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Chromosomes, Chromatids, Loci, and Alleles
During interphase, the cell is functioning normally and the DNA is unraveled and
impossible to see. Then, at some point in the cell’s life cycle, the cell will start to prepare
for cell division through either mitosis (somatic cells) or meiosis (sex cells). The DNA
will first replicate in the synthesis phase of the cell life cycle to produce two identical
copies of the chromosome. Then, the chromosomes will wind up around histones
(proteins) and form the X shape we recognize from genetics textbooks. It can often be
confusing to learn the terminology when just starting out, and there are subtle
differences between chromosomes, alleles, genes, and locus. The following simplified
diagram should help clarify the proper usage of these terms.
chromosome (after duplication—two copies of the same
Copy 1
Copy 2
homologous chromosome (after duplication)
two alleles for flower color
Copy 1
Copy 2
locus for a flower color gene (location on the chromosome)
The Saylor Foundation 1
Homologous chromosomes are chromosomes that are the same size and shape, and
they have the same genes on them, but they are not identical. In humans, we inherit
one from our mother and one from our father. Homologous chromosomes can have
different alleles on them. Alleles are variants of the same gene that occur on the same
place on a chromosome. (Through a mutation, they are different.) A locus refers to the
location on the chromosome where the gene is found. Loci is the plural form of locus.
The gene is located within a designated region on the chromosome and is composed of
the different base pairs (GATC) that will give genetic instructions to the plant. In this
case, the alleles will tell the plant what color of flower it will have.
The Saylor Foundation 2