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Lecture 22, part 2 of 4
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CHROMOSOMAL ABERRATIONS - HETEROPLOIDY


REFERENCE:
SINGH. CHAPTER 6B p.131
SCHULTZ-SCHAEFFER, J. 1980. Chapter 16 p.271-291



A. Definition of aneuploidy

B. Trisomy

C. Tetrasomy

D. Nullisomy

E. Monosomy




A. Definition of aneuploidy

Definition: cells, tissues and individuals having one or more whole chromosomes or segments of chromosomes absent from or in addition to a euploid complement.

Aneuploids: 

Aneuploidy is useful because it allows the analysis of meiotic phenomena and the construction of special genetic combinations.

Plants tolerate aneuploidy better than animals

As seen previously, plants seem to allow tremendous variation in genome size, and within a given species, many levels of ploidy are found in healthy plants. In fact, some tetraploids are heartier than their diploid counterparts. For example, commercially grown strawberries are octoploid. We will see in this section that just about every possible kind of aneuploidy is also tolerated in plants, often with little or no deleterious effect. 

Animals are far less tolerant of aneuploidy. In particular, all but a few trisomies or tetrasomies cause spontaneous abortion. That is, developmental abnormalities are so severe that the embryo does not survive. The few exceptions are virtually all deleterious.

Interestingly, human cells in culture can grow with triploid or greater chromosomal complements. For example, human carcinoma cell lines such as SK-BR-3 can have 80 to 84 chromosomes, compared to the normal diploid complement of 46. (Source: American Type Culture Collection, http://www.atcc.org ). The significance of this observation seems to be that aneuploidy in animals has less of an effect on the housekeeping functions of the cell, but a greater effect on the processes of development and differentiation.

B. Trisomy

Trisomic: cells, tissues of individuals with one or more extra chromosomes or chromosome segments as compared to the euploid.

- very common in plants.

Various types of trisomics:
 


Figure 6.23 Diagrammatic sketch of various types of trisomics
 

1. Primary trisomics: the extra chromosome(s) is completely homologous to one of the chromosome pairs in the complement (e.g. 2n = 2x+1)

2. Secondary trisomics: the extra chromosome(s) has two identical arms.

3. Tertiary trisomics: the extra chromosome(s) consists of two non homologous chromosome segments. They occur in the progeny of translocated heterozygotes. The chromosome ends are homologous with two different chromosomes.

4. Telotrisomics: The extra chromosome(s) is a telocentric chromosome.

5. Acrotrisomic: The extra chromosome(s) is an acrocentric chromosome.

6. Compensating trisomics: A chromosome is missing and is genetically compensated by two other modified chromosomes. These can be quite complex.

During zygotene pairing in normal diploids, the extra chromosome arms will attempt to participate in pairing as much as possible. After desynapsis, chromosomes remain paired at the ends at diakinesis.  Primary trisomics can form chains of three chromosomes in meiosis but never rings of three. Secondary trisomics can form rings of three chromosomes. Tertiary trisomics form chains of five but never rings of five while telosomic trisomics form a chain of three chromosomes but never a ring. 




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