GENETICS, Std-10

 

   Genetics
:- The term genetics was coined by Bateson in 1906.
 -> The basic principles were discovered by Gregor Johannes Mendel ( 1822 - 1884), therefore Mendel considered as the Father of genetics.
:-> The branch of science which deals with the mechanisms responsible for similarities and dissimilarities among closely related species is called genetics.

                                   Heredity
:-> The transmission of characters ( traits ) from the parents to their offsprings is called Heredity.
:-> It is also known as inheritance.

                                  Variations
:-> The differences in the characters among the individuals of a species is called Variation.
:-> e.g- Human height, Earlobes ( free or attached ), eye, etc.
:-> It increases the chances of survival of a species in a changing environment.

* Some Important terms in Genetics

a. Gene:- A small segment of DNA is called gene, also known as a hereditary unit.
b. Gametes:- Male and female reproductive units( sex cells ).
c. Inherited traits:- The characters or traits which are transmitted from parents to their offsprings or progenies are called inherited traits.
d. Alleles:- The forms of a gene or a pair of matching genes are called alleles.
e. Homozygous:- When both alleles of a gene are identical, called homozygous.
f. Heterozygous:-  When the two alleles of a gene are dissimilar  I.e one is dominant and the other is recessive for a particular trait is called heterozygous.
g. Homogametic:-  An individual with morphologically similar sex chromosomes known as homogametic.
h. Heterogametic:- An individual with morphologically dissimilar sex chromosomes known as heterogametic.
i. Dominant gene:- The gene which expresses itself in a heterozygous allele is called the dominant gene.
j. Recessive gene:- The gene which does not express itself in the presence of the dominant gene is called a recessive gene.
k. Genotype:- The genetic constitution of an organism is called the genotype of that organism.
 :- This characteristic is not visible in any organism.
l. Phenotype:- The characteristics which are visible in an organism is called its phenotype.
:-> e.g - Height , shape, and sizes etc.
m. Hybridization:- The process of mating organisms of different varieties or species to produce a hybrid is called hybridization.
n. Emasculation:- The process of removal of the stamen ( male part of the flower) from flowers is called emasculation.
:-> it is used to prevent/avoid self-pollination.
:-> This process is used to produce hybrid plants.

       Mendel's Experiment
 :-> It is also known as Mendel's law of variations/Inheritance.
:-> Duration of the experiment- 7 years ( 1856 - 1863 )
:-> Experiment plant- Pea plant I.e Pisum sativum.
:-> Mendel selected this plant for his experiment because  -
 a. It was easy to cultivate.                                   b. Self-pollination occurs, having bisexual flowers.
 c. Easily observable.
 d. And It has contrasting pairs of characters.
 e. It has a shorter life span of  3 - 4 months.
:-> He considered mainly seven contrasting pair of characters like-
a. Seed shape- Oval, Wrinkled
b. Seed color- Yellow, Green
c. Flower color- Purple, White
d. Pod shape- Inflated, Constricted
e. Pod color- Green, Yellow
f. Flower position- Axial, Terminal
g. Stem length- Tall, Dwarf

:-> During this experiment Mendel showed the process of transmission of characters from parent plants to their progeny.
:-> He designed his cross-breeding experiment taking various characteristics meticulously/ carefully.

* Based on considering traits during experiment he showed mainly two types of crosses.

a. MONOHYBRID CROSS
:- A cross between two parents representing contrasting forms of a single feature or trait is called monohybrid cross.
:- e.g- A cross between Tall(TT) and Dwarf(tt)/short plant ( trait is height ).

# - Explanation:-
                            The pollen grains from the flower of Tall plant were sprayed over the stigma of short plant's flower.
 -> After fertilization the seeds obtained of the F1 generation were carefully observed and the observation was recorded.
-> In this experiment Mendel observed that all the plants of the F1 generation were tall.
-> Mendel further planted the F1 seeds and allowed them to self pollinate to produce the F2 generation plants.
-> In the F2 generation, he found that a proportion of short plant.
   Ratio -> 3:1 ( Three were tall and one was short )

* Steps involved in Monohybrid cross ( Diagrammatically representation )

1. Cross between parents(Tall and short plants).
 ->.                      Tall                               short

Genotype-          T T               ×                    t t   

-> Gametes -     T      T                            t           t

                          Tt           Tt                Tt             Tt

:-> F1 generation- All offsprings were tall.

2. Cross b/w f1 progeny. 

                                 Tt                              Tt 
                                                   ×                           
 Gametes -          T       t                         T      t

  Progeny -            TT       Tt               Tt       tt

Generation - F2

Phenotypic ratio - 3:1 ( 3-tall TT,Tt,Tt short-tt,1 )

* Dihybrid Cross
:-   A cross in which alternating forms of two traits or features are simultaneously considered in the hybridization experiment is called the dihybrid cross.   
:-> e.g- Cross between two plants considering two alternating forms of traits(color and shape of seeds).
:-> Traits -
1. Yellow and Round seed ( genotype- RRYY )
2. Green and wrinkled seed (genotype-rryy )
:-> Mendel crossed these plants and observed that all seeds had the features of only one parental type in the F1 generation.
:-> He further crossed F1 progeny and found more variations.

 1. Parental Cross. 👇👇
  


2. Formation of F1 Hybrids
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3. Fusion of Gametes b/w F1 Hybrids
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* MENDEL'S LAW OF INHERITANCE   
:-> On the basis of experiment Mendel proposed mainly three laws and they are:-
a. Law of Dominance
b. Law of Segregation
c. Law of Independent Assortment

*Law of Dominance
:-> It states that:- When two homozygous individuals with one or more sets of contrasting traits have crossed the trait which appears in the F1 hybrids are Dominant and those which do not appear in the F1 generation are recessive.
:-> Explanation refers to the monohybrid cross. 

*Law of Segregation
:- It States that:- When a pair of alleles are brought together in a hybrid the members of the allelic pair remain together without mixing and separate or segregate from each other when the hybrids form gametes.
:-> Since, each gamete is pure for a characteristic.
:-> It is also known as the law of Purity.
:-> Explanation refers to the dihybrid cross.

*Law of Independent Assortment
:-> It states that- When a dihybrid organism forms gamete, each gamete receives one allele from each allelic pair ( or each characteristic) and the assortment of alleles of different characteristics during gamete formation is independent of their parental combination.
:-> Its explanation refers to the dihybrid cross.

* Sex Determination
:-> The process by which the sex of an organism is determined at the parental stage is called sex determination.
*Sex chromosomes:- The chromosomes which determine the sex of an organism are called sex chromosomes.
:-> e.g- Two types of sex chromosomes are found in humans I.e X and Y. Human Male- XY & Human female - XX

* Autosomes:- The 22 pairs of chromosomes are exactly similar and are called autosomes.

Note:- a. 23 pairs of chromosomes are found in humans.
        b. The sex of a child is depend on what happens at fertilization.
    
* Diagrammatically representation. 
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