Science [3 Mark Questions]

Sex of a child is dependent on the type of the male gamete fusing with the female gamete. Human beings possess 23 pairs of chromosomes. Out of these, 22 pairs are known as autosomes, while the remaining one pair comprises sex chromosomes (XX in females and XY in males). At the time of fertilisation, the egg cell fuses with the sperm cell, resulting in the formation of the zygote. If the egg cell carrying an X chromosome fuses with the sperm carrying an X chromosome, the resulting child would be a girl. If the egg cell carrying an X chromosome fuses with the sperm carrying a Y chromosome, the resulting child would be a boy.

It is explained by Mendel in the form of Law of Independent Assortment. when two or more characteristics are inherited, individual hereditary factors assort independently during gamete production, giving different traits an equal opportunity of occurring together. It was based upon the observation in the dihybrid cross.

We can say that light eye colour trait is dominant because only dominant traits are expressed in the first generation.

Different organisms with common ancestry in the past are more likely to have similar characteristics due to inheritance. These similar characteristics are used to determine evolutionary relationships. Basic structure of Forelimbs in human and birds is one such example of a characteristic which shows closeness of two species in evolutionary terms. The forelimbs in both the species have similar bones structure and similar origin, but, over period of time, spanning generations, they have been developed to perform different suitable functions as per the need. Such organs that have similar basic structure but perform different functions are called homologous organs. These are used to find out evolutionary relationships.

Methods of tracing evolutionary relationships.

1. Studying homologous organs: Organs having same structural plan or origin but are modified to perform different functions.

Example: Forelimbs of vertebrates.

2. Studying analogous organs: Organs have different structural plan or origin but are modified to perform same function.

Example: Wings of birds and wings of bat/wings of insects and wings of bat.

3. Study of fossils: By studying fossils,we can know about the species which once existed.

Example: Dinosaur skull/Invertebrate.

Homologous Organs	Analogous Organs
Homologous organs have similar origin n basic structure but perform different functions in different organisms.	Analogous organs are different in basic structure but perform same functions.
Homologous organs show divergent evolution.	Analogous organs show convergent evolution.
They develop in related orgainsms.	They develop in unrelated organisms.
Example: Forelimb of a frog and man seem to be built from the same basic design of bones but they perfrom different functions.	Example: Wings of birds and bats look similar. But in birds wings are covered bt feathers all along the arm but the wings of bats is skin folds between elongated fingers.

The wings of a bird and a bat are similar in function but are different in their origin. If we carefully look at these structures, then we will find that the wings of a bat are just the folds of skin that are stretched between its fingers whereas the wings of birds are present all along the arm. Hence, these organs can be classified as analogous organs.

1. Homologous Organs– The study of these organs suggests that these organisms with organs having same structure but performing different functions have evolved from a common ancestor, e.g. forelimbs of different vertebrates.
2. Analogous Organs– The study of these apparently similar organs suggests that the organisms with apparently similar organs do not share common ancestory. Similarity in these organs is superficial/Design and the structure of these organs are very different, e.g. Wings of bird and wings of butterfly.
3. Fossils– Provide the missing link between the species, e.g. Fossils of dinosaurs with feathers/fossils of prehistoric horse.

Certain traits cannot be passed on next generation because:

1. Acquired traits which occur in the somatic cells bring about some changes in them.
2. Changes in the somatic cells are not passed on to the offspring belonging to next generation.
3. Only the changes in the genetic material of the reproductive cells can be inherited.

Example: Muscles developed by strenuous exercise cannot be inherited to offspring as these changes occur in somatic cells. These are called as acquired traits.

1. Characters that are passed on from parents to offspring are inherited characters.

For example: colour of seeds, colour of eyes, etc.

2. Characters appearing in an individual’s lifetime but cannot be transmitted to next generation are acquired characters.

For example: obese body, loss of a finger in an accident, etc.

Acquired	Inherited
The acquired traits are the traits which are experienced by an indiavidul during his life time.	Inherited traits are distinguishing qualites of characteristics which one acquires from the ancestors.
The acquired traits of an organism cannot be passed on to its future generations.	The inherited traits of an organism are passed on to its future generations.
For e.g, 'low weight' of beetle. ' cut tail' of a mouse.	For e.g., red colour of beetles, fur coat of guinea pigs.

In sexual reproduction the offspring formed will not be exactly similar to the parent. The progeny formed will undergo various modification in his body according to the environment and when this progeny develops into an adult he will have many evolved characters which are not present in his parents. He when reproduces sexually then his offspring will undergo more modification and adapt to the environment. In this way sexual reproduction helps in evolution. This is the basis to Daltons theory of evolution.

DNA = Deoxyribo Nucleic Acid.
It is located inside the nucleus on the chromosomes as genes.
Basic event in reproduction is the creation of a DNA copy. Cells use chemical reactions to build copies of their DNA. This creates to build copies of their DNA. This creates two copies of the DNA in the reproducing cell. DNA copying is accompanied by the creation of an additional cellular apparatus and then the DNA copies separate with its own cellular apparatus. This way a cell divides to give rise to two cells.

• When a cross was made between a tall pea plant with round seeds and a short pea plant with wrinkled seeds, the F1 progeny plants are all tall with round seeds: this indicates that tallness and round seeds are the dominant traits.
• When the F1 plants are self-pollinated the F2 progeny consisted of some tall plants with round seeds and some short plants with wrinkled seeds which are the parental traits.
• There were also some new combinations like tall plants with wrinkled seeds and short plants with round seeds.
• Thus it may be concluded that tall and short traits and round and wrinkled seed traits have been inherited independently.

This is an example of artificial selection since humans have generated the different vegetables from cabbage by selectively breeding for specific characteristics.
Some farmers selected for very short distances between leaves, and have bred the cabbage we eat. Some have wanted to select for arrested flower development, and have bred broccoli, or for sterile flowers, and have made the cauliflower. Selecting swollen parts led to production of kohlrabi. Selecting the characteristic of slightly larger leaves led to producing a leafy vegetable called kale.

The simplest answer for this question is "using sequence of DNA" the relationship between the organism is determined by their DNA sequence The arrangement of the nucleotides in DNA of the organisms are specific and are determined by the scientist who use this information and compare the sequence and find the similarities thus helping in the defining the evolution relationship of organisms. These information also used to defines the difference between organisms. Scientist also used the gene sequence to find it.

1. Study of homologous organs suggests that the organisms having same structure of organs but performing different functions have evolved from a common ancestor.

eg. forelimbs of a frog, lizard, bird and man.

2. Analogous organs show adaptations of organs for common use.

eg. wings of butterfly and wings of bat.

3. The fossil Archaeopteryx looks like a bird but bears a number of other features found in reptiles. This observation provides a clue that the birds have evolved from reptiles.

Structures which have a common basic structure but perform different functions are called homologous structures.For example: Forelimbs of mammals, birds, reptiles and amphibians.
The forelimbs of humans and the wings of birds perform different functions, but their skeletal structures are similar.Yes, homology indicates common ancestry. Homologous organs follow the same basic plan of the organisation during their development but in the adult condition, these organs are modified to perform different functions as an adaptation to different environments.

The process by which a new species develops from the existing species is known as speciation. Factors that lead to speciation:

1. Genetic drift.
2. Natural selection.
3. Reduction in Gene flow.
4. Reproductive isolation.

Variation is the basis of heredity. When an organism reproduce through sexual reproduction copying of DNA take place which is not accurate, thereby causing variations. If these variations are favourable they help individuals to survive and pass these variations to their progeny. Variation acquires any trait in its genome, during a large course of time, it gets transmitted to its progeny. This forms heredity.

In India almost all the girls have their ears pierced still we never see any girl born with her ears pierced already. It happens because the piercing is done on the body of the organism and no effect is caused on the genetic material. The traits we inherit depends solely on the genetic material of the parents and not on the physical features. Piercing is an acquired trait, not inherited one.

Acquired	Inherited
The acquired traits are the traits which are experienced by an individual during his life time.	Inherited traits are distinguishing qualities or characteristics which one acquires from the ancestors.
The acquired traits of an organism cannot be passed on to its future generations	The inherited traits of an organism are passed on to its future generations.
For e.g., 'low weight' of beetle. 'cut tail' of a mouse.	For e.g., red colour of beetles, fur coat of guinea pigs.

Evolution is the process by which organisms develop and accumulate changes in them over time, so as to be better suited to the surrounding environment and survive. Evolution is a slow process and helps in the selection and survival of only those organisms that are best suited for the environment and can successfully reproduce. This is the essence of the idea of evolution.

• Sexual reproduction, asexual reproduction.
• Sexual reproduction is responsible for bringing in more variations.
• Because of process of DNA copying which may result in some error in it.
• Because it involves fusion of male and female gametes from two different parents.

1. Organs or features can be adopted to new. Functions during evolution Example - Feathers were developed form providing insulation in cold weather (in dinosaurs) but in birds they were adopted for flight.
2. “Older body designs are not inefficient”. Example -Simplest life forms like Bacteria, which inhibit most inhospitable habitats like, hot springs, deejs sea, thermal vents and ice in antarctica.

Some amount of variations are produced even during asexual reproduction but it is very small. The number of variations produced during sexual reproduction is, however, very large. It is due to these variations that no two human beings look alike. The number of successful variations is maximized by the process of sexual reproduction, so we can conclude that the variation is a necessity for organic evolution.
Example: Animal reproduced by sexual reproduction show large number of variations.

The two causes of speciation are:

1. Genetic drift: If the DNA changes are severe enough, such as a change in the number of chromosomes, eventually the germ cells of the two groups cannot fuse with each other.
2. Natural selections: If a new variation emerges in which females are only able to mate with few males due to their specific trait. This allow natural selection.

The evidence was given by Stanley L. Miller and Harold C. Urey in 1953. They assembled an apparatus to create an early earth atmosphere which was supposed to consist of gases like ammonia, methane and hydrogen sulphide, but no oxygen) over water. This was maintained by them at a temperature just below 100 C and electric sparks were passed through the mixture of gases to stimulate lightning. At the end of a week, they found that 15% of the carbon (from methane) had been converted to simple compounds of carbon including amino acids which make up protein molecules.

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.

The important factors which could lead to the rise (or formation) of a new species are the following:

1. Geographical isolation of a population caused by various types of barriers (such as mountain ranges, rivers and sea). The geographical isolation leads to reproductive isolation due to which there is no flow of genes between separated groups of population.
2. Genetic drift caused by drastic changes in the frequencies of particular genes by chance alone.
3. Variations caused in individuals due to natural selection.

Homologous organs are defined as the structures that differ in appearance or perform different functions but have similar basic structure or developmental origin.
For example: Wings of bats and forelimbs of lizards are homologous structures. They have same basic structure of bones, but perform different functions. Presence of homologous organs confirm divergent evolution, i.e., different organisms develop from a common ancestor. Presence of the same basic structure indicates that the organisms have common ancestral origin but evolved to perform different functions over the course of time. Therefore, homologous organs helps in establishing evolutionary relationship between organisms.

Speciation: The formation of new and distinct species in the course of evolution. Speciation involves the splitting of a single evolutionary lineage into two or more genetically independent lineages.
Natural selection is the process whereby organisms better adapted to their environment tend to survive and produce more offspring, whereas other less favourable traits tend to become eliminated. Genetic drift refers to random changes in gene frequencies that usually occurs in a small population and results from chance processes alone.
Example: A population of red beetles live in an area. Suppose a colour variation arises during reproduction, giving rise to one green coloured beetle instead of red. This beetle passes the colour on to its progeny, so that all its progeny beetles are green. Crows cannot see green-coloured beetles on the green leaves of the bushes, and therefore cannot eat them. So the progeny of green beetles is not eaten, while the progeny of red beetles continues to be eaten. This gives rise to more green beetles than red ones in the beetle population. Here the green coloured variation became common because it gave a survival advantage.

• When a cross was made between a tall pea plant with round seeds and a short pea plant with wrinkled seeds, the F1 progeny plants are all tall with round seeds: this indicates that tallness and round seeds are the dominant traits.
• When the F1 plants are self pollinated the F2 progeny consisted of some tall plants with round seeds and some short plants with wrinkled seeds which are the parental traits.
• There were also some new combinations like tall plants with wrinkled seeds and short plants with round seeds.
• Thus it may be concluded that tall and short traits and round and wrinkled seed traits have been inherited independently.

The experience and qualifications you earn during your lifetime are called acquired traits. The experience achieved during lifetime of an individual does not make any change in the gene. For example, if a person read books on birds the knowledge he earns by reading the book does not make any change in the gene, hence, this knowledge will not get automatically transmitted to his next generation. These traits earned during the lifetime make changes to the systematic cells in our body and not the germ cells. So they are not passed to the next generation.

Mendel learned about genetics. He figured out how traits carried by a parent would be passed onto their offspring. He didn't figure out that all organisms come from a single source through evolution and the formation of new species.
Darwin learned that when certain traits are passed from parents to offspring, this results in variations in individuals that are passes onto the succeeding generations. He realized that offspring which inherit traits that are a better "fit" for their environment would tend to breed more successfully than the less fit - and that this mechanism would result in evolution and the formation of new species.

Individuals with a particular trait may increase in a population due to the following factors-

1. Natural selection: The variations which give survival advantage to an organism are selected in nature and such traits increase in population.
2. Genetic Drift: It occurs due to change in gene frequency due to accumulation of a particular type of trait.
3. Geographical Isolation: It leads to change in gene frequency leading to expression of one type of traits in a geographically isolated population.

Fossils are the remains of plants, animals or other organisms which are in petrified form. These are formed when plants or animals buried in mud and soil. The soft tissues decay easily by leaving the hard bones and shells. Over a period of time sediment builds over these bones and shells, then hardens to form rock.
Fossils provide direct evidence for the history of evolution. Fossils help scientists to correlate the organisms from the past and present. Calculating the age of fossils gives idea about the occurrence of organism in a particular period.

A trait of an organism which is ‘not inherited’ but develops in response to the environment is called an acquired trait.
Example: If a beetle does not get sufficient food for a considerable time, its weight will be reduced due to starvation. The low weight of the beetle is an example of acquired trait. A trait of an organism which is caused by a change in its genes (DNA) is called an inherited trait.
Example: The change of colour from red beetle to green beetle is an example of inherited trait.

SPECIATION: A species may be defined as a group of organization capable of interbreeding and producing fertile off springs. The process of formation of new species from existing one by evolutionary means is known as speciation.
Natural selection: According to Darwin's theory of evolution., natural selection may be defined as the process by which the organisms which are better adaptive to a particular kind of environment tends to survive, leading in the elimination of the organism less adaptive to that environment.