Genetics: Inheritance and Molecular Genetics

Incomplete dominance is a type of inheritance in which one allele for a specific trait is not completely dominant over the other allele. This results in a combined phenotype (expressed physical trait). For example, if you cross pollinate red and white snapdragon plants, the dominant allele that produces the red color is not completely dominant over the recessive allele that produces the white color. The resulting offspring are pink.

Phenotype is any observable characteristic or trait of an organism: such as its morphology, development, biochemical or physiological properties, behaviour, and products of behaviour (such as a bird's nest). Phenotypes result from the expression of an organism's genes as well as the influence of environmental factors and the interactions between the two.

Polyploidy is the state where all cells have multiple sets of chromosomes beyond the basic set, for example, in triploids 2n = 3x, in tetraploids 2n = 4x. The chromosome sets may be from the same species or from closely related species.

Epistasis is an interaction between genotypes at two different gene loci, which sometimes resembles a dominance interaction at a single locus. Epistasis modifies the characteristic 9:3:3:1 ratio expected for two non-epistatic genes.

In case of autosomal dominant trait both the sexes are equally affected. Some autosomal dominant genes are preferentially expressed in one sex such as gout and pre-senile baldness affecting male.

This type of sex determination occurs in insects and mammals including human beings. Here, the females have two identical homologous sex chromosomes designated as XX and the males have two dissimilar sex chromosomes designated as X and Y. Thus, females are homogametic, producing only one type of ovum, and males are heterogametic, producing two types of gametes: one half carrying the X chromosome and the other half carrying the Y chromosome.

The abnormal genes located on X or Y chromosomes are called sex-linked genes. X-linked inheritance is more common and is expressed mostly as X-linked recessive traits

The genes distantly located in the chromosome show incomplete linkage because they have a chance of separation by crossing over and of going into different gametes and offspring.

Genetic recombination crossing over is the mutual exchange of the corresponding parts of the adjacent paternal and maternal chromatids of the pachytene of meiosisI, producing new combinations of genes. The chromatids resulting from the interchange of segments are known as the cross over recombinants and the chromatids that remain intact are called non-crossover parental chromatids.

The principle of linkage was discovered by Bateson and Punnet in 1906 in the sweat pea, plant, Lathyrus odoratus. However, linkage, as a concept was put forth by Thomas Hunt Morgan in 1910, based on his experiment on Drosophila melanogaster.

The double helix in DNA consists of two right-handed polynucleotide chains that are coiled about the same axis. The heterocyclic amine bases project inward toward the center so that the base of one strand interacts or pairs with a base of the other strand. It was given by D. Watson and Francis Crick .

Sometimes, a part of the chromosome is lost. This loss can be from one end or from chromosome parts between the ends. Accordingly, the former is known as terminal and the latter as interstitial deletion. Example:uniduchat syndrome: loss of half of the short arm of chromosome 5 in human.

This leads to variations in the population. Through the maternal gametes (ova) and paternal gametes (sperms), hereditary traits (characters) are passed on to child from parents. Genes are linearly arranged in chromosomes which are the carriers of hereditary traits. With the crossing over of two parental chromosome segments, a child inherits some of the traits from the mother and some from the father. This is why; two children from the same parents resemble each other in some characters and differ in others.

The term 'genetics' was used for the first time by W. Bateson in 1905. Genetics is often described as a science which deals with the mechanism of heredity and variations.

Heredity can be defined as the transmission of physical and mental characteristics from parents to offsprings. These characteristics can be called traits. These traits thus are transmitted from one generation to the next.

One full turn of the helix, called gyre, measures 34A⁰ in length. The distance between two successive sugar molecules is 3.4A⁰. Thus, each gyre accommodates 10 nucleotides.

Primase synthesizes a short RNA primer, which remains base-paired to template DNA that is extended at 3' end by DNA polymerase.

Conservative mode is a postulated type of DNA replication in which double-stranded helix remains intact and acts as a template for a new double-stranded DNA helix. This type of DNA replication is disproved model, in this type of replication one-half of the daughters DNA molecules will have both strands composed of newly polymerized nucleotides and the old parent double stranded helix will remain unchanged.

DNA can be twisted like a rope in a process called DNA supercoiling. With DNA in its relaxed state, a strand usually circles the axis of the double helix once every 10.4 base pairs, but if the DNA is twisted the strands become more tightly or more loosely wound.

It represents about 10 to 15% of the total RNA in the cell. It has the shortest molecule having only about 80 to 100 nucleotides. The polynucleotide chain is folded on itself to have the shape of a cloverleaf. The molecule has three lateral loops, a DHU loop, a t loop and an anticodon loop. The anticodon loop bears a triplet combination of nitrogen bases, called anticodon. It is complementary to a codon of mRNA.

Helicases are proteins that unwind DNA using energy of ATP and create two potential replication forks. 5' to 3' helicase unwind DNA from 5' single stranded end while 3' to 5' helicase unwind DNA at 3' single stranded end.

Endonucleases are enzymes that cleave the phosphodiester bond within a polynucleotide chain, in contrast to exonucleases, which cleave phosphodiester bonds at the end of a polynucleotide chain.

Transcription is the process of creating a complementary RNA copy of a sequence of DNA.Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes.

Ribose is an organic compound with formula C5H10O5.It is specifically, a monosaccharide (simple sugar) with linear form H-(C=O)-(CHOH)4-H, which has all the hydroxyl groups on the same side in the Fischer projection.

DNA replication is a biological process that occurs in all living organisms and copies their DNA; it is the basis for biological inheritance. The process starts with one double-stranded DNA molecule and produces two identical copies of the molecule.