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| Karyotype and Genetic Diseases Questions and Answers A Q&A Review of Genetic Diseases 1. What is karyotype? The name karyotype is given to the set of chromosomes of an individual, usually when visualized and identified under the microscope. The visualization generally is made with the cells in the initial phases of cell division for the chromosomes to be seen already replicated and condensed. Image Diversity: human chromosomes 2. Which type of genetic disease can be identified from the visual analysis of the number of chromosomes present in a karyotype? The counting and identification of chromosomes in the karyotype of an individual can diagnose the aneuploidies, diseases caused by alteration in the number of chromosomes in relation to the normal number of the species. 3. Why in the preparation of a karyotype analysis is the use of a substance like colchicine interesting? Colchicine is a substance that disallows the formation of microtubules and thus of the spindle fibers in cell division. Under the action of this drug the cells interrupt division at metaphase and the anaphase does not occur. Therefore the use of colchicine in the study of karyotypes is interesting because chromosomes will be seen replicated and condensed. 4. What is the karyotype found in Down syndrome? Down syndrome is an aneuploidy, i.e., a numeric alteration of chromosomes within the cells compared to the normal number of chromosomes of the species. Affected individuals have in their cells an additional chromosome 21 instead of only one pair. For this reason the condition is also called trisomy 21. The affected person has karyotype with 47 chromosomes: 45 + XY or 45 + XX. Image Diversity: trisomy 21 karyotype 5. What is aneuploidy? What are the conditions caused by the aneuploidies? Aneuploidy is an abnormal number of chromosomes in the cells of an individual. The main aneuploidies of the human species and their respective conditions are: the nullisomies (absence of any chromosome pair of the species, often incompatible with life); the monosomies (absence of a chromosome from a pair, for example, Turner’s syndrome, 44 + X); the trisomies (an extra chromosome, for example, the triple X syndrome, 44 + XXX, or the Edwards syndrome, trisomy 18, 45 + XY or 45 + XX). 6. In general what is the cause of the aneuploidies? Generally the aneuploidies are caused by impaired assortment of chromosomes during meiosis. For example, when the homologous chromosomes of the pair 21 do not separate gametes with two chromosomes 21 and gametes without chromosomes 21 form. If a gamete with two chromosomes 21 fecundates a normal gamete of the opposite sex the zygote will present trisomy (three chromosomes 21). If a gamete without chromosomes 21 fecundates a normal gamete of the opposite sex there will be a zygote with monosomy (with only one chromosome 21). The defects in the separation of chromosomes during cell division are called chromosomal nondisjunctions. During meiosis nondisjunctions may occur in the anaphase I (nondisjunction of homologous) as well in anaphase II (nondisjunction of sister chromatids). Image Diversity: chromosomal nondisjunctions 7. Do all genetic diseases result from alteration in the number of chromosomes of the cells? Besides aneuploidies there are other genetic diseases, other chromosomal abnormalities and also the genetic mutations. 8. How are genetic diseases classified? Genetic diseases classify into chromosomal abnormalities and genetic mutations. Among chromosomal abnormalities there are the aneuploidies, diseases caused by alterations of the normal (euploidy) number of chromosomes of the species. An example of aneuploidy is Down syndrome, or trisomy 21, in which there are three chromosomes 21 instead of the normal pair. In the group of chromosomal abnormalities there are also the deletions (absence of part of a chromosome), the inversions (in which a chromosome breaks and its pieces reconnect in inverse manner) and the translocations (pieces of a chromosome that exchange positions). In the genetic mutation group there are the deletions (one or more DNA nucleotide absent), the substitutions and the insertions. Image Diversity: chromosomal deletions chromosomal inversions chromosomal translocations 9. What are genetic mutations? Genetic mutations are alterations of the genetic material (compared to the normal condition of the species) involving modifications in the normal nucleotide sequence of a gene but without structural or numeric chromosomal changes. These modifications may be deletions (loss of nucleotides), substitutions (exchange of nucleotides by other different nucleotides) or insertions (placement of additional nucleotides in the DNA molecule). Image Diversity: genetic mutations 10. Does every gene mutation cause alteration in the protein the gene normally codifies? Not every gene mutation causes alteration in the composition of the protein the gene codifies. Since the genetic code is degenerated, i.e., there are amino acids codified by more than one different DNA nucleotide triplet, if by chance the mutation substitutes one or more nucleotides of a codifier triplet and the newly formed triplet still codifies the same amino acid codified by the original triplet there will be no modification in the protein made from the gene. 11. How do genetic mutations influence biological diversity? Too extensive or too frequent genetic mutations generally are deleterious for individuals and species. These mutations often cause important phenotypical changes or defects incompatible with the survival of the body and the continuity of the species. However small genetic mutations that do not cause the appearing of lethal changes are continuously accumulated in the genetic patrimony of the species. These mutations gradually add to each other giving birth to small phenotypical changes in individuals. These small changes are exposed to the selective criticism of the environment (natural selection) and the more favorable for survival and reproduction are preserved (the remaider are eliminated as their carriers have difficulty in surviving and reproducing). In this manner the combined processes of accumulation of small mutations and of natural selection incorporate new features in the species and they may even lead to speciation (formation of new species) and promotion of biological diversity. (Obviously only genetic mutations transmitted by cells that originate new individuals, in sexual or asexual reproduction, have evolutionary effect.) 12. What are mutagenic agents? Mutagenic agents, or mutagens, are physical, chemical or biological factors that can cause alteration in DNA molecules. Examples of well-known or believed to be mutagenic agents are: X, alpha, beta and gamma rays, ultraviolet radiation, nitrous acid, many dyes, some sweeteners, some herbicides, many substances of tobacco, some viruses, like HPV, etc. Small DNA fragments known as transposons can also act as mutagens when incorporated into other DNA molecules. 13. How are mutagenic agents related to cancer incidence in a population? Is cancer a disease transmitted to the individual offspring? The exposition of a population to mutagenic agents (for example, the people living in the surrounds of the Chernobyl nuclear power plant and exposed to the radiation from the nuclear accident in 1986) increases the cancer incidence in that population. This occurs because the mutagenic agents increase the rate of mutation and the probability of mutant cells to proliferate in pathological manner (cancer). Cancer itself is not a hereditarily transmissible disease. Genetic predispositions for the development of cancer, however, can be inherited. 14. How do the repairing enzymes of the genetic system act? There are enzymes within the cells that detect errors or alterations in DNA molecules and begin a repair of those errors. First, enzymes known as restriction endonucleases, specialized in cutting DNA molecules (also used in genetic engineering), cut the affected piece of DNA. Then polymerase enzymes build correct sequences of nucleotides correspondent to the affected piece taking as template the DNA chain complementary to the affected chain. Finally the new correct sequence is bound in the DNA under repair by specific enzymes. Image Diversity: DNA repairing system 15. What are some diseases or genetic abnormalities caused by recessive genes? Examples of recessive genetic diseases are: cystic fibrosis, albinism, phenylketonuria, galactosemia, Tay-Sachs disease. 16. What are some diseases or genetic abnormalities caused by dominant genes? Why are severe dominant genetic diseases rarer than recessive ones? Examples of dominant genetic diseases are: Huntington's disease (or Huntington’s chorea), neurofibromatosis, hypercholesterolemia, polycystic kidney disease. Severe and early autosomal dominant diseases are rarer than recessive autosomal diseases because in this last group the affected allele may be hidden in the heterozygous individuals and transmitted to the offspring until undergoing homozygosity (actual manifestation of the disease). In severe dominant diseases the heterozygous manifests the condition and often dies without having offspring. (Some genetic diseases are of later manifestation, like Huntington disease; in these cases the incidence is higher because many individuals have children before knowing that they are carriers of the dominant gene). 17. What is consanguineal marriage? Why is the appearing of genetic disease more probable in the offspring of a consanguineal marriage? Consanguineal marriage is the marriage between relatives, i.e., people having common near ancestors. The consanguineal marriage increases the probability of recessive genetic disease in the offspring since it is common for people from the same genetic lineage to be heterozygous carriers of alleles that condition recessive genetic diseases. 18. How is the early diagnosis of genetic diseases usually done? Genetic disease may be diagnosed in the prenatal period by karyotype analysis, in case of aneuploidies, or by DNA analysis, in case of other diseases. The test is performed by removal of material containing cells of the embryo by amniocentesis (extraction of amniotic fluid) or cordocentesis (puncture of the umbilical cord) or even by chorionic villus biopsy (that can be done earlier in gestation). Ultrasonography is a diagnostic procedure for some genetic diseases that produce morphological variations during the embryonic development. The study of genetic family trees is also an important auxiliary method in the early diagnosis of many genetic diseases. |