Genetics Terms Starting With S

Segregation is the separation of the two alleles of a gene into different gametes during meiosis, ensuring that each gamete carries only one allele for any given locus.

Segregation is the molecular consequence of homologous chromosomes separating during meiosis I. Each gamete produced by a heterozygote receives exactly one of the two alleles, so the allele ratio among gametes is always 1:1. This 1:1 gametic ratio produces the 3:1 phenotypic ratio Gregor Mendel observed in the F2 generation of his pea plant crosses in the 1860s, a pattern he described before the cellular mechanism was understood.

When two F1 heterozygotes are crossed, each parent contributes either allele with equal probability, yielding on average one homozygous dominant, two heterozygous, and one homozygous recessive offspring for every four progeny.

Selective Sweep is the reduction in genetic diversity at and around a chromosomal locus caused by the rapid spread of a strongly favored allele through a population, which carries neighboring linked variants to high frequency along with it.

When a new beneficial mutation arises on a single chromosome, natural selection increases its frequency in the population over successive generations. Because the mutation is physically linked to surrounding DNA on that chromosome, nearby variants hitchhike to high frequency alongside it, a process called genetic hitchhiking. Scanning genomes for unusually long stretches of low variation allows population geneticists to detect the resulting valley of reduced nucleotide diversity flanking the selected site.

Lactase persistence in Northern European populations swept to high frequency in fewer than 7,000 years following cattle domestication, producing one of the strongest and best-documented selective sweeps in the human genome.

Sex Chromosome is a chromosome that differs between the sexes of a species and carries genes involved in determining biological sex, designated X and Y in mammals.

In humans, females carry two X chromosomes and males carry one X and one Y. The Y chromosome carries the SRY gene, which triggers male sexual development in early embryos by activating a cascade of downstream transcription factors. Genes on the X chromosome are present in two copies in females but only one in males, leading to hemizygous expression of X-linked traits in males and making them more susceptible to X-linked recessive conditions such as hemophilia A.

To equalize X-linked gene dosage between the sexes, one X chromosome in each female somatic cell is inactivated early in development through a process called X-inactivation, leaving a condensed Barr body visible in the nucleus.

Sickle-Cell Disease is an autosomal recessive disorder caused by a point mutation in the beta-globin gene that produces an abnormal hemoglobin causing red blood cells to deform into a rigid sickle shape.

The sickle mutation substitutes valine for glutamic acid at position 6 of the beta-globin chain, causing deoxygenated hemoglobin S molecules to polymerize into fibers that distort red blood cells. Sickled cells block small blood vessels, causing painful episodes called vaso-occlusive crises, organ damage, and anemia. Heterozygous carriers are largely unaffected but show resistance to severe malaria, explaining the high frequency of the sickle allele in malaria-endemic regions of sub-Saharan Africa and the Mediterranean.

Silent Mutation is a nucleotide substitution that changes a codon to a synonymous codon encoding the same amino acid, producing no change in the protein sequence.

Silent mutations are possible because the genetic code is degenerate, with most amino acids encoded by more than one codon. Although the protein sequence is unchanged, silent mutations can sometimes affect gene expression by altering mRNA stability, splicing signals, or translational efficiency. Codon usage bias, well documented in organisms such as Escherichia coli, reflects selection pressure on synonymous codons that affect translation speed and accuracy.

Silent mutations accumulate at a higher rate than nonsynonymous mutations because they are largely invisible to natural selection.

SNP is a variation at a single nucleotide position in the genome that differs between individuals in a population and represents the most common form of genetic variation in most species.

The human genome contains roughly 10 million SNPs, occurring approximately once every 300 base pairs on average. SNPs in coding regions can alter amino acid sequences, while those in regulatory regions can affect gene expression levels or timing. Genome-wide SNP genotyping arrays are used extensively in genome-wide association studies to identify genetic variants associated with diseases, traits, and drug responses.

A single SNP in the CYP2C19 gene, for example, reduces the enzyme’s ability to convert the antiplatelet drug clopidogrel to its active form, and carriers of this variant have a measurably higher risk of cardiovascular events when treated with standard doses.

Somatic Mutation is a DNA sequence change that occurs in a non-germline body cell after fertilization and is not transmitted to offspring, though it can be passed to daughter cells through mitosis.

Somatic mutations accumulate throughout life because of replication errors, environmental mutagen exposure, and spontaneous DNA damage. When a somatic mutation strikes a proto-oncogene or tumor suppressor gene, it can initiate or promote cancer development. Whole-genome sequencing of human tumors has revealed that some cancers, such as UV-exposed melanomas, carry more than 100,000 somatic mutations per tumor, while others, such as pediatric leukemias, carry fewer than 100.

The accumulation of somatic mutations in aging tissues is increasingly recognized as a driver of age-related diseases beyond cancer, including clonal hematopoiesis.

Splice Site is a short conserved nucleotide sequence at the boundary between an exon and an intron that is recognized by the spliceosome to direct accurate intron removal from pre-mRNA.

The 5-prime splice site at the start of an intron almost always begins with GU, and the 3-prime splice site at the end almost always ends with AG, forming the GU-AG rule of canonical splicing. Mutations at these positions disrupt recognition by the spliceosome, causing intron retention, exon skipping, or activation of cryptic splice sites, all of which can produce aberrant proteins. Splice site variants account for roughly 15 percent of all point mutations that cause human genetic disease, making them one of the most clinically significant classes of pathogenic variant.

Stem Cell is an undifferentiated cell capable of self-renewal through cell division and differentiation into one or more specialized cell types.

Totipotent stem cells, present only in the earliest embryo, can give rise to any cell type including placental cells, while pluripotent embryonic stem cells can form all embryonic tissues but not the placenta. Adult stem cells are multipotent, restricted to producing the cell types of their resident tissue; hematopoietic stem cells in bone marrow, for example, generate all blood cell lineages, producing roughly 200 billion red blood cells per day in a healthy adult. Induced pluripotent stem cells, generated by reprogramming adult cells with defined transcription factors, have opened new avenues for disease modeling and regenerative medicine without requiring embryonic tissue.