Basics of Genetics for Competitive exams

Basics of Genetics

· Incomplete dominance in dihybrids – 3:6:3:1:2:1. Eg: Mirabilis jalapa.

  1. Complementory factors (Duplicate recessive epistatis) 9:7, Test cross ratio 1:3. Two separate factors governed the same character.

  2. Supplementory factors (Recessive epistasis) – 9:3:4, Test cross ratio 1:1:2.

  3. Epistatic factor (dominant epistaris) – 12:3:1. Test cross ratio 2:1:1.

The gene which suppresses the effect of other is known as “Epistatic gene” and other gene which was suppressed in its effect is known a “Hypostatic”.

  1. Inhibitory factor – 13:3, Test cross ratio 3:1.

  2. Duplicate factor (duplicate epistasis) 15:1.

  3. Additive factor – 9:6:1; Test cross ratio – 1:2:1.

· The ability of gene (or) gene combination to be expressed phenotypically to any degree is called “Penetrance”.

· The degree of effect produced by a penetrant genotype is called – expressivity.

· Quasi quantitative traits – The quantitative expression of certain qualitative character.

· 2 or more genes situated on the same chromosome to remain together in inheritence is known as “linkage”.

· The degree (or) intensity with which two independent genes are linked together is called the “linkage value”.

· The total percentage of recombined types indicates the percentage of cross over. It is a measure of “linkage”.

· Qualitative character genes linked with quantitative character gene. Such factor is called as “Marker gene”.

· Homologus chromosomes twist around each other in synapsis, then each of the two pairing chromosomes, itself become spirally wound up. This is called “Spiralisation”.

· The movement of chiasmata is called “Terminalisation”. It takes in “diplotene” stage.

· Cytological proof of crossing over – stern’s experiment.

· The tendency of one cross over to suppress the other in neighbourhood is known as “Interference”.

· “Coincidence” is the chance of crossing over as expected

Coincidence + Interference = 1

· Ca, Cobalt reduces crossover

· Irradiation and colchicine reduces the crossing over

Total recombinations
· 1 cross over % =________________________ X 100

Total progeny

The two point test cross give only distance between genes; but three point test cross give distance and order of genes.

· Progamic and Syngamic: Sex is determined before and at the time of fertilization. Male heterogametic:
1. XO – XX mechanism Eg: Vallisnaria, Cockroaches
2. XY – XX Mechanism Eg: Man, Rumex sp, Drosophila.

Female heterogametic:
1. ZZ – ZO Mechanism – Chicken, moths, butterflies.
2. ZZ – ZW Mechanism – Some fishes, some birds.

· Neurospora, Asparagus and maize are having genes influenced by sex.

· When twin zygotes are formed with male and female genotypes the hormones of male embryo make the female embryo as sterile (or) with non functional sex organ known as “Free martin”. Eg: OX.

· Sex chromosomes are also called as “heterochromosomes”.

· Ratio X/A 1/3 – Supermale, ½ – Normal male, 2/3 – intersex (or) sterile 2/2 – Normal female, 3/2 – super female.

· Inheritance of X- linked genes – Colour blindness, Haemophilia.

· Inheritance of Y- linked genes – Hypertrichosis (Presence of hair on ears).

· Sex influenced – Blindness X Pattern Baldness.

· Sex limited – Secondary sexual characters, Beard development, breast development.

· Genes located in the sex chromosomes – “Sex linked”

· Sudden heritable changes – “Mutations”.

· Occurrence of genetically different tissues in the same organisms are known as “Chimeras”. These are of 3 types.

1. Sectorial – ‘V’ Shape (or) pie shape.
2. Mericlinal – In epidermis mutation.
3. Periclinal – In deeper region.

· Agents which cause mutations are referred to as “Mutagens”.

· High frequency mutations result by “Mustard gas”.

· Induced sex linked lethal recessive mutation – ClB technique.

· Individual hereditary factor known in chromosomal inheritance as “genes” but in extra chromosomal inheritance as “Plasmagenes”.

· Plasmagenes are also called as “Kappa particles” (Paramoecium).

· Deletions survive in Heterozygous but lethal in homozygous condition.

· Non lethal deletion (or) deficiency leads to “Pseudodominance”.

· If a repeated segment is present besides normal segment and has the same gene order it is known as a “Tandom duplication”.

· Besides normal segment but have a reversed gene order – Reverse tandom.

· If the repeated segment is present on a non – homologous chromosome it is known as a “Displaced duplication.

· Reversal of the linear arrangement of genes in a segment without change in the total gene content, it is termed as Inversion”.

· Pericentric inversions produce unbalanced gamets and inviable zygotes.

· The transfer of a section of chromosome to a homologus chromosome is known as “translocation”.

· Haploid set of chromosomes – genome (term coined by Wrinkler).

· Haploid simply denotes the total number of chromosomes contained in a gamete of any sporophyte.

· Autopolyploids – A polyploid possessing similar (or) identical genome.
· Allo polyploids – A polyploid possessing dissimiklar genomes.
· Aneuploidy (Un even) ; The lowest haploid chrmose number is genome.

Nullisomics 2n – 2, Trisomic 2n+1
Monosomic 2n-1, Tetrasomic 2n+2.

· A series of genetic disturbances which ultimately lead to male sterility in plants are due to the presence of Accessory (or) B chromosome.

· Centromeres of bivalents are arranged on either side of equator – co orientation (in metaphase – 1)

· Animals having no sex hormones but with distinct areas of body showing male and female tissues is known as “Gynandromorphs”.

· Single stranded RNA in plant virus TMV.

· When the inverted segment of chromosome includes no centromere then such inversion is known as “Paracentric inversions”.

· The diploid chromosome number in Rice is 24.

· Crossing over takes place between non sister chromatids of homologous chromosomes.

· ‘N’ base containing single ring of atoms in their molecule – pyrimidine; double ring – purine.

· Expression of recessive gene in hemizygous condition – “Pseudodominance”.

· Genes at a given locus – multiple allele,

· Gene at different loci – Multiple factor.

· Both the sexes are well developed in one body but cannot function as either sex – “Pseudohermaphrodites”

· When an allele is absent from the homologous chromosome, it is termed as Hemizygous condition.

· The most accepted type of DNA replication is by “Semi conservative method”.

· Crossing over does not occur in the females of silkworm and in males of ‘Drosophila’.

· Genes present in Y- Chromosome, transmitted only from male to male are called as “Holandric”.

· The immediate effect of foreign pollen on the embryo and endosperm character is termed as xenia.

· Maximum frequency of crossing over between two genes regardless of multiple crossover is 50%.

· Quantitative traits are governed by “Polygenes”.

· Crossing of F1 with homozygous recessive – Test cross.

· Gene exchange in bacteria by viruses – Transduction.

· An example of man made allopolyploid – Triticale

· Raphano brassica – Raphanus sativus x Brassica oleraces.

· Cytoplasmically inherited characters are inherited through the maternal parent.

· Spindle fibres are formed by the polymerization of “Microtubules”.

· Autotriploid- Banana; Grape, seedless water melon.

· Autotetraploid – Potato, Autohexaploid – Sweet potato,

· Allotetraploid – Cotton, Allopolyploid – Triticale, Raphanobrassica.

· Man made autotriploid – Sugarbeet.

· Non sister chromatids break by “Endonuclease enzyme”.

· The tendency of one dominant gene and recessive gene to remain together on a chromosome is called “Repulsion phase”.

· Cells which undergo meiosis are called Meiocyte.

· The entire genetic constitution of an individual is called – genotype.

· Exception for cell theory – Virus.

· Lysosomes are tiny bags filled with “hydrolytic digestive enzyme”.

· Cell undergoes meiosis when the ratio of RNA and DNA is low.

· Non localized chromosomes are present in Luzula. i.e. Diffused Centromere.

· Telomeres showing active mobility is called Neocentric.

· Oil seed crop varitiesdeveloped by mutation breeding is castor.

· Amino acids are attached to the RNA at “Anticodon end”.

· Polygenic expression will be greatly influenced by environment.

· Chemical mutagen – Ethyl methane sulphonate (EMS).

· One allele mutagenic together – Paramutation.

· Barr eye character in drosophila is due to “duplication”.

· The number of tRNA kinds – 21.

· The end of tRNA which recognize “aminoacyl synthetases” is CCA.

· Starting codon on mRNA – AUG.

· Translocation heterozygotes produce viable gamets by “Altenative segregation”.

· Haploid – diploid sex determination is present in honeybee.

· Colchicin can depolymerise Microtubules.

· Chromatin = DNA + Nucleoprotein.

· Location of genes on chromosomes – is done by aneuprids.

· Striga resistant variety – N-13 of Sorghum.

· Paleolithic (or) early stone age – No domestication of plants and animals.

· Neolithic (or) late stone age – Domestication of plants occurred.

· Historical period – Domestication of industrial plants.

· Modern period – Domestication of industrial plants and drug plants.

· Introgression – Absorption of germplasm from one species by the other without impairing its taxanomical identity is known as “introgression”

· Corn has borrowed most of the characters from its wild relaties “Teosinte and Tripsacum”.

· Large proportion of cultivated crops originated through “allopolyploid”.

· According to Mendal variations in the population is created because of “Natural hybridization”.

· Natural selection – Darwin.

· Vavilov noted the existence of parallichism in variability among the related species and he called it as “Law of homologous series”.

· Largest and oldest independent centre of origin – The China centre of origin.

· Secondary crops – plants which are derived from weeds that grow among the primary crop plants.

· Bulbils – Modified flowers that develops into plants directly without formation of seed. Eg. Garlic.

· Somatic hybridization (Protoplasts of two different species may be fused with the help of poly ethylene glycol (PEG).

· Apomixis:- It is a form of asexual reproduction in which seed is produced but the embryo develop without fusion of male and female gamets.

· When sexual reproduction occurs besides apomixes, the apomixes is termed as “Facultative apomixes”.

· Apospory – Some vegetative cells of the ovule develop into unreduced embryo seed without meiosis.

· Apogamy – Synergids (or) antipodal cells develop into embryo.

· Pseudogamy – Development of apomictic seed by the stimulus of pollination and pollen tube growth without fertilization.

· Parthenogenesis – Development of a embryo without fertilization.

· Parthenocarpy – Development of fruit without seed.

· Monogamy – Both male and female flowers mature at the same time.

· Chasmogany – Flowers open only after pollination.

· Dichogamy – Male and female flowers mature at different times.

· Heterostyly – Flowers are bisexual but stamens and styles are of different lengths.

· In often cross pollinated crops cross pollination exceeds 5%.

· NBPGR – National Bureau of Plant Genetic Resources.

· The part of plant used in the propagation of the species is known as “Propagule”.

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