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. Most autosomal trisomies also fail to develop to birth; however, duplications of some of the smaller chromosomes 13, 15, 18, 21, or 22 can result in offspring that survive for several weeks to many years.
Trisomic individuals suffer from a different type of genetic imbalance: an excess in gene dose. Cell functions are calibrated to the amount of gene product produced by two copies doses of each gene; adding a third copy dose disrupts this balance. The most common trisomy is that of chromosome 21, which leads to Down syndrome. Individuals with this inherited disorder have characteristic physical features and developmental delays in growth and cognition.
The incidence of Down syndrome is correlated with maternal age, such that older women are more likely to give birth to children with Down syndrome Figure 7. Concept in Action Visualize the addition of a chromosome that leads to Down syndrome in this video simulation. 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.
In part, this occurs because of a process called X inactivation. Early in development, when female mammalian embryos consist of just a few thousand cells, one X chromosome in each cell inactivates by condensing into a structure called a Barr body. The genes on the inactive X chromosome are not expressed. The particular X chromosome maternally or paternally derived that is inactivated in each cell is random, but once the inactivation occurs, all cells descended from that cell will have the same inactive X chromosome.
By this process, females compensate for their double genetic dose of X chromosome. Females 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. When you see a tortoiseshell cat, you will know that it has to be a female.
In an individual carrying an abnormal number of X chromosomes, cellular mechanisms will inactivate all but one X in each of her cells. 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. Several errors in sex chromosome number have been characterized.
Individuals with three X chromosomes, called triplo-X, appear female but express developmental delays and reduced fertility. The XXY chromosome complement, corresponding to one type of Klinefelter syndrome, corresponds to male individuals with small testes, enlarged breasts, and reduced body hair. The extra X chromosome undergoes inactivation to compensate for the excess genetic dosage.
Turner syndrome, characterized as an X0 chromosome complement i. An individual with more than the correct number of chromosome sets two for diploid species is called polyploid.
For instance, fertilization of an abnormal diploid egg with a normal haploid sperm would yield a triploid zygote.
Polyploid animals are extremely rare, with only a few examples among the flatworms, crustaceans, amphibians, fish, and lizards. These individuals tend to have normal lifespans, though with sometimes major physiological and reproductive consequences.
Nondisjunction appears to be more common when homologous chromosomes fail to recombine. Interestingly X and Y chromosomes normally undergo less recombination compared to autosomes, perhaps explaining the frequency of nondisjunction in sex chromosomes.
Mutations in synaptonemal complex proteins, which attach homologous chromosomes, reduce crossing over but apparently increase nondisjunction. This suggests that proper chromosome recombination is an important step in normal meiosis. Nondisjunction is more frequent during oogenesis than during spermatogenesis. Mitotic nondisjunction is also a hallmark of many human cancers. Jones, Keith T. Hawley, R. Wenzel, Elizabeth S. To learn more about our GDPR policies click here.
If you want more info regarding data storage, please contact gdpr jove. Your access has now expired. Provide feedback to your librarian. If you have any questions, please do not hesitate to reach out to our customer success team. Login processing Chapter Meiosis. Chapter 1: Scientific Inquiry.
Chapter 2: Chemistry of Life. Chapter 3: Macromolecules. Chapter 4: Cell Structure and Function. Chapter 5: Membranes and Cellular Transport. Chapter 6: Cell Signaling. Chapter 7: Metabolism. Chapter 8: Cellular Respiration. Chapter 9: Photosynthesis. Chapter Cell Cycle and Division. Chapter Classical and Modern Genetics. Chapter Gene Expression.
Chapter Biotechnology. Chapter Viruses. Chapter Nutrition and Digestion. Chapter Nervous System. Chapter Sensory Systems. 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. Individuals with three X chromosomes, called triplo-X, are phenotypically female but express developmental delays and reduced fertility. The XXY genotype, corresponding to one type of Klinefelter syndrome, corresponds to phenotypically male individuals with small testes, enlarged breasts, and reduced body hair.
More complex types of Klinefelter syndrome exist in which the individual has as many as five X chromosomes. In all types, every X chromosome except one undergoes inactivation to compensate for the excess genetic dosage. This can be seen as several Barr bodies in each cell nucleus. Turner syndrome, characterized as an X0 genotype i. In addition to the loss or gain of an entire chromosome, a chromosomal segment may be duplicated or lost.
Duplications and deletions often produce offspring that survive but exhibit physical and mental abnormalities.
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