Aneuploidy and Polyploidy

Changes in Chromosome Number

• Aneuploidy and polyploidy are changes in chromosome number.
• Key terms include: aneuploidy, mosaic, euploid, polyploid, gynandromorph, and parthenogenesis are important terms.
• Objectives include explaining what leads to aneuploidy and polyploidy, relating these to viability and fertility, determining results of non-disjunction during meiosis/mitosis, and explaining X-inactivation

Aneuploidy

• Results from the loss or gain of one or more chromosomes.
• Aneuploids are individuals whose chromosome number is not an exact multiple of the haploid number (n) for the species.
• Monosomic individuals lack one chromosome from the diploid number (2n-1).
• Trisomic individuals have one extra chromosome in addition to the diploid set (2n+1).
• Tetrasomic individuals possess four copies of a particular chromosome (2n+2).
• Autosomal aneuploidy is generally harmful with monosomy typically lethal in humans.
• Trisomy is not always lethal in humans.
• Aneuploidy results from nondisjunction during meiosis.
• Trisomies of larger chromosomes are spontaneously aborted in early pregnancy, extra alleles result in excessive amounts of certain proteins and lead to lethal abnormalities.
• Trisomy 21 (small) is well tolerated.
• Trisomy 13 and 18 can survive birth, but have a short life expectancy.
• Other trisomies exist as mosaics.

X Chromosome Aneuploidy

• Instances: XX (female), XY (male), XO (Turner syndrome or female), XXX (female), XXY (Male with Klinefelter syndrome), XYY (male).
• Humans must have at least one X chromosome for viability and carries >1000 genes unrelated to sex determination and is needed for cell viability and development.
• It's generally okay to have 2 or even 3 X chromosomes.

X-Inactivation

• Most genes on all but one X chromosome are repressed in mammals.
• The inactivated X exists as a condensed Barr body near the nuclear membrane.
• Some genes near the telomere and centromere escape repression and are active, having equivalents on the Y chromosome.
• X inactivation: expression of each X chromosome at ~50%, resulting in mosaic expression.
• Occurs at random, early in the embryo (~800 cells)
• After mitosis, the same X remains inactivated in daughter cells.
• Calico patterns are almost always female (XBXb); B = black, b = orange. Examples: Males (XBY or XbY) and Females (XBXB or XbXb)
• 8% of males and 0.5% of females are affected by red-green color blindness

X-Inactivation in Drosophila

• Drosophila males (XY) increase gene product production from a single X to compensate.

Consequences of Aneuploidy

• In humans, 25-30% of pregnancies do not go to term, aneuploidy being a leading cause, often due to premature separation of sister chromatids in meiosis I.
• Aneuploidy can also happen with mitosis.
• XO (Turner Syndrome) is not viable if it happens during meiosis.
• Viable cases of Turner syndrome can be mosaics.

Aneuploid Mosaics

• Mitotic nondisjunction or chromosome loss during early embryo development can lead to mosaic aneuploidy.
• Chromosome loss in mitosis results in 50% diploid and 50% monosomic cells in the zygote, with proportions varying later in development.
• Non-disjunction in mitosis results in 50% trisomic and 50% monosomic cells in the zygote, with proportions of diploid, monosomic, and trisomic cells varying later.

Mosaicism Involving Sex Chromosomes

• Turner syndrome mosaic females carry both XX and XO cells.
• If an XX female loses an X chromosome during the first mitotic division after fertilization in Drosophila , the result is a gynandromorph, with equal parts male and female tissue, examples include bilateral and asymmetrical gynandromorphs.

Euploidy

• Only complete sets of chromosomes are present.
• Eukaryotes are mostly diploid (2n).
• Three or more complete sets of chromosomes are present in polyploids (3n, 4n, etc.).
• Only one complete set of chromosomes is in monoploids (1n) and are infertile.
• Monoploidy and polyploidy are not tolerated in many species.
• “somy” or “somic” refers to aneuploidy.
• “ploidy” or “ploid” refers to euploidy.

Euploidy: Haplodiploidy

• In ants, bees, and wasps, unfertilized (haploid) eggs develop into males, and fertilized (diploid) eggs develop into females.
• Parthenogenesis is reproduction without fertilization.
• Komodo dragons and male = ZZ and female = ZW. Haploid gametes Z and W. Diploid offspring males are ZZ and WW are lethal.

Triploids

• Almost always sterile because extra chromosomes can't pair at meiosis, resulting in unbalanced gametes.
• They are less likely to get a functional gamete set, as the more chromosomes there are.
• Seedless watermelons are triploid and triploidy has benefits for aquaculture.
• Triploids result from using pressure to retain the polar body or when a diploid is crossed with a tetraploid.

Tetraploids

• These can result in the chromosomes in a diploid tissue failing to segregate after replication during mitosis in germ cells.
• They can result through union of diploid gametes and are typically larger than their diploid ancestors.

Hybrids

• In plants, greater numbers of chromosomes often cause greater size because of a greater amount of gene product being present.
• The mating of a horse and a donkey results in a mule
• A mule's cells have 63 chromosomes, an odd number, as horse cells have 64 chromosomes and donkey cells have 62.
• The chromosomes from different species are not able to pair during the first part of meiosis, so mules are sterile.