hereditary components explained in human

hereditary components explained in human population

1. Question 1
Discontinuous variation refers to the variation in a characteristic within a population that has a
limited number of possible values, or categories. Individuals can only belong to one category.
These characteristics are usually controlled by alleles of one gene or a small number of genes,
and are not strongly influenced by the environment. An example of discontinuous variation is
blood type in humans. There are four categories (A, B, AB or O), and humans can only belong to
one of these categories. Some other examples of discontinuous variation are Eye colour and
Gender. Discontinuous variation, can lead to rapid adaptation when a particular phenotype is
highly advantageous. For instance, a mutation leading to antibiotic resistance in bacteria (a
discontinuous trait) can quickly become predominant in a population, demonstrating a significant
evolutionary advantage.
Question 2
Incomplete dominance
Incomplete dominance pertains to the genetic phenomenon in which the distinct gene products
from the two codominant alleles in a heterozygote blend to form a phenotype intermediate
between those of the two homozygotes. Incomplete dominance results from a cross in which
each parental contribution is genetically unique and gives rise to progeny whose phenotype is
intermediate. Incomplete dominance is also referred to as semi-dominance and partial
dominance. Mendel described dominance but not incomplete dominance. Had he worked
with Mirabilis jalapa rather than pea he would have pondered the cross of a plant with red
flowers by one with white flowers. All progeny were pink flowered, an intermediate phenotype.
An individual who is heterozygous for a gene has two different alleles, but it is not always
sufficient to produce an intermediate phenotype. Some genotypes that cause disease if both
alleles are recessive show no phenotype as heterozygotes. But, if each parent contributes the
mutated form of the gene, the resulting child will be homozygous, recessive, and sick. Other
diseases manifest as a phenotype that is intermediate to the parents. Incomplete dominance can
also result from the effect of one gene that masks the phenotype of another. Many color
pathways are complicated due to these epistatic interactions. The incomplete dominance
concerns the production of heterozygotes that possess intermediate traits between the two
homozygous traits. These heterozygous organisms have phenotypes that are a blend of the
phenotypes of their homozygous traits. The trait developed is neither dominant nor recessive.
Therefore, none of the alleles from the paired ones are expressed over the other for a specific
trait. The dilution of the dominant allele with respect to the recessive allele is also termed
incomplete dominance. There is a reduced ratio of dominant alleles. The variation of an
organism’s traits or properties is largely influenced by incomplete dominance.
Codominance
Codominance, as it relates to genetics, refers to a type of inheritance in which two versions
(alleles) of the same gene are expressed separately to yield different traits in an individual. As a
result, traits associated with each allele are displayed simultaneously. An example of
codominance is seen in the MN blood group system of humans. MN blood type is governed by
two alleles, M and N. Individuals who are homozygous for the M allele have a surface molecule
(called the M antigen) on their red blood cells. Similarly, those homozygous for the N allele have

2. the N antigen on their red blood cells. Heterozygotes—those with both alleles—carry both
antigens. An example of codominance for a gene with multiple alleles is seen in the human ABO
blood group system. Persons with type AB blood have one allele for A and one for B; the O allele
is recessive (its expression is masked by the other alleles). Examples of codominance in animals
include speckled chickens, which have alleles for both black and white feathers, and roan cattle,
which express alleles for both red hair and white hair. Codominance is also seen in plants. For
example, rhododendrons with simultaneous expression of red and white genes for flower colour
display flowers with both red and white petals. The codominance definition is an inheritance
pattern that allows both alleles to be expressed equally in the heterozygote. Inheritance patterns
are ways that different genes are inherited and expressed. Genes are sections of DNA that code
for a protein. Each gene comes in different versions, called alleles. The total combination of
alleles a person inherits for all genes is called the genotype. In humans, each person inherits two
alleles for each gene, although there can be more than two options for alleles for each gene. The
phenotype is how these alleles present and is the observable traits of the organism. Different
alleles can interact in different ways depending on the inheritance pattern.
Sex linkage
In humans, biological sex is determined by a pair of sex chromosomes: XX in females and XY in
males. The other 44 chromosomes are autosomes. Genes on either the X or Y chromosome
are sex-linked traits. Genes found on the X chromosome can be found in either males or females,
while genes found on the Y chromosome can only be found in males. Sex linkage refers to
characteristics (or traits) that are influenced by genes carried on the sex chromosomes. In
humans, the term often refers to traits or disorders influenced by genes on the X chromosome, as
it contains many more genes than the smaller Y. It applies to genes that are located on the sex
chromosomes. These genes are considered sex-linked because their expression and inheritance
patterns differ between males and females. While sex linkage is not the same as genetic linkage,
sex-linked genes can be genetically linked.
Question 3
Mapping of chromosomes
Our genetic information is stored in 23 pairs of chromosomes that vary widely in size and shape.
Chromosome 1 is the largest and is over three times bigger than chromosome 22. The 23rd pair
of chromosomes are two special chromosomes, X and Y, that determine our sex. Females have a
pair of X chromosomes (46, XX), whereas males have one X and one Y chromosomes (46, XY).
Chromosomes are made of DNA, and genes are special units of chromosomal DNA. Each
chromosome is a very long molecule, so it needs to be wrapped tightly around proteins for
efficient packaging. Near the center of each chromosome is its centromere, a narrow region that
divides the chromosome into a long arm (q) and a short arm (p). We can further divide the
chromosomes using special stains that produce stripes known as a banding pattern. Each
chromosome has a distinct banding pattern, and each band is numbered to help identify a
particular region of a chromosome. This method of mapping a gene to a particular band of the
chromosome is called cytogenetic mapping. For example, the hemoglobin beta gene (HBB) is
found on chromosome 11p15.4. This means that the HBB gene lies on the short arm (p) of
chromosome 11 and is found at the band labeled 15.4.With the advent of new techniques in DNA
analysis, we are able to look at the chromosome in much greater detail. Whereas cytogenetic
mapping gives a bird's eye view of the chromosome, more modern methods show DNA at a

3. much higher resolution. The Human Genome Project aims to identify and sequence the ~30,000
genes in human DNA.
Although a genetic map is an extension of the chromosome map described earlier, its creation is
a more detailed process by which the relationships among individual genes and their eventual
assignments to specific chromosome regions and linkage groups becomes possible. Sufficient
genes or polymorphic markers are mapped so as to effectively cover the whole genome.
Traditionally, extended kindreds are studied for the coinheritance of a particular phenotype,
whether physiologic or pathologic. Genes that cosegregate are established by this process; two
genes can be shown to be closely linked and, therefore, a component of a linkage group. This
sometimes involves genes of like function, the linkage group having arisen by gene duplication,
but often a cluster of genes do not have similar or related functions. Examples of functional
clusters are the HLA genes and the T-cell receptor genes, each of which each form close linkage
groups. The statistical analyses used in such situations are logarithm of the odds (lod) scores; a
score of 3.0 or more is the standard for establishing linkage between two genes. A particular
linkage group may be inferred from findings in other species, a concept known
as synteny. Although clustering of a particular set of genes may be shared across species, the
same linkage group may be found on a different chromosome in humans. For example,
the MHC linkage group is located on chromosome 6 in humans and on chromosome 17 in the
mouse. The recombination rate detected in families is a measure of the closeness of linkage.
Recombination may be current or ancestral and translates into map distance,
or centimorgans (cM). There are approximately 3500 cM in the human genome. This genetic
distance, which depends on recombination frequency, may be different from the physical
distance between linkage groups on the same chromosome. The HLA region itself amounts to
about 3.5 cM (i.e., about 0.1% of the genome).
Linked genes violate principles of independent assortment
Genetic linkage explains how two genes on a chromosome that are closely related are frequently
inherited together. Mendel's law of independent assortment does not apply to linkage. Inherited
genes are said to be independent of one another under the law. Since two genes are located on
the same chromosome, the linkage is an exception to this rule. Thus the independent assortment
is violated by linkage.
Although all of Mendel’s pea characteristics behaved according to the law of independent
assortment, we now know that some allele combinations are not inherited independently of each
other. Genes that are located on separate non-homologous chromosomes will always sort
independently. However, each chromosome contains hundreds or thousands of genes organized
linearly on chromosomes like beads on a string. The segregation of alleles into gametes can be
influenced by linkage, in which genes that are located physically close to each other on the same
chromosome are more likely to be inherited as a pair. However, because of the process of
recombination, or “crossover,” it is possible for two genes on the same chromosome to behave
independently, or as if they are not linked.

4. Homologous chromosomes possess the same genes in the same order, though the specific alleles
of the gene can be different on each of the two chromosomes. Recall that during interphase and
prophase I of meiosis, homologous chromosomes first replicate and then synapse, with like
genes on the homologs aligning with each other. At this stage, segments of homologous
chromosomes exchange linear segments of genetic material. This process is
called recombination, or crossover, and it is a common genetic process. Because the genes are
aligned during recombination, the gene order is not altered. Instead, the result of recombination
is that maternal and paternal alleles are combined onto the same chromosome. Across a given
chromosome, several recombination events may occur, causing extensive shuffling of alleles.
When two genes are located on the same chromosome, they are considered linked, and their
alleles tend to be transmitted through meiosis together. To exemplify this, imagine a dihybrid
cross involving flower color and plant height in which the genes are next to each other on the
chromosome. If one homologous chromosome has alleles for tall plants and red flowers, and the
other chromosome has genes for short plants and yellow flowers, then when the gametes are
formed, the tall and red alleles will tend to go together into a gamete and the short and yellow
alleles will go into other gametes. These are called the parental genotypes because they have
been inherited intact from the parents of the individual producing gametes. But unlike if the
genes were on different chromosomes, there will be no gametes with tall and yellow alleles and
no gametes with short and red alleles. If you create a Punnett square with these gametes, you will
see that the classical Mendelian prediction of a 9:3:3:1 outcome of a dihybrid cross would not
apply. As the distance between two genes increases, the probability of one or more crossovers
between them increases and the genes behave more like they are on separate chromosomes.
Geneticists have used the proportion of recombinant gametes (the ones not like the parents) as a
measure of how far apart genes are on a chromosome. Using this information, they have
constructed linkage maps of genes on chromosomes for well-studied organisms, including
humans.
Mendel’s seminal publication makes no mention of linkage, and many researchers have
questioned whether he encountered linkage but chose not to publish those crosses out of concern
that they would invalidate his independent assortment postulate. The garden pea has seven
chromosomes, and some have suggested that his choice of seven characteristics was not a
coincidence. However, even if the genes he examined were not located on separate
chromosomes, it is possible that he simply did not observe linkage because of the extensive
shuffling effects of recombination.