Introduction:
Genes are the basic unit of hereditary, and each gene has two alleles (version of DNA (deoxyribonucleic acid) sequence at a particular location), and each allele is received from each parent. Inheritance of traits is a complex part of genetics. There are two concepts of inheritance, Mendel and non-Mendel inheritance. Mendelian inheritance involves dominant and recessive alleles. The non-Mendelian inheritance involves complex inheritance and involves multiple alleles. It has codominance, where two alleles are expressed equally, and incomplete dominance, where complete dominance fails to occur.
What Are Genes and Alleles?
A cell is the smallest unit that makes up our body; inside the cell, it contains a nucleus, which contains 23 pairs of chromosomes. In each pair, one is inherited from the mother and another from the father. These chromosomes are thread-like structures and contain a protein and a single molecule of DNA (deoxyribonucleic acid). DNA is a molecule that carries genetic information, which is in the form of a twisted ladder, and one strand of DNA is around six meters. A segment of DNA is called a gene, and there are around 25,000 genes that determine the genotype (genetic composition of an individual).
Whereas an allele is a variant form of the gene and determines the phenotype (physical characteristic). If two alleles are the same in an individual, it is known as homozygous, and heterozygous if the alleles are different.
What Are the Types of Non-Mendelian Genetics?
The non-Mendelian inheritance involves complex inheritance and involves multiple alleles. The types include:
Dominance: When the allele is expressed over another allele, it is called dominance. The presence of multiple alleles leads to many subtypes of dominance. The subtypes of dominance involve:
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Complete Dominance: In this type of dominance, only one allele is expressed in the phenotype (physical characteristic). When two dominant alleles are expressed (AA) or one dominant allele is expressed (Aa), the phenotype expressed is the dominant type, and in the absence of the dominant allele (aa), the other trait is expressed.
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Codominance: This type involves the expression of both alleles equally in the phenotype. It is neither recessive nor dominant.
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Incomplete Dominance: Neither of the alleles is expressed completely in the phenotype. For example, a cross between a white flower and a red flower will result in a pink flower, therefore producing an intermediate phenotype.
Polygenic Inheritance: This is a type of inheritance where multiple alleles affect the resultant phenotype. They are controlled by two alleles called contributing alleles and non-contributing alleles. For example, skin color and pigmentation pattern, human height, and human eye color.
Linked Genes: Genes are said to be linked together only when they are located on the same chromosome, and they are inherited together, and they cannot be inherited if they are independent. For example, eye and hair colors are inherited together, like blonde and blue eyes or brown hair and eyes.
Sex-Linked: The phenotype is inherited from the genes which are located on the sex chromosome. It is often expressed by the genes located on the X chromosome because it is larger than the Y chromosome, and males are affected more. For example, hemophilia, some high blood pressure genes, congenital night blindness, red-green color blindness, Fragile X syndrome, and Duchenne muscular dystrophy.
Epistasis: It is a type of inheritance where the expression of the gene is suppressed by the other gene’s expression. For example, labrador retrievers are chocolate or black in color, but the recessive gene suppresses the color to reach the fur, which makes yellow labrador retrievers.
What Is Codominance?
Codominance is a type of inheritance where two alleles of the same gene are expressed separately, with different expressions in an individual instead of being supreme over another. To determine the codominance in the trait, a punnet square is used; it helps to see all possible allelic combinations, which determines the possible genotypes that can be inherited. Punnet square is a diagram with grids, and the alleles are represented with letters. The capital letters denote the dominant trait, and the recessive is denoted by a lowercase letter. It is used in the case of monohybrid crosses and dihybrid crosses.
Examples of codominance include;
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A cross between a yellow and blue flower will result in a blue flower with yellow spots, and if it results in a green flower, it shows incomplete dominance.
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The ABO blood grouping has two alleles for the A blood group and two alleles for the B blood group; the inheritance will be the AB blood group expressing codominance of both alleles.
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The MN blood group system (based on M, N, S, and s antigens present in the red blood cells) is determined by the presence of alleles. LM allele causes the M marker, and LN specifies the N marker; a cross between these will result in an equal number of M and N markers on the cell surface.
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A cross between a white fur coat and a black fur coat dog will result in the offspring of a black-and-white coat dog.
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Tabby cats are also a result of codominance.
How Is Codominance Different From Incomplete Dominance?
Codominance and incomplete dominance produce different phenotypic traits. In codominance, both alleles are equally expressed, whereas, in incomplete dominance, neither of the alleles is expressed completely. For example, a cross between a white cow and a black cow will result in a crow with black and white spots, which is codominance, whereas in incomplete dominance, the resultant cow will be grey in color. Another example includes flowers of red and white color; codominant results in white flowers with red spots, and incomplete dominance results in pink flowers.
Conclusion:
The fundamental building block of inheritance is the gene, which contains two alleles (versions of the DNA (deoxyribonucleic acid) sequence at a certain site), one of which is inherited from each parent. A challenging area of genetics is the inheritance of characteristics. Therefore inheritance is a complex pattern of genetics. The Mendelian trait involves dominant and recessive alleles, whereas the non-Mendelian trait involves multiple alleles and involves dominance, polygenic inheritance, linked gene, sex-linked, and epistasis. With the above-mentioned examples, codominance and other inheritance patterns can be understood to a great extent.
