Genetics Terms Starting With Z
Genetics Glossary: Z
Zolgensma
/ zol-GEN-smuh / · Coined name: from the gene therapy field
Zolgensma is a gene therapy that delivers a functional copy of the SMN1 gene using an adeno-associated virus vector to motor neurons, treating spinal muscular atrophy type 1 in children under two.
Spinal muscular atrophy type 1 is caused by biallelic mutations in the SMN1 gene, leading to loss of spinal motor neurons and progressive muscle weakness. A single intravenous infusion of Zolgensma delivers AAV9-SMN1 to motor neurons throughout the body, restoring survival motor neuron protein production. Children treated before symptom onset achieve developmental milestones including sitting, standing, and walking that were previously unattainable in this disease.
Zolgensma was approved by the FDA in 2019 and was at the time the most expensive drug ever approved, with a list price of approximately 2.1 million dollars per treatment for the one-time therapy.
Zolgensma does not cure spinal muscular atrophy by correcting the underlying mutation. It delivers an additional functional gene copy alongside the defective one, restoring sufficient SMN protein while the mutant gene remains present.
Infants with SMA type 1 who were treated with Zolgensma before six months of age in clinical trials went on to reach motor milestones such as sitting unassisted and some began walking. The treatment delivers a working SMN1 gene using an AAV9 vector.
Zygosity
/ zy-GOS-ih-tee / · Greek: zygos (yoked) + -ity
Zygosity is the degree of similarity between the two alleles an organism carries at a given locus, classifying the genotype as homozygous, heterozygous, or hemizygous.
In clinical sequencing, zygosity helps distinguish unaffected carriers from affected individuals in recessive disorders. Two pathogenic variants in the same gene may cause disease only when they occur on opposite homologous chromosomes, a configuration called trans. Hemizygosity is reported for X-linked genes in males and for chromosomal deletions, where only one allele remains.
Laboratories infer zygosity by combining read depth, allele fraction, pedigree data, and sometimes parental testing because sequencing alone may not show whether two variants lie on the same chromosome.
Twin zygosity testing using microsatellite or SNP panels can reliably distinguish identical monozygotic twins from fraternal dizygotic twins, information important for organ donation compatibility and medical history interpretation.
Zygosity is not the same as genotype. Genotype specifies which alleles are present, while zygosity describes the relationship between the two alleles at a locus, whether they are identical or different.
Loss of heterozygosity at the RB1 locus in retinoblastoma converts a cell heterozygous for a germline tumor suppressor mutation to homozygous or hemizygous, abolishing all functional retinoblastoma protein. This loss removes the remaining working copy of the gene.
Zygote
/ ZY-goht / · Greek: zygotos (yoked together)
Zygote is the diploid cell formed by the fusion of a male gamete and a female gamete at fertilization, representing the first cell of a new organism.
The zygote contains the full diploid complement of chromosomes, with one set contributed by each parent through the haploid gametes. After fertilization, the zygote undergoes rapid mitotic divisions called cleavage to produce the multicellular embryo, with each daughter cell inheriting the same genetic information. The totipotent zygote has the unique capacity to give rise to every cell type in the organism including extraembryonic placental tissues.
In humans, the zygote is about 100 micrometers in diameter, similar in size to the unfertilized egg from which it forms, as the sperm contributes almost no cytoplasm during fertilization.
A zygote is not the same as an embryo. The zygote is specifically the single-celled stage immediately after fertilization.
In vitro fertilization can create a zygote when a sperm fertilizes an egg in the laboratory or when a single sperm is injected directly into an egg. The zygote contains genetic material from both parents.