104 ❯ STEP 4. Review the Knowledge You Need to Score HighSex Determination and Sex Linkage
Mendel was not the only one to make progress in the field of heredity. In the early 1900s,
Thomas Morgan made key discoveries regarding sex linkage and linked genes.
In human cells, all chromosomes occur in structurally identical pairs except for two
very important ones: the sex chromosomes, X and Y. Women have two structurally identi-
cal X chromosomes. Men have one X, and one Y.Sex-Linked Traits
Morgan experimented with a quick-breeding fruit fly species. The fruit flies had four pairs
of chromosomes: three autosomal pairs and one sex chromosome pair. An autosomal
chromosomeis one that is not directly involved in determining gender. In fruit flies, the
more common phenotype for a trait is called the wild-type phenotype(e.g., red eyes).
Traits that are different from the normal are called mutant phenotypes(e.g., white
eyes). Morgan crossed a white-eyed male with a red-eyed female, and all the F 1 offspring
had red eyes. When he bred the F 1 together, he obtained Mendel’s 3:1 ratio. But, there
was a slight difference from what Mendel’s theories would predict—the white trait was
restricted to the males. Morgan’s conclusion was that the gene for eye color is on the X
chromosome. This means that the poor male flies get only a single copy, and if it is
abnormal, they are abnormal. But, the lucky ladies have two copies and are normal even
if one copy is not.
It is this male–female sex chromosomes difference that allows for sex-linked conditions.
If a gene for a recessive disease is present on the X chromosome, then a female must have
two defective versions of the gene to show the disease while a male needs only one. This is
so because males have no corresponding gene on the Y chromosome to help counter the
negative effect of a recessive allele on the X chromosome. Thus, more males than females
show recessive X-linked phenotypes. In a pedigree (see Figure 10.6 later in this chapter), a
pattern of sex-linked disease will show the sons of carrier mothers with the disease.
The father plays no part in the passage of an X-linked gene to the male children of a
couple. Fathers pass X-linked alleles to their daughters, but not to their sons. Do you
understand why this is so? The father does not give an X chromosome to the male offspring
because he is the one who provides the Y chromosome that makes his son a male. A mother
can pass a sex-linked allele to both her daughters andsons because she can pass only X chro-
mosomes to her offspring.
Three common sex-linked disorders are Duchenne’s muscular dystrophy, hemophilia,
and red-green colorblindness. Duchenne’s muscular dystrophyis a sex-linked disorder
that is caused by the absence of an essential muscle protein. Its symptoms include a pro-
gressive loss of muscle strength and coordination. Hemophiliais caused by the absence of
a protein vital to the clotting process. Individuals with this condition have difficulty clot-
ting blood after even the smallest of wounds. Those most severely affected by the disease
can bleed to death after the tiniest of injuries. Females with this condition rarely survive.
People afflicted with red-green colorblindnessare unable to distinguish between red and
green colors. This condition is found primarily in males.X Inactivation
Here is an important question for you to ponder while preparing for this exam: “Are all the
cells in a female identical?”
The answer to this question is “No.” Females undergo a process called X inactivation.
During the development of a female embryo, one of the two X chromosomes in each cellEmily (12th
grader):
“Be able to cate-
gorize diseases for
this exam!”BIG IDEA 3.A.4
The inheritance of
sex-linked traits
cannot be
explained by
simple Mendelian
genetics.