116 ❯ STEP 4. Review the Knowledge You Need to Score High
Sex-linked traits:passed along the X chromosome; more common in males than females
(males have only one X) (e.g., hemophilia [can’t clot blood], Duchenne’s muscular dystrophy
[muscle weakness], colorblindness).
X inactivation:one of two X chromosomes is randomly inactivated and remains coiled as a
Barr body.
Holandric trait: one that is inherited via the Y chromosome.Linked genes:genes that lie along the same chromosome and do not follow the law of
independent assortment.- Crossover:a form of genetic recombination that occurs during prophase I of meiosis.
- The further apart two genes are along a chromosome, the more often they will cross over.
Linkage map:genetic map put together using crossover frequencies. - Can determine the relative location of a set of genes according to how often they cross over.
- If two genes cross over in 20 percent of the crosses, they are 20 map units apart, etc.
Law of multiplication:To determine the probability that two random events will occur in
succession, multiply the probability of the first event by the probability of the second event.
(Useful in pedigree analysis!)
Pedigree:family tree used to describe genetic relationships (use pedigree diagram in review
question 14 for clearer understanding). To calculate the risk a couple faces of having a child
that has a recessive (bb) condition, first determine the probability that bothparents are Bb
(if neither have the condition), or the probability that one is Bb (if one hasthe condition).
Once determined, multiply this probability times the probability that a Bb ×Bb cross will
produce a bb (1 ⁄4) or that a bb ×Bb will produce a bb (1 ⁄ 2).Autosomal Recessive Disorders
Tay-Sachs:fatal, storage disease, lipid builds up in brain, mental retardation, increased
incidence in eastern European Jews.
Cystic fibrosis:increased mucus buildup in lungs; untreated children die at young age; one
in 25 Caucasians are carriers.
Sickle cell anemia:caused by error of single amino acid; hemoglobin is less able to carry
O 2 , and sickles when O 2 content of blood is low; one in 10 African Americans is a car-
rier. Heterozygous condition protects against malaria.
Phenylketonuria:inability to digest phenylalanine, which can cause mental retardation if
not avoided in diet.Autosomal dominant disorders:Huntington disease (nervous system disease) and achon-
droplasia (dwarfism).
Nondisjunction:error in which homologous chromosomes do not separate properly.- Monosomy:(one copy): Turner syndrome.
- Trisomy:(three copies): Down syndrome (21), Patau syndrome (13), Edwards syndrome (18).
Klinefelter syndrome:XXY; XYY males, XXX females.
Chromosome disorders:deletion (cri-du-chat), inversions, duplications, and translocations
(leukemia).