How does the possible impact of a chromosome

Here, capital letters represent the maternal chromosome, and lowercase letters represent the paternal chromosome:. When these chromosome pairs are reshuffled through independent assortment , they can produce eight possible combinations in the resulting gametes:.

A mathematical calculation based on the number of chromosomes in an organism will also provide the number of possible combinations of chromosomes for each gamete. In particular, Sutton pointed out that the independence of each chromosome during meiosis means that there are 2 n possible combinations of chromosomes in gametes, with "n" being the number of chromosomes per gamete.

Thus, in the previous example of three chromosome pairs, the calculation is 2 3 , which equals 8. Furthermore, when you consider all the possible pairings of male and female gametes, the variation in zygotes is 2 n 2 , which results in some fairly large numbers.

But what about chromosome reassortment in humans? Humans have 23 pairs of chromosomes. That means that one person could produce 2 23 different gametes. In addition, when you calculate the possible combinations that emerge from the pairing of an egg and a sperm, the result is 2 23 2 possible combinations.

However, some of these combinations produce the same genotype for example, several gametes can produce a heterozygous individual. Of course, there are more than 23 segregating units Hirsch, While calculations of the random assortment of chromosomes and the mixture of different gametes are impressive, random assortment is not the only source of variation that comes from meiosis. In fact, these calculations are ideal numbers based on chromosomes that actually stay intact throughout the meiotic process.

In reality, crossing-over between chromatids during prophase I of meiosis mixes up pieces of chromosomes between homologue pairs, a phenomenon called recombination. Because recombination occurs every time gametes are formed, we can expect that it will always add to the possible genotypes predicted from the 2 n calculation.

In addition, the variety of gametes becomes even more unpredictable and complex when we consider the contribution of gene linkage. Some genes will always cosegregate into gametes if they are tightly linked, and they will therefore show a very low recombination rate.

While linkage is a force that tends to reduce independent assortment of certain traits, recombination increases this assortment. In fact, recombination leads to an overall increase in the number of units that assort independently, and this increases variation. While in mitosis, genes are generally transferred faithfully from one cellular generation to the next; in meiosis and subsequent sexual reproduction , genes get mixed up.

Sexual reproduction actually expands the variety created by meiosis, because it combines the different varieties of parental genotypes. Thus, because of independent assortment, recombination, and sexual reproduction, there are trillions of possible genotypes in the human species.

During cell division, chromosomes sometimes disappear. This occurs when there is some aberration in the centromere , and spindle fibers cannot attach to the chromosome to segregate it to distal poles of the cell. Consequently, the lost chromosome never properly groups with others into a new nuclear envelope , and it is left in the cytoplasm , where it will not be transcribed.

Also, chromosomes don't always separate equally into daughter cells. This sometimes happens in mitosis, when sister chromatids fail to separate during anaphase. One daughter cell thus ends up with more chromosomes in its nucleus than the other. Likewise, abnormal separation can occur in meiosis when homologous pairs fail to separate during anaphase I.

This also results in daughter cells with different numbers of chromosomes. The phenomenon of unequal separation in meiosis is called nondisjunction. If nondisjunction causes a missing chromosome in a haploid gamete, the diploid zygote it forms with another gamete will contain only one copy of that chromosome from the other parent, a condition known as monosomy.

Conversely, if nondisjunction causes a homologous pair to travel together into the same gamete, the resulting zygote will have three copies, a condition known as trisomy Figure 3. The term " aneuploidy " applies to any of these conditions that cause an unexpected chromosome number in a daughter cell. Aneuploidy can also occur in humans. For instance, the underlying causes of Klinefelter's syndrome and Turner's syndrome are errors in sex chromosome number, and Down syndrome is caused by trisomy of chromosome However, the severity of phenotypic abnormalities can vary among different types of aneuploidy.

In addition, aneuploidy is rarely transferred to subsequent generations, because this condition impairs the production of gametes. Overall, the inheritance of odd chromosome number arises from errors in segregation during chromosome replication. Often, it is these very exceptions or modifications of expected patterns in mitosis and meiosis that enrich our understanding of how the transfer of chromosomes is regulated from one generation to the next.

Belling, J. On the attachment of non-homologous chromosomes at the reduction division in certain chromosome daturas. Proceedings of the National Academy of Sciences 12 , 7—11 Farmer, J. On the maiotic phase reduction divisions in animals and plants. Quarterly Journal of Microscopical Science 48 , — Gilbert, S.

Hirsch, J. Behavior genetics and individuality understood. Science 42 , — doi Uniqueness, diversity, similarity, repeatability, and heritability. International Journal of Comparative Psychology 17 , — Paweletz, N.

Walther Flemming: Pioneer of mitosis research. Nature Reviews Molecular Cell Biology 2 , 72—75 doi Chromosome Theory and the Castle and Morgan Debate. Changes in chromosome structure include the following: Translocations A translocation occurs when a piece of one chromosome breaks off and attaches to another chromosome. Deletions Deletions occur when a chromosome breaks and some genetic material is lost.

Duplications Duplications occur when part of a chromosome is abnormally copied duplicated. Inversions An inversion occurs when a chromosome breaks in two places; the resulting piece of DNA is reversed and re-inserted into the chromosome.

Isochromosomes An isochromosome is a chromosome with two identical arms. Dicentric chromosomes Unlike normal chromosomes, which have one centromere, a dicentric chromosome contains two centromeres.

Ring chromosomes Ring chromosomes usually occur when a chromosome breaks in two places, typically at the ends of the p and q arms, and then the arms fuse together to form a circular structure. Topics in the Variants and Health chapter What is a gene variant and how do variants occur?

How can gene variants affect health and development? Do all gene variants affect health and development? What kinds of gene variants are possible? Can a change in the number of genes affect health and development? Can changes in the number of chromosomes affect health and development? Can changes in noncoding DNA affect health and development? Can changes in mitochondrial DNA affect health and development?

What are complex or multifactorial disorders? What does it mean to have a genetic predisposition to a disease? How are gene variants involved in evolution?

The target DNA sequences may consist of either a single gene or a collection of genes spread out along the length of a chromosome. FISH procedures are now routinely employed in clinical cytogenetics. Spectral karyotyping provides an overview of any gross rearrangements and changes in chromosome number that have occurred in a patient's cells.

Using gene-specific probes, cytogeneticists can also positively identify the genes affected by chromosomal mutations. More recently, researchers have additionally begun to employ comparative genomic hybridization to analyze small quantitative differences between individuals' DNA, including copy number variations CNVs. Outside the clinic, FISH is one of many techniques biologists use to investigate the structure of chromosomes and their organization within the nucleus. Although chromosomes may appear to be static structures when viewed under a microscope, cytogeneticists know that chromosomes are actually dynamic assemblies made up of a DNA-protein complex called chromatin.

Chromatin undergoes dramatic changes in packing during the cell cycle, and its structure also varies locally along the length of each chromosome. Transcriptionally active chromatin, or euchromatin, has a different composition than silent chromatin, or heterochromatin. The inactive X chromosome in female mammals is a special case in which heterochromatin extends along the entire length of a chromosome.

Some chromatin specializations are essential for normal chromosome behavior. For example, centromeres contain a unique chromatin that is required for chromosome attachment to the mitotic spindle. Likewise, chromosome integrity depends on the assembly of a specialized chromatin found exclusively at the telomeres. Other less defined aspects of chromosome structure may also be important in positioning individual chromosomes with the nucleus.

For instance, mounting evidence seems to indicate that chromosomes occupy discrete territories in the interphase nucleus; this marks a significant departure from the previously accepted idea that chromosomes are randomly organized during interphase. In this era of comparative genomics, cytogenetics is also offering insights into evolution. Using cross-species FISH, scientists have identified groups of genes, called synteny groups , that maintain the same linkage relationships with each other across species boundaries.

Synteny data reveal numerous chromosomal rearrangements that have occurred during the course of evolution. Taken together with DNA sequence information, synteny data are proving useful for detecting genome duplications and for constructing phylogenetic trees.

The collection of articles in this topic room is intended to provide students with an introduction to chromosome biology and an appreciation of the experimental evidence that has led to the current state of understanding. Cytogenetics is a broad and growing field of research, and many topics have not been discussed in detail. The editors hope that this collection will grow over time as new discoveries are made and gaps in the current collection are filled. To this end, teachers and researchers are encouraged to contribute new articles to the collection after consultation with the editors.

Chromosome Mapping: Idiograms. Human Chromosome Translocations and Cancer. Karyotyping for Chromosomal Abnormalities.