DAV Public School

Sec.6, Dwarka, New Delhi-75

HEREDITY AND EVOLUTION

Biology                                                                          Class-10

Heredity is the transmission of characters or traits from parents to their offspring.

Genetics is the science of heredity, i.e., the study of inheritance of characteristics in offspring from their parents.

Variations – The difference in the character among the individuals of a species is called variation.

1.       Variations necessary for organic evolutions.

2.       It increases the chances of its survival in changing environment.

The free earlobes and attached earlobes are the two variation found in human population.

Accumulation of variations- The significance of variations shows up only if its continues to be inherited by the offspring for several generations.

Some important terms:-

1.       Genotype-the genetic make of an organisms

2.       Phenotype- the physical appearance of an organisms

3.       Genes- unit of heredity which transfer traits from parents to their offspring

4.       Diploid- two sets of chromosomes

5.       Haploid- one set of chromosomes

6.       Dominant genes- the gene which decides the appearance of organisms even in presence of alternative genes is known as dominant. Dominant gene represented by a capital letter.

7.       Recessive genes- the gene which decides the appearance of organisms only in presence of another identical gene is called recessive genes. Recessive genes represented by a small letter.

8.       F₁ generation- when two parents cross to produces to progeny then their progeny is called first filial generation.

9.       F₂ generation- when first filial progeny cross among themselves to produce second generation progeny, then this progeny is called second filial generation.

10.   Hybrid- a new form of plants resulting from a cross of different varieties of a plant is known as hybrid

11.   Monohybrid cross- crosses between one pair contrasting characteristics.

12.   Dihybrid cross- crosses between two pair of contrasting characteristics.

Rules of inheritance of traits given by GREGOR JOHANN MENDEL

Gregor Johann Mendel was an Austrian monk who grew garden peas (PISUM SATIVUM) in the church garden. He identified the number of characteristics showing contrasting traits follows-

1.       Pea plants are easy to cultivate and have large flowers which help in cross pollination under controlled conditions.

2.       These plants are self pollinated in nature.

3.       These plants show discrete variations in characters or traits.

4.       There are no intermediate forms produced.

1.       Mendel first crossed pure-bred tall pea plants with pure-bred dwarf pea plants and found that only tall plants were produces in the first generation or F₁ generation. No dwarf pea plant (or short pea plants) was obtained in the first generation of progeny. From this Mendel concluded that the first generation showed the traits of only one of the parent plants: tallness. The trait of other parent plant, dwarfness, did not show up in the progeny of first generation.

2.       Mendel then crossed the tall pea plants of the first generation and found that tall plants and dwarf plants were obtained in the second generation in the ratio of 3:1. In other words in the F₂ generation, three-fourth plants were tall and one- fourth were dwarf. Mendel noted that the dwarf trait of the parent pea plant which had seemingly disappeared in the first generation progeny reappeared in the second generation. Mendel said that the trait of dwarfness of one of the parent pea plant had not been lost; it was merely conceals or suppressed in the first generation to remerge in the second generation. Mendel also noted that all the pea plants produces from the hybrid tall parents of F₂ generation, were either tall or dwarf. There were no plants with intermediate height (or medium height) in between the tall and dwarf plants. In this way, Mendel’s experiment showed that the traits (like tallness and dwarfness) are inherited independently. 

Tall plants: Dwarf plants=3:1

Dihybrid inheritance involve the inheritance of two pairs of contrasting characteristics ( or contrasting traits)at the same time. The two pairs of contrasting characteristics chosen by Mendel were shape and color of seeds: round-yellow seeds, and wrinkled- green seeds.

Mendel first crossed pure-bred pea plants having round- yellow seeds with pure- bred pea plants having wrinkled green seeds and found that only round-yellow seeds were produces in the first generation. No wrinkled – green seeds were obtains in the F₁ generation from this it was concluded that round shape and yellow color of the seed were dominant traits over the wrinkled shape and green color of the seeds.

When the F₁ generation pea plants having round yellow seeds were cross-bred by self pollination, then four types of seeds having different combinations of shape and color were obtained in second generation. These were round-yellow, round- green, wrinkled- yellow and wrinkled- green seed. Mendel collected a total of 556 F₂ seeds and counted them shape wise and color wise. He got the following result:

Round- Yellow seeds-315

Round-green seeds- 108

Wrinkled- yellow seeds-101

Wrinkled-green seeds-32

The phenotypic ratio of different types of seed can be written as:

Round          Round                        Wrinkled                Wrinkled

Yellow:        Green:                        Yellow   :                Green   = 315 :108 :101 :32

Seeds           Seeds                          Seeds                      Seeds

                                                                                                        = 9 :3 :3 :1

Thus, the ratio of each phenotype (or appearance) of the seeds in the F₂ generation is 9:3:3:1. This known as the dihybrid ratio.

According to Mendel’s second law of inheritance: in the inheritance of more than one pair of traits in a cross simultaneously, the factors responsible for each pair of traits are distributed independently to the gametes.

How do traits get expressed?

Cellular DNA has the blueprint(information) to make cellular proteins. A fragment of DNA that gives information for synthesis of one protein is called a gene for that protein.

All biochemical reactions in the cell are controlled by protein/ enzyme. An enzyme is necessary for protein production.

The tallness or characteristics of the pea plant is controlled by hormone that triggers the growth. The height/ tallness will depend upon the amount of the particular hormone in the plant. The amount of the growth hormone will depend on the efficiency of the enzymes making it.

If the enzymes is working efficiently than the amount of growth hormone will be more, resulting in tall plant. And if the enzyme is less efficient then the amount will be less, resulting in short height plant. The gene controls the efficiency of the enzyme. Thus, genes control the characteristics or traits.

SEX DETERMINATION

The process by which the sex of a child is determined is called sex determination.

Genetic factors - genetics is involved in the determination of sex of a person. This can be explained as follows.

The chromosomes which determine the sex of a person are called sex chromosomes. There are two types of sex chromosomes, one is called X chromosome and the other is called Y chromosome.

                         A. male( man or father) has one X chromosome and one Y chromosome. This means                           that half the male gametes or half the sperms will have X chromosome and     the other                          half will have Y chromosome.

A female (women or mother) has two X chromosome (but no Y chromosome). This means that all the female gametes called ova (or eggs) will have only X chromosome.

The sex of a child depends on what happens at fertilization:

(a)    if a sperm carrying one chromosome fertilizes an ovum (or egg) which carries X chromosome, then the child born will be a girl (or female). This is because the child will have XX combination of sex chromosome.

(b)   If a sperm carrying Y chromosome fertilizes an ovum (or egg) which carries X chromosome, then the child born will be a boy (or male). This is because the child will have XY combination of sex chromosomes.

Please note that it is the sperm which determines the sex of a child. This is because half of the sperms have X chromosomes and the other half have Y chromosomes. Thus, there is 50% chance of a boy and 50% chance of a girl being born to the parents. This is why the human population is roughly half males and half females

Environmental factors –Environmental factors  play an important role in sex determination.

1.In turtles Chrysema picta High incubation temperature -- Results  in development of female.

2. In lizard Agama agama high incubation temperature – Results in development of female.

                                         Organic Evolution

It is the gradual changes in the structure and function of organism which may

Result in change in existing body design of the organism or result in formation of new species.  

Microevolution-  Structure and functions at small scale. Result in change in existing body design.

Macroevolution-  Structure and function changes at a large scale involving long period of time result  in formation of new species.

Factors/sources of evolution

1.      Variations during sexual reproduction

2.      Mutations

3.      Natural selection

4.      Genetic drift

Natural selection-the process by which nature selects organism with best possible variations to survive is called as natural selection .it occurs gradually and continuously in any population at any time.

Genetic drift- chance elimination of certain genes from a population due to an accident or natural calamity is called genetic drift .it occurs in a small population only .

Inherited trait                                                                                          Acquired trait

1.      The trait which a person inherit                                  1.The trait which a person acquire

      from his parents.                                                            from environment.

2.Inherited traits involve DNA of a                               2. They are not incorporated

    Person.                                                                             in the DNA of a person

    Eg. Eye colour etc                                                          eg. The son of the wrestler

                                                                                                not be a wrestler

Specition-  A process of formation of new species  from already existing one is called  Speciation.

1.      Existence of sub – populations which are separated by a geographical barrier.

Hence restriction to genus flow.

2.      Action of different forces of  natural selection and gemetic dirift  on sub-population ,in their   separate ways.

3.      Accumulation of  variations in the sub-population over a period of time making incompatable and unable to interbreed and therefore they form two different species.

Geographical isolation:

Two sub-population of the same species which are separated by some physical barrier like river ,mountain range etc. are unable to interbreed ,tough in the absence of physical barrier, interbreeding is possible. The sub-populations are said to be geographical.

Evidence of evolution

1.      Homologus organs (similar organs)- the organs of different organism having similar origin but they perform different functions are called homologus organs .these organs have same arrangement of bones ,similar blood vessels and nerves . Eg. hands of man ,floppres of whale ,, forelimbs of frog

2.      Analogous organs- The organs  that  differ in their origin and structural details but perform `the same function are called analogous organs.

e.g. the wings of birds and wings of insects are analogous organs.

                 3.  Evidences from fossils- Fossils 

Fossils are impressions/preserved traces of organisms that once lived on earth. These impressions can be on different materials such as rocks ,gums/resins.

Organisms that die ,get buried in moist soil ,clay or any other material. When the material hardens in due course of time ,the soft body parts decay. But impressions of hard body parts such as bones ,are left on material. It gets converted to fossil. Formation of fossils is called Fossilisation.

How Old is a Fossil?

The age of fossil is determined by:

1.The depth at which fossil is found. The fossils found close to the earth surface are of recent origin and the ones in deeper layers old.

2.Dating of the fossils. It is done by the detection of different isotopes of the same element found in the fossil material.

Evolution by Stages

Evolution is not all of by sudden. It takes through stages. For example ,from primitive to intermediate then to developed and then advanced stage. Like development and adaptation of wings in animals to fly ,eyes also developed.

1.Eyes. Eyes ,the sense organs were developed to’identify’. Some animals have rudimentary eyes just to detect light. Some have identification of objects ,in insects and image formation in vertebrates like man.

You know that flatworm has rudimentary eyes insects have compound eyes ,octopus has eyes ,structure of eye is different in each case. They have separate evolutionary origins.

2.Feathers on body. Feathers o body appeared for different purposes but later were used for other function also.

Initially ,it might have started for providing insulation in cold weather ,such as in some dinosaurs. But later ,it was used for flight by birds. Dinosaurs did not use it for flying. This means that bird are closely related to reptiles. Since dinosaurs were reptiles.

3.Descendants of wild cabbage. Over two thousand years farmers have cultivated wild cabbage as a source of food plant. They produced different vegetables from it by selection as we see today, six varieties of vegetables were produced by this artificial selection.

Some farmers selected for swollen stem and formed kohlrabi.

Some selected for sterile flowers and formed cauliflower.

Some selected for arrested flowers and formed broccoli.

Some farmers cultivated for short distance between the leaves cabbage.

Some preferred for colored red leaves for forming red cabbage.

While some others grew it for large leafy type called kale.

We can also trace the evolutionary relationship by studying the changes in DNA during the reproduction. Comparing the DNA of different species gives us a direct estimate of how much the DNA has changed from its original form during formation of new species. This method is now extensively used to define evolutionary relationships.

Evolution should not be equated with progress

Evolution as said is still in progress. The organisms observed today are the descendants of past and due accumulation of past changes. It does not mean that past organisms/ancestors were not good or the present ones are better. It all depends on environmental selection for survival. Still today we have very simple organisms like bacteria surviving in all habitats and inhospitable habitats. Neither the human beings are pinnacle of evolution. They are just one species of evolutionary life.

Human Evolution

Study of human evolution has been done by use of same tools for study of evolutionary relationships.

1.      Excavation-digging the soil.

2.      Time dating.

3.      Study of fossils.

4.      Study of DNA sequences.

There is a great diversity of human beings and features across the world. People used to talk about ‘races’. These races were identified on the basis of skin color as – white, black, brown and yellow. Actually all are one species.

Regardless the places where humans lived for past thousand years they all originated from Africa. Homo sapiens, the earliest human species lived there. From here our ancestors spread in all directions. Though they kept going there for mixing with each other. Some stayed there only and spread across Africa. The other people migrated from Africa to West Asia, Central Asia, Eurasia, South Asia and East Asia. They also travelled to Indonesia and Philippines to Australia and America. Like all other species, the humans also evolved accidently and were tried to live their lives the best as they could.