Chapter 4
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• A zygote is a single fertilized cell.
• A somatic cell is a body cell.
• A germ cell is a sex cell.
• Cell structure includes a cell membrane that surrounds the cell material (cytoplasm). The nucleus in the center of the cell houses the chromosomes.
• Deoxyribonucleic Acid (DNA) contains the blueprint of life and includes phosphates, sugars, and bases.
• Human Genome Project is a 10-year project that was designed to identify the exact chromosomal location of all human genes.
• There are 23 pairs of chromosomes that include all of the human genes involved in heredity.
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• Autosomes are the first 22 pair of chromosomes.
• Sex chromosomes are the 23rd pair of chromosomes; females have an XX combination, males have an XY.
• Karyotype is a magnified picture of the 23 pairs of chromosomes.
• Meiosis is when cell division within the sex cells occurs.
• Mitosis is when cell division in which chromosomes duplicate themselves before dividing occurs.
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AUTOSOMAL GENETIC DISORDERS
• Alleles are genes that produce different characteristics; an offspring inherits a specific allele from each parent.
• Homozygous is when an offspring inherits the same allele from both parents.
• Heterozygous is when an offspring inherits a different allele from each parent.
• Genetic inheritance can occur from a single gene or from multiple genes (quantitative trait loci).
• Interaction between genetic and environmental factors is known as multifactorial transmission.
• Autosomal Dominant Disorder is when an offspring inherits a disorder if either parent has the allele for that disorder.
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AUTOSOMAL GENETIC DISORDERS
• Nonpenetrance is a rare occurrence when an offspring does not inherit a dominant disorder.
• Punnett Square is a method to show various genetic combinations of offspring of parents with known allele pairs.
• Tuberous Sclerosis is an autosomal dominant disorder in which the gene responsible for inhibiting the growth of tumors is nonworking.
It results in tumors in the organs, including the brain. CIDs may or may not be present and the degree of symptoms varies from person to person.
• Neurofibromatosis is an autosomal dominant disorder involving growth of tumors in the peripheral nervous system and the central nervous system. There are two types: NF1 and NF2. NF1 leads to physical malformation. CIDs may or may not be present and the degree of symptoms varies from person to person.
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AUTOSOMAL GENETIC DISORDERS
• Autosomal Recessive Disorder is when an offspring inherits a disorder only when both parents have the allele for the disorder.
• Inborn errors of metabolism is an autosomal recessive disorder involving a missing enzyme that normally metabolizes certain substances.
• Galactosemia is an autosomal recessive disorder resulting in the inability to break down the sugar galactose. Buildup of galactose can cause brain damage and organ failure. Individuals should avoid foods containing lactose and galactose.
• Hurler syndrome is an autosomal recessive disorder resulting in the inability to break down complex carbohydrates called mucopolysaccharides. It results in distinctive physical features and severe CIDs; death usually occurs by early teenage years.
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AUTOSOMAL GENETIC DISORDERS
Inborn Errors of Metabolism (Continued)
• Maple Syrup Urine Disease is an autosomal recessive disorder resulting in the inability to metabolize the amino acids leucine, isoleucine, and valine. Urine has a distinctive sweet smell like maple syrup. If untreated, it results in brain damage. A diet is used that restricts those amino acids.
• Tay Sachs Disease is an autosomal recessive disorder found primarily in the Ashkenazi Jewish population. It is caused by the lack of an enzyme that breaks down a specific lipid, which accumulates in the brain. Serious complications result and death usually occurs at an early age.
• Phenylketonuria is an autosomal recessive disorder in which an enzyme is not available to metabolize the amino acid phenylalanine. PKU can be detected in the first week of life using routine blood screening. A diet is used to avoid the specific amino acid.
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SEX-LINKED DISORDERS
• Sex-linked disorders are those involving the 23rd pair of chromosomes
(usually the X chromosome).
• Lyonization is the process when only one X chromosome is randomly activated and the other one inactivated.
• Fragile X syndrome is when the FMR-1 gene on the X chromosome fails to produce a protein and that results in CIDs. It is more prevalent in males than females and males generally have more severe characteristics. It is the most common form of an inherited CID.
• Lesch-Nyhan disorder is associated with a gene on the X chromosome that results in self-injurious behavior, aggression, and CIDs.
• Rett disorder is a dominant neurodevelopmental disorder involving the X chromosome that results in decelerated head growth, loss of motor and language skills, and CIDs. It occurs almost exclusively in females.
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CHROMOSOMAL ABNORMALITIES
• Disorders related to the number of chromosomes usually occur when there are 47 chromosomes (trisomy) rather than the normal 46; less frequently there are 45 chromosomes
(monosomy).
• Nondisjunction is the failure of a chromosome to divide during meiosis. It results in three chromosomes going to one daughter cell and only one chromosome going to the other daughter cell.
• Translocation is an exchange of materials between two chromosomes or the fusion of a portion of one chromosome to a similar portion of another chromosome.
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CHROMOSOMAL ABNORMALITIES
• Mosaics is a situation in which some cells have 46 chromosomes and some have 47.
• Down syndrome is the most common biological cause of CIDs. It is most often caused by an extra 21st chromosome (trisomy 21). The syndrome is associated with distinctive facial features, frequent heart and other health problems, and mild to moderate CIDs.
• Edwards syndrome is caused by an extra 18th chromosome. It results in microcephaly (small head), health problems, and severe CIDs.
• Patau syndrome is caused by an extra 13th chromosome. It involves numerous physical malformations and very severe CIDs resulting in an early death.
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CHROMOSOMAL ABNORMALITIES
Mosaics (Continued)
• Klinefelter syndrome occurs when a male has an additional X chromosome (usually an XXY combination). Not all individuals will have CIDs; many have language-based and communication problems.
• XYY male is a once-sensationalized condition that was supposedly associated with criminal behavior.
• Turner syndrome occurs when a female has a missing X chromosome (XO). Not all individuals will have CIDs.
• Pentasomy X occurs when there are three extra X chromosomes resulting in an XXXXX combination. In general, the more X chromosomes the more severe the CID will be.
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CHROMOSOMAL ABNORMALITIES
• Disorders related to the structure of the chromosomes occur when the basic structure of the chromosome is altered.
• Deletion is when a large or small portion of the chromosome is missing.
• Centromere is the area on a chromosome that separates the long arm (q) and the short arm (p).
• Interstitial is when a deletion occurs toward the middle of the chromosome.
• Wolf-Hirshhorn syndrome is due to partial deletion of the short arm of the 4th chromosome; it usually results in microcephaly and significant CIDs.
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CHROMOSOMAL ABNORMALITIES
Interstitial (Continued)
• Cri-du-Chat is, literally, the cat’s cry syndrome, named after the characteristic high-pitch cry and is due to partial deletion of the 5th chromosome. It usually results in microcephaly and significant CIDs.
• Williams syndrome results from deletion on the 7th chromosome that results in “elfin-like” facial characteristics. CIDs with relative strengths in language and interpersonal skills are involved.
• Jacobsen syndrome results from partial deletion of the long arm of the
11th chromosome. It results in slow growth and physical abnormalities in addition to CIDs.
• Prader-Willi syndrome results from partial deletion of the long arm of the
15th chromosome; characteristics include short stature, insatiable appetite and obesity, and varying degrees of cognitive problems. PWS is paternally inherited.
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CHROMOSOMAL ABNORMALITIES
• Genome imprinting is when an allele has a different effect on an offspring based on the sex of the parent from whom it came.
• Angelman syndrome is associated with the same portion of the 15th chromosome as PWS but is maternally inherited.
Characteristics include microcephaly, seizures, and CIDs as well as behavior problems. Two unique characteristics are inappropriate laughter and preoccupation with water.
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• Some disorders are known to have multiple causes or are presumed to have a biological cause that is not identified.
• Hydrocephalus is the buildup of cerebrospinal fluid in the brain resulting in an enlarged head.
• Shunt and endoscopic ventriculostomy are two procedures used to divert the cerebrospinal fluid away from the brain in hydrocephaly.
• Macrocephaly is a condition of a larger head than is expected for the age and sex that has multiple causes.
• Microcephaly is a condition of a smaller head than is expected for the age and sex that has multiple causes.
• Sturge-Weber syndrome results in a facial port wine stain, seizures, and CIDs among its characteristics.
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• Alpha fetoprotein screening is a maternal blood test looking for abnormal amounts of alpha fetoprotein that is associated with certain disorders.
• Bart’s Test is a maternal blood test that screens for the presence of abnormal amounts of two maternal hormones in addition to alpha fetoprotein.
• Amniocentesis is a procedure in which a small amount of amniotic fluid is extracted; the fetal cells in the fluid are analyzed for abnormalities.
• Chorionic villus sampling is a procedure in which a small amount of placental tissue is extracted; the fetal cells in the tissue are analyzed for abnormalities.
• FISH technique, fluorescent in situ hybridization, is a technique to assist in detecting subtle chromosomal problems.
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• Multiplex ligation-dependent probe amplification (MLPA) and assay comparative genomic hybridization (ACGH) are both techniques used to detect copy number variations
(alterations of DNA resulting in abnormal number of copies of one or more sections of DNA).
• Carrier testing is used to determine if parents are carriers for a given disorder.
• Genetic counseling involves interpreting medical information and providing support for parents and informing parents of probabilities of having a child with a given disorder.
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