University:
California State University, Northridge
Course:
BIOL 360 | Genetics
Academic year:

2007
Views:

470

Pages:

22
Author:

Hugh Kaufman

Variations in Chromosome Structure & Function Ch. 8 1 Variation in Chromosome Number ! Chromosome numbers can vary in two main ways – Euploidy " Variation in the number of complete sets of chromosome " Variations occur occasionally in animals and frequently in plants – Aneuploidy " Variation in the number of particular chromosomes within a set " Variations are always regarded as abnormal conditions 2 Polyploid organisms have three or more sets of chromosomes Individual is said to be trisomic Individual is said to be monosomic 3 Aneuploidy ! The phenotype of every eukaryotic species is influenced by thousands of different genes – The expression of these genes has to be intricately coordinated to produce a phenotypically normal individual ! Aneuploidy commonly causes an abnormal phenotype – It leads to an imbalance in the amount of gene products 4 Aneuploidy In most cases, these effects are detrimental They produce individuals that are less likely to survive than a euploid individual 5 Aneuploidy ! Alterations in chromosome number occur frequently during gamete formation – ~ 5-10% of embryos have an abnormal chromosome number – ~ 50% of spontaneous abortions are due to such abnormalities ! In some cases, an abnormality in chromosome number produces an offspring that can survive 6 7 Aneuploidy ! The autosomal aneuploidies compatible with survival are trisomies 13, 18 and 21 – These involve chromosomes that are relatively small ! Aneuploidies involving sex chromosomes generally have less severe effects than those of autosomes – This is explained by X inactivation " All additional X chromosomes are converted into Barr bodies – The phenotypic effects listed in Table 8.1 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 8 Aneuploidy ! 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 9 Aneuploidy ! Down syndrome is caused by the failure of chromosome 21 to segregate properly – This nondisjunction most commonly occurs during meiosis I in the oocyte ! The correlation between maternal age and Down symdrome could be due to the age of oocytes – Human primary oocytes are produced in the ovary of the female fetus prior to birth " They are however arrested in prophase I until the time of ovulation – As a woman ages, her primary oocytes have been arrested in prophase I for a progressively longer period of time " This added length of time may contribute to an increased frequency of nondisjunction 10 Euploidy ! Most species of animals are diploid (2n) ! In many cases, changes in euploidy are not tolerated – Polyploidy in animals is generally a lethal condition ! Some euploidy variations are naturally occurring – Female bees are diploid – Male bees (drones) are monoploid " Contain a single set of chromosomes 11 Euploidy ! 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 diplay outstanding agricultural characteristics – They are often larger in size and more robust 12 Euploidy ! Polyploids having an odd number of chromosome sets are usually sterile – These plants produce highly aneuploid gametes " Example: In a triploid organism there is an unequal separation of homologous chromosomes (three each) during anaphase I Each cell receives one copy of some chromosomes and two copies of other chromosomes 13 Euploidy ! ! Sterility is generally a detrimental trait It is 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) 14 Natural And Experimental Ways To Produce Variations In Chromosome Number ! There are three natural mechanisms by which the chromosome number of a species can vary – – – 1. 2. 3. Meiotic nondisjunction Mitotic abnormalities Interspecies crosses 15 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 16 Meiotic Nondisjunction 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 All four gametes are abnormal 17 Meiotic 50 % Abnormal gametes Nondisjunction 50 % Normal gametes 18 Meiotic Nondisjunction ! 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 19 20 Mitotic Abnormalities ! Abnormalities in chromosome number often occur after fertilization – In this case, the abnormality occurs in mitosis not meiosis – 1. Mitotic disjunction " Sister chromatids separate improperly – This leads to trisomic and monosomic daughter cells – 2. Chromosome loss " One of the sister chromatids does not migrate to a pole – This leads to normal and monosomic daughter cells 21 Changes in Chromosome Number 22

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