Genetics - Heredity and beyond

General Science (Easy) Biology

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GENETICS

The secret of heredity

The members of a species give birth to their own kind. The offsprings show some basic similarities and some dissimilarities with their parents and with each other. The sexually reproducing organisms produce sex cells or gametes. Two gametes usually distinguishable as male and female fuse to form a zygote, which gradually develops into a young one. The gametes constitute the link between one generation and the next and pass on the paternal and maternal characteristics to the offspring. This relationship that continues to exist between successive generations is referred to as heredity. Although the off spring inherits the characteristics of the parent and resembles them very closely, the resemblances are not complete in all respects. The differences are referred to as variations, which are necessary for organic evolution. The significance of variations shows up only if they continue to be inherited by the progeny for several generations. Thus both heredity and variations are fundamental factors in the process of organic evolution. The study of the mode of transmission of characters from one generation to the next is known as Genetics.

Mendel laws of innertance

The Austrian monk Gregor Mendel is considered as the pioneer of modern genetics and appropriately called father of genetics. His experiments with Pisum sativum were spread over 9 years between 1856 and 1864. Although Mendel was not the first to record systematic observation and analyse them properly, he published his conclusion in 1866 in the “Annual Proceeding of the Natural History Society” of Brunn. Mendel’s original paper was republished in 1901. Bateson (who used the term ‘Genetics’ for the first time) confirmed Mendel’s work by a series of hybridization experiments. The approach of Mendel was simple, logical, scientific, mathematical and analytical. Mendel decided to experiment with Pisum sativum because it is an annual and its different varieties possess contrasting character and hybridization is easy in it. Its flower is complete and the structure of flower ensures self-fertilization. Anthers can be easily removed before dehiscing and flowers can be artificially cross-fertilized with desired pollen.
Actually Mendel himself did not postulate any genetically principles or laws. He simply gave conclusive theoretical and statistical explanation for his hybridization experiments in his research paper. However, it was Carrens who postulated three laws on the basis of the result of Mendel’s work
(1) Law of dominance: “When two plants with contrasting characters are crossed together only one of these characters is expressed in the first generation” eg. When a true breeding tall plant is crossed with a true breeding dwarf one, all the plants of F generation have the same height as the tall parent. Reciprocal cross gives the same result.
(2) Law of segregation: It is also known as “law of purity of gametes”. The law states that the hybrids or heterozygous of F generation have two contrasting characters or alieloniorphs of dominant and recessive nature. They do not mix with each other and are segregated at the time of gametogenesis so that each gamete receives only one character either dominant or recessive.
(3) Law of Independent assortment: When the parents differ from each other in two or more than two pairs of contrasting characters or factors then the inheritance of one pair of factors is independent of the other factors”.
If we crossed a round and yellow seeded plant with a wrinkled and green seeded one. In F1 hybrids all progenies have yellow round seeds when the F1 hybrids are allowed to cross among themselves they produced 4 types of seeds in the ratio of 9:3:3:1.

GENE

Genes are the units of heredity. They contain the hereditary information encoded in their chemical structure for transmission from generation to generation. They affect development and functions, both normal and abnormal. As genes are not seen with an electron microscope, much is known about by them indirect methods.
Genes are composed of Deoxyribonucleic Acid (DNA), which codes for production of specific amino acid residues. These amino acids are then joined to form the proteins that comprise living organism i.e. (both plant and animal).
The human genome consists of all DNA and consequently all the genes in one set of human chromosomes.
In humans, there are 23 chromosome set, each of which consists of approximately 3 x10⁹ base pairs.
It is said we inherit about 50, 000 genes from the father and 50, 000 from the mother.
The genes occupy a specific position or locus on the chromosome. For eg. The locus for the ABO blood groups in chromosome is 9 and the locus for the major histocompatibility complex is in chromosome 6. The Y-Chromosome contains genes that determine the normal development of testis. It is known that the human X chromosome carries the genes governing a blood group haemophilia, muscular dystrophy, height and gonadal development.

Chromosome

Chromosomes are rode-like condensation of chromatin. They become visible in the nucleus only during cell division. They occur in pairs – one number of each pair comes from the father and other from mother. The largest chromosome measures about 7 meter and is about 5 times the length of the shortest. Biochemically the chromosomes are made up of DNA genetically, they consists of genes arranged like the beads of a necklace.
There are 22 pairs of autosomal (non-sex) chromoses and a pair of sex chromosomes. Thus the total number of chromosomes for man is 46 i.e. 23 pairs.
Chromosomal Abnormalities: Diseases arises from chromosomal abnormalities are of two types.
Disorders of autosomal chromosomal changes
Disorders of sex chromosomal changes

Autosomal chromosomal changes

	Cause	Symptoms
Down’s syndrome	Tiresome of 21^st chromosome i.e., there are 3 homologus chromosomes instead of 2 at this number. Non disjunction during oogenesis is the cause of down’s syndrome	Rounded face short stature, flaccid muscle, mental deficiency, small ears, protruding tongue, eyelids folded as in Mongols, hence also called Mongoloids syndrome
Edwards syndrome	Trisomy of 18^th chromosome	Mental retardation, micrognathia (very small face), short sternum, malformed ears etc.
Patau’s syndrome	Trisomy of 13^th Chromosome	Hare lip, polydactyle etc. Death usually occurs soon after birth or may survive 3 months.

Sex Chromosomal Changes

	Cause	Symptoms
Klinefelter’s syndrome (XXY geneotype)	Union of a non disjunction XX egg and a normal X egg and abnormal XY sperm	Genetically male with undeveloped testes, azyospermia (less deformed sperms), mental retardation, gynaccomastia (enlarged breast) and occurrence of barr body.
Turner’s Syndrome (XO genotype) most common type female genetic disease	When abnormal egg without X chromosome is fertilized by a X sperm or normal egg fertilized by sperm without sex chromosome	Short Stature, under developed breasts, reduced ovary, infertility, webbing of neck and have primary amenorrhoea (non occurrence of menses since age 13yrs).
Super female	Due to presence of extra numerary chromosomes	Mentally retarded and congenital abnormalities like underdeveloped external genitalia, uterus and vagina.
Super Male	Due to presence of super numerary (means extra number) chromosomes	Over production of testosterone, unusual height, mental retardation

Chromosomal disorders due to breaks/deletion and recombination or DNA repair deficiency
Cat Cry Syndrome (Cri du chat syndrome) : It was discovered by Lejeune in 1963 and is due to the deletion of a large part of the small arm of one of the 5^th chromosome. Affected child during infancy has a characteristic high-pitched cry of a kitten. This syndrome is associated with malformation of the larynx.
Chronic Myeloid Leukemia (CML) : Patient of CML carry Philadelphia chromosome which is one of 22 autosomes that has last most of the distal part of its longer arm.
It was first reported from a patient in Phildelphia, USA in 1959. CML is characterized by an excess of granular leucocytes in the blood. With the increase in the number of leucocytes there is reduction in the number of RBCs resulting in severe Anemia.

Genotype and Phenotype

The genotype of an individual represents sum total of heredity whereas phenotype represents features, which are produced by interaction between genotype and environment. A genotype can thus exhibit different phenotypes under different conditions. Therefore, similar genotypes may not have the same phenotype. Conversely similar, phenotypes do not necessarily mean same genotype. In order to study the interaction of environment and heredity for the study of effect of different environments on a genotype would be to have individuals, which have same genotype. This can be done by using clones pure lines or inbred lines. A clone is the progeny of a single plant reproduced asexually eg. Bacterial population derived from a single cell. A pure line is the progeny of a single plant obtained due to continuous self-fertilization. Similarly inbred lines are obtained in cross-fertilizing individuals due to fertilization among closely related lines.
Photocopies: When two genotypes produce the same phenotypes due to different environments one is called the photocopy of the other because they differ genotypically.
Lethality: There are genes, which control certain phenotypic traits and at the same time also influence the viability of the individuals. For instance, white eyed and vestigial winged flies have lower viabilities than the wild types there are still other genes which have no effect on the phenotype but influence the viability this influence on viability may be of such an order that the individual may fail to survive such genes which cause the death of the individual carrying it are known as lethal genes. If the lethal effects is dominant over its normal allele all individuals carrying it will die and the gene cannot be transmitted to next generation such dominant lethal genes will therefore be lost in the same generation. On the other hand recessive lethals are carried in heterozygous condition and will express themselves only in the homozygous condition.
Complementary genes: The complementation between two genes meaning that both genes are necessary for the production of a particular phenotype, it gives 9:7 phenotypic ratio in f2 generation instead of 9:3:3:1.
Duplicate genes: When two different genes determine the same or nearly the same phenotype such genes are called duplicate genes in this case the 9:3:3:1 of f1 generation ratio is modified into a 15:1 ratio.
Suppressor gene: Also called ‘Inhibitory genes’, the suppressor or inhibitory genes themselves do not directly cause the expression of characters but suppress the expression of other genes; such genes are involved in the expression of many coloration of plant leaf etc.
Multiple genes: There are some characters, which are controlled by more than 3 gene pairs. For example body colour of mice is determined by six pairs of genes.
This type of inheritance is very common for quantitative characters and is called multiple genes. The multiple alleles are different alternative form of the same dominant gene. For example the different types of changes in the same dominant gene result in a series of alternative genes. Thus a single gene ‘A’ give rise to A1, A2, A3...... etc. such a group of alternatives is called multiple alleles.
Plciotropism: It is assumed that a specific gene has a specific effect upon specific phenotypic traits of an organism that is each gene has its relation with a single phenytopic trait but mostly a single gene often influences more than one phenotype trait. These genes are called pleiotropic genes and this phenomenon of multiple effects of a single gene is called Pleiotropism.

DEOXYRIBONUCLEIC ACID (DNA)

Genetic material of all cellular organisms and most viruses. DNA carries the information needed to direct protein synthesis and replication. Protein synthesis is the production of the proteins needed by the cell or virus for its activities and development. Replication is the process by which DNA copies itself for each descendant cell or virus, passing on the information needed for protein synthesis. In most cellular organisms, DNA is organized on chromosomes located in the nucleus of the cell.
A molecule of DNA consists of two chains, strands composed of a large number of chemical compounds, called nucleotides, linked together to form a chain. These chains are arranged like a ladder that has been twisted into the shape of a winding staircase, called a double helix. Each nucleotide consists of three units: a sugar molecule called deoxyribose, a phosphate group, and one of four different nitrogen-containing compounds called bases. The four bases are adenine (abbreviated A), guanine (G), thiamine (T), and cytosine (C). The deoxyribose molecule occupies the center position in the nucleotide, flanked by a phosphate group on one side and a base on the other. The phosphate group of each nucleotide is also linked to the deoxyribose of the adjacent nucleotide in the chain. These linked deoxyribose-phosphate subunits form the parallel side rails of the ladder. The bases face inward toward each other, forming the rungs of the ladder.
The nucleotides in one DNA strand have a specific association with the corresponding nucleotides in the other DNA strand. Because of the chemical affinity of the bases, nucleotides containing adenine are always paired with nucleotides containing thiamine, and nucleotides containing cytosine are always paired with nucleotides containing guanine. The complementary bases are joined to each other by weak chemical bonds called hydrogen bonds.
In 1953 American biochemist James Watson and British biophysicist Francis Crick published the first description of the structure of DNA. Their model proved to be so important for the understanding of protein synthesis, DNA replication, and mutation that they were awarded the 1962 Nobel Prize for Physiology or Medicine for their work.

Protein Synthesis

DNA carries the instructions for the production of proteins. A protein is composed of smaller molecules called amino acids, and the structure and function of the protein is determined by the sequence of its amino acids. The sequence of amino acids, in turn, is determined by the sequence of nucleotide bases in the DNA. A sequence of three nucleotide bases, called a triplet, is the genetic code word, or codon, that specifies a particular amino acid. For instance, the triplet GAC (guanine, adenine, and cytosine) is the codon for the amino acid leucine, and the triplet CAG (cytosine, adenine, and guanine) is the codon for the amino acid valine. A protein consisting of 100 amino acids is thus encoded by a DNA segment consisting of 300 nucleotides. Of the two polynucleotide chains that form a DNA molecule, only one strand, called the sense strand, contains the information needed for the production of a given amino acid sequence. The other strand aids in replication.
Protein synthesis begins with the separation of a DNA molecule into two strands. In a process called transcription, a section of the sense strand acts as a template, or pattern, to produce a new strand called messenger RNA (mRNA). The mRNA leaves the cell nucleus and attaches to the ribosomes, specialized cellular structures that are the sites of protein synthesis. Amino acids are carried to the ribosomes by another type of RNA, called transfer RNA (tRNA). In a process called translation, the amino acids are linked together in a particular sequence, dictated by the mRNA, to form a protein.
A gene is a sequence of DNA nucleotides that specify the order of amino acids in a protein via an intermediary mRNA molecule. Substituting one DNA nucleotide with another containing a different base causes all descendant cells or viruses to have the altered nucleotide base sequence. As a result of the substitution, the sequence of amino acids in the resulting protein may also be changed. Such a change in a DNA molecule is called a mutation. Most mutations are the result of errors in the replication process. Exposure of a cell or virus to radiation or to certain chemicals increases the likelihood of mutations.

Replication

In most cellular organisms, replication of a DNA molecule takes places in the cell nucleus and occurs just before the cell divides. Replication begins with the separation of the two polynucleotide chains, each of which then acts as a template for the assembly of a new complementary chain. As the old chains separate, each nucleotide in the two chains attracts a complementary nucleotide that has been formed earlier by the cell. The nucleotides are joined to one another by hydrogen bonds to form the rungs of a new DNA molecule. As the complementary nucleotides are fitted into place, an enzyme called DNA polymerase links them together by bonding the phosphate group of one nucleotide to the sugar molecule of the adjacent nucleotide, forming the side rail of the new DNA molecule. This process continues until a new polynucleotide chain has been formed alongside the old one, forming a new double-helix molecule.

Research and Applications

The study of DNA is still under way, and the results of such research are being applied in many disciplines. The Human Genome Project in the United States is a federally funded effort to determine the sequence of bases of the three billion pairs of nucleotides composing the human genetic material. The project will make possible the analysis of the mutations that cause genetic diseases and so will provide information needed to develop medicines and procedures for treating these diseases.
Forensic science uses techniques developed in DNA research to identify individuals who have committed crimes. DNA from semen, skin, or blood taken from the crime scene can be compared with the DNA of a suspect, and the results can be used in court as evidence.
Techniques of DNA manipulation are used in farming, in the form of genetic engineering and biotechnology. Strains of crop plants to which genes have been transferred may produce higher yields and may be more resistant to insects. Cattle have been similarly treated to increase milk and beef production, as have hogs, to yield more meat and less fat.

Genetic Engineering

The alteration of an organism’s genetic, or hereditary, material to eliminate undesirable characteristics or to produce desirable new ones. Genetic engineering is used to increase plant and animal food production; to diagnose disease, improve medical treatment, and produce vaccines and other useful drugs; and to help dispose of industrial wastes. Included in genetic engineering techniques are the selective breeding of plants and animals, hybridization (reproduction between different strains or species), and recombinant DNA.
For details consult Scince Vol-II

Facts about genetics

1. Twin pregnancy may result either from the fertilisation of two separate ova by two different sperms or of a single ovum (egg) by one sperm. The former is known as double ovum (dizygotic), or fraternal twins and the latter as single ovum, (monozyotic or identical twins.)

2. Single ovum twins as a rule, possess only a single placenta and two cords while double ovum twins may have two separate placentae or a fused one with no vascular connection between the two.

3. These are also called siamese twins. Siamese twins are often joined at the hop or the shoulder. They can sometimes be separated by an operation.

4. Fossills are the remains or impressions of animals and plants of the remote past which existed on earth. The study offossils is called palaeontology. Homologous organs are those which have similar origin and developmental pattern but different in functions, Vestigial organs-Similar evidence of common ancestral origin comes from the vestigial organs which are functioniess in some organisms but are otherwise functional in other related animals or plants. There are about 100 such vestigial organs in man can be explained only if we suggest that the organisms with vestigial organs and functional organs must have evolved from a common ancestor.

HUMAN GENOME PROJECT

Human Genome Project (HGP) was a coordinated effort to characterize all human genetic material by determining the complete sequence of the DNA in the human genome. It began in October 1990. Project goals were to:
Identify all the approximately 30,000 genes in human DNA.
Determine the sequences of the 3 billion chemical base pairs that make up human DNA.
Store this information in DATABASE.
Transfer related technologies to private sector.
Address the Ethical, legal and social issues (ELSI) that may arise from the project.
An ambitious schedule has been set to complete the full sequence by the end of 2003, 2 years ahead of previous projection. Though it was originally planned to last 15 years. However, the work is not yet complete as the human genome project is presently working to complete a finished version of the euchromatic portions (the portion containing most of the gene) by 2003.
The evolving plan includes goals for further improving sequently technologies, studying human genome sequence variation, both at the level of single nucleotides as well as entire chromosomal segments, referred to as haplotypes, continuing to sequence the mouse genome, as well as rat, frog, pufferfish and sea squirt genomes; and additional sequencing of microbial genomes.
All this support on going efforts in comparative genomics, the most powerful way to begin to elucidate the roles of the many related genes observed in the genomes of these model organisms.

The Human Genome

A genome is the complete collection of an organism’s genetic material. The human genome is composed of about 50,000 to 100,000 genes located on the 23 pairs of chromosomes in a human cell. A single human chromosome may contain more than 250 million DNA base pairs, and it is estimated that the entire human genome consists of about 3 billion base pairs.
The DNA being analyzed in the Human Genome Project typically comes from small samples of blood or tissue obtained from many different people. Although the genes in each person’s genome are made up of unique DNA sequences, the average variation in the genomes of two different people is estimated to be much less than 1 percent. Thus the differences between human DNA samples from various sources are small in comparison to their similarities.