Thanks for Asking!
Various human characteristics are determined by dominant and recessive
properties of genes. In the very simplest terms, each characteristic
is determined by genes contributed by each parent. Smooth cheeks are
'controlled' by recessive properties of a gene, dimples are controlled
by a dominant property of a gene. However, reproduction isn't quite
that simple. Genes and chromosomes also contain an ancestral history
going back generations. Both dominant and recessive characteristics
may "skip" generations.
An individual's characteristics such as eye color, nose shape or
dimples are determined by the principles of heredity discovered by
Austrian Monk, Gregor Mendel (1822-1884). Mendel's experimentation
involved pea plants. He studied flower characteristics such as height
variation, color of the flowers, and the different textures of the
seeds. From this, he further studied inheritance, which is the process
of passing characteristics to the further generation. His research
lead to several conclusions:
1. The characteristics of any organism are passed from parent to child
by pieces of information called genes. Every gene represents a single
bit of information containing one characteristic.
2. Alleles are two or more genes that carry a piece of information
about a single characteristic. One of the allele pair may be for
smooth cheeks, and the other may be for dimples. Alleles are usually
found in pairs, one of which is dominant, or overpowering, and one of
the alleles recessive, which is masked by the dominant.
3. In reproduction, each gamete, or reproductive cell, (sperm or egg)
has only one of the allele pair present.
4. If a dominant and a recessive allele are both present, the
individual will be affected by the dominant allele.
American scientist T.H.Morgan, later showed that Mendel's conclusions
also apply to animals and humans.
To examine "dimples":
If the correct heredity gene is found in a human, the muscles which
cause dimples to appear are present. Before reproduction, sex cells
are created through the process of meiosis. These cells will contain
either of two genes which produce dimples, or one that produces the
muscles controlling dimples and one that doesn't. In reproduction,
each parent will provide one of these genes to the child. In humans
the dimples gene is dominant; therefore, if a dimples gene is present
the child will have dimples. There are three possible outcomes:
1. Two "dimple genes" are present and the offspring develops the
muscles. Subsequently, all the offspring's children will have the
ability.
2. One of each gene is present. Because the dimples gene is dominant,
the muscles will develop. They will have 1/2 dimples and 1/2
non-dimples gametes.
3. Two non-dimple genes are present. The muscle therefore can not be
developed. Only recessive genes will be passed to offspring.
This simple principle would seem to rule out the possibility that
dimples could be passed along by two parents without them. However, an
additional principle is needed to understand the complete process.
Mendel's Principle of Independent Assortment:
"According to the principle of independent assortment, different pairs
of alleles are passed to offspring independently of each other. The
result is that new combinations of genes present in neither parent are
possible. "
The quote above is taken from a nicely illustrated essay by Dennis
O'Neil, of Palomar.edu, which explains both Mendel's First and Second
Principles in greater detail:
http://anthro.palomar.edu/mendel/mendel_1.htm
Additional causes for 'odd' inheritance of genetic characteristics may
be due to a single abnormal gene, a chromosomal abnormality or a
genetic predisposition allied to other factors. Though scientists have
come a very long way in mapping the human genome, there are still
interactive factors which need to be further studied and explained.
You can learn more about it reading the Department of Energy's online
brochure: "To Know Ourselves" at:
http://www.ornl.gov/hgmis/publicat/tko/
An additional illustrated explanation of the the principles of
independent assortment may be found by visiting:
http://bioweb.cs.earlham.edu/9-12/fundamentals/
To gain a deeper understanding of the genetics inheritance process,
you might enjoy this student coin-flipping experiment outlined at:
http://www.kumc.edu/gec/lpweiss.html
An Adobe PDF document, entitled "Inheritance: Genetic Heirlooms" might
also provide additional insight:
http://genetics.glaxowellcome.com/pdf/information_backgrounders/GSKIBInheritance%2004.pdf
A slightly more technical document, Mendelian Genetics, using genetic
notation in simple examples, might also be helpful to you:
http://tooldoc.wncc.nevada.edu/chap20.htm
Search terms used:
inheritance recessive characteristics
And just as an additional side note, I have two dark-haired
stepchildren. Both parents and children have been DNA tested and typed
for bone-marrow transplant matching, so parentage is quite certain.
Both parents have light blonde hair, which is is classed a double
recessive characteristic.
I hope this provides a better understanding of the topic of genetics
and inheritance. If you would like me to clarify any part of my
answer, please feel free to ask.
Thank you for using Google Answers.
~larre-ga |
