Chromosome
Definition
A discrete, condensed structure consisting of a single long DNA molecule complexed with histone proteins. Humans normally carry 46 chromosomes in 23 homologous pairs in somatic cells; germline cells (eggs and sperm) carry 23 unpaired chromosomes.
- Composition
- DNA molecule complexed with histone proteins
- Human somatic count
- 46, in 23 pairs
- Human germline count
- 23, unpaired
- Field
- Forensic genetics
Common questions
Why does the difference between somatic and germline chromosome counts matter forensically?+
Because eggs and sperm carry only 23 unpaired chromosomes, fertilisation restores the full 46, and this halving is why a child's DNA profile is a combination of both parents rather than a copy of either, a principle underlying paternity and kinship testing.
Where do the markers used in forensic DNA profiling sit relative to chromosome structure?+
Short tandem repeat markers used in profiling are located at specific positions along particular chromosomes, and because the autosomes come in matched pairs, a profile typically shows two values, one inherited from each parent, at each marker.
Can an abnormal chromosome count affect a DNA profile interpretation?+
Yes, conditions involving an atypical number of chromosomes, or degraded samples affecting how much material is recovered, can complicate standard interpretation and may require an analyst to note or account for the anomaly when reporting results.
Related terms
- Autosomal vs Mitochondrial DNA
- Autosomal DNA is nuclear, biparentally inherited, present as two copies per cell. Mitochondrial DNA is cytoplasmic, maternally inherited, present in hundreds to...
- Capillary Electrophoresis
- Size separation of fluorescently labelled PCR products through a polymer-filled capillary on an instrument like the ABI 3500. Output is an electropherogram.
- CODIS
- The FBI's Combined DNA Index System, the US national STR-based forensic DNA database. Contains profiles from convicted offenders, arrestees, and unidentified crime-scene...
- Gene
- A segment of DNA that encodes a functional product, most commonly a protein but also non-coding RNA molecules. Protein-coding genes occupy roughly...
- Heterozygosity
- The proportion of individuals in a population who carry two different alleles at a locus. A locus with high heterozygosity is more...
- Locus (Plural: Loci)
- A defined physical position on a chromosome. In forensic genetics, 'locus' usually refers to a specific STR site used in profiling. The...
- Multiplex STR Kit
- Single-tube PCR amplifying 15+ STR loci plus amelogenin in one reaction. AmpFISTR Identifiler Plus and PowerPlex are the workhorses in Indian DNA...
- Nucleotide
- The monomer unit of nucleic acids. Each nucleotide consists of a five-carbon sugar (deoxyribose in DNA, ribose in RNA), a phosphate group,...
- PCR
- Polymerase Chain Reaction. Kary Mullis 1983. Exponential amplification of a target sequence using a thermostable Taq polymerase, two primers and thermal cycling.
- Polymorphism
- The existence of two or more sequence variants (alleles) at a locus in a population at a frequency above one percent. High...
- RFLP
- Restriction Fragment Length Polymorphism. The first-generation DNA typing method using restriction enzymes and Southern blotting. Required nanograms of intact DNA.
- Short Tandem Repeat (STR)
- Short repetitive DNA sequences scattered throughout the genome, with the number of repeats varying between individuals at each locus. DNA profiling compares...
Explained in these topics
- Chromosomes, Genes and the Human Genome
- DNA in Forensic Science: Structure, Genetic Markers, Extraction and ProfilingPackaged DNA. Humans have 22 pairs of autosomes plus one pair of sex chromosomes (XX or XY), all housed in the nucleus, plus a small circular mitochondrial gen...