Genetics is a complex field that involves the study of genes, inheritance, and variation in living organisms. One of the central questions in genetics is determining which allele is dominant. This article will explore the concept of dominance in genetics and shed light on the mechanisms behind it.
What is an allele?
An allele is a variant form of a gene. Genes are segments of DNA that encode information about a specific trait or characteristic. Alleles can differ from each other in their DNA sequence, which can lead to variations in traits. For example, the gene that controls eye color has several different alleles that determine whether a person has blue, green, or brown eyes.
What is dominance in genetics?
When two different alleles are present in an individual for a particular gene, one allele may be expressed over the other. This phenomenon is known as dominance. The dominant allele is the one that is expressed in the phenotype (observable characteristic), while the recessive allele is masked and is not expressed. For example, in humans, brown eyes are dominant over blue eyes. If a person has one brown eye allele and one blue eye allele, they will have brown eyes because the brown allele is dominant.
Complete dominance
In complete dominance, the dominant allele completely masks the recessive allele, and only the dominant phenotype is observed. For example, the gene that controls flower color in snapdragons exhibits complete dominance. The dominant allele produces red flowers, and the recessive allele produces white flowers. If a plant has one red flower allele and one white flower allele, it will have red flowers because the red allele is dominant.
Incomplete dominance
In incomplete dominance, the heterozygous individual (having two different alleles) expresses a phenotype that is somewhere between the two homozygous phenotypes (having two identical alleles). For example, in humans, the gene that controls hair texture exhibits incomplete dominance. The dominant allele produces straight hair, the recessive allele produces curly hair, and the heterozygous individual will have wavy hair that is an intermediate between straight and curly.
Codominance
In codominance, both alleles are expressed equally in the heterozygous individual, and both phenotypes are observed. For example, the gene that controls blood type in humans exhibits codominance. The A allele produces glycoproteins (sugar-protein molecules) that are attached to red blood cells, and the B allele produces different glycoproteins. The heterozygous individual will produce both A and B glycoproteins, resulting in the AB blood type.
How is dominance determined?
The mechanism behind dominance is determined by the protein product of the gene. The dominant allele produces a protein that is functional and can be used by the organism, while the recessive allele produces a non-functional protein or no protein at all. Sometimes, the protein product of the dominant allele can interfere with the function of the recessive allele, leading to dominant expression. In other cases, the protein products of both alleles can interact in a way that produces the observed phenotype in the heterozygous individual.
Mendelian inheritance
Gregor Mendel was an Austrian monk who is known as the father of genetics. He discovered the basic rules of inheritance by performing experiments with pea plants. Mendelian inheritance is the term used to describe the patterns of inheritance that he discovered. Mendelian inheritance is based on the principle of dominance and the segregation of alleles during gamete formation. Each parent contributes one allele of each gene to their offspring, and the offspring inherit two alleles from their parents, one from each.
What is a dominant genetic disorder?
A dominant genetic disorder is a condition that is caused by a dominant allele. People who inherit one copy of the dominant allele will have the disorder, regardless of whether they also inherit a recessive allele for the same gene. Examples of dominant genetic disorders include Huntington’s disease, Marfan syndrome, and Neurofibromatosis.
What is a recessive genetic disorder?
A recessive genetic disorder is a condition that is caused by two copies of a recessive allele. People who inherit one copy of the recessive allele are called carriers, and they do not exhibit the disorder. However, if two carriers mate, there is a 25% chance that their offspring will inherit two copies of the recessive allele and exhibit the disorder. Examples of recessive genetic disorders include cystic fibrosis, sickle cell anemia, and Tay-Sachs disease.
How is dominance important in breeding?
Dominance is an important concept in breeding because it determines the outcome of genetic crosses. By knowing the dominant and recessive alleles for a particular trait, breeders can predict the phenotypic ratio of the offspring produced by a cross. Breeders can also use knowledge of dominance to produce plants and animals with desirable traits by selecting for individuals that have the desired dominant allele.
What happens when there is co-dominance in blood types?
When a person inherits two different alleles for the gene that controls blood type, they exhibit a phenotype that is the result of both alleles. In the case of codominance, both A and B glycoproteins are expressed in the red blood cells, resulting in the AB blood type. The O allele, which produces no glycoproteins, is recessive to both A and B alleles, and individuals with two O alleles have the O blood type.
What is the difference between complete and incomplete dominance?
In complete dominance, the dominant allele completely masks the recessive allele, and only the dominant phenotype is observed. In incomplete dominance, the heterozygous individual expresses a phenotype that is an intermediate between the two homozygous phenotypes. For example, in snapdragons, the red allele is dominant and produces red flowers, while the white allele is recessive and produces white flowers. If a plant has one red allele and one white allele, it will have pink flowers, which is an intermediate phenotype between red and white.
What happens when there is incomplete dominance in chickens?
The gene that controls feather color in chickens exhibits incomplete dominance. The dominant allele produces black feathers, the recessive allele produces white feathers, and the heterozygous individual produces grey feathers, which is an intermediate phenotype between black and white.
What is the difference between dominant and recessive genetic disorders?
A dominant genetic disorder is caused by a dominant allele, and people who inherit one copy of the dominant allele will have the disorder. In contrast, a recessive genetic disorder is caused by two copies of a recessive allele, and people who inherit one copy of the recessive allele are carriers but do not exhibit the disorder. Recessive disorders only occur when two carriers mate and their offspring inherit two copies of the recessive allele.
What is the role of dominance in evolution?
Dominance plays a role in evolution by influencing the frequency of alleles in a population. Strongly dominant alleles are likely to be fixed (occur in all members of the population), while recessive alleles may be maintained at low frequencies or eliminated from the population. Dominance can also affect the rate of evolution by creating new phenotypes that can be acted upon by natural selection.
Conclusion
Dominance is a fundamental concept in genetics that helps to explain the inheritance of traits and genetic disorders. Understanding how alleles interact can help breeders produce plants and animals with desirable traits and shed light on the mechanisms of evolution. The different types of dominance, such as complete dominance, incomplete dominance, and codominance, contribute to the diversity of traits observed in living organisms.
References
- Freedman, B. I., & Bowden, D. W. (2021). The genetics of renal disease. The Lancet Diabetes & Endocrinology, 9(1), 51-61.
- Griffiths, A. J. F., Miller, J. H., Suzuki, D. T., Lewontin, R. C., & Gelbart, W. M. (2015). Introduction to genetic analysis. W. H. Freeman and Company.
- Pierce, B. A. (2019). Genetics: a conceptual approach. Macmillan International Higher Education.
FAQs
- Q: What is an allele?
- A: An allele is a variant form of a gene.
- Q: What is dominance in genetics?
- A: Dominance is the phenomenon where one allele is expressed over another.
- Q: What is the difference between complete and incomplete dominance?
- A: In complete dominance, one allele completely masks the other, while in incomplete dominance, the heterozygous individual expresses an intermediate phenotype.
- Q: What is the difference between a dominant and recessive genetic disorder?
- A: A dominant genetic disorder is caused by a dominant allele, while a recessive genetic disorder is caused by two copies of a recessive allele.
- Q: How does dominance affect evolution?
- A: Dominance can influence the frequency of alleles in a population and create new phenotypes that can be acted upon by natural selection.