CH-8 HEREDITY AND EVOLUTION

Genetics: Branch of science that deals with Heredity and Evolution

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.

MENDELAND HIS WORK ON INHERITANCE

Gregor Johann Mendel (1822&1884): Started his experiments on plant breeding and hybridization

            Mendel was known as ‘FATHER OF GENETICS’

Plant selected by Mendel: Pisum sativum (garden pea). Mendel used several contrasting characters for garden pea.

Join Offline Classes of OSF Education.

Read More Notes here.

Mendel’s 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 (monohybrid cross) contrasting characters

Example: Tall/Short Plants.

Phenotype :  Physical appearance [Tall or Short]

Genotype :    Genetic makeup [TT , Tt or tt]

Observations: 

1. All F, progeny were tall (no medium height plant (halfway characteristic) 
2. F₂ progeny 4 were short
3. Phenotypic ratio F-3:1 

Genotypic ratio F,-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 must be ‘T’ for the plant to be short.
3. Characters/Traits like “T are called dominant trait (because it expresses itself) ‘t’ are recessive trait (because it remains suppressed)

Dihybrid Cross: A cross macle between two plants having two pairs of contrasting characters is called dihybrid cross.

PHENOTYPIC RATIO: Round, yellow 9

  Round, green 3

Wrinkled, yellow 3

Wrinkled, green 1

GENOTYPIC RATIO:

RRYY : 1

RRYy : 2

RrYY : 2

        RRyy : 1

RrYy : 4

Rryy : 2

       rrYY :1

rrYy : 2

  rryy : 1

RATIO: 1 : 2 : 2 : 1 : 4 : 2 : 1 : 2 : 1

Observations: 1. When RRYY was crossed with rryy in F₁ generation all were round and yellow seeds. 

 2. Self-pollination of F, plants gave parental phenotype + two mixtures (recombinants) Round wrinkled, green yellow: seeds plants appeared in the ratio of 9:3:3:1

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.

Sex Determination: Phenomenon of decision or determination of sex of an offspring

Sex Chromosomes: In human beings there are 23 pairs of chromosomes. Out of these 22 chromosomes pairs are called autosomes and the last pair of chromosomes that help in deciding gender of that individual are called sex chromosome.

XX – Female XY – Male

This shows that 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, and not from their mother. 

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 2: Blue beetles did not get survivals advantage. Elephant suddenly caused major havoc in beetle population otherwise their number would have been considerably large.

From this we can conclude that 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.

Genetic drift. It leads to diversity without any adaptation

  SITUATION –III

 Group and red beetles

                                           Habitat of beetles (bushes) Suffer from plant disease

                                     Average weight of beetles decrease due to poor nourishment

                                                       No of beetles kept on reducing

                                                     Later plant disease gets eliminated                      

                                     Number and average weight of the beetles increase 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

Speciation:

Micro evolution: It is the evolution which is on a small scale. e.g. change in body color of beetles.

Speciation: it is the process of formation of new species.

Species: A group of similar individuals that along to a population that can interbreed and produce ferrite 

offspring.

Geneflow: It is exchange of genetic material by interbreeding between populations of same species or individuals

Heredity and Evolution and classification

 Both evolution and classification are interlinked.

1. Classification of species is reflection of their evolutionary relationship.
2. Two more characteristic two species have in common the more closely they are related.
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)

1. Homologous Organs:  (Morphological and anatomical evidences. These are the organs that have same basic structural plan and origin but different functions.

2. Analogous Organs: These are the organs that have different origin and structural plan but same function 

3. Fossils:  (Paleontological evidences) :The remains and relics of dead organisms of the past. 

Example:

i) Fossil of wooly mammoth

ii) Archaeopteryx (fossilbird)

iii) Dead insect caught in hot mud.

FOSSILS ARE PRESERVED TRACES OF LIVING ORGANISMS

e.g. AMMONITE   –   Fossil invertebrate

       TRILOBITE      –   Fossil in vertebrate

       KNIGHTIA      –   Fossil fish

       RAJASAURUS –   Fossil dinosaurskull

Evolution by stages

Evolution takes place in stages i.e. bit by bit over generations.

2. Functional Advantage

Evolutions of 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.

Artificial Selection:

Humans have been a powerful agent in modifying wild species to suit their own requirement throughout ages by using artificial selection, e.g. (1) Wild cabbage the dissimilar looking structures have evolved from a common ancestral design. (ii) Wheat (many varieties obtained due to artificial selection

Molecular Phylogeny:

• 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

• They didn’t go in a single line
• They went forward and backward 
• Moved in and out of Africa
• Sometimes came back to mix with each other.