Mendel and the laws of inheritance

Gregor Mendel was one of the earliest people to systematically study genetics. He devised two basic Laws of Inheritance.

1). The Law of Segregation states that when an individual produces gametes, each gamete has only one of the two possible alleles from each parent; that is, the pairs of chromosomes (2N) from each parent separate into two different gametes; one set of parental chromosomes per gamete (1N). We now know that this happens during meiosis.

2). The Law of Independent Assortment states that different traits are inherited independently of one another. All the maternal chromosomes, or all the paternal chromosomes, do not end up in the same gamete, but rather the maternal and paternal pairs assort (separate into gametes) independently of each other.

Remember that each gamete has only one copy (1N) of its “parent” genome. When meiosis—the production of sperm or egg—is initiated within an organism, the set of chromosomes from each parent line up with their counterpart; i.e., chromosome 1 from the mother lines up with chromosome 1 from the father, and so on.  The chromosomes also pair such that the respective alleles on each chromosome are joined; e.g., the gene for eye color on Chromosome 15 from the mother will pair with the gene for eye color on Chromosome 15 from the father.  Once the chromosomes are paired, the segments of each chromosome are shuffled between them, resulting in a pair of Chromosome 15’s that are each a mixture of the egg and sperm DNA.  The architecture of this shuffling is such that segments of DNA are usually moved intact, moving linked segments (such as whole genes or regulatory sequences of the genome) together.  This helps to prevent getting an allele that is half a blue-eye gene from mom, and half a brown-eye gene from dad.  Once these ‘new’ chromosomes are formed, the two sets of chromosomes separate to form individual cells, or gametes.

The exception to the Law of Independent Assortment is linkage. When two genes or DNA segments are located near each other on the same chromosome, they are more likely to be inherited together. These linkages can also be shuffled, however, by recombination, where the DNA is actually broken apart and rejoined. Linkage and recombination are very useful for scientists trying to determine the chromosomal location of specific genes or DNA segments – called mapping -  as there is a direct statistical relationship between how far apart DNA segments are and how often they recombine.

To be sure, alleles for various phenotypes are sometimes misaligned, and genes and segments can be broken apart in the process. Although misalignment and breakage is relatively rare, one can imagine that this type of error can result in inheritance of, for instance, two copies of one allele or missing alleles, which could be lethal.  Or this phenomenon could result in new phenotypes that may be advantageous to the organism.  Fortunately evolution has designed this pairing and mixing of the genomes to be quite a successful process overall.  Sexual reproduction allows for a fantastic variation within a species, and contributes greatly to the resilience of a species to changes in its environment, disease, and other deleterious variables.

Resources:

You can learn more about eye color inheritance here:

Athro, Limited. Education on the Internet.

http://www.athro.com/evo/gen/genefr2.html
http://www.athro.com/evo/gen/inherit1.html