Sex Linked / "X" Linked Genetics Sex Linked : Sex chromosome abnormalities may also be caused by nondisjunction of one or more sex chromosomes. Any combination (up to XXXXY) produces maleness. Males with more than one X are usually underdeveloped and sterile. XXX and X women are known, although in most cases they are sterile. Nondisjunction can also occur with AUTOSOMAL CHROMOSOMES. Where there is an additional or loss of a chromosome numbered 1 through 22. An example of this is Down syndrome where there is a additional chromosome number 21. Syndrome = Group of disorders are expressed. Many individuals in the population have the disorder. The actual cause is not known, however correlational studies have shown that maternal age may be linked with the disorder. 1
Karyotype = Illustration or photograph of the chromosomes in the nucleus of a somatic cell in an organism. Below is an the karyotype for an individual with down syndrome. Trisomy: condition where every somatic cell has an additional chromosome. Monosomy: condition where every somatic cell expresses the loss of a single chromosome. 2
Deletion: Portion of the chromosome is lost. How does this occur? chemicals viruses Cri du chat Disorder: Loss of a portion of chromosome 5 > Child is born with abnormalities in facial construction and mentally handicapped. Duplication Loss of a portion of chromosome 5 > Child is born with abnormalities in facial construction and mentally handicapped. 3
Cystic Fibrosis Cystic fibrosis (CF) is caused by a defect in a gene called the cystic fibrosis transmembrane conductance regulator (CFTR) gene. This gene makes a protein that controls the movement of salt and water in and out of the cells in your body. In people with CF, the gene does not work effectively. This causes the thick, sticky mucus and very salty sweat that are the main features of CF. Carriers of a defective allele for the CFTR gene usually have no symptoms. Therefore, it is caused by recessive inheritance. Muscular Dystrophy There are several forms of this disease and they all involve the wasting away of muscle tissue. The most common form is Duchenne muscular dystrophy, which is caused by X linked recessive inheritance. Muscle cells engorge with fat and connective tissue deposits and eventually waste away. Symptoms start to develop between the ages of two and six. Their muscles begin to weaken and eventually the muscles associated with the lungs weaken leading to respiratory failure. 4
In mitosis or meiosis, there can be an error in the processes that results in the change in the Number of Chromosomes that the daughter cells receive. There of two types of this error condition: *Aneuploidy the gain or loss of whole chromosomes (this is the most common chromosome abnormality). It is caused by nondisjunction, the failure of chromosomes to correctly separate. Examples include: o Downs Syndrome (extra chromosome #21) o Turner Syndrome (only one X chromosome in a female) o Klinefelter Syndrome (extra X chromosome in a male) * Polyploidy is a condition of having too many sets (pairs) of chromosomes. For example, having one pair of a chromosome makes a cell diploid (2n), while having two pairs of a chromosome makes a cell that is tetraploid (4n) Nondisjunction: the failure of replicated chromosomes to segregate during Anaphase II. Both environmental and genetic factors have roles in the development of any disease. A genetic disorder is a disease caused by abnormalities in an individual s genetic material / DNA (genome). There are four different types of genetic disorders: 1. single gene 2. polygenic (multiple genes) 3. chromosomal (i.e. trisomy) 4. mitochondrial (relatively rare caused by mutations in the non chromosomal DNA of mitochondria) The normal function of a gene is to encode a protein, not cause illness. 5
Prenatal Testing for A person with a family history of genetic disorders may wish to undergo genetic screening before becoming a parent. Genetic screening is an examination of a person s genetic makeup. It may involve constructing a karyotype, a picture of an individual s chromosomes grouped in pairs and arranged in sequence. Prenatal Testing for Couples at risk for having children with genetic disorders may wish to undergo genetic counselling, a form of medical guidance that informs them about problems that could affect their offspring. Physicians can diagnose more than 200 genetic disorders in the fetus using a variety of tools and techniques. 1. Amniocentesis 2. Chorionic Villi Sampling 6
Prenatal Testing for During amniocentesis the physician removes a small amount of amniotic fluid from the amnion, the sac that surrounds the fetus, between the fourteenth and sixteenth weeks of pregnancy. Fetal cells and proteins from the fluid then can be analyzed and a karyotype can also be prepared. Prenatal Testing for During chorionic villi sampling the physician obtains a sample of the chorionic villi, a tissue that grows between the mother s uterus and the placenta, between the eighth and tenth week of pregnancy. The villi have the same genetic makeup as the fetus because they were both coded for by fetal DNA. Tissue samples from the villi can be used to produce a karyotype. 7