Genetics Terms Starting With M

Meiosis is a specialized type of cell division that reduces the chromosome number by half, producing four genetically distinct haploid cells from one diploid parent cell.

Meiosis consists of two successive divisions: meiosis I, which separates homologous chromosomes, and meiosis II, which separates sister chromatids. Crossing over during prophase I and independent assortment of homologs during meiosis I generate genetic diversity in the resulting gametes. Errors in meiosis, particularly nondisjunction, can produce gametes with incorrect chromosome numbers, leading to aneuploidy in offspring.

Nondisjunction during meiosis I is the cause of trisomy 21, the chromosomal basis of Down syndrome, and its frequency increases with maternal age.

Melanin is a pigment synthesized by melanocytes that gives color to skin, hair, and eyes while absorbing ultraviolet radiation and limiting DNA damage in underlying tissues.

Two chemically distinct forms exist: eumelanin, which is dark brown to black, and pheomelanin, which is yellow to red. The ratio of these forms, and total melanin output, is governed by several genes, with variants in MC1R being among the most studied regulators of melanocyte activity. People with a higher proportion of eumelanin have darker skin, hair, and eyes, while those with more pheomelanin tend to have red hair and freckles.

Albinism results from mutations that disrupt melanin synthesis entirely, leaving skin, hair, and eyes with little or no pigment regardless of ancestry.

Mendel's Laws are the foundational principles of heredity describing how traits are transmitted between generations, encompassing the law of segregation and the law of independent assortment.

The law of segregation states that the two alleles of a gene separate during gamete formation, so each gamete carries only one allele. Independent assortment, his second law, holds that alleles of different genes distribute to gametes independently when those genes reside on different chromosomes. Gregor Mendel derived both laws from controlled breeding experiments with garden peas (Pisum sativum) conducted between 1856 and 1863, tracking seven distinct traits across tens of thousands of plants.

His paper was published in 1866 but went largely unrecognized until 1900, when Hugo de Vries, Carl Correns, and Erich von Tschermak independently rediscovered his results.

Messenger RNA is a single-stranded RNA molecule transcribed from a DNA template that carries the genetic instructions for protein synthesis from the nucleus to the ribosome.

After transcription, pre-mRNA undergoes processing including 5-prime capping, 3-prime polyadenylation, and intron splicing before export from the nucleus as mature mRNA. Ribosomes read the mRNA sequence in triplet codons and assemble the corresponding amino acid chain according to the genetic code. The stability and abundance of mRNA molecules in a cell are major determinants of how much protein is produced from any given gene.

In human cells, mRNA half-lives range from a few minutes for unstable regulatory transcripts to more than 24 hours for highly stable messages such as those encoding globin proteins.

Metabolomics is the large-scale study of all small-molecule metabolites present in a biological sample, providing a snapshot of the biochemical activity of an organism at a given time.

Metabolomics uses analytical platforms including mass spectrometry and nuclear magnetic resonance spectroscopy to identify and quantify hundreds to thousands of metabolites simultaneously. Because metabolites are the downstream products of gene expression and enzymatic activity, the metabolome reflects the integrated output of the genome, transcriptome, and proteome. Metabolomic profiling is used in biomarker discovery, drug development, personalized medicine, and understanding metabolic diseases.

Phenylketonuria, one of the first metabolic disorders screened at birth, is detected by measuring elevated phenylalanine in blood, an early example of metabolite-based diagnosis now refined by modern metabolomic platforms.

Metagenomics is the direct sequencing and analysis of total genomic DNA extracted from an environmental sample, characterizing the genetic diversity and functional potential of all organisms present without culturing them.

By circumventing the requirement to culture microorganisms, metagenomics has revealed that the vast majority of microbial life on Earth has never been grown in a laboratory, the so-called microbial dark matter. Metagenomic studies have transformed understanding of the human gut microbiome, ocean microbial communities, and ancient DNA preserved in permafrost. Functional metagenomics screens environmental libraries for novel enzymatic activities, natural products, and metabolic pathways with industrial or biomedical value.

A 2022 study of deep-sea sediments off the coast of Puerto Rico recovered metagenomic sequences from more than 1,000 previously undescribed viral species in a single sampling expedition.

Methylation is the biochemical addition of a methyl group to a molecule, most commonly referring to the addition of a methyl group to cytosine bases in DNA or to lysine and arginine residues on histone proteins, with consequences for gene expression.

DNA methylation at CpG dinucleotides usually silences gene expression by preventing transcription factor binding and recruiting repressive chromatin-remodeling complexes. Histone methylation can either activate or repress transcription depending on which residue is methylated and how many methyl groups are added; for example, trimethylation of histone H3 at lysine 4 marks active promoters, while trimethylation at lysine 27 marks repressed regions. Abnormal methylation patterns are characteristic of cancer and aging, with global hypomethylation and promoter-specific hypermethylation both contributing to altered gene expression in tumors.

Methyl groups are donated by S-adenosylmethionine, linking dietary methionine and folate intake directly to the cell’s capacity for methylation.

Microsatellite is a short repetitive DNA sequence of one to six nucleotides repeated in tandem many times at a specific chromosomal locus, showing high variability in repeat number between individuals.

Microsatellites are highly polymorphic because DNA polymerase slippage during replication frequently alters the number of repeat units, generating new alleles at rates estimated between 10 to the negative 3 and 10 to the negative 4 mutations per locus per generation. Their variability, abundance throughout the genome, and ease of detection by PCR make microsatellites ideal markers for DNA fingerprinting, paternity testing, and forensic identification. Expansions of specific microsatellite repeats beyond a threshold copy number cause several human diseases, including Huntington disease, where a CAG repeat in the HTT gene normally runs fewer than 36 copies but expands to 36 or more in affected individuals.

Longer repeat expansions in Huntington disease generally correlate with earlier age of onset, a phenomenon called anticipation.

Missense Mutation is a single nucleotide substitution that changes one codon to another that specifies a different amino acid, potentially altering the structure and function of the encoded protein.

The functional impact depends on which amino acid is substituted, whether the change occurs at a functionally critical position, and how chemically different the new amino acid is from the original. Conservative missense mutations replace an amino acid with one of similar charge and size and often have minimal functional impact, while radical substitutions at active sites or structural cores can abolish protein function entirely. Sickle-cell disease results from a single missense mutation in the HBB gene, where adenine replaces thymine at codon 6, substituting valine for glutamic acid and causing hemoglobin to polymerize under low-oxygen conditions.

This one-nucleotide change alters the surface charge of the beta-globin subunit enough to drive the sickling of red blood cells at oxygen tensions common in capillary beds.

Mitosis is a type of cell division in which a single parent cell divides to produce two genetically identical daughter cells, each with the same chromosome number as the parent.

Mitosis proceeds through four stages: prophase, metaphase, anaphase, and telophase, followed by cytokinesis. Each stage is defined by the behavior of chromosomes: condensation and spindle formation in prophase, alignment at the cell equator in metaphase, separation of sister chromatids in anaphase, and nuclear envelope reformation in telophase. Unlike meiosis, mitosis does not reduce chromosome number or generate genetic diversity, relying instead on precise chromosome replication and spindle-mediated segregation to maintain genomic integrity.

In rapidly dividing human intestinal epithelial cells, a complete mitotic cycle takes approximately 12 to 16 hours, with mitosis itself occupying less than one hour of that interval.

Molecular Marker is a known DNA sequence at a specific chromosomal location whose variation between individuals can be detected and used to track inheritance, identify individuals, or map genes.

Molecular markers include microsatellites, single nucleotide polymorphisms, restriction fragment length polymorphisms, and insertion-deletion variants. Their applications span plant and animal breeding, forensic identification, population genetics, and disease gene mapping. An ideal marker is highly polymorphic, reproducible, and distributed throughout the genome.

Microsatellite markers, for example, can have dozens of alleles at a single locus, giving them high discriminating power in both forensic and breeding contexts.

Monohybrid Cross is a genetic cross between two individuals that differ in a single trait, used to study the inheritance pattern of one gene pair.

Mendel performed monohybrid crosses by crossing true-breeding pea plants (Pisum sativum) differing in one characteristic and recording phenotypic ratios in F1 and F2 generations. A monohybrid cross between two F1 heterozygotes produces offspring in a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio. Monohybrid crosses are the simplest experimental design for determining whether a trait follows Mendelian inheritance and whether one allele is dominant over the other.

Mendel analyzed nearly 7,000 F2 plants across seven traits to establish these ratios with statistical confidence.

Mutation is any heritable change in the nucleotide sequence of DNA, ranging from substitution of a single base pair to large-scale chromosomal rearrangements.

Mutations arise spontaneously from replication errors or DNA damage, or are induced by environmental mutagens such as ultraviolet radiation, chemical carcinogens, or ionizing radiation. Most mutations in somatic cells are harmless or repaired, but those that escape repair can alter gene function and contribute to cancer or other diseases. Germline mutations occurring in gametes are heritable and may be transmitted to offspring.

The human genome accumulates roughly 1 to 2 replication errors per cell division, yet the overall germline mutation rate is kept low by multiple proofreading and repair mechanisms.