Inheritance:
Gene and allel
Gene and allel
Gene and allele are basically what make us who
we are. They are genetic sequences of our DNA, although gene is a more general term than allele. To make an example:
humans have facial hair. It can be thick or patchy. The first statement is a
gene, the latter an allele.
Gene
Genes are the basic instructions for all life
forms. All living things are dependent on genes for survival, as genes specify
all proteins and functional RNA chains. Genes also contain the information and
instructions to build and maintain our cells, and pass them on to our
offspring. Genes not only tell us what we’ll look like, but also determine what
kind of diseases we will be more vulnerable to.
Allele
Alleles are variants of a gene. When someone
says this person got good genes, they are referring to an allele. They occur in
pairs and produce opposite phenotypes that, by nature, are
contrasting. If an allele has homogenous genes, then they are called
homozygous. If they are different, they are called heterozygous. And
heterozygous alleles will have a dominant and a recessive allele.
Difference between Gene and
Allele
Genes are the parts of the DNA that determine
what traits a person will have, while alleles are the different sequences of
that DNA and they determine what kind of characteristics those traits will
have. Alleles also occur in pairs, having a recessive and a dominant part.
Genes don’t have any pairing at all. Also alleles can be either homozygous or
heterozygous while genes don’t have such differentiation. Basically, an allele
is just different types of the same gene. If there is a gene for hair color,
one allele will be for black hair, the other for brown hair.
Alleles and genes are equally important in the
development of all forms of life, and their differences can be seen in every
living thing. The best example of how they manifest is you.
Incomplete Dominance: If two alleles have an incomplete dominance relationship, the phenotype of the heterozygote will be intermediate between the phenotypes of the two homozygotes. This is often described as "blending," though the alleles themselves do not blend. The phenotype of looks like the two traits have blended together. For example, in snapdragons, one of the various genes which control flower color has two alleles, one for red flowers and one for white flowers. The two homozygous plants will produce red and white flowers, respectively. But the heterozygote will produce pink flowers--as if the two homozygous conditions were blended together like paint. In this case, the actual flower color (phenotype) probably results from varying amounts of production of the red pigment. The homozygous red plant produces a lot of the pigment, the homozygous white plant produces none of the pigment, and the heterozygote produces half as much as the homozygous red. Note that there is no dominant allele here.
Every living thing—plant or
animal, microbe or human being—has a set of characteristics inherited from its
parent or parents. Since the beginning of recorded history, people have wanted
to understand how that inheritance is passed from generation to generation.
More recently, however, scientists have begun to appreciate that heredity holds
the key to understanding what makes each species unique. As a result, genetics, the scientific study of
heredity, is now at the core of a revolution in understanding biology.
INSTRUCTION
Gregor Mendel's Peas
The work of an Austrian monk
named Gregor Mendel was particularly important to understanding biological
inheritance. Gregor Mendel was born in 1822 in what is now the Czech Republic University of Vienna
Mendel carried out his work
with ordinary garden peas. He knew that part of each flower produces pollen,
which contains the plant's male reproductive cells, or sperm. Similarly, the
female portion of the flower produces egg cells. During sexual reproduction,
male and female reproductive cells join, a process known as fertilization. Fertilization
produces a new cell, which develops into a tiny embryo encased within a seed.
Pea flowers are normally self-pollinating, which means that sperm cells in
pollen fertilize the egg cells in the same flower. The seeds that are produced
by self-pollination inherit all of their characteristics from the single plant
that bore them. In effect, they have a single parent.
When Mendel took charge of the
monastery garden, he had several stocks of pea plants. These peas were true-breeding, meaning that if they
were allowed to self-pollinate, they would produce offspring identical to
themselves. One stock of seeds would produce only tall plants, another only
short ones. One line produced only green seeds, another only yellow seeds.
These true-breeding plants were the basis of Mendel's experiments.
Mendel wanted to produce seeds
by joining male and female reproductive cells from two different plants. To do
this, he had to prevent self-pollination. He accomplished this by cutting away
the pollen-bearing male parts as shown in the figure at right and then dusting
pollen from another plant onto the flower. This process, which is known as cross-pollination,
produced seeds that had two different plants as parents. This made it possible
for Mendel to cross-breed plants with different characteristics, and then to
study the results.
Genes and Dominance
Mendel studied seven different
pea plant traits. A trait is a specific characteristic, such as
seed color or plant height, that varies from one individual to another. Each of
the seven traits Mendel studied had two contrasting characters, for example,
green seed color and yellow seed color. Mendel crossed plants with each of the
seven contrasting characters and studied their offspring. We call each original
pair of plants the P (parental) generation. The offspring are called the F1,
or “first filial,” generation. Filius and filia are the Latin words for “son” and
“daughter.” The offspring of crosses between parents with different traits are
called hybrids.
What were those F1 hybrid plants like? Did the characters
of the parent plants blend in the offspring? Not at all. To Mendel's surprise,
all of the offspring had the character of only one of the parents, as shown at
right. In each cross, the character of the other parent seemed to have
disappeared.
From this set of experiments,
Mendel drew two conclusions. Mendel's first conclusion was that biological
inheritance is determined by factors that are passed from one generation to the
next. Today, scientists call the chemical factors that determine traits genes. Each of the traits Mendel
studied was controlled by one gene that occurred in two contrasting forms.
These contrasting forms produced the different characters of each trait. For
example, the gene for plant height occurs in one form that produces tall plants
and in another form that produces short plants. The different forms of a gene
are called alleles (uh-LEELZ).
Mendel's second conclusion is
called the principle of dominance. The principle of
dominance states that some alleles are dominant and others are recessive. An organism with a dominant allele for
a particular form of a trait will always exhibit that form of the trait. An
organism with a recessive allele for a particular form of a trait will exhibit
that form only when the dominant allele for the trait is not present. In
Mendel's experiments, the allele for tall plants was dominant and the allele
for short plants was recessive. The allele for yellow seeds was dominant, while
the allele for green seeds was recessive.
Segregation
Mendel wanted the answer to
another question: Had the recessive alleles disappeared, or were they still
present in the F1 plants?
To answer this question, he allowed all seven kinds of F1 hybrid plants to produce an F2 (second filial) generation by self-pollination.
In effect, he crossed the F1generation with itself to produce the
F2 offspring, as
shown in the figure at right.
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The F1 Cross The results of the F1 cross were remarkable. When Mendel
compared the F2plants, he discovered that the traits controlled by
the recessive alleles had reappeared! Roughly one fourth of the F2 plants showed the trait controlled by
the recessive allele. Why did the recessive alleles seem to disappear in the F1 generation and then reappear in the F2generation?
To answer this question, let's take a closer look at one of Mendel's crosses.
Explaining the F1 Cross To begin with, Mendel assumed that a dominant allele had masked
the corresponding recessive allele in the F1 generation. However, the trait
controlled by the recessive allele showed up in some of the F2 plants. This reappearance indicated
that at some point the allele for shortness had been separated from the allele
for tallness. How did this separation, or segregation, of alleles occur? Mendel
suggested that the alleles for tallness and shortness in the F1 plants segregated from each other
during the formation of the sex cells, or gametes (GAM-eetz). Did that suggestion make
sense?
Let's assume, as perhaps Mendel
did, that the F1 plants
inherited an allele for tallness from the tall parent and an allele for
shortness from the short parent. Because the allele for tallness is dominant,
all the F1 plants are
tall. When each F1 plant flowers and produces gametes, the two
alleles segregate from each other so that each gamete carries only a single
copy of each gene. Therefore, each F1 plant produces two types of gametes—those
with the allele for tallness and those with the allele for shortness.
Look at the figure below to see how alleles
separated during gamete formation and then paired up again in the F2 generation. A capital letter T represents a dominant allele. A
lowercase letter t represents
a recessive allele. The result of this process is an F2generation
with new combinations of alleles
Segregation of Alleles
During gamete formation, alleles segregate from each other so
that each gamete carries only a single copy of each gene. Each F1 plant produces two types of gametes—those with the allele for
tallness and those with the allele for shortness. The alleles are paired up
again when gametes fuse during fertilization. The TT and Tt allele combinations produce
tall pea plants; tt is the only allele
combination that produces a short pea plant.
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Mendel's Law of Independent Assortment
To this point
we have followed the expression of only one gene. Mendel also performed crosses
in which he followed the segregation of two genes. These experiments formed the
basis of his discovery of his second law, the law of independent assortment.
First, a few terms are presented.Dihybrid cross - a cross between two parents that differ by two pairs of alleles (AABB x aabb)
Dihybrid- an individual heterozygous for two pairs of alleles (AaBb)
Again a dihybrid cross is not a cross between two dihybrids. Now, let's look at a dihybrid cross that Mendel performed.
Parental Cross: Yellow, Round Seed x Green, Wrinkled Seed
F1 Generation: All yellow, round
F2 Generation: 9 Yellow, Round, 3 Yellow, Wrinkled, 3 Green, Round, 1 Green, Wrinkled
At this point, let's diagram the cross using specific gene symbols.
Choose Symbol
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Seed Color: Yellow
= G; Green = g
|
Seed Shape: Round = W;
Wrinkled = w
|
Let's now look at the cross using our gene symbols.
Now set up the
Female Gametes
|
|||||
GW
|
Gw
|
gW
|
gw
|
||
GW
|
GGWW
(Yellow, round) |
GGWw
(Yellow, round) |
GgWW
(Yellow, round) |
GgWw
(Yellow, round) |
|
Male
|
Gw
|
GGWw
(Yellow, round) |
GGww
(Yellow, wrinkled) |
GgWw
(Yellow, round) |
Ggww
(Yellow, wrinkled) |
Gametes
|
gW
|
GgWW
(Yellow, round) |
GgWw
(Yellow, round) |
ggWW
(Green, round) |
ggWw
(Green,<br)round)< td=""></br)round)<> |
gw
|
GgWw
(Yellow, round) |
Ggww
(Yellow, wrinkled) |
ggWw
(Green, round) |
ggww
(Green, wrinkled) |
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Phenotype
|
General Genotype
|
9 Yellow, Round Seed
|
G_W_
|
3 Yellow, Wrinkled Seed
|
G_ww
|
3 Green, Round Seed
|
ggW_
|
1 Green, Wrinkled Seed
|
ggww
|
Mendel's Second Law - the law of independent assortment; during gamete formation the segregation of the alleles of one allelic pair is independent of the segregation of the alleles of another allelic pair
As with the monohybrid crosses, Mendel confirmed the results of his second law by performing a backcross - F1dihybrid x recessive parent.
Let's use the example of the yellow, round seeded F1.
Female Gametes
|
|||||
GW
|
Gw
|
gW
|
gw
|
||
Male
Gametes |
gw
|
GgWw
(Yellow, round) |
Ggww
(Yellow, wrinkled) |
ggWw
(Green, round) |
ggww
(Green, wrinkled) |
- 1 Yellow, Round Seed
- 1 Yellow, Wrinkled Seed
- 1 Green, Round Seed
- 1 Green, Wrinkled Seed
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