Heredity ❮ 105
remains coiled as a Barr bodywhose genes are not expressed. A cell expresses the alleles
only of the active X chromosome. X inactivation occurs separately in each cell and involves
random inactivation of one of a female’s X chromosomes. But not all cells inactivate the
same X. As a result, different cells will have different active X chromosomes.
Why don’t females always express X-linked diseases when this X inactivation occurs?
Sometimes they do, but usually they have enough cells with a “good” copy of the allele to
compensate for the presence of the recessive allele.
One last sex-related inheritance pattern that needs to be mentioned is holandric traits,
which are traits inherited via the Y chromosome. An example of a holandric trait in humans
is ear hair distribution.
Linkage and Gene Mapping
Each chromosome has hundreds of genes that tend to be inherited together because the
chromosome is passed along as a unit. These are called linked genes.Linked genes lie on
the same chromosome and do not follow Mendel’s law of independent assortment.
Morgan performed an experiment in which he looked at body color and wing size on
his beloved fruit flies. The dominant alleles were G (gray) and V (normal wings); the reces-
sive alleles were g (black) and v (vestigial wings). GgVv females were crossed with ggvv
males. Mendel’s law of independent assortment predicts offspring of four different pheno-
types in a 1:1:1:1 ratio. But that is not what Morgan found. Because the genes are linked,
the gray/normal flies produce only GV or gv gametes. Thus, Morgan expected the ratio of
offspring to be 1:1, half GgVv and half ggvv. Morgan found that there were more wild-type
and double-mutant flies than independent assortment would predict, but surprisingly,
some Gv and gV were also produced.
How did those other combinations result from the cross if the genes are linked?
Crossover(also known as crossing over), a form of genetic recombination that occurs during
prophase I of meiosis, led to their production. The less often this recombination occurs, the
closer the genes must be on the chromosome. The farther apart two genes are on a chro-
mosome, the more often crossover will occur. Recombination frequency can be used to
determine how close two genes are on a chromosome through the creation of linkage maps,
which we will look at next.
Linkage Maps
Alinkage mapis a genetic map put together using crossover frequencies. Another unit
of measurement, the map unit(also known as centigram), is used to geographically
relate the genes on the basis of these frequencies. One map unit is equal to a 1 percent
crossover frequency. A linkage map does not provide the exactlocation of genes; it gives
only the relative location. Imagine that you want to determine the relative locations of
four genes: A, B, C, and D. You know that A crosses over with C 20 percent of the time,
B crosses over with C 15 percent of the time, A crosses over with D 10 percent of the
time, and D crosses over with B 5 percent of the time. From this information you can
determine the sequence (Figure 10.4). Gene A must be 20 units from gene C. Gene B
must be 15 units from C, but B could be 5 or 35 units from A. But, because you also
know that A is 10 unitsfrom D and that D is 5 units from B, you can determine that B
must be 5 units from A as well, if A is also to be 10 units from D. This gives you the
sequence of genes as ABDC.
KEY IDEA
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