The cells are then fixed to a slide. The geneticist then stains chromosomes with one of several dyes to better visualize the distinct and reproducible banding patterns of each chromosome pair.
Following staining, chromosomes are viewed using bright-field microscopy. An experienced cytogeneticist can identify each band. In addition to the banding patterns, chromosomes are further identified on the basis of size and centromere location. To obtain the classic depiction of the karyotype in which homologous pairs of chromosomes are aligned in numerical order from longest to shortest, the geneticist obtains a digital image, identifies each chromosome, and manually arranges the chromosomes into this pattern.
At its most basic, the karyogram may reveal genetic abnormalities in which an individual has too many or too few chromosomes per cell. Examples of this are Down syndrome , which is identified by a third copy of chromosome 21 , and Turner syndrome, which is characterized by the presence of only one X chromosome in women instead of two. Geneticists can also identify large deletions or insertions of DNA. For instance, Jacobsen syndrome, which involves distinctive facial features as well as heart and bleeding defects, is identified by a deletion on chromosome Finally, the karyotype can pinpoint translocations, which occur when a segment of genetic material breaks from one chromosome and reattaches to another chromosome or to a different part of the same chromosome.
Translocations are implicated in certain cancers, including chronic myelogenous leukemia. By observing a karyogram, geneticists can actually visualize the chromosomal composition of an individual to confirm or predict genetic abnormalities in offspring even before birth.
Of all the chromosomal disorders, abnormalities in chromosome number are the most easily identifiable from a karyogram. Disorders of chromosome number include the duplication or loss of entire chromosomes, as well as changes in the number of complete sets of chromosomes.
They are caused by nondisjunction , which occurs when pairs of homologous chromosomes or sister chromatids fail to separate during meiosis.
The risk of nondisjunction increases with the age of the parents. Nondisjunction can occur during either meiosis I or II, with different results Figure 7. If homologous chromosomes fail to separate during meiosis I, the result is two gametes that lack that chromosome and two gametes with two copies of the chromosome.
If sister chromatids fail to separate during meiosis II, the result is one gamete that lacks that chromosome, two normal gametes with one copy of the chromosome, and one gamete with two copies of the chromosome.
An individual with the appropriate number of chromosomes for their species is called euploid; in humans, euploidy corresponds to 22 pairs of autosomes and one pair of sex chromosomes.
An individual with an error in chromosome number is described as aneuploid, a term that includes monosomy loss of one chromosome or trisomy gain of an extraneous chromosome. Monosomic human zygotes missing any one copy of an autosome invariably fail to develop to birth because they have only one copy of essential genes.
Cell Respiration 9. Photosynthesis 3: Genetics 1. Genes 2. Chromosomes 3. Meiosis 4. Inheritance 5. Genetic Modification 4: Ecology 1. Energy Flow 3. Carbon Cycling 4. Climate Change 5: Evolution 1.
Evolution Evidence 2. Natural Selection 3. Classification 4. Humans display dramatic deleterious effects with autosomal trisomies and monosomies.
Therefore, it may seem counterintuitive that human females and males can function normally, despite carrying different numbers of the X chromosome. Rather than a gain or loss of autosomes, variations in the number of sex chromosomes are associated with relatively mild effects.
In part, this occurs because of a molecular process called X inactivation. Early in development, when female mammalian embryos consist of just a few thousand cells relative to trillions in the newborn , one X chromosome in each cell inactivates by tightly condensing into a quiescent dormant structure called a Barr body. The chance that an X chromosome maternally or paternally derived is inactivated in each cell is random, but once the inactivation occurs, all cells derived from that one will have the same inactive X chromosome or Barr body.
By this process, females compensate for their double genetic dose of X chromosome. Figure 4. In cats, the gene for coat color is located on the X chromosome. In the embryonic development of female cats, one of the two X chromosomes is randomly inactivated in each cell, resulting in a tortoiseshell pattern if the cat has two different alleles for coat color.
Male cats, having only one X chromosome, never exhibit a tortoiseshell coat color. Females that are heterozygous for an X-linked coat color gene will express one of two different coat colors over different regions of their body, corresponding to whichever X chromosome is inactivated in the embryonic cell progenitor of that region.
An individual carrying an abnormal number of X chromosomes will inactivate all but one X chromosome in each of her cells. However, even inactivated X chromosomes continue to express a few genes, and X chromosomes must reactivate for the proper maturation of female ovaries. As a result, X-chromosomal abnormalities are typically associated with mild mental and physical defects, as well as sterility. If the X chromosome is absent altogether, the individual will not develop in utero.
Several errors in sex chromosome number have been characterized. Biology Meiosis. Explanations 3 Gabi Slizewska. Nondisjunction During cell division, DNA is duplicated and then divided equally into two daughter cells.
Related Lessons. What Is Mitosis? View All Related Lessons. Aaron Lauer. Video Visualizing Nondisjunction by Jack Lewis. Deena Hauze. What's Nondisjunction? Vocabulary Review: Interphase : The longest part of the cell cycle where the cell grows and DNA is replicated to prepare for nuclear division Nuclear Division : The process of dividing the nucleus Meiosis : The process of dividing one nucleus into four unique haploid cells.
Only occurs in sex cells Cytokinesis : The final step of cell division where the cytoplasm is divided to form daughter cells Diploid Cell : Cell that contains two sets of chromosomes Haploid Cell : Cell that contains only half the number of chromosomes. You've reached the end.
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