DNA and Patterns of Inheritance

The genetic information of all living things is contained in the polymeric molecule known as deoxyribonucleic acid (DNA). This linear chain consists of three components- sugar, phosphate and nucleotide base. The chemical composition of the sugar in DNA is 2’-deoxyribose and the nucleotide bases are of four kinds—adenine (A), cytosine (C), guanine (G) and thymine (T).

Nucleotides are linked together through phosphodiester bonds in order to form a polynucleotide. The DNA in cells is double-stranded, wherein two polynucleotides are wrapped around each other to form a helical configuration.The two polynucleotides are maintained as a helix through the presence of hydrogen bonds between the nucleotide bases of the polynucleotides. These hydrogen bonds are base-paired, following the rule that the base A can only pair with T, and the base G can only pair with C. Such base-pairing ensures that the two DNA strands are complementary to each other. Since the DNA in an entire cell is very long, there is a need to package the DNA so it can fit into the nucleus of the cell.

DNA is therefore packaged into a smaller configuration through the help of histones or DNA-binding proteins, to form nucleosomes.This barrel-shaped DNA-protein structure is further packaged into a 30 nm fiber, which is then further wound into a chromosome. Chromosomes are therefore condensed chromatin material which may be visualized under a microscope, especially during cell division. Chromosomes contain different genes, which are the units of inheritance of different traits.

Gregor Mendel, the father of genetics, examined the patterns of inheritance of seven different traits in garden peas in 1866. He observed that certain characteristics of an offspring resembled one of the two parents. He called these specific characteristics alleles, or different forms of a gene.The allele that always appears in the offspring is the dominant allele, while the feature that is usually suppressed in the offspring is the recessive allele. He also defined that alleles exist as two copies in a cell, and that two identical copies of the same allele indicates a homozygous condition, while the combination of two different alleles designates a heterozygous condition.

In addition, he configured that the genetic combinations in a cell constitute its genotype, while the expression or appearance of the genetic combinations are observed as a cell’s phenotype.Genetic combinations can be predicted through the use of crosses, which is typically performed using a Punnett square. This procedure for genetic calculation involves the separation of alleles from each parent and combination each allele from one parent with an allele from the other parent. This type of monohybrid cross follows a genotypic ratio of 1:2:1 for three different genotypes.

It also shows a phenotypic ratio of 3:1 for two different genotypes. Should there be an instance when the genotype of an organism is unknown, a test cross may be used by crossing the unknown genotype with a homozygous recessive condition.If half of the offspring show the dominant trait, then the genotype in question is a heterozygous dominant, and if all of the offspring present the dominant trait, then the genotype in question is homozygous dominant. When combining at least two genes in a cross, a dihybrid pattern of inheritance can be observed. This mode follows the basic dihybrid ratio of 9:3:3:1 for four phenotypes.

There are certain instances when genes have several degrees of expression, unlike the straightforward dominant or recessive alleles.The condition of penetrance is determined by calculating the number of individuals the show the expected phenotype. The mode of expression of that phenotype is determined as the expressivity of that gene. One good example of incomplete penetrance and variable expressivity can be observed in individuals who have a bluish tinge in the sclera of their eyes. It has been observed that 90% of the time, the gene is expressed. Environmental factors may also affect the degree of penetrance and expressivity of certain genes.

Certain rabbit species grow a different color of fur if it is exposed to a temperature that is different from their usual habitat. Individuals who are malnourished or not given the proper nutrition do not grow tall enough because of poor environmental settings. There are certain traits that are influenced by more than two alleles. The human A-B-O blood type system is controlled by multiple alleles. Four general blood types are therefore existing in humans—A, B, AB and O.

These blood types are based on the presence or absence of an antigen on the surface of red blood cells.Red blood cells may or may not carry one or two antigens on its surface, so some red blood cells carry only the A antigen, or only the B antigen, or both A and B antigens, or do not carry any antigen at all. Individuals that have blood cells that only carry the A antigen will be classified to have blood type A, individuals that have blood cells that only carry the B antigen will be classified to have blood type B, individuals who have red blood cells that carry both antigens A and B will be classified as blood type AB, and individuals who have red blood cells that do not show any antigens are classified as blood type O.It is important to know an individual’s blood type for blood transfusion, to avoid rejection of the donor’s blood once introduced to the recipient. Blood types are also useful in determining the possible parentage of offsprings in hospital nurseries or vice versa.