Genetics
19. Genetics
Principles of inheritance
It is the scientific study of how physical, biochemical and behavioural traits are transmitted from parents to their offspring. Gene is the unit of inheritance and are carried by chromosomes in the nucleus and are arranges in a line along each chromosome. An Austrian monk, Gregor Mendel made the first truly scientific study of heredity by conducting experiments with garden peas. On the basis of these experiments, he formulated the following principles of inheritance.
It is the scientific study of how physical, biochemical and behavioural traits are transmitted from parents to their offspring. Gene is the unit of inheritance and are carried by chromosomes in the nucleus and are arranges in a line along each chromosome. An Austrian monk, Gregor Mendel made the first truly scientific study of heredity by conducting experiments with garden peas. On the basis of these experiments, he formulated the following principles of inheritance.
2. Law of segregation : when the factors that determine a pair of contrasting characters are brought together in a hybrid, they do not freely intermix but maintain their individualities. When the hybrid forms gametes, the factors of the pair become separated from each other and enter separate gametes.
3. Law of independent assortment : In the inheritance of more that one pair of characters considered together in a cross, the factors responsible for each pair of characters are distributed independently to the gametes i.e assorted independent of those of the other pair.
Fertilisation
Fertilisation is necessary to produce a single cell that contains a full complement of genes. When a cell undergoes meiosis, gametes are formed – a sperm cell or an egg cell. Each gamete contains only half of the genetic material of the original cell. During sperm and egg fusion in fertilisation , the full amount of genetic material is restored. Half contributed by male parent and half contributed by female. In humans there are 46 chromosomes in each human body cell –except in sperm and egg which each have 23 chromosomes. As soon as fertilisation is complete , the zygote that is formed has a complete set of 46 chromosomes containing genetic information from both parents. Male gametes (sperm) contains has only 22 paired homologous chromosomes and two unpaired chromosomes called X and Y. Each somatic cells of female has 23 pairs including a pair of X chromosomes. At fertilisation if a male gamete containing X chromosomes unites with a female gamete, a female organism is produced. If the male gamete containing a Y chromosomes unites with a female gamete a male organisms will be produced. The male is said to determine the sex of the off spring.
Fertilisation is necessary to produce a single cell that contains a full complement of genes. When a cell undergoes meiosis, gametes are formed – a sperm cell or an egg cell. Each gamete contains only half of the genetic material of the original cell. During sperm and egg fusion in fertilisation , the full amount of genetic material is restored. Half contributed by male parent and half contributed by female. In humans there are 46 chromosomes in each human body cell –except in sperm and egg which each have 23 chromosomes. As soon as fertilisation is complete , the zygote that is formed has a complete set of 46 chromosomes containing genetic information from both parents. Male gametes (sperm) contains has only 22 paired homologous chromosomes and two unpaired chromosomes called X and Y. Each somatic cells of female has 23 pairs including a pair of X chromosomes. At fertilisation if a male gamete containing X chromosomes unites with a female gamete, a female organism is produced. If the male gamete containing a Y chromosomes unites with a female gamete a male organisms will be produced. The male is said to determine the sex of the off spring.
Traits controlled by genes found on X and Y chromosomes are said to be sex linked. Recessive traits appear more frequently in malesthan females due to the fact that males have only one X chromosomes. A female will not exhibit the traits unless the recessive gene is on both her X chromosomes.
Genetic disorders
Congenital heart defects: a hole in the atrial septum that permits passage of blood from one atrial chamber to the other may be present at birth.
Congenital heart defects: a hole in the atrial septum that permits passage of blood from one atrial chamber to the other may be present at birth.
Erythroblastosis foetalis : This is a haemolytic disease of new born infants that occur due to incompatibility between mother and foetus.
Colour blindness : defect of vision affecting the ability to distinguish between red and green colours. This happens more commonly in males than females.
Downs syndrome (mongolism) : people with disease may have mild to severe learning disabilities and physical symptoms that include a small skull,extra folds of skin under the eyes, flattened nose bridge,a large,protruding tongue.such persons have 47 chromosomes.
Haemophilia: a hereditary blood disease characterised by inability of blood to clot or coagulate,leading to hemorrhage, or excessive bleeding even from minor injuries.
Harelip : It is abnormal clefts between upper lip and the base of nose. Cleft palate : It is the incomplete closure of palate or roof of mouth.
Sickle cell anaemia : Disease affecting black Africans . The RBC assume sickle shape when blood is deprived of oxygen.
Phenylketonuria: It causes inability to metabolise the amino acid Phenylpyruvic acid.
Albinism:Albinism (from Latin albus, meaning "white") is a lack of pigmentation in the eyes, skin and/or hair. Albinism is an inherited condition resulting from the combination of recessive alleles passed from both parents of an individual.
Cystic fibrosis : It causes body to secrete an abnormally thick, sticky mucus that clogs pancreas,lungs leading to problems with breathing, infection and ultimately death.
Hereditary material
Genes are chemically composed of deoxyribonucleic acid (DNA). It is the DNA that transmits the hereditary characters in a coded form one generation to the next in the living organisms. However in some virus ribonucleic acid carries the genetic information. The DNA and RNA are nucleic acids. The correct structure of DNA and RNA was first deduced by Watson and Crick in 1953. They proposed that DNA exits as a double helix in which the two polynucleotide chains are coiled about one another in a spiral. Each nucleotide chains consist of sequence of nucleotides. The nucleotides in each chain consists of
(i) pentose sugar(deoxyribose)
(ii) Phosphate molecule
(iii) nitrogenous bases (purines : adenine, guanine pyrimidines : cytosine,thymine).
Among these bases, adenine pairs specifically with thymine whereas cytosine does so with guanine. Hence the molecule is dependent on the sequence of bases in the other, The two chains are, therefore said to be complementary to each other. The base molecules of the two chains are linked through weak hydrogen bonds. Exceptionally,
DNA molecule may be single stranded and circular as in some bacteriophages.
The RNA molecule has generally a single stand of an unbranches polynucleotide chain. Its constituents are the same as in DNA except that the pentose sugar is ribose and among the nitrogenous bases,uracil is present as the pyrimidine in place of thymine. In some instances .the RNA strand is coiled on itself to form helices that may be connected by weak hydrogen bonds.
The genetic code is the sequence of nitrogen bases in deoxyribonucleic acid. The DNA carries the instructions for the production of protein. 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 DNA . A sequence of three nucleotide bases, called a triplet, is the genetic code word, or codon, that specifies a particular amino acid.
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