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 This graph shows the risk of Down syndrome in the fetus with increasing maternal age. Risk dramatically increases past a maternal age of 35.
The incidence of having a fetus with trisomy 21 increases dramatically with maternal age.

Visualize the addition of a chromosome that leads to Down syndrome in this video simulation .


An individual with more than the correct number of chromosome sets (two for diploid species) is called polyploid    . For instance, fertilization of an abnormal diploid egg with a normal haploid sperm would yield a triploid zygote. Polyploid animals are extremely rare, with only a few examples among the flatworms, crustaceans, amphibians, fish, and lizards. Polyploid animals are sterile because meiosis cannot proceed normally and instead produces mostly aneuploid daughter cells that cannot yield viable zygotes. Rarely, polyploid animals can reproduce asexually by haplodiploidy, in which an unfertilized egg divides mitotically to produce offspring. In contrast, polyploidy is very common in the plant kingdom, and polyploid plants tend to be larger and more robust than euploids of their species ( [link] ).

 Photo shows an orange daylily
As with many polyploid plants, this triploid orange daylily ( Hemerocallis fulva ) is particularly large and robust, and grows flowers with triple the number of petals of its diploid counterparts. (credit: Steve Karg)

Sex chromosome nondisjunction in humans

Humans display dramatic deleterious effects with autosomal trisomies and monosomies. Therefore, it may seem counterintuitive that human females and males can function normally, despite carrying different numbers of the X chromosome. Rather than a gain or loss of autosomes, variations in the number of sex chromosomes are associated with relatively mild effects. In part, this occurs because of a molecular process called X inactivation    . Early in development, when female mammalian embryos consist of just a few thousand cells (relative to trillions in the newborn), one X chromosome in each cell inactivates by tightly condensing into a quiescent (dormant) structure called a Barr body. The chance that an X chromosome (maternally or paternally derived) is inactivated in each cell is random, but once the inactivation occurs, all cells derived from that one will have the same inactive X chromosome or Barr body. By this process, females compensate for their double genetic dose of X chromosome. In so-called “tortoiseshell” cats, embryonic X inactivation is observed as color variegation ( [link] ). Females that are heterozygous for an X-linked coat color gene will express one of two different coat colors over different regions of their body, corresponding to whichever X chromosome is inactivated in the embryonic cell progenitor of that region.

 Photo shows a tortoiseshell cat with orange and black fur.
In cats, the gene for coat color is located on the X chromosome. In the embryonic development of female cats, one of the two X chromosomes is randomly inactivated in each cell, resulting in a tortoiseshell pattern if the cat has two different alleles for coat color. Male cats, having only one X chromosome, never exhibit a tortoiseshell coat color. (credit: Michael Bodega)

An individual carrying an abnormal number of X chromosomes will inactivate all but one X chromosome in each of her cells. However, even inactivated X chromosomes continue to express a few genes, and X chromosomes must reactivate for the proper maturation of female ovaries. As a result, X-chromosomal abnormalities are typically associated with mild mental and physical defects, as well as sterility. If the X chromosome is absent altogether, the individual will not develop in utero.

Questions & Answers

what difference between animal cell and plant cell
Lazarus Reply
what is fertilization?
Muhamed Reply
What kind of nutrients is composed of plants
Annie Reply
what is hormones
Igwe Reply
hormon is the chemical messanger
Genes can make someone dull?
21ecological instrument and their diagrams
Ayomide Reply
cell biology
I am sorry
for wat
no reason
nothing but speculate
what are the kidney disease
Immaculate Reply
kidney stones
it is a disease that affects the kidney
what are some lung diseases
what is micro-organism
Jackson Reply
what is the hypothesis
hypothesis is a proposed explanation for a phenomenon
hypothesis is raw materials
what is biology
biology is the study of living things and their interaction with their environment
what is zyogot?
what is fertilization?
zygote is an unfertilized eggs
fertilization refers to the fusion of a sperm and ovum
what does mean stigma
Amira Reply
what is the full of the MOST dangerous disease in the world where one stops sleeping and just dies :Hint ; FFI
God Reply
fatal familial insomnia which affects the thalamus
there are other dangerous diseases like CAD i.e coronary artery disease
what is matter
Thomas Reply
it is any thing that has weight and occupies space
matter is any substances that occupies spaces and has mass
describe photosynthesis
Mavis Reply
What is equilibrium
What is equilibrium
like corporal intern balance right?
on my own understanding is just a balanced state
photosynthesis is the process by which plants and other organisms convert light energy to chemical energy
what is a chromosome?
Wise Reply
Are thread-like structures located inside the nucleus of animal and plant cells.
what are the difference between Biotic community and Ecological nitche.
Ganiyat Reply
what is the celll
A cell is the simplest bit of living matter that exist independently
cell is the basic unit of life
what is ecdysis
what is genetics
Sebastian Reply
The cell is the simplest bit of living matter that can exist independently.

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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