Genetics : Genetics deals with the study of of inheritance. Every living organism inherits traits from parents. These traits are transferred from generation to generation. All living being have genetic characters.
Bateson in 1905 used for the first time the term ‘Genetics’. Mendel is considered the father of genetics. Genetics was revised in 1900.
Genetics can be divided into six branches.
(a) Plant genetics : Deals with the study of plants.
(b) Animal genetics : It deals with the genetics of animals.
(c) Microbial genetics : In this branch we study the genetics of micro-organisms like bacteria, viruses etc.
(d) Human genetis : It is concerned with the study of genetics of man.
(e) Cytogenetics : It deals with the study of cytological explanation of genetical principles.
(f) Molecular genetics : How chemical molecules are related to geneties, that we study under molecular geneties.
Eugenics
Golton in 1885 coined the term eugenics. It deals with the application of the laws of genetics to the improvement of human race. In fact eugenics deals with the methodology of improving inborn qualities of a race, Eugenics is of two types namely—
1. Positive Eugenics and 2. Negative Eugenics.
Positive Eugenics deals with early marriage of those having desirable traits. While Negative Eugenics deals with the marriages of undesirable persons.
MENDELIAN PATTERNS OF INHERITANCE
Laws of heredity : There are three laws of heredity, which Mendel established after doing the experiments of pea plants.
Law of Dominance : It states that, out of the contrasting characters of a pair, the one which expresses itself in the F1 generation is called a dominant character and the other which is prevented from expressing itself is called a recessive character.
Law of Segregation : It states that when a pair of contrasting characters occur together in a hybrid, they remain together without mixing with each other and segregate during the formation of gametes.
Law of Independent Assortment : This law states that when two parents differing from each other in two or more pairs of contrasting characters are crossed, then the inheritance of one pair of characters is independent of that of the other pair of characters.
Variations : The differences which occur in closely related organisms are called variations.
Allele : Alternative forms of a gene—which occur at the same locus on homologous chromosomes.
Dominant Allele : An allele that exerts its phenotype effect in the heterozygote; it hides the expression of the recessive allele.
Gene Locus : The location of a particular gene on homologous chromosomes is called gene locus.
Genotype : The genes of an organism for a particular trait are called genotype.
Heterozygous : Possessing unlike alleles for a particular trait.
Homozygous : Possessing two identical alleles for a particular trait.
Phenotype : The visible expression of a genotype—for example, brown eyes or attached earlobes.
Punnett Square : A grid that enables one to calculate the results of simple genetic crosses by lining up alleles within the gametes of two parents on the outside margin and their recombination in boxes inside the grid.
Recessive Allele : An allele that exerts its phenotypic effect only in the homozygote; its expression is masked by a dominant allele in heterozygote.
Monohybrid Ratio : A cross which involves a single pair of alleles is called a monohybrid cross.
A phenotypic ratio of 3 : 1 obtained in the F1 generation of a monohybrid cross is called a monohybrid cross characteristics intermediate between two extreme parental characteristics—for example, a red and a white flower producing pink offspring.
Linkage Group : Alleles of different genes that are located on the same chromosome and tend to be inherited together.
Multiple Allele : A pattern of inheritance in which there are more than two alleles for a particular trait.
Mutation : An alteration in chromosome structure or number and also an alteration in a gene due to a change in DNA composition.
Polygenic Inheritance : A pattern of inheritance in which a trait is controlled by several allelic pairs; each dominant allele contributes in an additive and like manner.
Polyploid (Polyploidy) : A condition in which an organism has more than two complete sets of chromosomes.
Sex Chromosome : A chromosome that determines the sex of an individual in humans females have two X chromosomes and males have an X and Y chromosome. Combination of X and X give birth to a daughter while X and Y produce a boy.
Dihybrid Ratio : A cross which involves two pairs of alleles is called a dihybird cross and a phenotypic ratio of 3 : 3 : 1 obtained in the F2 generation of a dihybrid cross is called a dihybrid ratio.
Back Cross : A cross between the F1 of fspring with either of its parents is called a back cross.
Test Cross : A cross between an individual with the dominant phenotype and an individual with the recessive phenotype to see if the individual with the dominant phenotype is homozygous or heterozygous.
Types of chromosomes
Each chromosome has a centromere. The type of chromosome is decided by the position of centromere. Thesse are of following types.
(a) Telecentric : In these chromosomes the centromere is on the proximal end.
(b) Acrocentric : In this, centromere is near to one end.
(c) Submetacentric : In this centromere is near the centre.
(d) Metacentric : In this, centromere is in the centre.
Giant Chromosomes
Polytene chromosomes : These are found in the salivary glands.
Lampbrush chromosomes : These are found in oocytes of vertebrates.
Supernumeray chromosomes:
Human chromosomes
(1) Autosome : They are large in number and are not the sex chromosomes.
(2) Sex chromosomes : These determine the male or female and are known as X and Y chromosome.
Tijo and Leven (1965) for the first time demonstrated 46 chromosomes or 23 pairs in human cells in which 22 pairs are autosomes and one pair is sex chromosomes. In females the sex chromosomes are X-X whereas in males sex chromosomes are X-4. The females contain XX (diploid) chromosomes and male with XY (diploid) sex chromosomes. The X and Y chromosomes exhibit structural differences. The X chromosome is mostly straight rod like and comparatively larger than Y. The Y chromosome is smaller in size. Combination of X of man and X of woman give birth to a female sex while Y of man and X of woman give birth to a male sex.
CHROMOSOMAL ABNORMALITIES IN HUMAN
1. Klineflter’s syndrome (47, XXY)—Sterile male born with 44+XXY diploid chromosomes. Though the child have two X chromosomes but still his a male. External genitalia normal, testes small, sperms not produced, mentally retarded, arms longer than average etc. They are sex chromatin positve.
2. Turner’s syndrome (45, X) (i.e. AAXO)—Abnormal females are born with 44 + X chromosomes. Grown up child has short stature, shield shaped chest, slightly mentally retarded, breast absent, pubic hair reduced, genitalia infantile. They have negative sex chromatin.
3. Down’s syndrome (Mongolism) (2n = 47)—They are mentally retarded height is below average upper eyelid peculiar slopping forehead, flattened nose, short hands, sexual maturity not attained, males sterile, females give birth to children. Addition in 21st pair of chromosome.
4. Edward’s syndrome (Trisomy-18)—They have multiple malformation, mentally retarted, more pronounced in females, death occures genereally at 3-4 months of age. This is due to addition of extra 18th chromosome.
DETERMINATION OF SEX
For the purpose of sexual reproduction the animal may be either monoceious or dioecious.
In normal human beings males are XY and females are XX. Thus males with 44 + XY and females with 44 + XX.
Other abnormal combinations are also there.
There are two types of genetic material RNA and DNA found in genes of humans.
LINKED GENES
Each chromosome have large number of genes. These genes are placed over chromosome in a liner fashion. Some genes are very closely situated and some are far away from each other. Two closely situated genes would not show the independent assortment. They are linked together in heredity by virtue of their common chromosome and all the genes on one chromosome are said to be linked genes. These genes tend to remain on the same chromosome after meiosis.
One character may be controlled by a single gene or it may be controlled by more than one gene. It is also quite possible that more than one character may be controlled by one gene. The chromosomes move as units during meioses and all the genes have a specific number of linkage groups of genes.
Linkage strength : The genes situated very closely have great affinity to remain on the same chromosome. This is due to a linkage force. The greater linkage force will express the greater linkage strength towards each other.
CHROMOSOMAL MAP
Studies regarding chromosal map show that
(i) Genes are located on chromosomes in the linear fashion.
(ii) The number of gene groups are equal to the chromosome pairs in a cell.
(iii) Crossing over and recombination frequencies of genes depend upon the distance between two linked genes.
(iv) Linkage strength is inversely proportional to the distance between two genes.
(v) Chromatid determines the position of genes.
SEX LINKED INHERITANCE
Some physiological and morphological characters are controlled by genes present over sex chromosomes and are inherited through generation to generation. These are known as sex linked characters.
Humans have X and Y chromosomes which are not entirely homologous. The genes that occur only on the X chromosome will be presented twice in females and once in males. Genes located exclusively on the X chromosome are called sex linked, while the genes that occur only on the Y chromosome can produce their effects only in males, these are called holandric genes.
Characteristics of sex linked characters :
(1) Recessive sex linked diseases are found in males. The genes of these are present on X chromosome only. Thus it can be expressed in males. The recessive sex linked characters in females are expressed when homozygous condition (Both the X bear the recessive genes for disease) is obtained by the female.
(2) Sex linked genes are inherited to the daughter, but these genes are dominated by the gene present on the maternal X chromosome. Then it is transferred to the 2nd generation. This inheritance is known as criss-cross inheritance. The daughters are the carrier of the disease.
INHERITANCE OF SEX LINKED CHARACTERS IN MAN
In man about 50 sex linked characters have been reported the genes of which present over X chromosome. The most important and common sex linked characters in man are—
1. Colour or red-green blindness : Man suffering with this kind of blindness cannot differentiate the red and green colour. This is sex linked character.
2. Heamophilla : This is also known as bleeder’s disease. In this disease blood in which blood has no blood clotting property. Even from a small injury the blood continues to come out from wound for hours together. It may lead to death. It is found in males but females are the carrier of this disease.
3. Anhidrotic ectoderma (No functional sweat glands) : These diseases are associated with X-linked recessive genes and are most common in man. Human beings have 46 chromosomes (23 pairs) present in each somatic cell. Female has 22 pairs + XX while male has 22 pairs + XY chromosomes. Since female will produce only one type of gamete, the gametes from the male individual will determine the sex of the progeny.
Sex influenced or sex linked traits : Hereditary baldness is due to recessive autosomal allele gene pair (Bb) but genes BB are dominant. It is (BB) the condition in females and thus baldness is not present in female but the heterozygous genes express the character of baldness.
BLOOD
Blood is the homogeneous fluid which continues circulating in the body. It is composed of a large variety of dissolved and suspended inorganic and organic substances as well as 3 kinds of cells.
In man, the normal adult contains 5-6 litres blood. The osmotic pressure of human blood averages about 5900 mm Hg or 7.8 atmospheres. The osmotic pressure is mainly due to the various salts, waste products, sugar and other substances dissolved in plasma.
The viscosity is about five times greater than that of water.
The sp. gravity of blood is in the range of 1.035-1.075.
When freshly drawn blood, to which an anticoagulating agent is added and placed stationary for some time, the erythrocytes start sedimenting. The rate at which these cells sediment is known as erythrocyte sedimentation rate (E.S.R.). The ESR is expressed in mm/hr and normally it varies from 4-10 mm/hr. During transfusion it is essential to prevent coagulation by some substances. There are certain substances or processes which can prevent the blood coagulation. These are—
(a) Heparin—Best and powerful anticoagulant.
(b) Antithromboplastin
(c) Antithrombic activity
(d) Oxalates and citrates
(e) Defibrination
BLOOD GROUPS IN HUMAN
Blood groups represent the presence of specific antigens in the blood. ABO system of blood is universally accepted. The other system is rhesus system which is found in the rhesus monkey.
ABO system : It is divided into four types which can easily be represented by a table—
Blood groups with their antigens and antibodies
Blood group Antigens in Antibodies
RBC in Plasma
A A Anti B
B B Anti B
AB A and B Absent
O Absent Anti A and
Anti B
Blood groups and their possible combinations in blood transfusions
Blood group Can be Can receive
given to blood from
A A, AB A and O
B B, AB B and O
AB AB All groups
O All groups O
AB = Universal recipient, O = Universal donor.
Blood group determination
Blood group Serum A Serum B
A – –
B + –
AB + +
O – –
– no agglutination
+ means agglutination (clumping)
Rhesus system : Landsteiner and Weiner (1940) found certain specific antigen in RBCs of rhesus monkey. The name was given to this antigen as rhesus factor (RH factor). Later on it was shown that most human beings (85%) also have this factor and said to be Rh positive, while only 15% are Rh negative. 93% Indian population is RH+, while the rest 7% is Rh-, and Rh- bloods are incompatible and cannot be mixed, thus they cannot be given to a patient having different RH factor.
Significance of blood groups :
1. It is very important in blood transfusion.
2. It helps in solving the disputed parentage problem in medico-legal cases.
3. It furnishes the best example of multiple alleles.
4. Haemolytic disease of the new born.
5. Study of physical anthropology.
6. Relationships of blood groups and susceptibility to various diseases.
BLOOD BANKS
Genetic Engineering
Genetic Engineering is a very important branch of zoology. It is relatively a new branch.Gene cloning is very useful.
Following are the outcomes of genetic engineering.
1. Insulin production : It is a very important hormone for the regulation of blood sugar in the body. It is produced in the pancreas. In diabatic patients insulin is given for the control of sugar in the body. At present insulin is prepared by the extraction of pancreas of cows and pigs.
2. Production of human growth hormone : Pituitary gland secretes the growth hormone in the body. Growth hormone when produced less it brings dwarfism. At present this hormone is synthesized by engineered bugs.
3. Interferon Production : Interferon is an antibody protein. It is a virus induced protein. It is produced by virus infected animal cells. It is an important natural defence system in human body.
4. Tissue Culture and Organ Transplant : Tissues and skin can be transplanted in the body. In 1870 Reverdin transplanted a small piece of skin. There are four types of transplant (a) Isograft, (b) Allograft, (c) Autograft and (d) Xenograft. Isograft transplant is done between two twins (identical). Allografting same species. In Autografting donor and recipient of grafting is the same person. Xenografting transplantation is in between two different species.
Graft rejection takes place due to antigens, immune products are formed against the host.
The following are the antirejection mechanisms—antimiotic drugs and introduction of antibodies.
Gene Mutation
The change in number and structure of chromosomes and changes in genetic combination of a person bring mutation in human body. Gene mutation is due to alteration in very small part of DNA molecule. It is of three types (a) Deletion mutation (b) Addition mutation and
(c) Substitution mutation.
Mutagenic chemicals, radiations and temperature are the main causes of gene mutation.
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