INHERITANCE OF ONE GENE
The inheritance of a single gene, also known as monogenic inheritance, follows Mendel’s laws of inheritance. Monogenic inheritance involves the transmission of traits that are controlled by a single gene with two or more alleles. Let’s explore how this type of inheritance works:
Gene Alleles: A gene is a specific segment of DNA that codes for a particular trait or characteristic. In monogenic inheritance, there is usually one gene responsible for the trait in question, and this gene may have multiple versions or alleles. Alleles are different forms of the same gene and can be dominant or recessive.
Dominant and Recessive Alleles: In monogenic inheritance, the alleles come in pairs, with one allele inherited from each parent. These alleles can be classified as dominant or recessive. The dominant allele typically expresses its trait when present, whereas the recessive allele is only expressed when two recessive alleles are inherited (homozygous for the recessive allele).
Homozygous and Heterozygous: An individual can be either homozygous or heterozygous for a particular gene. Homozygous individuals have two identical alleles for the gene (e.g., two dominant alleles, denoted as “AA,” or two recessive alleles, denoted as “aa”). Heterozygous individuals have one dominant and one recessive allele (denoted as “Aa”).
Mendel’s Laws in Monogenic Inheritance: a. The Law of Segregation: During the formation of gametes (sperm and egg cells), the two alleles of a gene segregate so that each gamete carries only one allele. For example, in a heterozygous individual (Aa), the “A” and “a” alleles segregate into separate gametes.
b. The Law of Dominance: In a heterozygous individual (Aa), the dominant allele “A” will be expressed, while the recessive allele “a” remains hidden. However, the recessive trait is still carried in the genotype.
c. The Law of Independent Assortment (applies if considering multiple genes): If you are looking at the inheritance of one gene, this law may not apply. It becomes relevant when studying the inheritance of multiple genes on different chromosomes.
Phenotype and Genotype: In monogenic inheritance, the phenotype is the physical or observable trait that results from the combination of alleles (e.g., blue eyes or brown eyes). The genotype represents the genetic makeup of an individual, indicating which alleles they carry (e.g., AA, Aa, or aa).
Examples of monogenic inheritance include traits like Mendel’s pea plant experiments with seed color (yellow or green) and flower color (purple or white). In these cases, the inheritance of a single gene with two alleles results in specific phenotypic ratios in the offspring, following Mendel’s laws.
Monogenic inheritance is just one aspect of genetics, and many traits are influenced by the interaction of multiple genes (polygenic inheritance). Nevertheless, understanding how single genes are inherited provides a foundational framework for more complex genetic studies.
Contrasting Traits Studied by Mendel in Pea
| Trait | Contrasting Characteristics |
|---|---|
| Seed Color | Yellow vs. Green |
| Seed Shape | Round vs. Wrinkled |
| Flower Color | Purple vs. White |
| Flower Position | Axial vs. Terminal |
| Pod Color | Yellow vs. Green |
| Pod Shape | Inflated vs. Constricted |
| Flower Position | Axial vs. Terminal |
| Stem Length | Long vs. Short |
| Pod Color and Flower Color | Both traits were studied simultaneously in some experiments. |
Monohybrid Cross
A monohybrid cross is a genetic cross that involves the inheritance of a single trait controlled by a single gene with two alleles. Here’s a tabular representation of a monohybrid cross:
| Parental Generation (P) | Genotype | Phenotype |
|---|---|---|
| Male (Father) | AA | Dominant Trait |
| Female (Mother) | aa | Recessive Trait |
| F1 Generation (First Filial Generation) | Genotype | Phenotype |
|---|---|---|
| Offspring (Hybrids) | Aa | Dominant Trait |
| F2 Generation (Second Filial Generation) | Genotype | Phenotype |
|---|---|---|
| Offspring (Recessive Trait Expression) | aa | Recessive Trait |
| Offspring (Dominant Trait Expression) | Aa and AA | Dominant Trait |
In this example, the parental generation (P) consists of a male parent with a homozygous dominant genotype (AA) and a female parent with a homozygous recessive genotype (aa). They are crossed to produce the F1 generation, where all the offspring are heterozygous (Aa) and exhibit the dominant trait. Finally, when the F1 generation is self-fertilized or allowed to interbreed, the F2 generation shows a 3:1 phenotypic ratio with three individuals expressing the dominant trait and one expressing the recessive trait. This follows Mendel’s Law of Segregation and demonstrates the inheritance of a single gene with dominant and recessive alleles.
Test Cross
A test cross, also known as a back cross or a tester cross, is a genetic cross used to determine the genotype of an individual with a dominant phenotype (i.e., an individual expressing a dominant trait). This type of cross is particularly useful when the genotype of the dominant individual is not known. The test cross involves breeding the individual with the dominant phenotype with an individual that is homozygous recessive for the same trait. The results of the offspring can reveal the genotype of the dominant individual.
Here’s a description of a test cross in a tabular form:
Dominant Phenotype Individual (Unknown Genotype) Test Cross:
| Dominant Phenotype Individual | Genotype (Unknown) |
|---|---|
| Dominant Trait Expression | A_ (unknown) |
Homozygous Recessive Individual (Tester) Used in Test Cross:
| Recessive Phenotype Individual | Genotype (rr) |
|---|---|
| Recessive Trait Expression | rr |
Offspring of the Test Cross (F1 Generation):
| Offspring Genotype | Phenotype |
|---|---|
| A_ (unknown) | Dominant Trait |
| rr | Recessive Trait |
In this test cross, if all the offspring exhibit the dominant trait (e.g., express the dominant phenotype), it suggests that the individual with the dominant phenotype has a homozygous dominant genotype (AA). If some offspring display the recessive trait, it indicates that the individual with the dominant phenotype is heterozygous (Aa).
The test cross is a valuable tool for determining whether an organism with a dominant phenotype is homozygous dominant (AA) or heterozygous (Aa) for a particular trait, and it is a common technique used in genetics to uncover hidden or unknown genotypes.
Scenario: You have a plant with purple flowers (which is a dominant trait) and you want to determine whether it is homozygous dominant (PP) or heterozygous (Pp) for the flower color gene. To do this, you perform a test cross with a known homozygous recessive plant (pp).
Dominant Phenotype Individual (Unknown Genotype) Test Cross:
| Dominant Phenotype Individual (Purple Flowers) | Genotype (Unknown) |
|---|---|
| Dominant Trait Expression (Purple Flowers) | P_ (unknown) |
Homozygous Recessive Individual (Tester) Used in Test Cross:
| Recessive Phenotype Individual (White Flowers) | Genotype (pp) |
|---|---|
| Recessive Trait Expression (White Flowers) | pp |
Offspring of the Test Cross (F1 Generation):
| Offspring Genotype | Phenotype |
|---|---|
| Pp | Purple Flowers |
| pp | White Flowers |
In this test cross, if all the offspring have purple flowers (the dominant trait), it suggests that the individual with purple flowers (the unknown genotype) is heterozygous (Pp). This indicates that it carries one dominant allele (P) and one recessive allele (p). On the other hand, if any of the offspring have white flowers (the recessive trait), it indicates that the individual with purple flowers is likely homozygous dominant (PP), as the presence of a recessive allele (p) would only be possible if it was inherited from the homozygous recessive parent (pp).
So, by analyzing the phenotype of the offspring in the test cross, you can determine the genotype of the individual with the dominant trait.
Law of Dominance
(i) Characters are controlled by discrete units called factors.
(ii) Factors occur in pairs.
(iii) In a dissimilar pair of factors one member of the pair dominates (dominant) the other (recessive).
Law of Segregation
The Law of Segregation, formulated by Gregor Mendel, describes how alleles segregate or separate during the formation of gametes (sperm and egg cells). Here are the key points of the Law of Segregation:
Allele Pairs: An individual inherits one allele for each gene from each parent, resulting in a pair of alleles for each gene.
Segregation: During gamete formation, the two alleles of a gene segregate or separate from each other, with one allele going into each gamete.
Genotype and Phenotype: The combination of alleles an individual carries (genotype) determines their observable traits (phenotype).
Basis of Genetic Variation: The Law of Segregation is fundamental to genetic diversity and the inheritance of traits in sexually reproducing organisms. It underlies Mendel’s observed 3:1 phenotypic ratio in monohybrid crosses.
Incomplete Dominance
Incomplete dominance is a type of inheritance pattern in genetics where neither of the two alleles for a particular trait is completely dominant over the other. Instead, the heterozygous individual (having one of each allele) displays an intermediate or blended phenotype that is distinct from the phenotypes of the homozygous individuals. Here are the key characteristics of incomplete dominance:
Heterozygous Intermediate Phenotype: In incomplete dominance, when an individual has two different alleles for a specific trait, neither allele is completely dominant over the other. Instead, the heterozygous individual exhibits a phenotype that is a blend or intermediate between the two homozygous phenotypes.
Genotype and Phenotype Relationship: The phenotype is a direct result of the genotype. For example, in a flower color trait, if “R” represents a red allele and “W” represents a white allele, “RW” (heterozygous) might result in pink flowers, a phenotype intermediate between the red (RR) and white (WW) homozygous phenotypes.
1:2:1 Phenotypic Ratio: In a monohybrid cross involving incomplete dominance, the offspring of heterozygous parents will typically display a 1:2:1 phenotypic ratio. This means that 25% of the offspring will have the dominant phenotype, 50% will have the intermediate phenotype, and 25% will have the recessive phenotype.
Molecular Basis: In many cases of incomplete dominance, the molecular basis lies in the incomplete production of the gene product, which affects the expression of the trait. For example, in the case of flower color, the production of pigment may be reduced in heterozygotes, resulting in a lighter color.
Examples: Common examples of incomplete dominance include flower color in snapdragons (red x white results in pink flowers) and coat color in some animals like Andalusian chickens (black x white results in blue-gray feathers).
Scenario: Let’s consider the cross between two flowers with different alleles for petal color. In this case, “R” represents the allele for red petal color, and “W” represents the allele for white petal color. The cross is between a red-flowered plant (RR) and a white-flowered plant (WW), resulting in a heterozygous offspring (RW) with an intermediate pink color.
| Parental Generation (P) | Genotype | Phenotype |
|---|---|---|
| Male (Father) | RR | Red Flowers |
| Female (Mother) | WW | White Flowers |
| F1 Generation (First Filial Generation) | Genotype | Phenotype |
|---|---|---|
| Offspring (Hybrids) | RW | Pink Flowers |
In this case, the offspring in the F1 generation (hybrids) display an intermediate pink petal color due to incomplete dominance. This intermediate phenotype is distinct from the homozygous red and white parental phenotypes. The genotype of the F1 individual is heterozygous (RW), and the resulting phenotype represents the blending of the red and white colors, illustrating incomplete dominance.
Co-dominance
Codominance is a genetic inheritance pattern in which both alleles for a specific trait are fully expressed in the heterozygous individual. This means that neither allele is dominant or recessive, and both are visibly present in the phenotype. Here are the key characteristics of codominance:
Equal Expression: In codominance, both alleles contribute to the phenotype, and neither suppresses the other. This results in the simultaneous and equal expression of both alleles.
Distinct Phenotypes: The heterozygous individual exhibits a distinct phenotype that combines the traits associated with both alleles. It is not an intermediate or blended phenotype like in incomplete dominance.
Molecular Basis: Codominance often occurs when multiple alleles at a single gene locus produce different protein variants, and both are fully functional and expressed. Each allele contributes its own unique characteristic to the phenotype.
Blood Type Example: One of the classic examples of codominance is the ABO blood group system. In this system, there are three alleles: IA (codes for A antigen), IB (codes for B antigen), and i (codes for no antigen). When an individual inherits IA and IB alleles, they express both A and B antigens on their red blood cells, resulting in the AB blood type.
Punnett Square: In a Punnett square for codominance, the genotypes are represented as follows:
- IAIA or IAi: A phenotype
- IBIB or IBi: B phenotype
- IAIB: AB phenotype
Codominance is distinct from both complete dominance (where one allele is fully dominant) and incomplete dominance (where heterozygotes display an intermediate phenotype). In codominance, the heterozygous phenotype represents a combination of both alleles’ effects, making them equally and distinctly visible in the individual’s phenotype.
Scenario: In the ABO blood group system, there are three alleles: IA (codes for A antigen), IB (codes for B antigen), and i (codes for no antigen). A person can inherit two of these alleles, one from each parent. Let’s consider a cross between an individual with blood type A (IAIA) and an individual with blood type B (IBIB) to illustrate codominance in their offspring.
| Parental Generation (P) | Genotype | Blood Type |
|---|---|---|
| Male (Father) | IAIA | Blood Type A |
| Female (Mother) | IBIB | Blood Type B |
| F1 Generation (First Filial Generation) | Genotype | Blood Type |
|---|---|---|
| Offspring | IAIB | Blood Type AB |
In this cross, the offspring (F1 generation) inherits one A antigen allele (IA) from the father and one B antigen allele (IB) from the mother. The resulting genotype IAIB corresponds to blood type AB, where both A and B antigens are expressed on the red blood cells. This illustrates codominance, where both alleles are fully and distinctly expressed in the phenotype of the individual. Blood type AB is different from blood types A and B and displays characteristics of both parent’s blood types.
Pleiotropy
The example of starch synthesis in pea seeds provides a clear illustration of the concept of pleiotropy, where a single gene can have multiple effects on different traits or aspects of an organism’s phenotype. In this case, the “B” and “b” alleles control both the size and shape of the starch grains produced in pea seeds, resulting in a combination of pleiotropic effects.
Here’s a summary of the effects of the “B” and “b” alleles on different traits:
Starch Grain Size:
- BB (homozygous for B): Large starch grains are produced.
- bb (homozygous for b): Small starch grains are produced.
- Bb (heterozygous): Intermediate-sized starch grains are produced.
Seed Shape:
- BB (homozygous for B): Seeds are round.
- bb (homozygous for b): Seeds are wrinkled.
- Bb (heterozygous): Seeds are round.
Starch Synthesis Efficiency:
- BB (homozygous for B): High efficiency in starch synthesis.
- bb (homozygous for b): Low efficiency in starch synthesis.
- Bb (heterozygous): Intermediate efficiency in starch synthesis.
When considering the size of starch grains as the phenotype, the alleles “B” and “b” exhibit incomplete dominance, as the heterozygous individuals do not show an intermediate phenotype but instead produce starch grains of intermediate size. However, it’s important to note that the same gene (with its alleles “B” and “b”) has pleiotropic effects, influencing both starch grain size and seed shape, resulting in various combinations of these traits in different genotypes.
However, when the “B” and “b” alleles are heterozygous (Bb), the starch grains produced are of intermediate size. This demonstrates incomplete dominance, where the heterozygous individuals do not display the same phenotype as the homozygous dominants or homozygous recessives but instead show a unique, intermediate phenotype.
This example highlights the complexity of genetics and the interconnectedness of traits influenced by a single gene. It also demonstrates how different aspects of an organism’s phenotype can be affected by the same genetic locus and its alleles.
Concept of pleiotropy in genetics
Pleiotropic Genes: Some genes have the ability to influence multiple phenotypic expressions. These genes are known as pleiotropic genes. Rather than affecting a single trait, they have far-reaching effects on various aspects of an organism’s phenotype.
Underlying Mechanism: The underlying mechanism of pleiotropy often involves the gene’s impact on metabolic pathways. These pathways contribute to different phenotypic traits, and alterations in the gene can result in multiple, seemingly unrelated effects.
Example: Phenylketonuria (PKU): The passage provides an example of pleiotropy in humans. In the case of phenylketonuria (PKU), a single gene mutation affects the gene that codes for the enzyme phenylalanine hydroxylase. This mutation leads to phenotypic expressions characterized by:
- Mental retardation: The inability to metabolize phenylalanine, an amino acid, results in the accumulation of toxic phenylalanine in the body, which can lead to intellectual disabilities.
- Reduction in hair and skin pigmentation: The same mutation can impact the production of pigments, leading to a reduction in hair and skin pigmentation.
In this example, a single gene mutation has pleiotropic effects, influencing both intellectual development and pigmentation. This illustrates how a single gene can have diverse consequences on an organism’s phenotype, affecting seemingly unrelated traits. Pleiotropy is an important concept in genetics and helps us understand the complexity of genetic interactions and their impact on an organism’s overall phenotype.
