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.
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Bio 2. The Success Code. Why Science Matters. The Beyond. Plant ChemCast. Postcards from the Universe. Brain Metrics. Mind Read. In a study of oral cancer, SNPs were analyzed in patients and control persons [7]. Scanning the material revealed , combinations of two SNP genotypes, including , combinations that were common to both patients and controls, and 46, present only in patients.
Combinations from these clusters were present in the genomes of of the oral cancer patients, and not in the genomes of any of the control persons. The two clusters contained 52 and 43 combinations, and were very different from each other, with no overlap between the represented SNP genotypes, indicating two completely different genetic subgroups of patients with oral cancers.
One cluster contained combinations of SNP genotypes from a single biological pathway, and the patients in this cluster harbored relatively large numbers of these combinations in their genomes. The other cluster contained combinations from three different biological pathways, and patients in this cluster showed relatively few combinations in their genomes.
These findings suggest that the accumulation of few genetic variants in several pathways can carry the same disease risk as the accumulation of many genetic variants in a single pathway. There are several methods of scanning a dataset of genetic variants for combinations of these variants. Small datasets can be directly scanned for combinations containing only a few variants. In larger datasets, it may be necessary to scan subsets of the variants to identify combinations.
When a dataset is obtained from groups of patients and control persons, it can be helpful to separate the combinations occurring exclusively in patients from the combinations found in both controls and patients and those occurring exclusively in control persons.
Combinations occurring exclusively in patients may be significantly associated with the investigated disorder. However, in four studies of such combinations, no single combination was found to be significantly associated with the investigated disorder [7] , [8] , [10] , [25]. Obviously, a combination that occurs only once in the study material will be present in either a patient or a control person, and such a combination will not be statistically significantly associated with a disorder.
However, even combinations common among several patients and not present in controls are sometimes not found to be significantly associated with the disorder. This may be at least partly because the groups of patients having a common combination are too small to obtain statistical significance.
To analyze larger groups of patients, it is sometimes possible to extract clusters of combinations that show some similarity, for example, where each combination in a cluster contains a common SNP genotype. Such clusters may show significant association with a disorder. Patients having one or more of the combinations from a cluster in their genome are considered to belong to that cluster. Investigations of clusters of combinations occurring exclusively in patients have found that, although clusters are significantly associated with the disorder, individual combinations from these clusters do not show significant association with the disorder [7] , [8] , [10] , [25].
These findings raise questions regarding the interpretation of a cluster of combinations that is significantly associated with a disorder. It is possible that a cluster of combinations that is significantly associated with a disorder could represent a general risk factor for the disorder, whereas the accumulation of combinations from the cluster in the genome of a patient may be regarded as a personal risk factor.
In this respect, it would be interesting to assess whether the accumulation of many combinations in the genome results in higher risk or more severe disease, compared to the accumulation of fewer combinations from the clusters. There are also unanswered questions regarding the generalizability of the findings from the few studies of combinations of genetic variants occurring exclusively in patients [7] , [8] , [10] , [25].
Is it a coincidence that, in all four studies, the groups of patients harboring a common combination are too small for any single combination to achieve statistical significance? Or is this high degree of genetic heterogeneity typical for polygenic disorders?
Answering this question will require more studies of combinations of genetic variants that occur exclusively in patients.
Fortunately, it may be relatively easy to perform such studies as a supplement to new or ongoing studies, or by analyzing the genetic variants already reported in previous studies. National Center for Biotechnology Information , U.
Comput Struct Biotechnol J. Published online Mar Author information Article notes Copyright and License information Disclaimer. Erling Mellerup: kd.
This article has been cited by other articles in PMC. Abstract In studies of polygenic disorders, scanning the genetic variants can be used to identify variant combinations. Keywords: Genetic variants, Polygenic disorder, Combinations of genetic variants, Patient-specific combinations.
Introduction A polygenic disorder is caused by the combined effects of multiple genes. Methods for Studying Combinations of Genetic Variants 2. Technical Methods Genome-wide association studies and studies of selected genes can produce datasets that include billions of possible genetic variant combinations.
Non-technical Methods If a study of genetic variants includes too many combinations to allow analysis with the available technical tools, various methods can be applied to select smaller subgroups of combinations.
Open in a separate window. Statistics For the analysis of polygenic disorders, chi-square, z-test or similar tests can be applied to determine whether the distribution of a genetic variant combination significantly differs between patients and control subjects. Table 2 A cluster of 16 combinations of four SNP genotypes.
Combinations of Genetic Variants in Clinical Studies Clinical studies of genetic variant combinations have primarily focused on potential associations between two-variant combinations and the disorder of interest.
Discussion There are several methods of scanning a dataset of genetic variants for combinations of these variants. This process is also known as recombination. During prophase I, chromosomes condense and become visible inside the nucleus. As the nuclear envelope begins to break down, homologous chromosomes move closer together.
The synaptonemal complex, a lattice of proteins between the homologous chromosomes, forms at specific locations, spreading to cover the entire length of the chromosomes. The tight pairing of the homologous chromosomes is called synapsis.
In synapsis, the genes on the chromatids of the homologous chromosomes are aligned with each other. The synaptonemal complex also supports the exchange of chromosomal segments between non-sister homologous chromatids in a process called crossing over. The crossover events are the first source of genetic variation produced by meiosis.
A single crossover event between homologous non-sister chromatids leads to an exchange of DNA between chromosomes.
Following crossover, the synaptonemal complex breaks down and the cohesin connection between homologous pairs is also removed. At the end of prophase I, the pairs are held together only at the chiasmata; they are called tetrads because the four sister chromatids of each pair of homologous chromosomes are now visible.
During metaphase I, the tetrads move to the metaphase plate with kinetochores facing opposite poles. The homologous pairs orient themselves randomly at the equator. This event is the second mechanism that introduces variation into the gametes or spores. In each cell that undergoes meiosis, the arrangement of the tetrads is different.
The number of variations is dependent on the number of chromosomes making up a set. As we age, experience changes us so that we are different as well. And lots of the changes happen at a DNA level similar to what happened to the yellow mice. The best way to see these effects is to look at identical twins.
Identical twins have the same DNA but are certainly not exactly alike. One of the reasons is that as they age, these twins start using their DNA differently. In one twin, gene A may be going full blast while in the other, gene A is barely on. The bases are the same, they still have an identical DNA sequence. The difference is that they are using their genes differently -- their "dimmer switches" are set to different levels.
Same DNA, different people. So how many different variants of human are there? Lots and lots. Our variation has to do with our DNA, the genes we use and how we use them, our experiences, what we eat.
Maybe there are an infinite number of people possible! By Dr. Rama Balakrishnan, Stanford University. Original article about the mice with different colored hair A related question on how many different kinds of people are possible Why are identical twins different? DNA changes outside the gene make someone lactose tolerant. The Tech Interactive S. Market St. San Jose, CA The Tech is a registered c 3. Federal ID Its content is solely the responsibility of the authors and does not necessarily represent the official views of Stanford University or the Department of Genetics.
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