Explanation Of Genetics

Genetics :

Genetics is described as the study of transmission of body features from parents to offsprings and the laws relating to such transmission. It is the science which deals with the mechanism of heredity and variations.

Heredity :

Heredity is the study of transmission of genetically based characters from parents to offsprings.

Variations :

These are small differences among individuals. All human beings are similar but they show so many types of races due to the variations in their characters. These are observed in animals and plants.

the chances of survival in a changing environment is increase because of variation in a species.

Genetic engineering :

Genetic constitution of an organism is introduced by new genes into its chromosomes. is Called Genetic engineering.

We known Gregor Johann Mendel as "Father of Genetics". He conducted breeding experiments on

garden pea (Pisum sativum). The reasons for

selecting garden pea for his experiments were that :-

1. Many varieties were available having contrasting characters.

2. Varieties were available in pure forms.

3.Peas are self pollinated since it is bisexual.

4. Self pollination can be prevented, If we remove corresponding reproductive parts of the flower.

5. Cross pollination could be done artificially.

6. Reproductive span of pea plant is very small.

The seven pairs of contrasting characters used by Mendel in his work :

Deoxyribonucleic Acid (DNA) is a substance within the chromosomes that carries the hereditary instructions for producing proteins while RNA is a substance similar to DNA that transmits DNA's instructions for making proteins from the nucleus to the cytoplasm of the cells. There are two sets of chromosomes in Diploid cells, one from each parent.

Homologous Chromosomes : These are a pair of corresponding chromosomes of the same shape and size, one from each parent. All genes are specific parts of chromosomes which determine hereditary characters.

Alleles or Allelomorphs :

These are alternative forms of gene occupying same position on homologous chromosomes and affecting the same characteristic but in different ways.

Two genes are present in a homologous pair for any one particular trait.

Normally one gene in every pair is dominant. It hides the effects of its recessive partner.

Recessive genes show their effect only if there are two of them. It is not expressed in the presence of a dominant allele.

Homozygous :

It means individuals having similar gene pairs, e.g. RR, rr.

Heterozygous :

It means individuals having dissimilar gene pairs, e.g. Rr.

Genotype :

Genotype is the genetic constitution of an organism with regards to one or more characters, e.g. The genetic composition of RR, Rr or rr may have by an organism .

Phenotype :

Phenotype refers to the observable characters of an organism which is genetically controlled. e.g. RR and Rr both are red (externally they look same). This mean that they are phenotypically same but they carry different genes and are therefore said to be differ in genotype.

Monohybrid crosses :

These are Mendelian crosses in which only one pair of contrasting character is taken into consideration at a time.

A sketch of monohybrid cross.

Dihybrid Crosses :

These are Mendelian crosses in which two pairs of contrasting characters are taken into consideration at a time.

Mendels laws of Inheritance :

1.Law of Dominance : In heterozygous condition out of two contrasting alleles one expresses itself morphologically and the other remains unexpressed. The allele which expresses itself phenotypically in the presence of its contrasting allele is called dominant and the other which remains unexpressed is called recessive.

2. Law of segregation : The two factors controlling one character segregate without influencing each other during the formation of gametes so that each gamete receives one factor for each character.

3. Law of Independent assortment: The factors controlling different characters assort independently without influencing each other during the formation of gametes.

Sex linked inheritance :

Colourblindness, haemophilia and nightblindness are some of the common X linked sex characters which are controlled by the recessive genes present on X chromosomes. These diseases are common in males than in females because both the X chromosomes may not carry the abnormal gene in the females. The gene which may be abnormal on one X chromosome being recessive, its influence will be hidden by the normal gene on the other X chromosome. The male with one X chromosome has only one gene for these diseases. If that is the abnormal gene then there is nothing on Y chromosome to mask it and hence these diseases can occur easily.

Hypertrichosis (long hair on the pinna):

It is present on the Y chromosome only. Since Y chromosomes are found in males only that is why such traits are restricted in males only and such genes are called holandric genes. Another example is baldness in males.

Mutation : It is a sudden change in one or more genes. It alters the hereditary material of an organism's cells thereby causing changes in certain traits (characters). A mutation can effect an individual gene or an entire chromosome.

The gene mutation is caused even by slight chemical changes in DNA. e.g. sickle cell anaemia (a blood disease).

A chromosomal mutation occurs if the number or arrangements of genes on chromosome changes, for example,

Downs syndrome also called mongolism is a disorder which occurs if a person is born with one extra chromosome.

(47 chromosome).

Colourblindness is checked by Ishihara cards.

Crosses between two different animals :

1. Horse + Zebra = HEBRA

2. Tiger + Lioness = TIGON

3. Lion + tigress = LIGER

4. Bison + Buffalo = BIFFALO

5. Zebra + Horse = ZORSE

6. Zebra + Donkey = ZENKEY

Definitions :

1. Alleles (= Allelomorphs) : Genes or factors controlling contrasting traits, occupying same locus on the homologous chromosomes.

2. Allosomes (= Idiosomes Sex chromosomes) :

Chromosomes controlling sex of an individual, i.e.. X and Y-chromosomes.

3. Autosomes: All the chromosomes other than sex chromosomes.

4. Dihybrid cross : Breeding between two individuals having two pairs of pure-breeding contrasting traits, producing progeny hybrid for both the characters.

5. Dominant gene : The gene which expresses itself phenotypically in the presence of its contrasting allele.

6. Gene: A small segment of chromosome that helps in the transmission of one character from parent to offspring

7. Genetics : Branch of biology concerned with the

study of mechanism of heredity and variations.

8. Genotype : Genetic constitution for any trait of an individual.

9. Hemizygous : When any trait is controlled by a single gene in diploid condition. e.g., genes present on Y - chromosome in human males.

10. Heterogametic : When the gametes produced are of different types. e.g. sperms with X or Y chromosomes.

11. Heterozygous : A condition in which the two genes or factors controlling one trait are dissimilar. e.g. Tt.

12. Homologous chromosomes: A pair of chromosomes

which are exactly similar and bear same genes at same loci.