Introduction:
In
genetics, dominance refers to the relationship between two alleles of a gene,
where one allele masks the expression of the other allele in a heterozygous
individual. However, the concept of dominance is not always straightforward, as
there can be variations in dominance relationships that influence the
phenotypic expression of traits. Understanding these variations is crucial for
predicting the outcomes of genetic crosses and studying the inheritance of
traits.
Types of Dominance Relationships:
Complete Dominance:
Complete
dominance is a type of dominance relationship in genetics where one allele
completely masks the expression of the other allele in a heterozygous
individual. This means that the dominant allele is always expressed
phenotypically, while the recessive allele remains hidden in the presence of
the dominant allele. In a monohybrid cross involving complete dominance, the
phenotypic ratio of the offspring is typically 3:1.
Example
of Complete Dominance:
One
classic example of complete dominance is the inheritance of flower color in pea
plants studied by Gregor Mendel. In this case, Mendel observed that the gene
controlling flower color had two alleles: one for purple flowers (dominant
allele, denoted as P) and one for white flowers (recessive allele, denoted as
p).
When a pea
plant with the genotype PP (homozygous dominant) is crossed with a pea plant
with the genotype pp (homozygous recessive), all the offspring will have the
genotype Pp (heterozygous). Despite having one dominant allele and one
recessive allele, the heterozygous offspring will display the purple flower
phenotype, as the dominant allele (P) completely masks the expression of the
recessive allele (p).
|
Genotype |
Phenotype |
|
PP
(homozygous dominant) |
Purple
flowers |
|
Pp
(heterozygous) |
Purple
flowers |
|
pp
(homozygous recessive) |
White
flowers |
The
phenotypic ratio of the offspring in this monohybrid cross will be 3 purple
flowers (Pp) to 1 white flower (pp), demonstrating the principle of complete
dominance. This classic example of complete dominance in pea plants provided a
foundational understanding of genetic inheritance and laid the groundwork for
the field of genetics as we know it today.
Incomplete Dominance
Incomplete
dominance is a type of genetic inheritance where the phenotype of a
heterozygous individual is intermediate between the phenotypes of the two
homozygous parents. Unlike complete dominance, where one allele completely
masks the other, incomplete dominance results in a blending of traits,
producing a third, distinct phenotype. This phenomenon is also known as partial
dominance or semi-dominance.
Mechanism
of Incomplete Dominance
In
incomplete dominance, neither allele is completely dominant over the other.
Instead, the heterozygous genotype produces a phenotype that is a mix or
intermediate of the two homozygous phenotypes. This occurs because the dominant
allele does not fully express itself in the presence of the recessive allele,
leading to a partial expression of both alleles.
Example
of Incomplete Dominance
A classic
example of incomplete dominance is the inheritance of flower color in
snapdragons (Antirrhinum majus). In snapdragons, the gene for flower color has
two alleles: one for red flowers (R) and one for white flowers (r).
When a
homozygous red-flowered plant (RR) is crossed with a homozygous white-flowered
plant (rr), the resulting F1 generation consists of heterozygous plants (Rr).
These heterozygous plants display pink flowers, an intermediate phenotype
between the red and white flowers of the parent plants.
The
phenotypic and genotypic ratios in the F2 generation (when F1 plants are
self-crossed) are as follows:
|
Genotype |
Phenotype |
Ratio |
|
RR |
Red
flowers |
1 |
|
Rr |
Pink
flowers |
2 |
|
rr |
White
flowers |
1 |
This
results in a phenotypic ratio of 1:2:1, where one-quarter of the plants have
red flowers, half have pink flowers, and one-quarter have white flowers.
Co-dominance
Co-dominance
is a genetic concept where two different alleles for a gene are both expressed
in the phenotype of an individual. This means that neither allele is dominant
over the other, and they both contribute to the phenotype in a distinct way. In
co-dominance, the heterozygous individual will exhibit a phenotype that shows
characteristics of both alleles.
Mechanism
of Co-dominance
One
classic example of co-dominance is the ABO blood group system in humans. In
this system, there are three alleles for the gene that determines blood type:
A, B, and O. The A and B alleles are co-dominant, meaning that if an individual
inherits both A and B alleles, they will express both A and B antigens on their
red blood cells.
To
illustrate this concept, let's consider the following scenario:
- Allele A (IA) codes for the A
antigen on red blood cells.
- Allele B (IB) codes for the B
antigen on red blood cells.
- Allele O (i) does not code for
any antigen.
When an
individual inherits the IAIB genotype (heterozygous for A and B alleles), they
will express both A and B antigens on their red blood cells, resulting in blood
type AB. This is an example of co-dominance because both alleles are expressed
equally in the phenotype.
Here is a
chart to illustrate the ABO blood group system and co-dominance:
|
Genotype |
Phenotype |
|
IAIA or IAi |
Blood type A (expresses A antigen) |
|
IBIB or IBi |
Blood type B (expresses B antigen) |
|
IAIB |
Blood type AB (expresses both A and B
antigens) |
|
ii |
Blood type O (does not express A or B
antigens) |
In
summary, co-dominance is a genetic phenomenon where both alleles for a gene are
expressed in the phenotype of an individual, resulting in a unique combination
of traits. The ABO blood group system is a classic example of co-dominance in
humans.
Overdominance
Overdominance,
also known as heterozygote advantage or heterosis, is a genetic phenomenon
where the heterozygous genotype (having two different alleles for a gene)
exhibits a phenotype that is superior to either of the homozygous genotypes. In
other words, the heterozygous individual shows a trait that is more
advantageous or beneficial compared to individuals with two identical alleles.
example of
overdominance is the resistance to certain diseases in plants. One well-known
example is the gene responsible for resistance to the fungal disease called
wheat leaf rust in wheat plants.
In wheat
plants, there is a gene that can have two alleles: one allele (R) confers
resistance to wheat leaf rust, while the other allele (r) does not provide
resistance. When a wheat plant has the genotype RR (homozygous for the
resistant allele), it is resistant to the disease. However, interestingly, when
a wheat plant has the genotype Rr (heterozygous), it can exhibit even greater
resistance to the disease compared to the homozygous resistant genotype.
Here is a
chart to illustrate the overdominance in disease resistance in wheat plants:
|
Genotype |
Phenotype |
Trait |
|
RR |
Resistant to wheat leaf rust |
- |
|
rr |
Susceptible to wheat leaf rust |
- |
|
Rr |
Enhanced resistance to wheat leaf rust |
Greater disease resistance |
In this
example, the heterozygous genotype (Rr) for disease resistance in wheat plants
exhibits a phenotype with enhanced resistance to wheat leaf rust, providing a
greater level of protection compared to the homozygous resistant genotype (RR).
This demonstrates the concept of overdominance, where the heterozygous genotype
confers a beneficial trait that is superior to the homozygous genotypes.
In
conclusion, dominance relationships in genetics play a crucial role in
determining the expression of traits in individuals. Understanding dominance
relationships is essential for predicting the inheritance patterns of traits
and can have significant implications in various fields, including agriculture,
medicine, and evolutionary biology. By studying these genetic concepts, researchers
can gain insights into the complexity of genetic inheritance and the diversity
of traits observed in populations.
Frequently
Asked Questions
1. What
is a dominance relationship in genetics?
Answer: A dominance relationship in
genetics refers to the interaction between alleles of a gene, where one allele
(the dominant) masks the expression of another allele (the recessive) in the
phenotype. For example, in Mendelian inheritance, the allele for brown eyes (B)
is dominant over the allele for blue eyes (b), so an individual with one B and
one b allele (Bb) will have brown eyes.
2. How
does incomplete dominance differ from complete dominance?
Answer: Incomplete dominance occurs when
neither allele is completely dominant over the other, resulting in a blended
phenotype. For example, in snapdragons, a cross between red (RR) and white (WW)
flowers produces pink (RW) offspring. In contrast, complete dominance results
in the dominant allele completely masking the recessive allele in the
heterozygous state.
3. What
is codominance, and how is it different from dominance and incomplete
dominance?
Answer: Codominance occurs when both
alleles in a heterozygous organism are fully expressed, leading to a phenotype
that shows both traits simultaneously. A classic example is the AB blood type
in humans, where both A and B alleles are expressed equally. This differs from
dominance (where one allele masks another) and incomplete dominance (where the
phenotype is a blend).
4. Can
you give an example of a dominance relationship in humans?
Answer: One example of a dominance
relationship in humans is the inheritance of freckles. The allele for having
freckles (F) is dominant over the allele for not having freckles (f).
Therefore, individuals with the genotypes FF or Ff will have freckles, while those
with the genotype ff will not.
5. What
are the different types of dominance relationships?
Answer: The main types of dominance
relationships include complete dominance, incomplete dominance, and
codominance. Complete dominance occurs when one allele completely masks
another. Incomplete dominance results in a blended phenotype, and codominance
results in both alleles being expressed equally in the phenotype.
6. How
is epistasis related to dominance?
Answer: Epistasis is a form of genetic
interaction where one gene affects the expression of another gene. Unlike
dominance, which involves interactions between alleles of the same gene,
epistasis involves interactions between different genes. For example, in Labrador
retrievers, one gene determines the pigment color (black or brown), while
another gene determines whether the pigment is deposited in the fur, leading to
yellow labs.
7. What
is overdominance, and how does it differ from regular dominance?
Answer: Overdominance, also known as
heterozygote advantage, occurs when the heterozygous genotype has a higher
fitness or better phenotype than either homozygous genotype. This is different
from regular dominance, where the heterozygous phenotype resembles one of the
homozygous phenotypes. An example of overdominance is seen in the sickle cell
trait, where heterozygous individuals (AS) are more resistant to malaria than
either homozygous individuals (AA or SS).
8. Can
dominance relationships change over time?
Answer: Dominance relationships can change
over time due to mutations, environmental changes, or changes in selective
pressures. For example, a previously recessive allele may become advantageous
in a new environment and increase in frequency, potentially altering its
dominance relationship if the fitness benefits lead to changes in gene
interactions.
9. How
do dominance relationships affect genetic diversity?
Answer: Dominance relationships can
influence genetic diversity by affecting the frequency of alleles in a
population. Dominant alleles tend to mask recessive ones, which can lead to the
persistence of recessive alleles in a population at low frequencies. This hidden
genetic variation can contribute to a population's overall genetic diversity.
10. Why
is understanding dominance relationships important in breeding programs?
Answer: Understanding dominance
relationships is crucial in breeding programs because it helps predict the
traits of offspring. By knowing which alleles are dominant or recessive,
breeders can make informed decisions to achieve desired phenotypes, improve
crop yields, enhance animal traits, and manage genetic disorders effectively.
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