150 ■ CHAPTER 08 Chromosomes and Human Genetics
GENETICS
from the type of virus used to insert the healthy
gene into their cells. Three of the boys died,
either from leukemia or from complications
after bone marrow transplants. For this reason
the trial was halted. Together with other physi-
cians and researchers, Klein is investigating the
causes of leukemia after gene therapy, and he is
looking for safer ways to insert the healthy gene
into cells.A Happy Ending for Felix
Numerous gene therapy trials continue around
the world for other genetic diseases, using other
methods to insert new genes as a way to avoid
risks like leukemia. In 2012, European healthofficials green-lit gene therapy for a recessive
genetic disorder called lipoprotein lipase defi-
ciency (LPLD), the first commercially approved
gene therapy treatment in either Europe or the
United States. Many others nip at its heels, includ-
ing treatments for hemophilia and ADA-SCID,
also known as the “bubble boy disease.”
As for Felix, a year after the gene therapy,
he returned to the hospital for tests. The treat-
ment had worked. Today, his body is produc-
ing healthy, functional blood cells, and most
of his symptoms are completely resolved. Felix
now enjoys a normal life. His inherited genetic
disorder was cured by gene therapy. “The most
beautiful moment is when my son smiles, hugs
me, and says, ‘Mum, I love you so much,’” said
Felix’s mother after the treatment. “We are
grateful that there was a treatment that helped
our son.”REVIEWING THE SCIENCE
● (^) A genetic disorder is a disease caused by an inherited
mutation in a gene, passed down from a parent to a
child. Genetic disorders can be caused by mutations
in individual genes or by chromosomal abnormalities
(changes in chromosome number or structure).
● (^) Every person has two sex chromosomes: males have
one X and one Y chromosome, and females have two
X chromosomes. The SRY gene on the Y chromosome
is required for human embryos to develop as males.
● (^) The physical location of a gene on a chromosome
is called its locus. Genes found solely on the X or Y
chromosome are said to be sex-linked.
● (^) A dominant genetic disorder is caused by a dominant
allele on an autosome (non–sex chromosome).
Dominant genetic disorders are more rare than
recessive genetic disorders, which are caused by
two autosomal recessive alleles. Several thousand
human genetic disorders are inherited as recessive
traits on autosomes.
● (^) A family pedigree can be used to determine whether a
given condition is recessive, dominant, or sex-linked.
● (^) Gene therapy is a genetic engineering technique for
correcting defective genes responsible for disease
development.
(^1) Use these terms correctly in the following
sentence: alleles, chromosomes, genes, loci.
Two homologous contain the same
, found at the same ,
but may have the same or different copies
of.
THE QUESTIONS
The Basics
(^2) A particular person is said to be a carrier of a
genetic trait. What does this tell you about their
phenotype?
(a) They physically show the trait.
(b) They physically show the trait more than a
noncarrier would show the trait.