Notes of Heredity and Evolution- Part I

CHAPTER – 9

HEREDITY AND EVOLUTION 

˜ Genetics : Branch of science that deals with Heredity and variation.

˜ Heredity : It means the transmission of features/ characters/ traits from

one generation to the next generation.

˜ Variation : The differences among the individuals of a species/

population are called variations.

Mendel and His Work on Inheritance

˜ Gregor Johann Mendel (1822-1884) : Started his experiments on

plant breeding and hybridization and proposed the laws of inheritance

in living organisms.

Mendel ® was known as Father of Genetics

˜ Plant selected by Mendel : Pisum sativum (garden pea). 

Mendel used a seven pairs of contrasting characters for garden pea.

(TABLE OF CONTRASTING CHARACTERS. SEVEN PARTS)
CHARACTER	DOMINANT  TRAIT	RECESSIVE  TRAIT
Flower colour Flower position Seed colour Seed shape Pod shape Pod colour Height of plant	Violet Axial Yellow Round Inflated Green Tall	White Terminal Green Wrinkled Constricted Yellow Dwarf/Short

  

˜ Mendels Experiments : Mendel conducted a series of experiments in

which he crossed the pollinated plants to study one character (at a time)

Monohybrid Cross

Cross between two pea plants with one pair of contrasting characters

is called a monohybrid cross.

Example : Cross between a tall and a dwarf plant (short).

                                  MONOHYBRID CROSS

PARENT                  à                          Tall plant        ´        Dwarf plant   

ALLELIC PAIR     à                               TT              ´                tt

OF GENES

GAMETES              à                        T             T                      t              t

F1 GENERATION à

(first filial generation)                                                  Tt        (All tall plants)

SELF POLLINATION  à                          Tt           ´          Tt

of F1 gametes

GAMETES            à                              T          t                T       t

F2 GENERATION     à                     (Punnet Square)

Gametesà	T	t
T	TT Tall	Tt Tall
t	Tt Tall	tt Dwarf

Phenotypic ratio    à                  3 : 1      

Genotypic ratio     à                1: 2 : 1

TT                        Both dominant alleles           Pure or Homozygous condition

tt                         Both recessive alleles

Tt                         One dominant,           

                           One  recessive trait                 Heterozygous condition-Hybrid

Phenotypic ratio : 3:1

Genotypic ratio : 1:2:1

Phenotype ® Physical appearance [Tall or Short]

Genotype ® Genetic make up [TT, Tt or tt]

Observations of Monohybrid Cross

1. All F1 progeny were tall (no medium height plant (half way characteristic)

2. F2 progeny ¼ were short, ¾ were tall

3. Phenotypic ratio F2 à 3:1 (3 tall : 1 short)

Genotypic ratio F2 à   1:2:1 (TT : Tt : tt )

                                                      1 : 2 : 1

Conclusions

1. TT and Tt both are tall plants while tt is a short plant.

2. 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.

3. Characters/Traits like 'T' are called dominant trait (because it express

itself) and ‘t’ are recessive trait (because it remains suppressed)

Law of Dominance- Each trait is controlled by two forms of a gene i.e alleles. 
One of the allele is dominant and expresses itself whenever present irrespective
 of other one. Recessive allele is one which will express itself only in absence of
 dominant allele.
Law of segregation: Two alleles controlling a trait in a cell/ individual will
 get separated during gamete formation so that one gamete has only one of the allele 
in it.

DIHYBRID CROSS : A cross made between two plants having two pairs of

contrasting characters is called dihybrid cross.

PARENT                        ROUND                 x                   WRINKLED

GENERATION        GREEN SEEDS                           YELLOW SEEDS

                                     

                                         RRyy                     x                      rrYY

GAMETES                        Ry                                                 rY

   

F1                                                                                                                                    RrYy             

                                                                                                                                   Round Yellow

                                                     F1                                ´                        F1

 Selfing F1 ®                                                                               

                                                    RrYy                                  RrYy          

                                                                                                    

 Gametes-à             Ry    RY     rY       ry   ( Both male and female)  

F2 :

Gamete	Ry	RY	rY	ry
Ry	RRyy	RRYy	RrYy	Rryy
RY	RRYy	RRYY	RrYY	RrYy
rY	RrYy	RrYY	rrYY	rrYy
ry	Rryy	RrYY	rrYy	rryy

Phenotypic Ratio in F2

Round, yellow : 9

Round, green : 3

Wrinkled, yellow : 3

Wrinkled, green : 1

Observations

1. When RRyy was crossed with rrYY, in F1 generation all were Rr Yy

round and yellow seeds.

2. Self pollination of F1 plants gave parental phenotype and two mixtures

(recombinants round yellow & wrinkled green) seeds plants in the ratio

of 9:3:3:1

(9 )  : ( 3 ) :  ( 3 ) :   ( 1 )

Round       Round     Wrinkled    Wrinkled

Yellow       Green        Yellow       Green

Conclusions

1. Round and yellow seeds are DOMINANT characters

2. Occurrence of new phenotypic combinations show that genes for round

and yellow seeds are inherited independently of each other.

Law of Independent assortment: Two pairs of alleles controlling two traits
get separated during gamete formation independent of each other.

MECHANISM OF HEREDITY

Characters or traits of an organism are controlled by the genes

             A section of DNA i.e. Gene provides information for synthesis of Proteins

             These proteins control a character.

Example :

Gene T  à responsible for       à          More          à     Results in 

                synthesis of efficient        production of           tall plants

                enzyme (Protein)              growth hormone

Gene t à responsible for       à          Less          à     Results in 

                synthesis of less             production of           short plants

                efficient enzyme            growth hormone

SEX DETERMINATION

Phenomenon of decision or determination of sex of an offspring

                                  

  Factors Responsible for Sex Determination

Environmental	Genetic
In some animals the temperature at which the fertilised eggs are kept decides the gender. eg. in Turtle, crocodiles	In some animals like humans gender or individual is determined by a pair of chromosome called sex chromosome XX – Female XY – Male

Sex Chromosomes : Cell of Human beings have 23 pairs of chromosome.

 22 chromosomes pairs are autosomes + 1 pair of sex chromosome (decide the gender of the child).

                                                                 i.e.   XX – female

                                                                         XY – male

Sex determination in Human beings

PARENTS :                        FATHER                                                              MOTHER

                                                 XY                                                                          XX

GAMETES

(Reproductive cells)          X        Y                                                                 X      X

Zygote

formed

after fusion

of gametes                           XX                           XX                           XY                XY     

                                           FEMALE               FEMALE                 MALE             MALE

                                                   

 offspring                              50% probability                                  50% probability

                                               of a female child                                 of a male child

Half the children will be boys and half will be girls. 

All children will inherit an X chromosome from their mother regardless whether they are 

boys or girls. Thus sex of children will be determined by what they inherit from their father.

EVOLUTION

Evolution is the sequence of gradual changes which takes place in the primitive 

organisms, over millions of years, and as a result new species are produced.

Evolutionary Forces: Natural selection, Genetic drift

Situation-I   Natural selection

                                  Group of red beetles

                     

                    Colour variation arises during reproduction

All beetles red except                              One beetle Green

one that is green                                                           Reproduction

                                                                Progeny bettles green

   Progeny beetles red                                                                  

Crows feed on red beetle                       Crow could not feed on

                                                             green beetles as they got

                                                            camouflaged in green bushes

    

No. of beetles reduces                           Number of green

                                                                beetles increases

Situation 1: Green beetles got the survival advantage or they were

naturally selected as they were not visible in green bushes. This natural selection

is exerted by crows resulting in adaptations in the beetles to fit better in their

environment.

Situation-II    Genetic Drift

                                            

                                             Group of red beetles

                          Colour variation arises during reproduction

All beetles are red except one                               One blue beetle  

       that is blue                                                 

                                               Reproduction

Number of red beetle increases                           No. of blue beetle increases

  

Ø  Crows can see both blue and red beetles and can eat them

Ø  Number reduces but still red beetles are more and blue ones are few

Ø  Suddenly elephant comes and stamps on the bushes

Ø  But now beetles left are mostly blue.

Situation 2 : Blue beetles did not get survival advantage. Elephant suddenly 

caused major havoc in beetle population otherwise their number would have 

been considerably large.

Conclusion: Accidents can change the frequency of some genes even if they

 do not get survival advantage: This is called genetic drift and it leads to

 variation or diversity without any adaptation.

SITUATION-III (Acquired Traits)

Ø  Group of red beetles

Ø  Habitat of beetles (bushes) suffer from plant disease

Ø  Average weight of beetles decreases due to poor nourishment

Ø  Number of beetles kept on reducing

Ø  Later plant disease gets eliminated

Ø  Number and average weight of the beetles increases again

Situation 3 : No genetic change has occurred in the population of beetle.

The population gets affected for a short duration only due to environmental

changes.

ACQUIRED AND INHERITED TRAITS

Acquired Traits	Inherited Traits
1. These are the traits which  develop in an individual due to special conditions	1. These are the traits which  develop in an individual due to changes in DNA (genetic changes)
2. They cannot be transferred to the progeny	2. They get transferred to the progeny
3. They cannot direct evolution	3. They are helpful in evolution.
eg. Low weight of starving beetles	eg. Colour of eyes and hair

Effects of Evolutionary forces i.e. natural selection and genetic drift:

1.Micro evolution : It is the evolution which is on a small scale. eg.

change in body colour of beetles.

2.Speciation : It is the process of formation of new species.

   Species : A group of similar individuals within a population that can

   interbreed and produce fertile offspring.

Ways by which speciation takes place

Speciation takes place when variation is combined with geographical

isolation.

Gene flow : It is exchange of genetic material by interbreeding between

populations of same species or individuals. It occurs between population that 

are partly but not completely separated.

POPULATION Z  spread in a big area.

Sub Population     GEOGRAPHICAL         Sub Population

      Z1                        BARRIER                             Z2

                                 ISOLATION

                            (River, Mountain)

                   

                 Over many-many generations

          

     Results in Accumulation of different variations in

              Sub population Z1 and Z2 

                                           i) Natural selection*   
                                           ii) Genetic drift*

               Sub population Z1 and Z2  incapable

                         of interbreeding

                         Reproductive

                             Barrier

Formation of                            Formation of

new Species 1                          new Species 2

Natural Selection : The process by which nature selects and consolidate those

 organisms which are more suitably adapted and possesses favorable variations.

Genetic Drift: It is the random change in the frequency of alleles (gene pair in a

population over successive generations.

      Genetic drift takes place due to

         (a) Severe changes in the DNA

         (b) Change in number of chromosomes