When does nondisjunction occur in meiosis

Cell Structure 3. Membrane Structure 4. Membrane Transport 5. Origin of Cells 6. Cell Division 2: Molecular Biology 1. Metabolic Molecules 2. Water 3. Protein 5. Enzymes 6. 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. The isolation and microscopic observation of chromosomes forms the basis of cytogenetics and is the primary method by which clinicians detect chromosomal abnormalities in humans. A karyotype is the number and appearance of chromosomes, including their length, banding pattern, and centromere position. The karyotype is a method by which traits characterized by chromosomal abnormalities can be identified from a single cell. In the laboratory, the isolated cells are stimulated to begin actively dividing.

A chemical is then applied to the cells to arrest mitosis during metaphase. 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. 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.

Non-nuclear Genes and Their Inheritance. Polygenic Inheritance and Gene Mapping. Sex Chromosomes and Sex Determination. Sex Determination in Honeybees. Test Crosses. Biological Complexity and Integrative Levels of Organization.

Genetics of Dog Breeding. Human Evolutionary Tree. Mendelian Ratios and Lethal Genes. Environmental Influences on Gene Expression. Epistasis: Gene Interaction and Phenotype Effects. Genetic Dominance: Genotype-Phenotype Relationships. Phenotype Variability: Penetrance and Expressivity. Citation: Miko, I. Nature Education 1 1 Although mitosis and meiosis both involve cell division, they transmit genetic material in very different ways.

What happens when either of these processes goes awry? Aa Aa Aa. Gene Transmission in Mitosis. Figure 1. Gene Transmission in Meiosis. Figure 2: Examples of polytene chromosomes. 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. Keywords Keywords for this Article. Save Cancel. Flag Inappropriate The Content is: Objectionable. Flag Content Cancel. Email your Friend. Submit Cancel. This content is currently under construction. Explore This Subject. Gene Linkage. The Foundation of Inheritance Studies.

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