(CHAPTER 8- Brooker Text)

Chromosomal Number

Variations

October 30, 2007BIO 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 to1. The expression of X-linked genes prior to embryonic X-inactivation2. 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 8.24All 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 8.24

50 % 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 (Xb)• One from its father that carries an allele conferring black coat

color (XB)

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