Genetics is the study of heredity, a process by which children inherit certain characterstic from their parents.
Heredity is controlled by genes and environmental factors. Various environmental factors may cause anomalies in chromosomes.
Genes - Gene , the functional unit of DNA, is the basic unit of heredity in a living organism. All living things depend on genes. Genes hold the information to build and maintain an organism's cells and pass genetic traits to offspring.
Properties of Genes
1- To determine traits,e.g. colour of skin,. intelligence, height, etc.
2- Undergo replication
3- May undergo mutation
4- Homeobox genes are group of regulatory genes that control the expression of other genes involved in the normal development,growth and differentiation. Teratogens like retinoic acid can activate these genes to cause abnormal gene expression.
Functions of Genes
1- Maintain the genetic specificity of an individual.
2- Play key role in transmission of traits from the parents to the offspring.
3- Synthesise various proteins and enzymes of the cell.
Types of Genes- Each gene occupies a specific locus on the chromosome. Both chromosomes of a given pair contain similar genes. The genes occupying the same locus on the homologous chromosomes are called "alleles".
In females, the two sex chromosomes (XX) are identical in length, hence these are homologous.
In males, the two sex chromosomes (XY) are unequal in length. There are so alleles on the Y chromosomes, for most of the loci are on the X chromosome.
Types of Genes
According to Mendelian pattern of inheritance-
1- Dominant gene- An allele which is always expressed in both the homozygous and the heterozygous combination.
2- Recessive gene - When an allele is expressed only in the homozygous state, it is known as recessive gene.
3- Carrier gene - In the hetrozygous state, the recessive gene acts as a carrier gene which is not expressed in the individual but may be expressed in the subsequent generations.
4- Co-dominant gene - when both the allelic genes are dominant but of two different types, both traits may have concurrent expression, e.g. blood group AB.
5- Sex-linked genes- The genes located on X or Y chromosomes are sex-linked genes.
6- Sex-limited genes- These genes are borne by the autosomes, but the trait borne is preferentially in one sex only,e.g.baldness seen predominantly in males.
7- Structural genes- These are segments of DNA which code for specific sequence of amino acids in the protien.
8- Regulatory genes- These are segments of DNA which control functions of structural genes.
Some important terms
1- Inheritance- It is process of transmission of characters/traits from generation to generation.
2- Reproduction- It is essential requisite for inheritance to take place. The inheritance of traits from parents to offspring takes place through genes which carry all information about all types of traits.
3- Locus- The position of a gene in the chromosome is called locus.
4- Alleles- Genes occupying identical loci in a pair of homologous chromosomes.
a)- Homozygous alleles- When both allelic genes regulating a particular character are similar.
b)- Hetrozygous alleles- When both allelic genes regulating a particular character are dissimilar.
c)- Multiple alleles- When in a population, more than two different alleles exist at a given locus of a chromosome.
d)- Mutation- It is a phenomenon which results in alteration in base pair in DNA. Under abnormal conditions, adenine may pair with cytosine or guanine, instead of thymine. This forms the basis of mutation.
Modes of inheritance (Mendel's laws of inheritance)
Law of uniformity- The crossing over between two homozygous of different types results in offspring that are identical and hetrozygotic. The inherited characters do not blend.
Law of segregation- Segregation of alleles occurs during the process of gamete formation and randomly united at fertilization.
Law of independent assortment- This law states that the traits are transmitted to the offspring independently of one another.
Chromosomes
Structure of Chromosomes
1- All chromosomes consist of two parallel identical filaments called chromatids joined together at a narrowed constriction called centromere.
2- There are three types chromosomes, Metacentric,Subcentric and Acrocentric .
Groups of chromosomes: The chromosomes are arranged in descending order of length. The first pair is the longest and 22nd pair is the shortest. Sex chromosomes somes are grouped separately. The chromosomes are divided into 7 groups. They are denoted as A and G .
Each cell contains fixed number of chromosomes which is characterstic of that species or organism. In somatic cell of human,the number is 46, which is diploid number. In gametes, i.e. ovum and sperm it is 23 , called haploid number. During fertilization, union of two haploid cells restores diploid number of chromosomes.
Classification of Chromosomes
According to functions:
a)- Autosomes: 22 pairs in humans
b)- A pair of sex chromosomes which decides the sex of the individual:
1-) In male- XY
2-) In female - XX
Groups of chromosomes:
Group A-) it is chromosomes number is 1 to.
3 and it is large and Metacentric.
Group B-) it is chromosome number is 4 to 5
. and it is Large and.
submetacentric
Group C-). It is chromosomes number is 6 to
. 12 and X and it is medium in size ,
. it is submetacentric and X.
chromosomes.
Group D-) It is chromosomes number is 13
. to 15 and it medium in size,
. acrocentric with satellite.
Group E-) 16 to 18 chromosomes number of
. group E and it is short Subcentrics.
Group F-). 19 and 20 chromosomes number
. of group F and it is short.
Metacentric .
Group G-). 21,22 and Y chromosomes
. number of group G and it is very
. short,Acrocentric and Y
. chromosomes.
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