Video Transcript
Autosomes are chromosomes that do
not determine the sex of a human. What is meant by autosomal
linkage? (A) Autosomal linkage is the
expression of an allele that is located on an X or Y chromosome. (B) Autosomal linkage refers to two
or more genes being carried on the same autosome. (C) Autosomal linkage occurs when
two or more genes on an autosome merge together. (D) Autosomal linkage occurs when
two or more autosomes are structurally altered.
In most of our cells, we contain 23
pairs of chromosomes or 46 chromosomes in total. We have two copies of each
chromosome. One copy comes from our biological
father, shown in blue, and the other comes from our biological mother, shown in
pink. Chromosomes numbered one to 22 are
called autosomes. They are arranged based on their
size. Chromosome 1 is the largest and
chromosome 22 is the smallest. The 23rd pair of chromosomes, X and
Y, are called the sex chromosomes. This is because they determine our
biological sex. Having two copies of the X
chromosome results in a biological female. And one copy of X and Y results in
a biological male.
Let’s zoom in on a chromosome pair
so we can see what is meant by autosomal linkage. Here’s the pair of chromosomes for
chromosome 1. These chromosomes contain genes
scattered throughout the DNA. Let’s label a few genes as
uppercase letters on one of the chromosomes. These chromosomes are the same and
contain the same genes in the same positions. But some of the genes might be
slightly different versions called alleles. Let’s indicate these alleles as
lowercase letters on the second chromosome. So there’s six genes labeled A to F
and two alleles for each gene, either an uppercase letter or a lowercase letter.
To explain autosomal linkage, we
also need to remind ourselves of a special kind of cell division called meiosis. You’ll recall that meiosis is the
cell division that makes our gametes. Before meiosis takes place, a
process called interphase takes place. This is where the 46 chromosomes
are duplicated. So now each chromosome takes on
this duplicated chromosome structure. You’ll also notice that all the
genes are duplicated also. During the first stage of meiosis,
the arms of these chromosomes can cross over. Crossing-over is when two
chromosomes in a pair exchange segments of the chromosome. When this happens, parts of the
chromosome are exchanged. Now these chromosomes contain
alleles that they didn’t have before. This increases genetic variation of
the offspring.
When genes are close together on
the same chromosome, they’re said to be linked and are more likely to cross over
together. So if we had another gene that
we’ll label using the letter G, we can see that this would cross over as well
because it’s contained in the same chromosomal segment that was exchanged. Since they’re close together,
they’re more likely to cross over together and to be inherited together. So genes F and G are said to be
linked. On the other hand, the gene labeled
as E wouldn’t cross over because it’s too far from where the crossover occurred. So gene E is not linked to genes F
and G.
Autosomal linkage maps can be
constructed based on how genes are inherited. This can show how far apart genes
are from each other. Genes that are inherited together
frequently are closer on the chromosome than genes that are inherited together
rarely. If we go back to our question, the
best definition of autosomal linkage is given by answer choice (B). Autosomal linkage refers to two or
more genes being carried on the same autosome.
