Eli5 : Why do some genes (like skin color) mix and shade upon inheritance while some don’t (like eye color) ?
In: 455
In school you were probably the Mendellion theory of genes. Each parent passes down to their offspring one of the two alleles (versions of a gene) they have for each trait. You have dominant genes (capital) and recessive genes (lowercase). You can use a punnet square to calculate the chance of inheritance like this. Aa x Aa => 25% AA, 50% Aa, 25% aa. This is known as Mendelian inheritance, named after the scientist Gregor Mendel who conducted experiments on pea plants to discover the basic principles of genetics.
However, in reality, inheritance it much more complicated than this.
Incomplete Dominance – Sometimes a dominant gene doesn’t completely mask the other gene. Red (R) + White (r) = (Rr) Pink
Codominance – Sometimes both genes are expressed. That’s how you get the AB blood type. One parent contributes the A gene and the other B gene, both are expressed. A + B = AB
Polygenic – Mendelian genetics assume all traits are caused by 1 gene, but in reality, most traits are created by multiple genes. This includes skin color and is the reason there are so many variations. ABCDEF (Black skin) + abcdef (White skin) = AbCdeF (Light skin)
It gets even more complicated when you take into account the fact that genes occur on certain chromosomes.
Sex Linked genes – Some traits only appear on the X or only on the Y chromosome (usually X). This messes with inheritance because males only need to inherit one bad X chromosome from the mom to get a disorder.
Linked genes – genes that are located near each other on the same chromosome. Because they are so close together, they tend to be inherited together during the formation of gametes (sex cells) through a process called recombination. Recombination is when chromosomes exchange genetic material during the formation of gametes, which increases genetic diversity. However, the closer two genes are on a chromosome, the less likely they are to be separated during recombination, leading to their being inherited together more frequently than would be expected by chance.
Epistasis – This occurs when one gene influences the expression of another gene. The effect of one gene can be masked or altered by the effect of another gene. For example, in Labrador retrievers, coat color is determined by the interaction of two genes. One gene determines the pigment production, while the other determines whether the pigment will be deposited in the fur or not.
Aneuploidy – Sometimes during meiosis, when gametes are being created, you have nondisjunction. This causes one of the gametes to have too many chromosomes and the other to have too few. This is what created Down syndrome (trisomy-21), a condition where someone has an extra 21 chromosome. 47 total chromosomes instead of 46.
Pleiotropy – Some genes can have multiple effects on different traits. A single gene can influence multiple aspects of an organism’s appearance, physiology, or behavior. For instance, the gene that causes sickle cell anemia also provides resistance to malaria in carriers of the gene.
Latest Answers