(CHAPTER 8- Brooker Text) Chromosomal Number Variations October 30, 2007 BIO 184 Dr. Tom Peavy

Genome mutations –changes in the number of Sets of chromosomes –OR Numbers of individual chromosomes in a set Chromosome numbers can vary in two main ways –Euploidy (Polyploidy) Variation in the number of complete sets of chromosome –Aneuploidy Variation in the number of particular chromosomes within a set VARIATION IN CHROMOSOME NUMBER

Aneuploidy commonly causes an abnormal phenotype –It leads to an imbalance in the amount of gene products Alterations in chromosome number occur frequently during gamete formation –About 5-10% of embryos have an abnormal chromosome number –Indeed, ~ 50% of spontaneous abortions are due to such abnormalities Aneuploidy

The Sex-linked phenotypic effects may be due to 1. The expression of X-linked genes prior to embryonic X-inactivation 2. An imbalance in the expression of pseudoautosomal genes

Some human aneuploidies are influenced by the age of the parents –Older parents more likely to produce abnormal offspring –Example: Down syndrome (Trisomy 21) Incidence rises with the age of either parent, especially mothers Figure 8.19

Meiotic Nondisjunction Nondisjunction refers to the failure of chromosomes to segregate properly during anaphase Meiotic nondisjunction can produce haploid cells that have too many or too few chromosomes –If such a gamete participates in fertilization The resulting individual will have an abnormal chromosomal composition in all of its cells

Figure 8.24 All four gametes are abnormal During fertilization, these gametes produce an individual that is trisomic for the missing chromosome During fertilization, these gametes produce an individual that is monosomic for the missing chromosome

Figure % Abnormal gametes 50 % Normal gametes

In rare cases, all the chromosomes can undergo nondisjunction and migrate to one daughter cell This is termed complete nondisjunction –It results in a diploid cell and one without chromosomes –The chromosome-less cell is nonviable –The diploid cell can participate in fertilization with a normal gamete This yields a triploid individual

Randomly, one of the two X chromosomes is inactivated by the DNA becoming highly compacted –Most genes on the inactivated X cannot be expressed When this inactivated X is replicated during cell division –Both copies remain highly compacted and inactive X inactivation of the same chromosome is passed along to all future somatic cells The Mechanism of X inactivation (formation of Barr bodies)

Examples of Barr body formations –white and black variegated coat color is found in certain strains of mice –A female mouse has inherited two X chromosomes One from its mother that carries an allele conferring white coat color (X b ) One from its father that carries an allele conferring black coat color (X B )

Figure 7.4 At an early stage of embryonic development The epithelial cells derived from this embryonic cell will produce a patch of white fur While those from this will produce a patch of black fur

In contrast to animals, plants commonly exhibit polyploidy –30-35% of ferns and flowering plants are polyploid –Many of the fruits and grain we eat come from polyploid plants In many instances, polyploid strains of plants display outstanding agricultural characteristics –They are often larger in size and more robust Euploidy

Sterility is generally a detrimental trait However, it can be agriculturally desirable because it may result in –1. Seedless fruit Seedless watermelons and bananas –Triploid varieties Asexually propagated by human via cuttings –2. Seedless flowers Marigold flowering plants –Triploid varieties Developed by Burpee (Seed producers)

Interspecies Crosses Complete nondisjunction can produce an individual with one or more sets of chromosomes –This condition is termed autopolyploidy How could this occur?

Interspecies Crosses A much more common mechanism for changes in the number of sets of chromosomes is alloploidy –It is the result of interspecies crosses allodiploid

Allopolyploidy can arise due to a combination of nondisjuction, autopolyploidy and alloploidy An allotetraploid: Contains two complete sets of chromosomes from two different species