Banking Biology Genetics Genetics and Evolution

Introduction

• Study of heredity and variation is called genetics.
• Term genetics was given by - Bateson.
• Father of genetics - Gregor Johann Mendel.
• Father of experimental genetics - Thomas Hunt Morgan.
• Father of human genetics - Archibald Garrod.

• Some Terms in Genetics
• Gene: It is segment of DNA. It is basic unit of heredity
• Back cross: It is cross which is performed between hybrid and one of its parents.
• Test cross: Test cross is crossing of offspring with unknown dominant phenotype with the individual homozygous recessive for the trait.
• Monohybrid cross: It is a cross between two organisms of a species which is made to study the inheritance of a single pair of alleles or factors of a character.
• Monohybrid ratio: Monohybrid ratio is usually 3:1 (phenotypic ratio) or 1: 2:1 (genotype ratio) in which 25% of the individuals carry the recessive trait, 25% pure dominant and 50% have hybrid dominant trait.
• Dihybrid cross: It is a cross between two organisms of a species which is made to study the inheritance of two pairs of factors or alleles of two genes.
• Dihybrid ratio: Dihybrid ratio is 9: 3: 3: 1 (phenotypic ratio) where 9/16 first recessive and second dominant and 1/16 carry both the recessive traits.
• Mendel conducted cross hybridization experiments on Garden Pea plant (Pisum sativum). The first was the Principle of segregation, which claimed that each trait was specified by paired hereditary determinants (alleles of genes) that separate from each other during gamete formation. This law is also called Law of purity of gametes or Law of splitting of hybrids.
• Gregor Mendel was the first individual to apply a modem scientific approach to the study of heredity. Mendel proposed two basic principles of trasmission genetics.
• Mendel's second basic conclusion was the Principle of independent assortment, which stated that the segregation of one pair of genes-controlling a given trait - was not influenced by the segregation of other gene pairs. The chormosome theory provided a physical basis for the principle of independent assortment. Genes located on different chromosomes move to gametes independently of each other during meiosis.

• Human Blood Groups and Multiple Allele
• The system of blood groups in humans was discovered by Karl Landsteiner in 1900s.
• There are four phenotypes of Blood namely A, B, AB and O produced by three different alleles.\[{{I}^{A}}\],\[{{I}^{B}}\]and i of a gene.
• The allele\[{{I}^{A}}\] and \[{{I}^{B}}\] are equally dominant and do not interfere with expression of each other hence the allele \[{{I}^{A}}{{I}^{B}}\] are said to be co-dominant because both are expressed in the phenotype AB.
• Linkage is the phenomenon of certain genes staying together during inheritance through generations without any change or separation due to their being present on the same chromosomes.
• Linkage in the genes can be identified by test cross.
• The rearrangements of linked genes due to crossing over is known as recombination. Recombination also occurs due to chance separation of chromosomes during gametogenesis and their random coming together during fertilization.

• Sex Determination
• Henking discovered X body in spermatogenesis of few insects and it was given name of X chromosome. Due to involvment of X and Y chromosomes in determination of sex, they were called sex chromosomes.
• Rest of the chromosomes which determine other metabolic character of the body are called autosomes.

• Mutation
• Phenomenon that results in alteration of DNA sequence and consequently results in change in genotype and phenotype of an organism is called mutation.
• Mutagens are various chemical and physical factors that induce mutations, e.g., UV radiations, carcinogenic chemicals like nicotine, nitric oxide (NO).

Genetic Disorder

• Genetic Disorder
• A genetic disorder is a disease that is caused by an abnormality in an individual's DNA.

• Haemophilia
• A protein involved in clotting of blood is affected in an affected individual; if person gets a cut, will result in non-stop bleeding.
• Females are heterozygous and carriers of haemophilia.

• Sickle Cell Anaemia
• It is due to inheritance of defective allele coding for P-globin. It results in the transformation of \[H{{b}^{A}}\] into \[H{{b}^{S}}\]in which glutamic acid is replaced by valine at 6th position in each of two \[\beta \]-chains of haemoglobin.
• It is an excellent example of single mutation.

• Phenylketonuna
• Affected individual lacks enzyme phenylalanine hydroxylase that converts ammo acid phenylalanine to tyrosine.
• It is characterized by severe mental retardation, hypopigmentation of skin & hair, eczema, etc.

Chromosomal Mutation

• Deoxyribonucleic Acid
• DNA is a long chain polymer of deoxyribonucleotides.
• Nucleotide is made up of 3 chemical groups \[\to \]\[\underbrace{A\,\,\operatorname{int}rogen\,\,base\,\,moiety\,\,+\,\,Pentose\,\,sugar}_{Nucleoside}\] + Phosphate group
• Nitrogenous base are of two types - purines (9 membered double rings with nitrogen at 1, 3, 7 & 9th positions) and pyrimidines (6 membered rings with nitrogen at 1 and 3rd position).
• Purines are of two types - adenine (A) and guanine (G) and pyrimidines are of three types - thymine (T), cytosine (C) and uracil (U).
• A characteristics that differentiate DNA from RNA is that DNA contains all of the nitrogen bases except uracil and RNA contains all of the nitrogen bases except thymine.
• Wilkins-Franklin carried out X ray diffraction (X-ray crystallography) on the basis of which Watson and Crick suggested secondary structure of DNA in 1953.
• 2 DNA strands are organized in antiparallel and complementary arrangement [i.e., 2 strands run in opposite orientation (one in 5' - 3' and allies in 3' - 5')].
• Adenine pairs with thymine with 2 hydrogen bonds and guanine pairs with cytosine with three hydrogen bonds.
• The helix is generally right handed. e. its mm run clockwise looking along the helix axis. The pitch of helix is 3.4 nm (1 nm =\[{{10}^{-9}}\]) and there are 10bp in each turn.
• The concept of central dogma in molecular biology was proposed by Francis Crick (1958). It proposes unidirectional or one way flow of information from DNA to RNA & then to protein.

\[DNA\xrightarrow{Transcription}mRNA\xrightarrow{Translation}Polypeptide(protein)\]

Evolution

• It is the formation of newer types of organisms from the pre-existing ones through modification. Evolution is therefore, often called descent with modification.
• Earth originated 4600 million years ago.
• Life appeared 3.7 billion years ago. This is indicated by the discovery of microfossils of cyanobacteria like organisms.
• Theory of spontaneous generation (Abiogenesis/autogenesis)
• Given by Aristotle.
• Life originated from non-living things in spontaneous manner.
• Theory of Biogenesis
• Life originated from pre-existing life.
• Given by Francesco Redi.
• Oparin- Haldane theory was supported experimentally by Miller-Urey experiment in 1953.

Discharge Apparatus - a large flask containing mixture of \[C{{H}_{4}},N{{H}_{3}},{{H}_{2}}\] and\[{{H}_{2}}O\] with electric source and boiling of water for a week.

• Miller observed dark condensed liquid which was analysed. Analysis reports concluded, that it was a mixture of ammo acids like alanine, glycine, glutamic acid, aspartic acid, valine and leucine and number of other organic compounds like HCN, aldehyde and cyano compounds.

• Theories of Evolution

Darwinism

• Darwinism is the term coined for the explanation offered by Darwin for the origin of species by Natural selection in 1858.
• Darwinism or theory of natural selection is a theory of organic evolution which states that new species evolve over a long period of time through accumulation of small variations which provide the organism with structural and functional superiority over others in their survival and differential reproduction.

• Genetic Equilibrium (Hardy Weinberg Law)
• Law states that "both gene frequency and genotype frequency" will remain constant from generation to generation in an infinitely large interbreeding population in which mating is random and no selection, migration or mutation occurs.
• Hardy Weinberg formula or binomial expression is given as \[{{P}^{2}}+2pq+{{q}^{2}}=1\] for two alleles A-and a.
• Genetic drift refers to chance elimination of gene(s) of certain traits when a section of population migrates or dies of natural calamity. It eliminates certain alleles and fixes other alleles.
• Genetic drift in a new colony is called founder effect.
• Natural calamity like earthquake greatly reduces the size of population, killing the individuals randomly. The genetic pool of surviving population decreases. This condition of reduced genetic variability is called bottleneck effect.

• Vestigial Organs
• The useless and functionless degenerate structures which were large and functional in some other animals.
• Examples: Vermiform appendix, coccyx, pinna muscle, wisdom tooth in humans, rudiments of hindlimbs in python.

• Adaptive radiation
• It is a special evolutionary pattern, characterized by a rapid increase in number of kinds of closely related species.
• A good example of adaptive radiation is found among the Finches of Galapagos Islands. Another example is Australian Marsupials.

• Biological evolution
• It is the process of change over time, in the heritable characteristics, or traits of a population organisms.
• Many experts believe that Australopithecus garhi or a similar species gave rise to the genus Homo.
• Early hominids-members of the genus Homo-lived contemporaneously with Australopithecines for perhaps a half million years.
• The oldest fossil remains of a member of the genus Homo were discovered in Tanzania. It was named H. habilis.
• Homo erectus is the only other known extinct species of the genus Homo.
• H. erectus was replaced in tropical regions by Homo sapiens about 200,000 years ago.