Genetics Notes
Chapter 9 - Chromosome Variation

These notes are provided to help direct your study from the textbook. They are not designed to explain all aspects of the material in great detail; that is what class time and the textbook is for. If you were to study only these notes, you would not learn enough genetics to do well in the course.

Chromosomal mutations

Changes in chromosome number

Most organisms are diploid, or have two sets of chromosomes. Many others are haploid, or have one set of chromosomes.

  • euploidy - a general term that refers to any number of sets of chromosomes
  • aneuploidy - a general term that refers to at least one chromosome more or less than the diploid number
  • polyploidy - refers to a cell having more than two sets of chromosomes
    1 set = haploid
    2 sets = diploid (the "normal" condition for most eukaryotic cells)
    3 sets or more = polyploid
    3 sets = triploid,
    4 sets = tetraploid
    Euploidy
    As it turns out, many species of plants are polyploid descendants of diploid ancestors. Polyploidy is tolerated rather well in many species of plants.

    In animals, polyploidy is not tolerated and very few polyploid species are known to exist. Those that do exist are usually asexual, parthenogenetic, or hermaphroditic. Most of the problems resulting from polyploidy occur during synapsis of homologues during prophase I.

    As plants do not have a chromosomal mechanism for sex determination, synapsis and subsequent disjunction is not as great a problem. In fact, most plants are monoecious.

    Also, organisms that have an odd number of sets, for example triploid or pentaploid, are usually sterile because during prophase I, three homologues may synapsis. Disjunction at anaphase can result in two one way and one the other way. This leads to variable numbers of chromosomes in the gametes. Another possibility is two of the three homologues synapse and the third does not synapse at all. This also leads to unbalanced gametes (Figure 9.28).

    Those organisms that have an even number of sets, for example tetraploid or hexaploid, will have an even number of homologues synapsis, and have a better chance of getting the same number of chromosomes to each gamete, though still not very likely.

    Any individual that has multiple sets from the same genome. . . 2N ----> 4N is called an autopolyploid. This individual will have four homologues synapsis during prophase I of meiosis. This is better than three or five, but there are still some problems.

    Polyploids can be created artificially by treatment with colchicine, which blocks spindle formation so that the chromosomes do not separate. Eventually, the nucleus reforms but with double the number of chromosomes. Within somatic cells, for example liver cells, many individual cells may be tetraploid, with no apparent ill effects for the individual. If, however, the sets represent separate genomes, things are better in terms of how meiosis proceeds and ultimately in terms of fertility. With two sets from two different genomes (allopolyploidy), the synapsis involves only two chromosomes, and works well (Figure 9.29).

    Allopolyploidy can come about in several ways.
    1.
  • a) Haploid gametes of two distinct species combine to form a hybrid.
  • The hybrid reproduces vegetatively until somatic doubling occurs in a cell of the floral meristem, which produces a flower stucture that has all chromosomes existing as homologous pairs.
  • At this point, meiosis is normal and sexual reproduction can occur via self-fertilization.

    2. Two distinct species produce unreduced (dipliod) gametes, which fuse to produce an allopolyploid, which is fertile if it can self-fertilize.

    Aneuploidy

    Aneuploidy refers to at least one more or one less chromosome than the diploid number. If an individual has 2N+1, in which one of the chromosomes has three copies, the individual is trisomic. Two extras (2N+2) is tetrasomic and only one copy (2N-1) is monosomic. Aneuploidy results from nondisjunction or failure of either homologous chromosomes in anaphase I or sister chromatids in anaphase II to separate at some stage of meiosis. This produces a gamete with one fewer or one extra chromosome than the normal haploid number (Figure 9.20).

    Upon fertilization, a monosomic or trisomic results. In humans, the most common viable aneuploids involve the sex chromosomes, giving rise to XXX, XO, or XXY individuals.

    There are no clear deleterious effects associated with the karyotype XYY. However, XYY men tend to be taller. Interestingly, there is a twenty-fold higher incidence of XYY males among the prison population than in the population as a whole, though XYY males in the non-prison population do not show an increased tendency for criminal behavior.

    Within the autosomal complement, the only aberrations to have survived to birth are:
    trisomy-21 = Down's syndrome (1/1000)
    trisomy-13 = Patau syndrome (1/15,000)
    trisomy-18 = Edward syndrome (1/7500, mostly female)

    Apparently, the deleterious effects of trisomic conditions are due to overproduction of certain proteins that lead to a number of developmental problems. The only ones to have been detected in live births involve the smaller chromosomes in the human complement.

    The only cases of monosomy found in live births involves the X chromosome, where XO individuals are viable, but are compromised in certain mental and physical characteristics. Monosomy in the autosomal complement is always inviable in humans, probably due to the unmasking of lethal mutations that exist in a recessive state, as well as disruption of developmental processes because of an underproduction of regulatory proteins.

    30% of all fertilizations spontaneously abort within the first three months. 50% of spontaneous abortions in the United States have some sort of obvious chromosomal rearrangement, such as monosomic or trisomic conditions. These two facts indicate that nondisjuction during gametogenesis or possibly during early mitotic division is a quite common occurrence and that most aneuploid fetuses fail to survive to birth.

    Centric fusion/fission
    These are also referred to as Robertsonian events. It occurs when two acrocentrics fuse to produce one metacentric. The process can also go backwards. The metacentric chromosome created by a centric fusion may retain both centromeres.

    These fusion/fission events change the chromosome number, but do not change the number of arms. The number of arms in the autosomal complement is referred to as the fundamental number. A centric fusion will change the number of chromosomes but will not change the fundamental number.

    As a rule, an individual that is heterozygous for a centric fusion will still be fertile. Disjunction will proceed normally. Thus, some species will have individuals with different diploid numbers, but the same fundamental number.

    Change in chromosomal structure
    1. deletions
    2. duplications
    3. inversions
    4. translocations

    Deletions can result from one or two breaks from a single chromosome.

    If a single break occurs across both chromatids after replication, a dicentric fragment can result (Figure 8.1). The fragment will be lost due to the lack of a centromere. The chromosome will not move properly during anaphase and will either break or be totally lost.

    If two breaks occur, the ends may rejoin and the interior fragment will be lost (Figure 9.10).

    The deletion can be detected due to mismatched pairs in a standard karyotype. It can be confirmed by the appearance of a bulge during synapsis at prophase I. Also, similar structures in the polytene chromosomes of fruit flies can be seen, and the absence of certain bands can be detected (figure 9.10.

    G- and C-banding
    G-banding - dark stains of AT rich regions
    C-banding - dark stain for heterochromatin
    G- and C-banding are quite useful in detecting chromosomal changes that occur within a species, as well as chromosomal changes that characterize differences between species (figure 9.4).

    Deletions are almost always detrimental. Most species cannot tolerate the loss of any chromosomal material. The deletion of a piece of one chromosome probably allows the unmasking of lethal recessive genes present on the homologue, as seen in a monosomic individual. In additions, underproduction of regulatory proteins probably disrupts fetal development. An example in humans is cri-du-chat, a deletion in the short arm of chromosome 5 (table 9.1). The infant makes a meowing sound like a cat, and is severely retarded.

    Duplications are usually detected by repeated band sequences, especially in polytene chromosomes. They can be tandem repeats such as ABCABC, or inverted repeats such as ABCCBA. Besides being caused by chromosomal breaks, they are usually caused by unequal crossing-over between chromatids. This results in one chromatid with an excess and the other with a deletion (figure 9.8). Serially duplicated genes are thought to have given rise to the moderately and highly repetitive DNA found in eukaryotes.

    The genes alpha-globin and beta-globin of hemoglobin represent a duplication that occurred in the genome leading to the vertebrates. The alpha-globin gene duplicated itself and the repeated sequence became the beta-globin gene. Both are very similar to each other in the placement of introns and exons and both genes share many amino acid positions.

    Inversions involve two breaks in a chromosome, where a piece is cut out, flipped end over end and reinserted. Crossing-over occurs between chromatids within the inversion loop (figure 9.12). The dicentric and acentric pieces are either lost totally or give rise to abnormal gametes that produce inviable offspring. An individual heterozygous for an inversion has a 50% reduction in fertility because of inviable gametes. An inversion represents a strong barrier to interspecies reproduction, a post-mating isolating mechanism. We will examine this in more detail when we get to population genetics.

    If the centromere is included within the inverted sequence, it is called a pericentric inversion (figure 9.14). If the centromere is not included within the inverted sequence, it is called a paracentric inversion (figures 9.12 and 9.13). An inversion can be detected either by a change in banding pattern or by the presence of inversion loops formed during synapsis in the heterozygous state. If the species has a mechanism to suppress crossing-over within the loop (as in Peromyscus), or a mechanism to sequester malformed chromosomes into polar bodies (as in Drosophila), the heterozygous state of the inversion loop is not detrimental. However, because the inverted segment of the chromosome has suppressed crossing-over, this region of the chromosome does not recombine and behaves as one large gene, called a supergene.

    Translocation is the transfer of a part of one chromosome to another nonhomologous chromosome. If it involves one break and a transfer to another chromosome, it is called a simple translocation. If the chromosomes exchange segments, the exchange is called a reciprocal translocation.

    An individual that is heterozygous for a reciprocal translocation, such as the hybrid that may result from the mating of two different species, will have a reduced fertility because of problems during meiosis (figure 9.17).

    Some chromosomal rearrangements (such as translocations) may change the position of a gene such that is finds itself in a highly active region of a chromosome. This results in increased expression of the gene. This change in expression is know as position effect. An example is in Burkitt's lymphoma, in which a translocation between chromosome 8 and 14 moves a growth factor gene close to the antibody genes, which are very active in a lymphocyte. This leads to the cell becoming cancerous (figures 9.32 and 9.31).

    Last update on 22 October 2004
    Provide comments to Dwight Moore at mooredwi@emporia.edu
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