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An allele [1] (or allelomorph) is a variant of the sequence of nucleotides at a particular location, or locus, on a DNA molecule. [2]Alleles can differ at a single position through single nucleotide polymorphisms (SNP), [3] but they can also have insertions and deletions of up to several thousand base pairs.
Humans have a total of 46 chromosomes, but there are only 22 pairs of homologous autosomal chromosomes. The additional 23rd pair is the sex chromosomes, X and Y. Note that the pair of sex chromosomes may or may not be homologous, depending on the sex of the individual. For instance, females contain XX, thus have a homologous pair of sex ...
The words homozygous, heterozygous, and hemizygous are used to describe the genotype of a diploid organism at a single locus on the DNA. Homozygous describes a genotype consisting of two identical alleles at a given locus, heterozygous describes a genotype consisting of two different alleles at a locus, hemizygous describes a genotype consisting of only a single copy of a particular gene in an ...
A graphical representation of the typical human karyotype The human mitochondrial DNA. Human genetic variation is the genetic differences in and among populations. There may be multiple variants of any given gene in the human population , a situation called polymorphism. No two humans are genetically identical.
When it comes to insects' DNA, humans have a bit less in common. For example, fruit flies share 61 percent of disease-causing genes with humans, which was important when NASA studied the bugs to ...
With the exception of identical twins, all humans show significant variation in genomic DNA sequences. The human reference genome (HRG) is used as a standard sequence reference. There are several important points concerning the human reference genome: The HRG is a haploid sequence. Each chromosome is represented once.
Each chromosome carries many genes, with each gene occupying a different position or locus; in humans, the total number of protein-coding genes in a complete haploid set of 23 chromosomes is estimated at 19,000–20,000. [2] Genes may possess multiple variants known as alleles, and an allele may also be said to reside at a particular locus.
Though polyploidy in humans is not viable, mixoploidy has been found in live adults and children. [45] There are two types: diploid-triploid mixoploidy, in which some cells have 46 chromosomes and some have 69, [46] and diploid-tetraploid mixoploidy, in which some cells have 46 and some have 92 chromosomes. It is a major topic of cytology.