Inheritance and Variation
Mendel’s Law of Inheritance
(1) Mendelism means experiments performed by Mendel on genetics.
(2) Mendel’s experiment involved 4 steps as selection, hybridization, selfing and calculations. His results led to the formation of laws of genetics later.
(3) Mendel performed monohybrid and dihybrid crosses and gave three principles of inheritance.
(4) Mendel’s three principles of inheritance are:
(i) Law of dominance
(ii) Law of segregation or law of purity of gametes
(iii) Law of independent assortment
(4) Law of Dominance – The dominant characters are expressed when factors are in heterozygous condition.
(5) The recessive characters are only expressed in homozygous conditions. The characters never blend in heterozygous condition. A recessive character that was not expressed in heterozygous condition may be expressed again when it becomes homozygous.
(6) Law/Principle of segregation states that when a pair of contrasting factor or gene is brought together in a hybrid, these factors do not blend or mix up but simply associate themselves and remain together and separate at the time of gamete formation.
(7) Principle of independent assortment states that genes of different characters located in different pairs of chromosomes are independent of one another in this segregation during gamete formation.
(8) Test Cross: A cross between F1 hybrid (Aa) and its homozygous recessive parent (aa) is called Test Cross. This cross is called test cross because it helps to find out whether the given dominant phenotype is homozygous or heterozygous.
(9) Monohybrid cross – When we consider the inheritance of one character at a time in a cross, this is called monohybrid cross.
(10) Dihybrid Cross – A cross made to study the inheritance of two pairs of contrasting traits.
Exceptions of Conclusions of Mendel
Incomplete Dominance
(1) When neither of the alleles of a character is completely dominant over the other and the F1 hybrid is intermediate between the two parents, the phenomenon is called incomplete dominance.
(2) Incomplete dominance was first discovered by Correns in Mirabilis jalapa. The plant is called as 4’O clock plant or ‘Gul-e-Bans’. Homozygous red (RR) flowered variety of the plant was crossed with white (rr) flowered variety. F1 offspring had pink flowers (Rr). This is called incomplete dominance.
(3) Incomplete dominance is also known to occur in snapdragon. The phenotypic ratio and genotypic ratio in F2 generation in case of incomplete dominance is 1:2:1.
Co-dominance
(1) In co-dominance both the gene expressed for a particular character in F1 hybrid progeny. There is no blending of characters, whereas both the characters are expressed equally.
(2) Co-dominance is seen in animals for coat colour. When a black parent is crossed with white parent, a roan color in F1 progeny is produced.
Sex determination
(1) Fixing the sex of an individual as it begins life is called sex determination. The various genetically controlled sex-determination mechanisms have been classified into following categories
(2) Chromosomal theory of sex determination: The X-chromosome was first observed by German biologist, Henking in 1891 during the spermatogenesis in male bug and was described as X-body.The chromosome theory of sex determination was worked out by E.B. Wilson and Stevens (1902-1905).
(3) They named the X and Y chromosomes as sex-chromosomes or allosomes and other chromosomes of the cell as autosomes.
(4) Sex chromosomes carry genes for sex. X-chromosomes carries female determining genes and Ychromosomes has male determining genes.
(5) The number of X and Y chromosomes determines the female or male sex of the individual,Autosomes carry genes for the somatic characters. These do not have any relation with the sex.
Sex Determination by chromosomes:
Those chromosomes which are involved in the determination of sex of an individual are called sex chromosomes while the other chromosomes are called autosomes.
1) XX – XY type: In most insects including fruit fly Drosophila and mammals including human beings the females possess two homomorphic sex chromosomes, named XX. The males contain two heteromorphic sex chromosomes, i.e., XY. Hence the males produce two types of gametes / sperms, either with X-chromosome or with Y-chromosome, so they are called Heterogamety.
2) ZZ – ZW type: In birds and some reptiles, the males are represented as ZZ (homogamety) and females are ZW (heterogamety).
3) XX – XO type: In round worms and some insects, the females have two sex chromosomes, XX,while the males have only one sex chromosomes X. There is no second sex chromosome.Therefore, the males are designated as XO. The females are homogametic because they produce only one type of eggs. The males are heterogametic with half the male gametes carrying X-chromosome while the other half being devoid of it.
Numerical aberrations of chromosomes: Each species has a characteristic number of chromosome. Variations or numerical changes in
chromosomes (Heteroploidy) can be mainly of two types:
(1) Turner’s syndrome: Such persons are monosomic for sex chromosomes i.e. possess only one X and no Y chromosome (XO). In other words they have chromosome number 2n – 1 = 45. They are phenotypic females but are sterile because they have under developed reproductive organs.They are dwarf about 4 feet 10 inches and are flat chested with wide spread nipples of mammary glands which never enlarge like those in normal woman. They develop as normal female in childhood but at adolescence their ovaries remain under developed. They lack female hormone estrogen. About one out of every 5,000 female births results in Turner’s syndrome.
(2) Klinefelter’s syndrome: Since 1942, this abnormality of sex is known to geneticists and physicians. It occurs due to Trisomy of sex chromosomes which results in (XXY) sex chromosomes. Total chromosomes in such persons are 2n + 1 = 47 in place of 46. Klinefelter (1942) found that testes in such male remain under developed in adulthood. They develop secondary sex characters of female like large breasts and loss of facial hair. Characters of male develop due to Y chromosome and those like female due to XX chromosomes. About one male child out of every 5,000 born, develops Klinefelter’s syndrome.
Molecular Basis of Inheritance
DNA
(1) DNA is a long polymer of deoxyribonucleotides.
(2) The length of the DNA depends on the number of nucleotide pairs present in it.
(3) Bacteriophage lambda has 48,502 base pairs.
Central dogma of molecular biology
(1) Crick proposed the Central dogma in molecular biology
(2) It states that the genetic information flows from DNA à RNA à Protein.
(3) In some viruses like retroviruses, the flow of information is in reverse direction, which is from RNA à DNA à mRNA à Protein.
Structure of polynucleotide chain:
(1) A nucleotide has three components-
(a) A nitrogen base
(b) A pentose sugar (ribose in RNA and deoxyribose in DNA)
(c) A phosphoric acid.
(2) There are two types of nitrogen bases:
(a) Purines (Adenine and Guanine)
(b) Pyrimidines (Cytosine, Uracil and Thymine)
(3) Adenine, Guanine and Cytosine are common in RNA and DNA.
(4) Uracil is present in RNA and in DNA in place of Uracil, Thymine is present.
(5) In RNA, Pentose sugar is ribose and in DNA, it is Deoxyribose.
(6) Based on the nature of pentose sugar, two types of nucleosides are formed - ribonucleoside and deoxyribonucleotides.
(7) Two nucleotides are joined by 3’-5’ Phosphodiester linkage to form dinucleotide.
(8) More than two nucleotides join to form polynucleotide chain.
(9) The two strands of DNA (called DNA duplex) are antiparallel and complementary, i.i., one in 5’->3’ direction and the other in 3”->5” direction.
History of DNA
(1) DNA is an acidic substance in the nucleus.
(2) It was first identified by Friedrich Meischer in 1869. He named it as ‘Nuclein”
(3) In 1953 double helix structure of DNA was given by James Watson and Francis Crick, based on Xray diffraction data produced Maurice Wilkins and Rosalind Franklin.
Packaging of DNA Helix
(1) The basic unit into which DNA is packed in the chromatin of eukaryotes.
(2) Nucleosome is the basic repeating structural (and functional) unit of chromatin, which contains nine histone proteins.
(3) Distance between two conjugative base pairs is 0.34nm
(4) The length of the DNA in a typical mammalian cell will be 6.6 X109 bp X 0.34 X10-9 /bp, it comes about 2.2 meters.
(5) The length of DNA is more than the dimension of a typical nucleus (10-6m)
DNA Replication
(1) DNA is the only molecule capable of self duplication so it is termed as a living molecule.
(2) All living beings have the capacity to reproduce because of DNA.
(3) DNA replication takes place in S-phase of the cell cycle. At the time of cell division, it divides in equal parts in the daughter cells.
(4) Delbruck suggested three methods of DNA replication i.e.
(i) Dispersive
(ii) Conservative
(iii) Semi-conservative
(5) The process of DNA replication takes a few minutes in prokaryotes and a few hours in eukaryotes.
RNA
(1) RNA is the first genetic material.
(2) RNA is a non hereditary nucleic acid except in some viruses (retroviruses).
(3) RNA used to act as a genetic material as well as catalyst.
(4) It is a polymer of ribonucleotide and is made up of pentose ribose sugar, phosphoric acid and nitrogenous base (A,U,G,C).
(5) RNA may be of two types – genetic and non-genetic.
Genetic Code
(1) Term genetic code was given by George Gamow (1954). He was the first to propose the triplet code (one codon consists of three nitrogen bases).
(2) The relationship between the sequence of amino acids in a polypeptide chain and nucleotide sequence of DNA or mRNA is called genetic code.
(3) There occur 20 types of amino acids which participate in protein synthesis. DNA contains information for the synthesis of any types of polypeptide chain. In the process of transcription, information transfers from DNA to m-RNA in the form of complementary N2-base sequence.
(4) A codon is the nucleotide sequence in m-RNA which codes for particular amino acid; whereas the genetic codeis the sequence of nucleotides in m-RNA molecule, which contains information for the synthesis of polypeptide chain.
(5) 61 out of 64 codons code for only 20 amino acids.
(6) The main problem of genetic code was to determine the exact number of nucleotide in a codon which codes for one amino acid.
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