in utero and arrests in diplotene stage of meiosis I, prior to the formation of primordial follicles (circa 20 weeks of gestation). Meiosis I in females resumes after puberty in response to hormonal stimulation. Meiosis II completion requires fertilization. In males, meiosis I commences at the time of puberty, and spermatogonial stem cells continue to supply male gonads with spermatocytes that continually enter meiosis to generate haploid spermatozoa. Females on the other hand do not have comparable stem cells and are born with a finite pool of oocytes that is significantly depleted by age 30 and only a few oocytes remain by the time of menopause (51 years of age).

In oogenesis, after the first meiotic division, two daughter cells greatly differ in size. A large secondary oocyte enters meiosis II, while a small cell (polar body) degenerates. A second polar body is produced after fertilization and completion of the second meiotic division. In females, one oocyte gives rise to one mature egg for fertilization and two polar bodies, while one spermatocyte gives rise to four spermatozoa.

Meiotic Nondisjunction

Meiotic chromosome segregation errors increase dramatically with maternal age. Meiosis I errors are the predominant cause of aneuploidy (80%–90%) (Figure 2-3). Although pregnancy loss can occur at any gestational age, most conceptions are clinically unrecognized pregnancies in which a fertilized egg fails to implant in the uterus, or a pregnancy is lost shortly after implantation (biochemical pregnancy).^2,3 Meiotic nondisjunction is usually a random error in the oocyte or sperm and, therefore, the recurrence risk is approximately 1%. However, some couples may carry a genetic predisposition for nondisjunction due to mutations in genes that regulate meiosis, and they will suffer higher rates of recurrent miscarriages and aneuploid pregnancies. In other couples that have recurrence of aneuploidy with the same chromosome, germline mosaicism may be the cause. Germline mosaicism, also known as gonadal mosaicism, means that a subset of germ cells is genetically abnormal. The risk of transmission will depend on the percent of affected germ cells in the gonad. In approximately 5% of young couples with a previous child with Down syndrome, one of the parents will have germline mosaicism for trisomy 21.^4,5

Uniparental Disomy

Uniparental disomy (UPD) is a condition in which both homologues of a chromosome or a chromosome segment are inherited from only one parent. UPD is encountered in approximately 1:3500 newborns. In general, it requires two independent nondisjunction events to produce UPD. As a result of nondisjunction during meiosis I, both chromosomes are transmitted to an egg or sperm (Figure 2-3A). These disomic gametes have a pair of nonidentical chromosomes (nonidentical due to recombination) originated from the two homologues (heterodisomy). In contrast, errors in meiosis II result in isodisomy, due to a nondisjunction of sister chromatids (Figure 2-3B). Fertilization of the disomic gametes would result in trisomy; however, loss of one of the three chromosomes early in mitosis (trisomy or embryo rescue) can lead to UPD (Figure 2-4). UPD may also occur as a result of a monosomy rescue, when a nullisomic gamete is fertilized by a normal haploid gamete, or when a normal zygote loses a chromosome due to mitotic nondisjunction. A single parental homologue is replicated postzygotically, leading to whole chromosome isodisomy. UPD can involve an entire chromosome or a segment (segmental UPD). UPD is classified as maternal (mat) or paternal (pat), depending on the origin of the disomic chromosome. It is further classified as uniparental heterodisomy or uniparental isodisomy.⁶ In approximately 70% of UPD cases the karyotype is normal, while in 30% of patients UPDs are seen in association with numerical or structural chromosomal abnormalities.⁷

UPD caused by isodisomy is associated with an increased risk for a recessive disorder, while heterodisomy for the majority of chromosomes does not cause health problems. A few chromosomes (6, 7, 11, 14, 15, and 20) contain genes and regulatory elements that are differentially expressed depending on whether the chromosome was inherited from the father or mother. If a paternal or maternal imprinted chromosome is missing due to UPD, the individual will be affected with one of the various imprinting disorders (Table 2-1. As an example, Prader-Willi syndrome is caused by a deletion of the 15q12 region on the paternal chromosome or by maternal UPD (due to absence of paternal chromosome 15).

The diagnosis of UPD requires molecular testing. Microarrays containing SNP probes allow genotype analysis and can detect long contiguous regions with homozygosity (absence of heterozygosity or AOH) suggestive of identical DNA sequences. The presence of AOH regions on a single chromosome is consistent with uniparental isodisomy (Figure 2-4). SNP-containing microarrays can identify isodisomy, but they are unable to detect whole chromosome uniparental heterodisomy. Although both chromosomes are inherited from the same parent, they are not identical, so genotype comparison between the child and parent is required to detect uniparental heterodisomy.

Multiple constitutional regions of homozygosity on the same chromosome may represent UPD and can be confirmed by methylation analysis of the imprinted genes, microsatellite or microarray SNP genotype analysis of parental and patient samples. In contrast, multiple AOH regions on multiple chromosomes are characteristic of consanguineous relationship (Figure 2-5).