How does nondisjunction result in variation

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. 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. Nondisjunction is the failure of homologous chromosomes or chromatids to segregate during mitosis or meiosis.

Nondisjunction during mitosis can result in cancer. Nondisjunction during meiosis results in one daughter cell having both of a pair of parental chromosomes with the other having none.

It causes a mutation in the number of chromosomes, either extra chromosomes or missing chromosomes. Nondisjunction during meiosis can cause chromosomal disorders, such as trisomy 21 Down syndrome , Turner's syndrome, and Klinefelter's syndrome.

Trisomy 21, also called Down syndrome, occurs when an individual inherits three copies of chromosome 21, rather than the usual two. An affected individual has 47 chromosomes rather than the normal Trisomy 21 occurs due to the nondisjunction of chromosome 21 during meiosis.

It can occur during either the formation of ova or spermatozoa, resulting in an ovum or sperm cell with 22 chromosomes, and which lacks a chromosome 21, and another with 24 chromosomes, which has an extra copy of chromosome When a normal sperm cell or ovum unites with a sperm cell or ovum with an extra copy of chromosome 21, the result in the zygote is trisomy Pairing of homologous chromatids results in hundreds to thousands of individual chromatid copies aligned tightly in parallel to produce giant, "polytene" chromosomes.

High-pressure treatment of polytene chromosomes improves structural resolution. Nature Methods 4, All rights reserved. Aberrations That Alter Chromosome Number. Figure 3: Nondisjunction results in daughter cells with unusual chromosome numbers. Nondisjunction, in which chromosomes fail to separate equally, can occur in meiosis I first row , meiosis II second row , and mitosis third row. These unequal separations can produce daughter cells with unexpected chromosome numbers, called aneuploids.

When a haploid gamete does not receive a chromosome during meiosis as a result of nondisjunction, it combines with another gamete to form a monosomic zygote.

When a gamete receives a complete homologous chromosome pair as a result of nondisjunction, it combines with another gamete to form a trisomic zygote. Genetics: A Conceptual Approach , 2nd ed. Figure 4: Jimsonweed seed pod shapes. Trisomy in any of Jimsonweed's 12 chromosomes will cause seed pods to deviate from a wild-type, spherical shape. References and Recommended Reading Belling, J. Genetics: A Conceptual Approach W. Freeman, New York, Article History Close. Share Cancel. Revoke Cancel.

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