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Genetics: The Science
of Heredity
1.- Introduction
2.- Mendelian genetics
3.- Chromosomal theory of inheritance
4.- Mutations
5.- Human Inheritance
Cell division
Cell division
All cells are derived from preexisting cells.
Cell division is the process by wich cells produce new
cells.
Reasons for cell division
Cell growth
Repair and replacement of damaged cell parts: some
tissues must be repaired often such as the lining of gut,
white blood cells, skin cells with a short lifespan. Other
cells do not divide at all after birth such as muscle and
nerve.
Reproduction of the species.
Cell Cycle
During a cell’s life cycle there are various different phases.
The Cell Cycle includes two main parts:
Interphase: is the longest part of a cell’s life cycle and
is called “the resting stage” because the cell isn´t
dividing. during interphase. During interphase cell grows,
develops, makes a copy of its DNA, prepares to divide into
two cells and carry on all their normal metabolic functions.
Cell division: includes Mitosis (nuclear division) and
Cytokinesis (division of the cytoplasm).
Hydrogen bonds
Structure of DNA
DNA (The Double Helix)
Sugar
Deoxyribonucleic Acid (DNA) is a double-stranded,
helical molecule consisting of two sugar-phosphate
backbones on the outside, held together by hydrogen
bonds between pairs of nitrogenous bases on the inside.
The bases are of four types (A, C, G & T): pairing always
occurs between A & T and C & G (complementary base
pairing). This structure was first described by James
Watson and Francis Crick in 1953.
Phosphate
Sugar-phosphate
backbone
Base
A: adenine
C: cytosine
G: guanine
T: thymine
Watson & Crick
Replication of DNA
Since the instructions for making cell parts are encoded in
the DNA, each new cell must get a complete set of the
DNA molecules. This required that the DNA be copied
(replicated, duplicated) before cell division. This process
takes place during the Interphase stage of the Cell Cycle.
Each strand of the original molecule acts as a template for
the synthesis of a new complementary DNA molecule. The
two strands of the double helix are first separated by
enzymes. With the assistance of other enzymes, spare
parts aivalable inside the cell are bound to the individual
strands following the rules of complementary base
pairing : adenine (A) to thymine (T) and guanine (G) to
cytosine C. Finally, two strands of DNA are obtained from
one, having produced two daughter molecules which are
identical to one another and to the parent molecule.
MITOSIS
Mitosis is the process by which somatic cells divide and
multiply. It results in the production of two daughter cells
from a single parent cell. The two daughter cells are
identical to one another and to the original parent cell. In a
typical animal cell, mitosis can be divided into four
principal stages:
Prophase: The chromatin, diffuse in interphase,
condenses to form double-rod structures called
chromosomes. Each chromosome has duplicated and
now consists of two sister chromatids (the two rods).
Each chromatid in a chromosome is an exact copy of the
other. The two chromatids are held together by a structure
called centromere. At the end of the prophase, the
nuclear envelope breaks down.
Metaphase: The chromosomes align at the equatorial
plate and are held in place by microtubules attached to the
mitotic spindle and to part of the centromere.
Anaphase: The centromere divide. Sister chromatids
separate and move toward the corresponding poles.
Telophase: Daughter chromosomes arrive at the poles
and the microtubules dissapear. The condensed
chromatin expands and the nuclear envelope reappears.
Cytokinesis: The cytoplasm divides, the cell membrane
pinches inward ultimately producing two daughter cells.
Centromere
Sister
Chromatids
Mitosis phases as seen with microscope
Prophase
Anaphase
Metaphase
Telophase
MEIOSIS
Meiosis is a type of cell division by which sex cells (eggs and sperm) are
produced. Is the process by which a single parent diploid cell (both
homologous chromosomes) divides to produce four daughter haploids
cells (one homologous chromosome of the pair). Meiosis involves a
reduction in the amount of genetic material. It comprises two successive
nuclear divisions with only one round of DNA replication. Four stages can be
described for each nuclear division:
Interphase: before meiosis begins, genetic material is duplicated.
First division of meiosis:
Proohase 1: duplicated chromatin condenses. Each chromosome consists
of two, closely associated sister chromatids. Crossing over can occur during
the latter part of this stage.
Metaphase 1: Homologous chromosomes align at the equatorial plate.
Anaphase 1: Homologous pairs separate with sister chromatids remaining
together.
Telophase 1: two daughter cells are formed with each daughter containing
only one chromosome of the homologous pair.
Second division of meiosis:
Prophase 2: DNA does not replicate.
Metaphase 2: Chromosomes align at the equatorial plate.
Anaphase 2: centromeres divide and sister chromatids migrate separately to
each pole.
Telophase 2: cell division is complete.Four haploid daughter cells are
obtained.
Daughter cells have half the number of chromosomes found in the original
parent cell and with crossing over, are genetically different.
Comparison Meiosis and Mitosis
2.- Mendelian Genetics
Genetics Terms
Genetics: this is the part of Biology which studies the transmission
of characteristics from one individual to its descendants.
Character or trait: this is each one of the characteristics which
are inherited from parents by offspring (colour of eyes, skin, etc.)
Gene: Each piece of DNA of the nucleus of a cell in which the
information for a character is located.
Cross
This symbolised the sexual union of a
pair and the probable descendants:
Genotypes
Allele: they are the different forms of a gene.
Dominant allele: allele that is always expressed. A trait controlled
by a dominant allele always shows up in the organism when the
allele is present. It is symobolised with capital letters: A, B, C,etc.
Recessive allele: allele that is expressed only if dominant allele is
not present. A trait controlled by a recessive allele will only show
up if the organism does not have the dominant allele. It is
symobolised with lower case letters: a, b, c,etc.
X
Phenotypes
Gametes
Genotypes
black
white
Bb
bb
B
50%
b
50%
b
100%
Bb
bb
50% black
50% white
Phenotypes
Austrian botanist monk. Considered to be
the father of classical genetics.
He spent many years studying pea plants
(Pisum sativum) in the garden of the
monastery. He wanted to find out how
particular qualities are inherited when
plants are cross-fertilized.
Barely acknowledged during his lifetime,
Mendel’s work was rediscovered in 1900
and his laws were recognized.
Homozygotic: this is the individual which has two equal alleles for
a specific character. It is symbolised with the same letters: AA, aa,
BB, bb, etc.
Heterozygotic: this is the individual which has two different alleles
for a specific character. It is symbolised with one upper an one
case letter: Aa, Bb, Cc, bb, etc.
Genotype: this is the set of genes which a living being has in each
one of its cells.
Phenotype: this is the set of characteristics that are expressed or
manifested in a living being.
Punnet Square
Probability diagram ilustrating the
possible offspring of a mating:
Gregor Mendel
1822-1884
Yellow
Green
Parental
generation
Mendel’s Work
X
Gametes
Mendel’s First Law (of uniformity):
AA
aa
A
a
X
F1 generation
Aa
Gametes
F2 generation
75%
3
Aa
A
a
A
a
50%
50%
50%
50%
Gametes
A
a
A
AA
Aa
a
Aa
aa
25%
:
1
Punnet Square
The first thing Mendel discovered was that if he crossed two
different but homozygotic individuals, their descendants were
uniform (all the same). By crossing a homozygotic plant with
yellow seeds with another which was also homozygotic, but with
green seeds, the resulting plants only produced yellow seeds. The
AA plant only produces A gametes and the aa plant only a
gametes. The green colour of one of the parents did not appear in
the descendants. This is known as dominance: the “colour of
seed” character is inherited by means of a pair of alleles, one
dominant, which corresponds to “yellow” (A) and the other
recessive, which corresponds to “green” (a); the parents were
homozygotic AA and aa (yellow and green), which means that the
offspring would be heterozygotic Aa and yellow, because the
dominant allele does not allow the expression of the recessive
allele.
Mendel’s Second Law (independent segregation):
When Mendel crossed the descendants obtained (F1) together, he
found that the two kinds of seeds appeared in the second
generation (F2), three yellow and one green (3:1). The green
seeds appear again, which meant that F1, despite being yellow,
carried information for the colour green. In fact the seeds of the F1
generation were heterozygotic (Aa) and produced gametes of two
kinds, A and a. The two hereditary factors that provide
information on the same character did not fuse, and during
the process of fertilization of the gametes, they segregated, or
separated.
Smooth yellow
Rough green
Mendel’s Third Law (independent combination):
When studying the behaviour of two characters at the same time,
such as colour (yellow and green) and the texture of the surface
(smooth and rough), Mendel found that, if he began with smooth
yellow homozygotic seeds (AABB) and rough green seeds (aabb),
in the first generation he obtained uniform descendants which
were smooth and yellow (AaBb) but in the second generation he
obtained all the possible combinations of phenotypes in the
following proportions: 9:3:3:1. When he checked the characteres
separately, he saw that there were 12/16 yellow seeds as opposed
to 4/16 green ones, and 12/16 smooth ones as opposed to 4/16
rough ones, which means 75% and 25% (3:1) as happened in
accordance with the Law of independent segregation. Thus he
deduced that when various characters combine together,
heredity is independent and the proportions of phenotypes were
due to the dominance of the colour yellow and the smooth texture
as opposed to the colour green and the rough texture.
X
P
AABB
aabb
X
F1
AaBb
AaBb
AB Ab aB ab
25% 25% 25% 25%
G
Gametes
AB
Ab
aB
ab
AB
AABB
AABb
AaBB
AaBb
Ab
AABb
AAbb
AaBb
Aabb
aB
AaBB
AaBb
aaBB
aaBb
ab
AaBb
Aabb
aaBb
aabb
F2
Smooth
yellow
9
Rough
yellow
:
3
Smooth
green
:
3
Rough
green
:
1
Codominance
For all of the traits that Mendel studied, one allele was dominant
while the other was recessive. This is not always the case. For
some alleles, an inheritance pattern called codominance exists. In
codominance, the alleles are neither dominant nor recessive.
As a result, both alleles are expressed in the offspring.
Look the picture. Mendel’s principle of dominant and recessive
alleles does not expalin why the heterozygotic chickens have both
black and white feathers. The alleles for feather color are
codominant. As you can see, neither allele is masked in the
heterozygotic chickens. Notice also that the codominant alleles are
written as capital letters with superscripts (FB for black feathers
and Fw for white feathers. As the Punnet square shows,
heterozygotic chickens have the FB FW allele combination.
3.- Chromosomal Theory of Inheritance
Chromosomes
When Mendel made his discoveries he didn’t know where the genetic information
was to be found, nor what material it carried. Now we know that it is in the nucleus of
the eucaryotic cells, more specifically in the deoxyribonucleic acid or DNA.
In the nucleus of the cell, the DNA molecules are practically invisible during the
interphase period due to their thickness. However, during mitosis, each one of the
DNA molecules rolls itself up several times and combines with proteins in such a
way that it becomes a structure known as a chromosome, and it is visible under
microscope.
These are human chromosomes taken from a scanning electron microscope
Number of chromosomes
The number of chromosomes an organism has depends on its species. All
species have a characteristic number of chromosomes. The more complex an
organism is, the more chromosomes it will have. For example, humans are
complex organisms and have 46 chromosomes when bacteria have only one.
Chromosomes can be counted and are visible only during the cell division
(metaphase) because that is when the DNA is supercoiled and condensed to
facilitate distribution into daughter cells becoming into individual chromosomes.
They can be coloured using specific techniques to differentiate one from
another.
The parts of a chromosome are:
Chromatid: one of the two identical parts of the chromosome after DNA
replication.
Centromere: the point where the two chromatids and microtubules attach.
In higher organisms each cell usually contains two similar copies of each
chromosome. One of this copies is a maternal contribution and the other is a
paternal contribution. Together, these are called a homologous pair and each
alone is called a homologue.
Human Male Karyotype. The black and white banding pattern is due to a
particular staining technique used to visualize and identify the
chromosomes.
The haploid number of a cell refers to the total number of homologous pairs
in a cell (or number of unique chromosomes). In humans it is 23. The diploid
number of a cell refers to the total number of chromosomes in a cell and is
equal to two times the haploid number. In humans it is 46. If the haploid number
is thought of as n, the diploid number would be 2n.
Gametes are haploid (n) cells, because they have only one set of
chromosomes. Somatic cells are diploid (2n) cells because they have two sets
of chromosomes, one from the mother, one from the father. When a male and
female gamete join (fertilization), a new diploid organism is formed (n + n = 2n).
The Karyotype is the representation of entire metaphase chromosomes in a
cell, arranged in order of size.
chromosome
centromere
chromatids
Genes on Chromosomes
Remember that a gene is a segment of DNA. Each gene controls a trait. The alleles
are different forms of a gene. Genes are located on chromosomes, which are made
up of thousands of genes, there are about 35.000 in a single cell. Every cell in a body
contains an identical set of 46 chromosomes, grouped in 23 pairs. Because genes
are a part of chromosomes, they also come in pairs, and each gene pair works
together to control a specific function or activity within cell. In other words, each one
of us has two copies of every gene. One set of copies is inherited from our mother,
the other from our father. Each chromosome in a pair has the same genes but may
have different alleles for some genes and the same alleles for others.
The molecular gene is a definite sequence of bases in the DNA chain
wich together code for the production of a particular protein. A
difference in the sequence of bases between two copies of a gene would
mean that these two copies are different alleles.
Notice that each chromosome in the pair has the same genes. This
genes are lined up in the same order on both chromosomes. However,
the alleles for some of the genes might be different. For example, the
organism has the A allele on one chromosome and the a allele on the
other. As you can see, this organism is heterozygotic for some traits
and homozygotic for others.
Structural
mutations
4.- MUTATIONS
Mutations
Mutation is a change in the DNA of a cell, which is produced
spontaneously and randomly. Mutations can cause a cell to
produce an incorrect protein during protein synthesis. As a result,
the organism’s trait, or phenotype, may be different from what it
normally would have been.
Mutations appear naturally, but their frequency can be significantly
increased by the action of chemical products or radiations. These
factors are known as mutagenic agents.
Types of mutations
Structural: some mutations are the result of small changes in an
organism’s hereditary material. For example, a single base may be
substituted for another , or one or more bases may be removed from
a section of DNA. This type of mutation can occur during the DNA
replication process.
Numerical
mutations
Numerical: they involve the loss or gain of one or more
chromosomes. This type of mutation may occur when chromosomes
don’t separate correctly during meiosis. The cell could also end up
with extra segments of chromosomes. When this type of mutation
occurs, the individual suffers a series of alterations and symptoms
which are known by the name of syndrome. The most well- known
are:
Down’s syndrome or trisomy 21: an extra chromosome number 21.
Klinefelter: 44 + XXY
Turner: 44 + X0
Effects of Mutations
Because mutations can introduce changes in an organism, they can be a source of genetic variety.
A mutation is harmful to an organism if it reduces the organism’s chance for survival and reproduction. Whether a mutation is harmful or not depends pertly on the
organism’s environment. The mutation that led to the production of a white animal (albinism) would probably be harmful to an organism in the wild.The animal’s white
colour would make it more visible, and thus easier for predators to find. However,,a white animal in a zoo has the same chance for survival as a brown animal. In a zoo,
the mutation neither helps nor harms the animal.
Helpful mutations, on the other hand, improve an organism’s chances for survival and reproduction. Antobiotic resistance in bacteria is an example. Antibiotics are
chemicals that kill bacteria. Gene mutations have enabled some kinds of bacteria to become resistant to certain antibiotics, that is, the antibiotics do not kill the bacteria
that have the mutations. The mutations have improved the bacteria’s ability to survive and reproduce.
Albinism: lack of pigment in the skin, eyes
as a result of a mutation.
Different morphological mutations in Fruit Flies (Drosophila melanogaster). This fly
is a favorite “model” organism for genetics research.
5.- HUMAN INHERITANCE
Patterns of Human Inheritance
Single genes with two alleles: a number of
human traits are controlled by a single gene with
one dominant allele and one recessive allele.These
human traits have two distinctly differents
phenotypes, or physical appearances. For example,
a widow’s peak is a hairline that comes to a point in
the middle of the forehead.
Single genes with multiple alleles: some human
traits are controlled by a single gene that has more
than two alleles. Such a gene is said to have
multiple alleles, three or more forms of a gene that
code for a single trait. Human blood type is
controlled by a gene with multiple alleles. There are
four main blood types: A, B, AB and O. Three alleles
control the inheritance of blood types. The allele for
blood type A and the allele for blood type B are
codominant. The allele for blood type O is recessive.
There are six possible genotypes which give rise to
the four blood groups.
Traits controlled by many genes. Polygenic
inheritance: some human traits show a large
number of phenotypes because the traits are
controlled by many genes. For example, at least
four genes control heigh in humans, so there are
many possible combinations of genes and alleles.
Skin colour is another human trait that is controlled
by many genes.
Widow’s peak Punnet Square
This Punnet Square shows a cross
between two parents with widow’s
peaks who are heterozygotics for this
trait. The allele for a widow’s peak is
dominant (W) over the allele for a
straight hairline.
Blood groups in humans
Genotypes
Phenotypes
AA
Blood group A
AO
Blood group A
BB
Blood group B
BO
Blood group B
AB
Blood group AB
AO
Blood group O
Many phenotypes
Skin colour in humans
is determined by three
or more genes.
The sex chromosomes
As this cross shows,
there is a 50%
probability that a child
will be a girl and a 50%
probability that a child
will be a boy.
A human somatic cell contains two sets of homologous
chromosomes, which may be divided into two types: there
is a pair with different chromosomes, the sex
chromosomes or heterochromosomes. The other
chromosomes are the same and are called autosomes.
The sex chromosomes carry genes that determine
whether a person is male or female. They also carry
genes that determine other traits.
Girl or Boy?
The sex chromosomes are the only chromosome pair that
do not always match. If you are girl, your two sex
chromosomes match. The two chromosomes are called X
chromosomes. If you are boy, your sex chromosomes
do not match. One of them is an X chromosome, and the
other is a Y chromosome. The Y chromosome is much
smaller than the X chromosome.
Sex chromosomes and fertilization
Since both of a female’s sex chromosomes are X
chromosomes, all eggs carry one X chromosome. Males,
however, have two different sex chromosomes. Therefore,
half of a male’s sperm cells carry an X chromosome, while
half carry a Y chromosome. When a sperm cell with an X
chromosome fertilizes an egg, the egg has two X
chromosomes. The fertilizated egg will develop into a girl.
When a sperm with a Y chromosome fertilizes an egg, the
egg has one X chromosome and one Y chromosome. The
fertilized egg will develop into a boy. This means that,
depending on the sperm which intervenes inthe
fertilization of the egg, the future individual will be male or
female.
The Sex-Linked Genes
The genes for some human traits are carried on the sex
chromosomes. Genes on the X and Y chromosomes are
often called sex-linked genes because their alleles are
passed from parent to child on a sex chromosome. Traits
controlled by sex-linked genes are called sex-linked traits.
One sex-linked trait is red-green colorblindness. A
person with this trait cannot distinguish between red and
green.
Unlike most chromosome pairs, the X and Y
chromosomes have different genes. Most of the genes on
the X chromosome are not on the Y chromosome.
Therefore, an allele on an X chromosome may have no
corresponding allele on a Y chromosome.
Like other genes, sex-linked genes can have dominant
and recessive alleles. In females, a dominant allele on the
other X chromosome will mask a recessive allele on the
other X chromosome. But in males, there is usually no
matching allele on the Y chromosome to mask the allele
on the X chromosome. As a result, any allele on the X
chromosome, even a recessive allele, will produce the
trait in a male who inherits it. Because males have only
one X chromosome, males are more likely than females to
have a sex-linked trait that is controlled by a recessive
allele.
Hemophilia: It is agenetic disorder in which a person’s
blood clots very slowly or not at all. People with this
disorder do not produce one of the proteins needed for
normal blood clotting. Hemophilia is also caused by a
recessive allele on the X chromosome. Because it is a
sex-linked disorder, it occurs more frequently in males
than in females.
One important tool that genetics use to trace the inheritance of traits in humans is a pedigree. It is a chart or “family tree” that
tracks which members of a family have a particular trait. The figure above shows the Queen Victoria-Family Tree tracing the
inheritance of hemophilia in this family. Hemophilia played an important role in Europe’s history. It became known as the
“Royal disease” bacause it spread to the royal families of Europa through Victoria’s descendants.