Genetics is deals with the study of Heredity and Variation.
Heredity: It refers to the transmission of characters from parents to offspring i.e. one generation to the next generation.
Variation: It refers to the differences in the characters or traits between the parent and offspring. This variation genetically determined feature that distinguishes a person from the others.
Example: Attached or free ear, lobes in human beings.
Variations are of two types:
Variations during Reproduction are of following types:
Significance or Importance of variation:
- According to the nature of variations different individuals may have different kinds of advantages/ benefits.
- Main advantages to variation in species are that it increases the chances of its survival in a changing environment.
|Gene||:||Gene is the functional unit of heredity that control one or several particular characteristic features in living organism.|
|Genotype||:||The complete set of genes in an organism’s genome is known as genotype.|
|Phenotype||:||The observed characters (physical appearance) in an organism is called Phenotype|
|Clones||:||The exact copies of an organism are called Clones.|
The rules for inheritance of traits in human beings refers to the fact that mother and father contribute an equal number of genetic material i.e. DNA to their offspring. Therefore offspring will get two versions of that trait from the two parents.
To find the inheritance of these traits, Gregor Johan Mendel (1800-1884) “Father of genetics” performed his experiments with garden peas ( Pisum sativum) in the garden behind his monastery. He observed a number of contrasting characters in Pisum sarivum and observed their inheritance.
He choose garden pea (Pisum sativum) as his experiment materials because,
- This plant has short life span.
- It produces large number of seed.
- This plant has several detectable contrasting characteristics.
- Easy to Self-fertilization but cross fertilization can also be carried out.
Seven pairs of contrasting characters in Garden Pea:
|Character Trait||Dominant Trait||Recessive Trait|
|1. Flower colour||:||Violet||White|
|2. Flower position||:||Axial||Terminal|
|3. Seed colour||:||Yellow||Green|
|4. Seed shape||:||Round||Wrinkled|
|5. Pod shape||:||Inflated||Constricted|
|6. Pod colour||:||Green||Yellow|
|7. Height of plant||:||Tall||Dwarf/ Shot|
(A) Monohybrid cross:
Cross between two pea plants with one pair of contrasting characters is called monohybrid cross.
- First generation or F1 progeny are all tall plants.
- Second-generation or F2 are progeny (descendant) of the F1 tall plants are not all tall.
- Both the tallness and shortness traits were inherited in the F1 plants, but only the traits are expressed.
- Thus, two copies of the traits are inherited in each sexually reproducing organism.
- These two may be identical or may be different depending on the parentage.
i. Pure or homozygous condition:
TT: Both are dominat traits.
tt: Both are recessive alleles.
ii. Heteroxygous condition (hybrid):
Tt: one is dominat, one is recessive traits.
- Phenotypic ratio:- 3 : 1 (three tall and one short)
- Genotypic ratio:- 1 : 2 : 1 (TT- one, Tt-two, tt-one)
- All F1 progeny were tall, no medium height plant.
- F2 progeny ¼ were short, ¾ were tall.
- TT and Tt both are tall plant, tt is a small plant.
- A single copy of T is enough to make the plant tall, while both copies have to be t for the plant to be short.
- Characters or trait like T are called dominant trait and t are recessive traits.
- From the above observations, Mendel put forward the rules of inheritance.
LAW OF SEGREATION: each individual possesses a pair of alleles for a particular trait. In gamete formation, gametes receive only a single trait from the alleles. A particular trait can be dominant or recessive in a particular generation.
LOW OF DOMINANCE: If parents of pure contrasting characters are crossed then only one character expresses itself in the F1 generation. This always is a dominant characters and the character which is not express is called the recessive character.
(B) Dihybrid cross:
Mendel observed the inheritance of two pairs of contrasting characters, which is known as dihybrid cross.
- He cross breed pea plants bearing round green seed (RRyy) with plant bearing wrinkles and yellow seeds (rrYY).
- In the F1 generation he obtained all round and yellow seed, which refers that round and yellow traits of seeds are dominant traits and wrinkled and green are recessive.
- He self pollinated the F1 generation to obtain the F2 generation.
- RRyy was crossed with rrYY in f1 generation were RrYy i.e. Round and yellow
- After self pollination of F1 generation in f2 generation the phenotypic appearance is 9 : 3 : 3 : 1.
- Phenotype Ratio:
|Round, yellow: 9|
|Round, green: 3|
|Wrinkled, yellow: 3|
|Wrinkled, green: 1|
- Round and yellow seeds are showing Dominant characters.
- Occurrence of new phenotype combinations shows that genes for round and yellow seed are inherited independently of each other.
Sex determination in human beings
- Human beings have total 23 pair of chromosome out of which 22 pair of autosomes and one pair of sex chromosome (X and Y).
- The X and Y chromosome is the sex determining chromosome.
- Male have a single X and a single Y; so it is written as XY. The Y chromosome is determining the sex of a male. Similarly in female have a pair of X- chromosome i.e. XX.
- All gametes or ova formed by homozygous female gametes i.e have X-chromosome. Males have heterogenetic form i.e. half of X-chromosome and another half of Y-chromosome.
Autosomes: These chromosomes don’t have any role in sex determination.
Sex chromosome: these are plays an important role in sex determination of new born baby.
- If a sperm having X chromosome fertilises with ovum with X-chromosome then the new born baby have XX- chromosome and it will be female.
- If the sperm having Y-chromosome fertilizes with ovum with X-chromosome then the baby will have XY-chromosome and it will be female.
- This show 50% of children will be boys and 50% will be girls. Single X-chromosome from mother is always inherited to boys or girls.
- Thus the sex of children will be determined by what they inherit from their father not from mother.
Q.1 How the mendelian traits get expressed?
- DNA present in cell provides the information source to make protein in the cell.
- A site of DNA that provides information for one protein is called the gene for this protein.
- Plant height can also depend on the amount ‘of a particular plant hormone. The amount of plant hormone made will depend upon the efficiency of the process of making it.
Q.2 How the offspring get single set of genes from parents and who have two copies in them?
- DNA is not containing a single set of gene, but as it separate independent pieces each called a chromosome.
- Each cell has two copies of chromosome, one from mother and one from father.
- Each gem cell or gametes (mother or father) has one copy of chromosome, because there is reductional division in the sex organs at the time of formation of gametes.
- When fertilization takes place normal number of chromosome is restored in the progeny ensuring the stability of DNA of the offspring.
Q. 3 How is the sex determination is different in different individual?
- Different species use different strategies for sex determination.
- In some organisms the temperature at which fertilized egg are kept determines whether the developing organism will be male or female.
- Some animals like snails can change sex, which is indicating that sex is not genetically determined.
- In human being gene inherited from parents to determine the sex of offspring.