Lining Up of Homologous
Chromosomes
Genetic Variation
The reason each offspring, and more generally, each individual has their
own unique genetic code is due to genetic variation. First, there is
independent assortment. This occurs when homologous
chromosome pairs line up during Metaphase I of Meiosis. There is
a fifty-fifty chance of each daughter cell getting one of two chromosomes
from the pair. Therefore, there are two options for each chromosome,
a total of 2n options. Humans have a total of 23 chromosomes, creating
a total of 8 million different gametes arising from a single germ line
cell.
Second, the meeting of a sperm and an egg is a somewhat haphazard process, producing random fertilization. For each woman, each female gamete (egg) is one of 8 million different options and for each man, each male gamete (sperm) is also one of 8 million. Because fertilization is random as to which sperm fertilize which eggs, there are a total of approximately 64 trillion different offspring that can potentially be created by one man and one woman. There is even more genetic variation due to crossing over of chromosomes discussed in more detail in the processes of meiosis. (35)
Mendelian Genetics
A study of Mendelian genetics explains how during the process of meiosis
this genetic variation affects the characteristics of the offspring.
For an organism, each character has two alleles,
one inherited from their mother (maternal) and one from their father
(paternal). For most characters, there are two kinds of alleles. Dominant
alleles are expressed as a capital letter-A and recessive alleles are
expressed as a lowercase letter-a. During formation of gametes, alleles
are split so that only one of the two is expressed in each gamete. Mendel's
Law of Separation states that if you had one recessive and one dominant
allele for a specific character, then half your gametes would have one
set of alleles while the other half had the other. Mendel's Law of Independent
Assortment states that each set of alleles for a specific gene segregate
independently from other sets of alleles into gametes.
For every genotype, there is a specific phenotype. A genotype is the
specific gene that are expressed, while the phenotype is the trait that
is expressed. For instance, in normal cases, if the genotype is made
up of two dominant alleles (AA) or one dominant and one recessive (Aa)
then the dominant phenotype is expressed. However, if there are two
recessive alleles (aa), then the recessive phenotype is expressed.
Genetics is a much more complicated system than these basic rules illustrate.
Some genes produce incomplete dominance where a hybrid offspring with
two different alleles expresses a third phenotype as a blend of the
other two phenotypes. For example, genes from white and black mice that
produce grey mice exhibit incomplete dominance. Some genes exhibit codominance,
where in the same situation as incomplete dominance, the offspring expresses
a phenotype that has both of the other two phenotypes. For example,
if those genes from white and black mice produced white and black spotted
mice, they would display codominance.
Also, many genes (such as blood types) have more than two alleles. And
many traits come from more than one set of genes.
Genes can be traced through family lines in a chart called a pedigree.
Pedigrees have been important in following the inheritance of many debilitating
or fatal diseases. Many genetic disorders are passed on as recessive
traits. Alleles of genetic diseases either code for the wrong protein
of lack of protein, but the disease is only displayed when both recessive
alles are present (aa). A carrier, who has only one recessive allele
(Aa), could possibly have an offspring with the disease if their mate
was either a carrier (Aa) or had the disease (aa). There are also diseases
from dominant alleles. (35)
DNA-Genetics at a Molecular Level
DNA is the basic molecular structure of all genetic material. DNA makes
up each chromosome. Through DNA replication followed by the process
of meiosis, DNA patterns, which make up different inherited traits,
are passed on from parents to their offspring.
DNA Strand
The original strand is split apart almost like a zipper and new complementary strands are added on to each original one to create two duplicate strands. Many different proteins regulate this process in order to make sure that there are no mistakes made during replication. Even one single mistake in a base pairing may alter whole traits.
DNA replication occurs during Interphase, before the beginning of mitosis or meiosis. In mitosis, the DNA and the unique genetic code of the original cell is preserved in each of two daughter cells. However in meiosis, each of four gametes ends up with half the amount of DNA as the original cell, with differing genetic codes between each other and with the original. This difference allows organisms to create unique offspring.
DNA Replication