Genetics:-Defn :- “Genetics is the branch of bioscience which deals with the study of the underlying principles of heredity.”Importance-• Parent to offspring• Inherited diseases / medical genetics• Fertility genetics---- programmed baby• First test tube baby---- Durga
Historical events: Mendel (1865) : discovered principles of heredity. Karl Landsteiner (1930): blood groups & immunology William Bateson (1903):coined the term Genetics Herman Joseph Muller (1946): induction of mutation by X- rays Arthur korenberg, Severo Ochoa(1959): study on chemistry of DNA & RNA
James watson, Francis Crick: Maurice wilkins(1961): structure of DNA Marshell Nirenberg, Har Gobind Khorana, Robert Holley (1968): “cracking the genetic code” & revelling the means by which a gene determines the sequence of amino acid in a protein. And many more researcher who added the drops of their knowledge into ocean of genetics.
Mendel & Mendelism 1853 conducted experiments on garden peas (pisum sativum) 7 pairs of contrasting characters Crossed these varieties considering one pair of contrasting character F1 generation: allowed to self pollinate F2 generation : allowed to self pollinate.
On analysis of progeny of generations in garden pea, Mendel proposed his concepts.Mendel’s Laws:1. Law of unit inheritance:- it states that the characters do not blend ; if they do not expressed in the first generation they can reappear without change in the subsequent generations.
2. Law of Segregation: It states that the members of gene pair segregate and pass to different gametes.3. Law of independent assortment: It states that the members of different gene pairs assort independently of one another during gametogenesis.
Terms in Genetics Allele: One of alternative forms of a gene at a particular locus. Aneuploid: A set of chromosomes which does not contain an exact multiple of haploid sets of chromosomes. Centromere: Constricted region where sister chromatids are attached in mitotic chromosomes. The centromere is generally flanked by repetitive DNA sequences and it is late to replicate. The centromere is an A-T region of about 130 bp. It binds several proteins with high affinity to form the kinetochore which is the anchor for the mitotic spindle.
Codon: A group of three consecutive nucleotides in mRNA which specifies an amino acid to be incorporated in the polypeptide product of the gene. Deletion: A mutation resulting in the loss of normal DNA sequence. A deletion may be of any size from 1 nucleotide pair to the loss of most of a chromosome. Dominant: An allele is dominant if its effect can be observed in the phenotype of a heterozygote. Genome: The complete DNA sequence of an organism (though it may sometimes be used in the sense of mitochondrial genome or nuclear genome).
Haploid: In the human case, having exactly one copy of each of the autosomes and one sex chromosome. Chimaera (chimera): An animal (or plant) made up from a mixture of cells from more than one species. Also (in human genetics), an individual made up from cells derived from two or more zygotes. Mosaic: An individual composed of more than one genetically distinguishable cell population derived from a single zygote.
Plasmid: A short circular DNA sequence which is replicated within a host bacterium (or yeast) and which usually confers a selective advantage (such as antibiotic resistance) to the host. Plasmids have been engineered to serve as vectors for the propagation of DNA which is foreign to the host species. Polymerase chain reaction (PCR): A technique by which a relatively small piece of DNA of known sequence can be amplified (often from a complex mixture) by successive cycles of strand separation followed by DNA synthesis.
Human chromosome It is a combination of two words, i.e., “Chroma”- means „colour‟ and “Somes”-means „body‟. So the coloured thread like bodies present in the nucleoplasm of the living cells, which helps in the inheritance (transmission) of characters in form of Genes from generation to generation are known as CHROMOSOMES. 1956- Tjio & Levan : demonstrated 46 chromosomes in human 22 pairs – autosomes 1 pair - sex chromosome (XX / XY)
Morphology Uncoiled in interphase chromatin granules : some remain coiled Euchromatin / heterochromatin Rod like – end of interphase Centromere p- arm , q- arm Two sister chromatid- in late prophase Telomers Satellite bodies
Classification of ChromosomesAccording to position of centromere
According to “Denever system” of classificationGroup Chromosomes DescriptionA 1–3 Largest; 1 and 3 are metacentric but 2 is submetacentricB 4,5 Large; submetacentric with two arms very different in sizeC 6–12,X Medium size; submetacentricD 13–15 Medium size; acrocentric with satellitesE 16–18 Small; 16 is metacentric but 17 and 18 are submetacentricF 19,20 Small; metacentricG 21,22,Y Small; acrocentric, with satellites on 21 and 22 but not on the YAutosomes are numbered from largest to smallest, except that chromosome 21 issmaller than chromosome 22.
Paris nomencleature (1971,75) More accurate way of identification of chromosome based on various banding pattern.
Karyotyping / KaryotypeDefinition:- Photomicrograph of an individual‟s chromosomes arranged in a standard manner.Indications:1) Turner Syndrome (45,X and variants)2) Klinefelter Syndrome (47,XXY and variants)3) Down Syndrome (Trisomy 21, translocation and mosaicism)4) Couples with infertility of unknown cause
5) Couples with recurrent spontaneous abortions6) Children with ambiguous genitalia7) Female children with inguinal hernia8) Children with mental subnormality and dysmorphic features9) Suspected cases of Fragile X syndrome, Fanconi anaemia, Ataxia, Telangiectasia, Bloom syndrome etc.10) Bone marrow analysis in leukemias (blood cancer).11) Prenatal Diagnosis of fetal chromosome disorders in high-risk pregnancies.
Karyotyping Procedure Blood sample 5ml of venous blood (centrifuge) Nutrient medium (phytohaemagglutinin) Stimulates T- lymphocyte to divide
Culture at 370C for 3 daysColchicine added to arrest cell cycle in metaphase Add hypotonic saline- causes swelling of cells Cell lyses Mounted on slide & stained(cells are dropped on a cold, clean slide, to cause rupture and stained with giemsa)
Photomicrograph Arrange chromosome indecreasing order of their length
Methods of study of chromosomesA) Chromosome Banding• Q banding• G banding• R bandingB) Flow cytometryC) Molecular cytogenetics: FISH
Barr body / Sex ChromatinIntroduction1949- Barr & Bertram.Found chromatin mass in female cat neurons.They found it only in females sex.Best site to examine - Buccal mucosa.
Procedure of examining Barr body Scrapping from inner side of cheek. Smeared evenly on slide Fixed in alcohol Stained with thionin Observed under microscope.
No. of Barr bodies = no. of X chromosomes – 1.eg. Individual with 47,XXX complementNo. of barr bodies are 3-1= 2.In Turners syndrome (45,XO) it is 0. Barr body represents one of two X chromosomes of female cell. It remains condensed & inactive in interphase.
Lyon’s hypothesisIt states that:1) In female somatic cells, only one X chromosome is active. The second is inactive, condensed and appears in the form of sex chromatin in interphase.2) Inactivation occurs early in embryonic life.
3) Inactivation is random but fixed. inactive X can be maternal or paternal in different cells of same individual. but once decision is made then it is followed by it‟s descendent cells.• It is detected in blastocyst at 9-12 days.• First in syncytotrophoblast, then chorionic villi then in yolk sac• In embryo proper- after 18 days.
Mechanism of inactivation of X chromosomeMethylation of DNA. Mainly cytosine.5- methyl cytosine at certain sites in DNA.Inactivation centre:-Long arm of X chromosome.In some translocation involving X chromosome & autosomes. The intact X chromosome forms Barr body.
Genetic significance of X inactivation1) Dosage compensation2) Variability of expression3) Mosaicism