Is it Possible that DNA Genetic Material itself
to fool the Definite Determination? Part
VIII
Final Episode
Real Fathers may escape responsibility of
fathering; Lawyers shall stand on their toes on floor to question the protocols
used by laboratories, to determine a real father.
Whatever the forensic DNA test, it
should stand as a supporting evidence only, and cannot be depended upon as sole
evidence to conclude a case; conventional system of penal code procedure shall sum
up the results of the entire evidence along with the results of DNA test, whether
it is negative or positive, however modern technique science could be.
Further enhancing the knowledge of
genetics will further the strengthen us to understand the pros and cons of the
applications of DNA studies for any related fields.
Pairing
All animals and plants consists of
chromosomes in pairs within their each cell nucleus, and are known to be diploid
chromosomes or diploid chromosomal pattern. But the sperm cell and ova
of all animals would only contain the half the number of chromosomes in order
to ensure only the component quota within the Zygote, the first cell of next generation
– the fertilized ova, that goes on a division spree to attain the embryonic
status.
|
Common
Name
|
Genus and Species
All Consisting of 3.3 billion Base Pairs
|
Diploid Chromosome
|
|
Buffalo
|
Bubalus bubalis Riverine
|
23+1 = 48
|
|
Bubalus bubalis Swamp
|
24+1 = 50
|
|
|
Camel
|
Camelus dromedaroius
|
33+3+1 = 74
|
|
Cat
|
Felis catus -
all felines
|
18+1 = 38
|
|
Cattle
|
Bos taurus, B. indicus
|
29+1 = 60
|
|
Dog
|
Canis familiaris – most of the canines
|
38+1 = 78
|
|
Donkey
|
Equis asinus
|
30+1 = 62
|
|
Goat
|
Capra hircus
|
29+1 = 60
|
|
Horse
|
Equis caballus
|
31+1 = 64
|
|
Human
|
Homo sapiens
|
22+1 = 46
|
|
Pig
|
Sus scrofa
|
18+1 = 38
|
|
Rabbit
|
Oryctolagus cuniculus
|
21+1 = 44
|
|
Sheep
|
Ovis aries
|
26+1 = 54
|
Though it is said each and every cell of
the animals carry diploid set chromosomes, as in the above list, red blood
cells (RBC) of all mammals including human void of nucleus and do not carry any
chromosomes. Camals & Camelidae family mammals RBC do carry nucleus as well as chromosomes
is an exception. Except camels all listed above consists of one pair as sex
chromosomes X Y and the rest are known as autosomal chromosomes. Though
camels are also have one pair little sized sex chromosomes but adjoined with
another 3 pairs of autosomal chromosomes.
Human Sex
Determination
Paired non-sex chromosomes
22 are, for practical purposes, identical in size, shape, and position and
number of genes. Because each member of a pair of non-sex chromosomes contains
one of each corresponding gene, there is in a sense a backup for the genes on
those chromosomes. These are known as autosomal chromosomes.
The 23rd pair is the sex chromosomes (X and Y).
A chromosome is made of a very long strand of DNA and contains many genes (hundreds to thousands).
The pair of sex chromosomes determines whether a fetus becomes male or female. Males have one X and one Y chromosome, called Heterogamete XY. A male’s X comes from his mother and the Y comes from his father.
Homogamete XX of Females would have one X from mother and another X chromosome from father.
In all the mammals, the female is XX and the male is XY.
Primary
sex determination is
the determination of the gonads the precursor cells of Testicles or Ovaries.
In
mammals, primary sex determination is strictly chromosomal and is not usually
influenced by the environment.
Every individual must
have at least one X chromosome.
Since the female is XX,
each of her eggs has a single X chromosome.
The
male, being XY, can generate two types of sperm: half bear the X chromosome,
half the Y. If the egg receives another X chromosome from the sperm, the
resulting individual is XX, forms ovaries, and is female.
If
the egg receives a Y chromosome from the sperm, the individual is XY, forms
testes, and is male. The Y chromosome carries a gene that encodes a
testis-determining factor. This factor organizes the gonad into a testis rather
than an ovary.
The mammalian Y
chromosome is a crucial factor for determining sex in mammals. A person with
five X chromosomes and one Y chromosome (XXXXXY) would be male. Furthermore, an
individual with only a single X chromosome and no second X or Y (i.e., XO)
develops as a female and begins making ovaries, although the ovarian follicles
cannot be maintained. For a complete ovary, a second X chromosome is needed.
In mammalian primary sex
determination, there is no “default state” (as of, there is no definite either
male or female). The formation of ovaries and testes are both active,
gene-directed processes. Moreover, as we shall see, both diverge from a common
precursor, the bi-potential gonad.
The gonads cells
function uniquely from all the other embryonic cells. All other organ rudiments
can normally develop into only one type of organ. A lung rudiment can become
only a lung, and a liver rudiment can develop only into a liver.
The gonadal rudiment,
however, has two normal options. When it differentiates, it can develop into
either an ovary or a testis. The path of differentiation taken by this rudiment
determines the future sexual development of the organism. But, before this
decision is made, the mammalian gonad first develops through a bipotential (probable
for both) stage, an indifferent stage, during which time it has
neither female nor male characteristics.
In humans, the gonadal
rudiments appear in the intermediate mesoderm during the 4th week
and remain sexually indifferent until 7th week. From 8th
weekly begin to differentiate into female if the carried chromosome pair is XX
or into a male if the carried one are XY
Secondary sex
determination affects
the bodily phenotype (appearance) outside the gonads. A male mammal has a
penis, seminal vesicles, and prostate gland. A female mammal has a vagina,
cervix, uterus, oviducts, and mammary glands. In many species, each sex
has a sex-specific size, vocal cartilage, and musculature. These secondary
sex characteristics are usually determined by hormones secreted from the
gonads. However, in the absence of gonads, the female phenotype is
generated. When Jost
(1953) removed fetal rabbit gonads before they had
differentiated, the
resulting rabbits had a female phenotype, regardless of whether they were XX
or XY. They each had oviducts, a uterus, and a vagina, and each
lacked a penis and male accessory structures.
The general scheme of
mammalian sex determination is that, if the Y chromosome is absent, the gonadal
primordia develop into ovaries. The ovaries produce estrogen, a
hormone that enables the development of the Müllerian duct into the
uterus, oviducts, and upper end of the vagina.
If the Y chromosome is
present, testes form and secrete two major hormones. The first hormone—anti-Müllerian
duct hormone (AMH; also referred to as Müllerian-inhibiting
substance, MIS)—destroys the Müllerian duct. The second hormone—testosterone—masculinizes
the fetus, stimulating the formation of the penis, scrotum, and other portions
of the male anatomy, as well as inhibiting the development of the breast
primordia. Thus, the body has the female phenotype unless it is changed by the
two hormones secreted by the fetal testes.
I have refrained from going into the
details of DNA based explanation for sex determination, and have used very
conservative, still scientific chromosome based explanation, which is very
fitting and enough to understand the basics of sex determination.
DNA Replication is an important process in
life and is almost similar
to the DNA transcription for the Gene Expressing. Cells reproduce by splitting in two. Because each new cell
requires a complete set of DNA molecules, the DNA molecules in the original
cell must reproduce (replicate) themselves during cell division. Replication
happens in a manner similar to transcription, except that the entire
double-strand DNA molecule unwinds and splits in two. After splitting, bases on
each strand bind to complementary bases (A with T, and G with C) floating
nearby. When this process is complete, two identical double-strand DNA
molecules exist.
Mutation
To prevent
mistakes during replication, cells have a “proofreading” function to help
ensure that bases are paired properly. There are also chemical mechanisms to
repair DNA that was not copied properly. However, because of the billions of
base pairs involved in and the complexity of the protein synthesis process,
mistakes can happen. Such mistakes can occur for numerous reasons including
exposure to radiation, drugs, or viruses or drastic environment or for no
apparent reason. Minor variations in DNA are very common and occur in most
people. Most variations do not affect subsequent copies of the gene. Mistakes that are duplicated in
subsequent copies are called mutations. Mutations that affect the reproductive
cells may be passed on to offspring. Mutations that do not
affect reproductive cells affect the descendants of the mutated cell (for
example, becoming a cancer) but are not passed on to their offspring. Mutations
may be unique to an individual or family, and most mutations are rare.
Mutations that become so common that they affect more than 1% of a population
are called polymorphisms (for example, the human blood types A, B, AB, and O).
Most polymorphisms have no effect on the phenotype (appearance).
Mutations may
involve small or large segments of DNA. Depending on its size and location, the
mutation may have no apparent effect or it may alter the amino acid sequence in
a protein or decrease the amount of protein produced. If the protein has a
different amino acid sequence, it may function differently or not at all. An
absent or nonfunctioning protein is often harmful or fatal.
Natural selection refers to the concept that mutations
that impair survival in a given environment are less likely to be passed on to
offspring (and thus become less common in the population), whereas mutations
that improve survival progressively become more common. Thus, beneficial
mutations, although initially rare, eventually become common. The slow changes
that occur over time caused by mutations and natural selection in an interbreeding
population collectively are called evolution. The environment we live will play a role in the distant long
run, what organ to be improved and what to be lost for best survival chances.
Abnormalities-: We know that females have two X chromosomes, one from the
mother and one from the father. In certain ways, sex chromosomes function
differently than non-sex chromosomes. The smaller Y chromosome carries
the genes that determine male sex as well as a few other genes. The X chromosome
contains many more genes than the Y chromosome, many of which have functions
besides determining sex and have no counterpart on the Y chromosome. In
males, because there is no second X chromosome, these extra genes on the
X chromosome are not paired and virtually all of them are expressed.
Genes on the X chromosome are referred to as sex-linked, or X-linked,
genes. Normally, in the non-sex chromosomes, the genes on both of the pairs of
chromosomes are capable of being fully expressed. However, in females, most of the genes on one of the two X
chromosomes are turned off through a process called X inactivation
(except in the eggs in the ovaries). X inactivation occurs early
in the life of the fetus. In some cells, the X from the father becomes
inactive, and in other cells, the X from the mother becomes inactive.
Thus, one cell may have a gene from the person’s mother and another cell has
the gene from the person’s father. Because of X inactivation, the
absence of one X chromosome usually results in relatively minor
abnormalities. Thus, missing an X chromosome is far less harmful than missing a
non-sex chromosome. If a female has a disorder in which she has more than
two X chromosomes, the extra chromosomes tend to be inactive. Thus,
having one or more extra X chromosomes causes far fewer developmental
abnormalities than having one or more extra non-sex chromosomes. For example,
women with three X chromosomes (triple) XXX syndrome are often
physically and mentally normal. Most but in contrast males who have more than
one Y chromosome XYY are not physically and mentally normal.
Science will not bury the truth, unless
it is buried voluntarily!
In birds, along with some
reptiles, fishes and in some invertebrates sex determination chromosomes are
termed Z and W system, in which homo-gametes is male
ZZ and female is hetero-gametes as such completely reversed to that of
mammals’ XY system.
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