Heredity and Evolution notes

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 Fgametes

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 F®                                                                               
                                                    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 Fplants gave parental phenotype and two mixtures
(recombinants round yellow & wrinkled green) seeds plants in the ratio
of 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



Evolution

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 Z
                                           i) Natural selection*   
                                           ii) Genetic drift*

               Sub population Z1 and Z 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




Origin of Life:
Evolution can explain formation of many species ultimately from a single
 species in the past. But origin of life (first species) needs further explanation.
Oparin and Haldane proposed biochemical theory of evolution:
Chemical evolution: Formation of simple organic substances (amino acids,
 glucose, nucleotides) from inorganic substances CH4, NH3, H2S on 
primitive Earth.
Biological/ Organic Evolution: Formation of first primitive cell using organic
 molecules. Evolution of different forms of life forms from simple unicellular
 prokaryotic organism

Miller and Urey experiment: They simulated primitive earth.
Primitive reducing atmosphere             Gases like H2S, NH3, CH4
Evaporation from hot oceans                Boiling water in flask
Lightening                                             Electrodes
Rainfall                                                 Condensation using cold water
They showed formation of simple substances in water taken in experimental
 set up and confirmed Harold and Urey’s theory.

Evolution and classification
Both evolution and classification are interlinked.
1. Classification of species is reflection of their evolutionary relationship.
2. The more characteristic two species have in common the more closely
     they are related (and will be placed in same group during classification).
3. The more closely they are related, the more recently they have a common
    ancestor.
4. Similarities among organisms allow us to group them together and to
    study their characteristic.


TRACING EVOLUTIONARY RELATIONSHIPS
(Evidences of Evolution)
I.Morphological and anatomical evidences. External and internal structures 
are compared to find out evolutionary relationships.
A.Homologous Organs : These are the organs that have same basic structural
plan and origin but different functions.
Homologous organs provides evidence for evolution by telling us that 
they are derived from the same ancestor. (Divergent Evolution)
Examples :
Organs                                          Function
Forelimb of Horse                          (Running)                      
Wings of bat                                    (flying)                            
Paw of a cat                            (walk/scratch/attack)       

Example from plants
Tendrils in cucurbits                 (climbing/support)
Thorns in bougainvillea                  (protection)
(both are stem modifications)

Tendrils in pea plant                       (climbing support)     
Spines of cactus                            (protection)
(both are leaf modifications)

B. Analogous Organs : These are the organs that have different origin
and structural plan but perform same function.
Analogous organs indicate towards Convergent Evolution i.e. structures derived
 from different ancestors adapt to perform same function.
Example :  Organs                                                          Function 
Wings of bat ® elongated fingers                      
                           with skin folds                                        Flying
Wings of bird ® Feathery covering               
                             along the arm                      
                                                                        
Example from plants 
Spines in Cactus (modified leaf) 
Thorns in Bouganvilea (modified stem)                          Protection
                                   
II. Fossils : (Palaeontological evidences)
Fossils are preserved body parts or traces of living organisms of the past.
ü  To study and compare structural features of extinct organisms in order to
 find evolutionary relationship, we study fossils.
ü  Fossil Archeaopteryx possess features of reptiles as well as birds. This
             suggests that birds have evolved from reptiles.
Examples of Fossils
AMMONITE - Fossil-invertebrate
TRILOBITE - Fossil-invertebrate
KNIGHTIA - Fossil-fish
RAJASAURUS - Fossil dinosaur skull

Determining age of fossils:
I. Deeper the fossil, older it is.
              1. ...........................
              2. ...........................
 Recent 3. ...........................

             4. ...........................
             5. ..........................
             6. .......................... Older


II. Detecting the ratios of different isotopes of the same element in the 
Fossil material i.e. Radio-carbon dating (C-14 dating)

III. Molecular Phylogeny (Molecular Evidence)
˜ It is based on the idea that changes in DNA during reproduction are the
    basic events in evolution
˜ Organisms which are more distantly related will accumulate greater
    differences in their DNA than closely related organisms.
So we look for differences in DNA to find out evolutionary relationships.

Evolution of New organs/ structures 
Evolution takes place in stages i.e. bit by bit over generations.
 A. Usefulness/Fitness advantage in Intermediate stages.
Evolution of Eyes
Evolution of complex organs is not sudden it occurs due to minor 
changes in DNA, however takes place bit by bit over generations.
Flat worm has rudimentary eyes        
Insects have compound eyes
Humans have binocular eyes
But these eyes have differnces in structure and hence have different
 ancestoral origin.

B.Preadaptation/ Functional Advantage of Intermediate stages.
Evolutions of feathers
Feathers ® provide insulation in cold weather but later they might
become useful for flight.
Example : Dinosaurs had feathers, but could not fly using feathers.
Birds seem to have later adapted the feathers to flight.

Evolution by Artificial Selection : Humans have been a powerful
agent in modifying wild species to suit their own requirement through
out ages by using artificial selection. eg (i) From wild cabbage many
varieties like broccoli, cauliflower, red cabbage, kale, cabbage and
kohlrabi were obtained by artificial selection. (ii) Wheat (many varieties
obtained due to artificial selection).

Wild cabbage:
    Broccoli: Arrested flower development
    Cauliflower: Has sterile flowers
    Cabbage:With short distance between the leaves
    Kale: With larger leaves
    Kohlrabi: With swollen parts

Evolution: Ladder vs Tree
Evolution can not be equated with progress (ladder) but aims at producing
 more diversity (Tree).

 HUMAN EVOLUTION
Although there is great diversity of human forms all over the world yet
all humans are a single species.

GENETIC FOOTPRINTS OF HUMANS
Hundreds/thousand of years ago Earliest members arose in Africa.
They migrated to many places namely East Asia, South Africa, Philippines, 
Island of Indonesia, West Asia, Central Asia, Australia, Eurasia.
˜ They did not go in a single line
˜ They went forward and backward
˜ Moved in and out of Africa
˜ Sometimes came back to mix with


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