There are six different types of patterns of inheritance. They include simple Mendelian genetics, codominance, polygenetic traits, incomplete dominance, multiple alleles, and sex-linked recessive inheritance. Simple Mendelian genetics is when a dominant allele is always present, and the recessive allele is only expressed when in the absence of a dominant allele. Codominance means the two alleles are both dominant and expressed and are together. Polygenic traits are the control of a trait by more than one gene. Incomplete dominance is similar to codominance but not alike; incomplete dominance, both traits are partially expressed without one overpowering the other, which causes them to blend. Multiple alleles mean that there are genes that have multiple alleles. Sex-link recessive inheritance still follows simple Mendelian genetics predictions but is just on the sex chromosomes.
What is Simple Mendelian Genetics?
If you have ever done a Punnett square, you most likely know simple Mendelian genetics. An example, a purebred white dog is bred with a pure breed black dog, the color black is dominant while white is recessive. In this case, 100% of the offspring will have black fur color; however, they will all be heterozygous. Therefore, if one of the offspring is bred with a purebred white dog, 50% of the offspring is predicted to be white while the other 50% is black. Homozygous dominant is written with two capital letters, homozygous recessive is written with two lower case letters, and heterozygous is written with one capital letter and one lower case. A dominant allele will always be expressed in heterozygous and homozygous dominant while the recessive allele will only be present in homozygous recessive.
What is Codominance?
An example of codominance is blood type. There are A, B, AB, and O blood types a person can have. Blood type AB is an example of codominance as that is an inheritance with blood types A and B from parents. However, AB are both dominant, and therefore, one allele is not expressed but both. Type O is recessive. The genotype for blood A is AA or AO, blood B is BB or BO, blood AB can only be AB, and blood O is OO. Codominance is different from regular genetics because even with a recessive allele, the genotype is still present; however, the phenotype is just the blood type.
What are Polygenic Traits?
Polygenic traits are traits that are controlled by more than one gene. Skin color is considered a polygenic trait as there are many factors and genes that determine a person's skin color. Each gene product is additive to the others. There are two different hallmarks of a polygenic trait’s phenotype expression: bell curve distribution and continuous distribution. For example, two parents with a darker skin color can still have children who are way lighter than the parents. This is because of the distribution; there is a higher chance that the offspring will have similar skin color to parents, but there are still chances of the children having lighter skin color.
What is incomplete dominance?
As we talked about codominance earlier, incomplete dominance is similar, but instead, it is blended. Incomplete dominance will only occur if the alleles are heterozygous. Both traits are expressed partially without one trait overpowering and expressing the other. Flower color traits are incomplete dominance because the traits separate during meiosis and are seen in the F2 generation. To show, if a red and white flower are bred together, in codominance, the flower might be red and have white spots. However, in incomplete dominance, if the alleles are heterozygous, the offspring will be pink. This is because red and white blended together create the color pink. There will still be offspring that are just red and white.
What are Multiple Alleles?
Eyes, hair, and fur colors all have multiple alleles. Multiple alleles usually exhibit some sort of incomplete dominance. The blood type ABO is also an example of a trait that has more than two alleles. Multiple alleles are somewhat similar to polygenic traits but different because in polygenic traits there are several genes that determine one trait. There are more than two forms of a gene in multiple alleles but are located on the same homologous chromosomes. Multiple alleles are complete dominance or codominance, while polygenic traits are incomplete dominance or codominance.
What is Sex-Linked Recessive Inheritance?
Sex-linked recessive inheritance is the same as simple Mendelian genetics but are just with sex-linked disorders. The affected chromosomes are found on the X chromosomes, which means that men are at a higher chance of inheriting while women are carriers. The Russian Tsar family had hemophilia that ran in the family and only affected most males. This is because women have two X chromosomes; even if one chromosome was recessive, one would still be dominant, and they would only be a carrier. With men, they only have one X chromosome, if that X chromosome inherited the recessive gene, they are affected with the disease.
How to find out if you have specific genes?
A genetic or DNA test will test if you have different alleles. Many websites 23andme or AncestryDNA will also test for your traits as well your DNA and where you came from. Many testing can include lactose intolerant, taster or non-taster, and many genetic disorders and diseases.
Written by: Amber Truong
Source:
“Access Denied - NCBI Bookshelf.” NCBI, www.ncbi.nlm.nih.gov/books/NBK132145. Accessed 27 May 2021.
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