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 [Show Less]