Understanding inheritance patterns in Genetics
Although many things about heredity and genetics have been known since prehistoric times, it wasn't until Gregor Mendel began his genetics experiments in the mid 19th century that modern genetics came to be.Through his studies, Mendel was able to discover that organisms inherit traits through units of inheritance now known as genes.
During the course of his study of pea plants Mendel was able to determine the genetic patterns for color, plant height, shape, size, pod color, flower color, seed color and location. Using what Mendel was able to learn about gene alleles (different versions of the same gene); we are now able to understand the inheritance of human traits.
Over the years, through all of his experimentation, Mendel was able to develop the laws of genetics. The first of Mendel's three laws is the Law of Dominance. This law states that when an organism has two alleles for a particular trait (for example eye color) one allele will dominate. The second law is Mendel's Law of Segregation, which states that during the formation of gametes (sex cells) the pair of alleles for each trait (for example the X or Y chromosome) separate out during meiosis (gamete formation). The third law which is called Mendel's Law of Independent Assortment states that a gene pair separates during meiosis and that they separate independently from any other gene pairs. So in other words, your traits separate individually and are not linked to your traits for anything else.
Heredity or how we inherit our characteristics and traits is very complicated. One of the simpler means is by dominance. A dominant gene is usually shown as two capital letters such as BB for brown eyes, where the recessive blue eyed trait would have a genotype of bb. Simply put, an allele is said to be dominant when it is expressed in the appearance of the organism (the organism's phenotype). For example; if a person inherits brown eyes, and his or hers parents have brown and blue eyes respectively, then the brown allele is said to be dominant. For the offspring to have blue eyes the parents would each have to be heterozygous for brown eyes (each one would have to be Bb) and the offspring would have to have the genotype (bb) to have blue eyes. The blue eyes would be considered to be the recessive trait.
Another type of inheritance is incomplete dominance. In incomplete dominance, a red flower crossed with a white flower produces flowers which are pink. So both the red and white color are expressed to a certain degree and neither one is dominant over the other. In humans, a similar type of inheritance can occur with blood groups; however this situation is a little different. A person who is blood type AB has received an A allele and a B allele from their parents with neither allele being dominant. This is an example of co-dominance. In Mendel's experiments, co-dominance with red and white flowers would have showed up as red and white spotted flowers.
This is just a basic explanation of how inheritance patterns work. In reality they are really much more complex. There are at least three genes that code for eye color, maybe more, and there are many variables for skin color. Truth be told however, there is something to the fact that most blondes have blue eyes and most brunettes, brown. This leads you to believe that the law of independent assortment doesn't always work when it comes to eye and hair color. Hair color and eye color may somehow be linked more closely than originally thought and the Law of Independent Assortment may not work with these traits.