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Vocabulary List
What do you think the Based on the readings,
word means?
what does the word
mean? Examples?
1.
Genetic Drift
2.
Codominance
3. Incomplete
Dominance
4.
Polyploidy
5.
Multiple Alleles
6.
Polygenic Traits
7. Sex-Linked
Traits
8.
Karyotype
9.
Nondisjunction
10. Independent
Assortment
11. Pleiotropy
12. Recombination
Frequency
Genetic Variability
1. Genetic Drift  Genetic drift is evolution, or change in gene pool frequencies, resulting from random
chance. Genetic drift occurs most rapidly in small populations. In large populations, random deviations
in allele frequencies in one direction are more likely to be cancelled out by random changes in the
opposite direction.
It is the process of change in the genetic composition of a population due to chance or random events
rather than by natural selection, resulting in changes in allele frequencies over time. The effect of genetic
drift in large populations is usually negligible whereas in small populations, it predominates. In a small
population, genetic drift results in some alleles become more common while others become less common
over time.
2. Codominance  This is the situation in which two different alleles for a trait are expressed
unblended in the phenotype of heterozygous individuals. Neither allele is dominant or recessive, so that
both appear in the phenotype or influence it. Type AB blood is an example. Such traits are said to be
codominant.
It is where both alleles in a heterozygote are fully expressed. (E.g. red + white=red & white patches). It
is a condition in which the alleles of a gene pair in a heterozygote are fully expressed thereby resulting in
offspring with a phenotype that is neither dominant nor recessive. A typical example showing
codominance is the ABO blood group system. For instance, a person having A allele and B allele will have
a blood type AB because both the A and B alleles are codominant with each other.
3. Incomplete Dominance  This is where an allele for a specific trait is not completely dominant over
the other (E.g. Red+White = pink).
This is a kind of dominance occurring in heterozygotes in which the dominant allele is only partially
expressed, and usually resulting in an offspring with an intermediate phenotype. In incomplete
dominance, a heterozygous organism carrying two alleles wherein one is dominant and the other one is
recessive, (e.g. Aa), the dominant allele will only be partially expressed. Hence, the heterozygote (Aa) will
have an intermediate phenotype. A typical example is the color of the flower in which R symbolizes the
dominant allele for red pigment and r is the recessive allele for no pigment. In incomplete dominance, the
heterozygous plant carrying both alleles, Rr, will not be able to produce enough red pigment (since the
dominant allele is only partially expressed) and therefore will appear pink.
4. Polyploidy  Polyploidy is having one or more extra sets of chromosomes. It is the condition in which
a normally diploid cell or organism acquires one or more additional sets of chromosomes. In other words,
the polyploid cell or organism has three or more times the haploid chromosome number. Polyploidy
arises as the result of total nondisjunction of chromosomes during mitosis or meiosis. Polyploidy is
common among plants and has been, in fact, a major source of speciation in the angiosperms. Particularly
important is allopolyploidy, which involves the doubling of chromosomes in a hybrid plant. Normally a
hybrid is sterile because it does not have the required homologous pairs of chromosomes for successful
gamete formation during meiosis. If through polyploidy, however, the plant duplicates the chromosome
set inherited from each parent, meiosis can occur, because each chromosome will have a homologue
derived from its duplicate set. Thus, polyploidy confers fertility on the formerly sterile hybrid, which
thereby attains the status of a full species distinct from either of its parents. It has been estimated that up
to half of the known angiosperm species arose through polyploidy, including some of the species most
prized by man. Plant breeders utilize this process, treating desirable hybrids with chemicals, such as
colchicine, that are known to induce polyploidy. Polyploid animals are far less common, and the process
appears to have had little effect on animal speciation.
5. Multiple Alleles  This is a situation in which a gene has more than two alleles. The ABO blood type
system is an example. Multiple-allele series only partly follow simple Mendelian genetics.
This is where three or more alleles can determine the phenotype. Typically, there are only two alleles
for a gene in a diploid organism.
In Mendel's studies, he proposed that there are two alleles for every gene, the dominant of the two
having its phenotype expressed in a heterozygote. However, a gene can have more than two allelic forms
segregating within a population.
These genes are referred to as having multiple alleles. This does not mean that the gene in a particular
individual possesses more than two alleles. An individual can only have a maximum of two of the alleles,
one maternal and one paternal, no matter how many alleles exist in the population.
An example of multiple alleles of a gene is the C series in dogs. C is required for color while cc yields an
albino. The genotypes and phenotypes are as follows; C is a dog with color series expressed, cch is a
chinchilla patterned dog, cd is a white dog with dark eyes, cb is a pale gray dog, and c is an albino dog (pale
eyes and nose)
The series works in such a way that C>cch>cd>cb>c. When an allelic series is written like this, it means
that the allele to the left, in this case the C allele, is dominant to every allele to its right, while the next
allele (cch) is dominant to everything but the allele to its left (C), and so on. In this system, the c allele is
recessive to all of the other alleles.
6. Polygenic Traits  This is a trait that is controlled by a group of nonallelic genes. Polygenic traits are
controlled by two or more than two genes (usually by many different genes) at different loci on different
chromosomes. These genes are described as polygenes. Examples of human polygenic inheritance are
height, skin colour and weight. Polygenes allow a wide range of physical traits. For instance, height is
regulated by several genes so that there will be a wide range of heights in a population.
This is another exception to Mendel’s rules is polygenic inheritance. Often these traits are in fact
controlled by many genes on many chromosomes. Each dominant allele has an additive effect, so the
resulting offspring can have a variety of genotypes, from no dominant alleles to several dominant alleles.
In humans, some examples of polygenic traits are height and skin color. People are neither short nor tall,
as was seen with the pea plants studied by Mendel, which has only one gene that encodes for height.
Instead, people have a range of heights determined by many genes. Similarly, people have a wide range of
skin colors. Polygenic inheritance often results in a bell shaped curve when you analyze the population.
That means that most people are intermediate in the phenotype, such as average height, while very few
people are at the extremes, such as very tall or very short.
There may be 4 or 6 or more alleles involved in the phenotype. At the left extreme, individuals are
completely dominant for all alleles, and at the other extreme, individuals are completely recessive for all
alleles. Individuals in the middle have various combinations of recessive and dominant alleles. Other
polygenic traits in dairy cattle are of extreme economic importance in agriculture .
Most polygenetic traits are partially influenced by the environment. For example, height is partially
influenced by nutrition in childhood. If a child is genetically programmed to be average height but does
not get a proper diet, he or she may be below average in size.
7. Sex-linked Traits  Genes that are found on the sex chromosomes are called sex-linked genes. These
genes can be on the X chromosome or on the Y chromosome. If a gene is located on the Y chromosome, it
is a Y-linked gene. These genes are only inherited by males because, in most instances, males have a
genotype of (XY). Females do not have the Y sex chromosome. Genes that are found on the X chromosome
are called X-linked genes. These genes can be inherited by both males and females. Since genes for a trait
may have two forms or alleles, one allele is usually dominant and the other is recessive. Dominant traits
mask recessive traits in that the recessive trait is not expressed in the phenotype.
In X-linked recessive traits, the phenotype is expressed in males because they only contain one X
chromosome. The phenotype may be masked in females if the second X chromosome contains a normal
gene for that same trait. An example of this can be seen in hemophilia. Hemophilia is a blood disorder in
which certain blood clotting factors are not produced. This results in excessive bleeding that can damage
organs and tissues. Hemophilia is an X-linked recessive trait caused by a gene mutation . It is more often
seen in men than women. The image above depicts the inheritance pattern of the hemophilia trait when
the mother is a carrier and the father does not have the trait. The sons have a 50/50 chance of inheriting
the trait and the daughters have a 50/50 chance of being carriers of the trait. If a son inherits an X
chromosome with the hemophilia gene, the trait will be expressed and he will have the disorder. If a
daughter inherits the mutated X chromosome, her normal X chromosome will compensate for the
abnormal chromosome and the disease will not be expressed.
In X-linked dominant traits, the phenotype is expressed in both males and females who have an X
chromosome that contains the abnormal gene. If the mother has one mutated X gene (she has the
disease) and the father does not, the sons and daughters have a 50/50 chance of inheriting the disease. If
the father has the disease and the mother does not, all of the daughters will inherit the disease and none
of the sons will inherit the disease.
There are several disorders that are caused by abnormal sex-linked traits. In addition to hemophilia,
color blindness, Duchenne muscular dystrophy, and fragile-X syndrome are examples of X-linked
recessive disorders. A common Y chromosome linked disorder is male infertility.
8. Karyotype  A karyotype is the characterization of the chromosome complement of a species (such
as the shape, type, number, etc. of chromosomes). The karyotype of an organism is usually displayed in
photomicrographs wherein chromosomes are arranged in homologous pairs, and in descending order of
size and relative position of the centromere. Karyotype is used to study chromosomal aberrations,
cellular function, or taxonomic relationships, or to gather information about past evolutionary events.
Chromosomal karyotyping, in which chromosomes are arranged according to a standard classification
scheme, is one of the most commonly used genetic tests. To obtain a person’s karyotype, laboratory
technicians grow human cells in tissue culture media. After being stained and sorted, the chromosomes
are counted and displayed. The cells are obtained from the blood, skin, or bone marrow or by
amniocentesis or chorionic villus sampling, as noted above. The standard karyotype has approximately
400 visible bands, and each band contains up to several hundred genes. When a chromosomal aberration
is identified, it allows for a more accurate prediction of the risk of its recurrence in future offspring.
Karyotyping can be used not only to diagnose aneuploidy, which is responsible for Down syndrome,
Turner syndrome, and Klinefelter syndrome, but also to identify the chromosomal aberrations associated
with solid tumours such as nephroblastoma, meningioma, neuroblastoma, retinoblastoma, renal-cell
carcinoma, small-cell lung cancer, and certain leukemias and lymphomas. Karyotyping requires a great
deal of time and effort and may not always provide conclusive information. It is most useful in identifying
very large defects involving hundreds or even thousands of genes. (Genetic Testing)
9. Nondisjunction  (In mitosis) The failure of sister chromatids to separate during and after mitosis.
(In meiosis) The failure of homologous chromosomes to segregate or to separate during and after
meiosis. This could result to a condition wherein the daughter cells have an abnormal number of
chromosomes; one cell having too many chromosomes while other cell having none. Examples of
nondisjunction: Down syndrome, Triple-X syndrome, Klinefelter's Syndrome, Turner's Syndrome.
10.
Independent Assortment  This is where pairs of alleles of different genes separate
independently. The process of random segregation and assortment of chromosomes during anaphase I of
meiosis resulting in the production of genetically unique gametes. Gregor Mendel, a monk, who came up
with the Laws of Inheritance, including the Law of Independent Assortment (which refers to the random
assortment of alleles of unlinked loci) to describe the transmission of genes from parent organisms to
their offspring. The Law of Independent Assortment speaks of alleles of a gene separating independently
from alleles of another gene. Hence, the inheritance pattern of one trait will not affect the inheritance
pattern of another. For instance, the gene for the eye color is inherited independently from the gene for
hair color. That is, not all individuals with brown eyes will always have a black hair color; others may still
have a different hair color. It is because the gene coding for the eye color separates independently (and
randomly) from the gene coding for the hair color during formation of gametes (meiosis). Independent
assortment of genes is important to produce new genetic combinations that increase genetic variations
within a population.
11.
Pleiotropy  This is the phenomenon of one gene being responsible for or affecting more than
one phenotypic characteristic.
A trivial instance would be that the gene influencing the length of the left leg also influences the length
of the right leg. The growth of legs probably takes place through a growth mechanism controlling both
legs.
Pleiotropy exists because there is not a one-to-one relationship between the parts of an organism that
a gene influences and the parts of an organism that we recognize as characters.
Genes divide up the body in a different way from the human observer. Genes influence the developmental
process, and a change in development will often change more than one part of the phenotype. This
sometimes places a developmental constraint on the adaptation of organisms.
A Hawaiian spider illustrates the concept of pleiotropy. The length of matching legs is controlled by
the same genes, giving the spider a symmetrical appearance.
12.
Recombination Frequency  In the standard Mendelian dihybrid cross involving two genes
that undergo independent assortment (i.e., the two genes are on different chromosomes), we know that
homozygous parents (AABB and aabb) produce heterozygotic offspring (AaBb), and then these F1s
produce gametes that are 1/4 AB, 1/4 Ab, 1/4 aB, and 1/4 ab. Half of these gametes (AB and ab) are the
same genotype as those produced by the original homozygous parental (P) organisms, and the other half
(Ab and aB) are different. So, we say that the gametes produced by the F1 organisms are 50% "parentaltype" and 50% "non-parental-type".
If, however, the two genes are on the same chromosome, we would expect to get only two types of
gametes, AB and ab, produced by the F1s. These gametes are genetically the same as those produced by
the original parents P; that is, 100% of the F1 gametes are parental-type. Often, this situation is depicted
using a "+" (meaning wildtype) in place of A and B allele designations, and using the word
"nonrecombinant" to mean the same thing as parental-type.
Now we are ready to start considering the genetic effects of crossing-over. This produces two
"recombinant" chromatids, while leaving the other two chromatids unaltered. Thus, the four haploid
gametes have genotypes of four types, two of which are recombinant and two of which are
nonrecombinant. Note that these are the same four types that would have resulted from independent
assortment of genes on separate chromosomes.
So, for the case of independent assortment OR for the case of two genes on the same chromosome but
with a crossing-over event occurring between them during meiotic prophase I, we say we have a
"recombination frequency" of 50% (because half the gametes are non-parental type and half are parental
type).
Two genes that are close together, or a moderate distance apart, on the same chromosome (i.e., close
enough such that it will not always be that a crossing over event occurs between them) are said to be
"linked". That is, they will segregate together in meiosis more often than if they were on different
chromosomes ("unlinked").
So, for any two genes in any organism, there is a numerical value for the recombination frequency
between them, and this number must be between 0% and 50%. Genes on separate (non-homologous)
chromosomes have a recombination frequency of 50% and are "unlinked". Genes that are very close
together on the same chromosome have a recombination frequency very close to 0% and are "tightly
linked".
In simpler terms, when two genes are located on the same chromosome, the chance of a crossover
producing recombination between the genes is related to the distance between the two genes. Thus, the
use of recombination frequencies has been used to develop linkage maps or genetic maps.