Study Guide Exam 1
1. Understand the process of meiosis related to homologous chromosomes.
During the first meiotic stage, the arms of homologous
... [Show More] chromosome pairs intertwine and sometimes
exchange portions of their DNA in a process known as crossover. During crossover, new combinations
of alleles can be formed. For example, two loci on a chromosome have alleles A and a and alleles B and
b. Alleles A and B are located together on one member of a chromosome pair, and alleles a and b are
located on the other member. The genotype of this individual is denoted as AB/ a0b. Genetic Results of
Crossover. A, No crossing over. B, Crossing over with recombination. The allele pairs would be
transmitted together when no crossover occurs. However, when crossover occurs, all four possible pairs
of alleles can be transmitted to the offspring. The process of forming such new arrangements of alleles is
called recombination. Loci that are located very close to one another are unlikely to experience
recombination and are said to demonstrate linkage. The frequency of recombination can be assessed in
families and is used to determine the relative positions of loci on chromosomes.
2. Define chromosomal mosaic
Trisomies may occur in only some cells of the body. Individuals thus affected are said to be
chromosomal mosaics, meaning that the body has two or more different cell lines, each of which has a
different karyotype. Mosaics are often formed by early mitotic nondisjunction occurring in one
embryonic cell but not in others. An example of this is Downs Syndrome
3. Causes/common traits of genetic disorders
a. Cri du chat
Broken chromosomes and lost DNA cause deletions. Usually, a gamete with a deletion unites
with a normal gamete to form a zygote. The zygote thus has one chromosome with the normal
complement of genes and one with some missing genes. Because many genes can be lost in a
deletion, serious consequences result, even though one normal chromosome is present. The most
often cited example of a disease caused by a chromosomal deletion is the cri du chat syndrome.
The term literally means “cry of the cat” and describes the characteristic cry of the affected child.
Other symptoms include low birth weight, severe intellectual disability, microcephaly (smaller
than normal head size), and heart defects. The disease is caused by a deletion of part of the short
arm of chromosome 5.
b. Turner’s
Turner’s Syndrome (45, X) Monosomy of X Chromosome IQ Not considered to be intellectually
disabled, although some impairment of spatial and mathematical reasoning ability is found Male/ female
findings Found only in females Musculoskeletal system Short stature common, characteristic webbing of
neck, widely spaced nipples, reduced carrying angle at elbow Systemic disorders Coarctation (narrowing)
of aorta, edema of feet in newborns, usually sterile and have gonadal streaks rather than ovaries; streaks
are sometimes susceptible to cancer Mortality About 15%-20% of spontaneous abortions with
chromosome abnormalities have this karyotype, most common single-chromosome aberration; highly
lethal during gestation, only about 0.5% of these conceptions survive to term Causative factors 75%
inherit X chromosome from mother, thus caused by meiotic error in father; frequency low compared with
other sex chromosome aneuploidies (1 : 5000 newborn females); 50% have simple monosomy of X
chromosome; remainder have more complex abnormalities; combinations of 45, X cells with XX or XY
cells common
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c. Fragile X
number of areas on chromosomes develop distinctive breaks and gaps (observable microscopically) when
the cells are cultured. Most of these fragile sites do not appear to be related to disease. However, one
fragile site, located on the long arm of the X chromosome, is associated with fragile X syndrome. The
most important feature of this syndrome is intellectual disability. With a relatively high population
prevalence (affecting approximately 1 in 4000 males and 1 in 8000 females), fragile X syndrome is the
second most common genetic cause of intellectual disability (after Down syndrome). In fragile X
syndrome, females who inherit the mutation do not necessarily express the disease condition, but they can
pass it on to descendants who do express it. Ordinarily, a male who inherits a disease gene on the X
chromosome expresses the condition because he has only one X chromosome. An uncommon feature of
this disease is that about one-third of carrier females are affected, although less severely than males.
Unaffected transmitting males have been shown to have more than about 50 repeated DNA sequences
near the beginning of the fragile X gene. These trinucleotide sequences, which consist of CGG sequences
duplicated many times, cause fragile X syndrome when the number of copies exceeds 200.3 The number
of these repeats can increase from generation to generation. More than 20 other genetic diseases,
including Huntington disease and myotonic dystrophy, also are caused by this mechanism. [Show Less]