+VARIATION IN CHROMOSOME NUMBER PART 1+

VARIATION IN CHROMOSOME NUMBER

May be result of:

• Change in numbers of sets of chromosomes.
• Variation in numbers of chromosomes within a set.

TERMINOLOGY FOR VARIATION IN CHROMOSOME NUMBER

• Euploid = organism that have chromosome numbers that is an exact multiple of the chromosome set.
• Triploid = organism with three multiples of the chromosome set.
• Polyploidy = organism with three or more sets of chromosomes.

ANEUPLOIDY AND GENE EXPRESSION

• Aneuploidy = change in numbers of a specific chromosome:

1. Trisomic (2n+1) = an extra copy of one chromosome.
2. Monosomic (2n-1) = a missing copy of one chromosome
   

• Aneuploidy commonly causes an abnormal phenotype usually due to change in production of gene product.

ANEUPLOIDY AND HUMANS

• Approximately 5-10% of fertilized human eggs have abnormal chromosome numbers.

1. Almost 50% of spontaneous abortions are due to to aneuploidy.
2. Changes in numbers of X chromosomes are usually non-lethal due to formation of inactive Barr bodies.
3. May be phenotypic consequences due to the expression of genes early in embryonic development (prior to X activation).
   

• Age of parents may also influence formation of aneuploid condition.
• An example is Down Syndrome in which chromosomes do not separate correctly during anaphase.
• May be due to age of the oocyte, but other factors may also contribute.
  

VARIATIONS IN EUPLOIDY IN ANIMALS

• Most animals are diploid and changes in numbers of chromosome sets are not tolerated.
• Exceptions are:

1. Male bees which are monoploid. Some vertebrate animals such as amphibians and reptiles are polyploid.
   

VARIATIONS IN EUPLOIDY IN ANIMAL TISSUES

• Tissues of body may have abnormal variations in the numbers of chromosome sets.

1. Liver cells in humans are triploid, tetraploid or octoploid – called endopolyploidy.
   

• Polytene chromosomes in the salivary glands of Drosophila consist of bundled replications of chromosome that lie together in a parallel fashion.

1. Can contain up to 9 sets of chromosomes.
2. The central location of aggregation is called chromocenter.
3. Polytene chromosomes consist of dark bands called chromomeres. About 95% of DNA is found in chromomeres.
   

VARIATIONS IN EUPLOIDY IN PLANTS

• Unlike animals, plants commonly exhibit euploidy.

1. Polyploidy plants often have outstanding agricultural characteristics or produce large flowers.

• Polyploids with an odd numbers of chromosome sets are usually sterile due to production of aneuploid gametes.

1. Sterility is often selected for in modern agriculture.