Heredity and evolution Class 10 notes

Heredity and evolution

Recognisable feature of a human being like height, complexion, shape of hair, colour of eyes and shape of nose, and chin Are called characters or traits. The transmission of characters from the parents to their of springs is called heredity. Two parents a male and female are involved in sexual reproduction. The sexually producing organisms produce sex sales. The male gamet called as form uses with a female gamet called ovum form as I got which gradually develops into a young one. Actually the hereditary information is present in the sex cells of the parents. Does gamuts constitute the link between one generation and the next and pass on the paternal and maternal characters or traits to the offspring. This relation that continues to exist between successive generations is referred to as heredity.

Variations

All the office springs in harit the characters of the parents and resemble them very closely but the resemblance is not complete in all respects. The office springs are never a true copy of the parents full stop in fact note to individuals are exactly like and the members of anyone species before from one another in some characters or  the other. This differences are known as variations. Variation is the occurrence of difference among the individual of a species. The difference in the characters among the individuals of a species is called variation. in most of the people the earlobe is hanging and it is called free earlobe. In some people however the earlobe is closely attached to the side of the head and it is called attached earlobe. So, the free earlobes and attached earlobes are the two variation found in human population. 

Accumulation of variations

The reproduction of organisms reproduce variations full stop the variations produced in organisms during successive generation get accumulated in the organisms full stop the significance of a variation source of only if it continuous to be inherited by the of spring for several generation full stop this will become clear from the following example. Suppose a bacterium produces two bacteria by a sexual reproduction. Again suppose that one of the of spring bacteria has a variation due to which it can tolerate a little higher temperature then the other one. Now this variation of little more heat resistance will go on accommodating in the of springs of successive generation of this bacteria. And this will ultimately give rise to a variant of bacteria which will be highly heat resistant and able to survive even at very high temperature. The great advantage of variation to a species is that it increases the chances of it survival in a changing environment.

What is chromosomes?

Chromosome is a thread like a structure in the nucleus of a cell formed of DNA which carries the genes. Different organisms have different number of chromosomes in their nuclei. A gean is characteristic of an organism. There are thousand of geans of a chromosome which control various characteristic of an organism. Genes are actually units of heredity which transfer characteristics from parents to their office springs during reproduction. They works in pairs. In diagram and example of heredity genes are represented by letters. 

The gene which decides the appearance of an organism even in the presence of an alternative gene is known as a dominant gene full stop it dominate the recessive gene for the same characteristic on the other chromosome of the pair. The Jean which can decide the parents of an organism only in the presence of another identical gene is called a recessive gene. A single recisive gene cannot decide the appearance of an organism full stop the dominant gene is represented by a capital letter and the corresponding recessive gene is represented by the corresponding small letter. Genotype sauce the genetic constitution of an organism. In simple words genotype is the description of jeans present in an organism full stop the characteristic which is visible in an organism is called its phenotype. When to parent cross breed to produce progeny than their progeny is called first filial generation. When the first generation progeny cross among themselves to produce second generation progeny, then this progeny is called second generation or F2 generation. The generation produced by crossing to F1 progeny is called F2 generation. Gregor Mendel was the first scientist to make a systematic study of patterns of inheritance which involved the transfer of characteristic from parents to progeny. Hey did this by using different varieties of pea plants. Which hegrive in his garden full stop some of the characteristics of the pea plants whose transmission to progeny was investigated by Mendel were height of pea plant or length of stem of pea plant shape of seeds and colour of seeds. I yet another contrasting characteristic investigated where colours of flower.

Mendal choose pea plants for studying inheritance because pea plants had a number of clear cut differences which were easy to tell apart. For example round yellow seeds where as other produced wrinkled green seeds full stop another reason for choosing pea plants was that they were self pollinating which enable them to produce next generation of plants easily. And finally mental choose pea plant to study inheritance because many generation of pea plants can be produced in a comparatively short time spend and their study is much simpler than that of animals. A new form of plant resulting from across breeding of different varieties of a plant is known as hybrid full stop when we breed to p plants having one contrasting characteristic each to obtain new plants than it is called a monohybrid cross. In monohybrid cross we will study the inheritance of one pair of contrasting characteristic toldness and doughnut of the pea plants by their first generation and second generation progeny. On the other hand if you breed to p plants having two contrasting characteristic to obtain new plants than it is called a die hybrid cross full stop in the diehydrate cross we will study the inheritance of two pairs of contrasting characteristic of pea plant such a round yellow seeds and wrinkled green seeds.

Rules for the inheritance of Traits: mendel's contribution

Inheritance is the transmission of genetically controlled characteristic from one generation to the next. We will now describe how Mendel studied the inheritance of characters or traits in various generation of pea plants cultivated by him. first we will discuss monohybrid inheritance which concerned the inheritance of a single characteristic such as plant height. After that we will describe the hybrid inheritance which inverse the inheritance of two characteristic such as seed shape and seed colour.

1. Monohybrid inheritance and the law of segregation

In order to trace the inheritance of a single pairs of contrasting characteristic among the pea plants Mendel crossed the pure-bred pea plants defearing in this traits and. noted their occurrence in the progeny of succeeding generation.

• mendal first crossed pure bread toll free plants with pure bread dwarf pea plants and found that only tall pea plants were produced in the first generation full stop no dwarf p plants where obtain in the first generation of progeny. From this mandal concluded that first generation sort the traits of only one of the parent plants that is tallness full stop the trait of other parent plants vs did not show up in the progeny of first generation.

• mendal than crossed the tall pea plants of the first generation and found that tall plants and dwarf planets were obtained in the second generation in the ratio of 3:1. In other words in the F2 generation 3 /4th plants where tall and 1/4th were dwarf. Mental noted that the dwarf threads of the parents appeared in the first generation reappeared in the second generation. Mendel called the refreshed trades of dwarfness as recessive trade and the expressed trait of tallness as the dominant trait. In this way mendel's experiments with tall and dwarf pea plants so that the traits maybe dominant or recessive.

2. Dihybrid inheritance and the law of independent assortment

Dihybrd inheritance involves the inheritance of two pairs of contrasting characteristic at the same time. The two pairs of contrasting characteristics chosen by Mendel were safe and colour of seeds. In order to trace the inheritance of two pairs of contrasting trades mental crossed pea plants having round yellow seeds with pea plants having wrinkled green seeds and noted there occurrence in the succeeding generation of pea plants.

•Mendel first crossed pure-bred pea plants having round yellow seeds with pure-bred peaplants having wrinkled green seeds and found that only round yellow seeds were produced in the first generation. No wrinkled green seeds found. From this it was concluded that round shaped and yellow colour of the seeds were dominant traits over the wrinkled safe and green colour of the seeds.

•when the F1 generation pea plants having around yellow seeds were cross breed by self pollination then four types of seeds having different combination of safe and colour where obtain in second generation. This where round yellow round green wrinkled yellow and wrinkled green seeds full stop mental collected a total of 5:56 F2 seats and counted them shape wise and colour wise. He got the following result:

Roun-yellow seeds  315

Round-Green seeds 108

Wrinkled-yellow seeds 101

Wrinkled-Green seeds 32

The fino typic ratio of different type of seed are 9:3:3 :1.

Mendel observe that he had started with two combinations of characteristic in seeds round yellow and wrinkled green and two new combination of characteristic head appeared in the F2 generation round green and wrinkled yellow full stop on the basis of this observation mental concluded that the two pairs of original characteristic combine in the F1 generation but they separate and behave independently in subsequent generation. The result of dihybrid cross emerald mandal to formulate his second law of inheritance which is called the law of independent assortment full stop according to Mendel second law of inheritance: in the inheritance of more than one pair of traits in across simultaneously, the factors responsible for each pair of traits are distributed independently to the gametes.

How are characteristics transmitted to progeny

Genes are responsible for the characteristic features of an organism full stop the characteristic of parents are transmitted to their progeny through Genes present on their chromosomes during the process of sexual reproduction. Genes work in pairs full stop there is a pair of Genes  for each characteristic of an organism. One is dominant and other is recessive gene. Each parent poses appear of genes for which characteristic on a pair of chromosomes. however each parent passes only one of the two Genes of the pair of for which characteristic to its progeny through gamets. 

How blood groups are inherited

A person has only one of the four blood groups: A, B, AB, O. This blood group system is controlled by a gene which has three different forms denoted by the symbol IA. Hello virgins of A and B both are dominant over the gene o. All though there are three gene for blood but any one person can have only two of them. So the blood group of a person depends on which two forms of the genes he poses. 

Sex determination

A person can have a male sex or a female sex. The process by which the sex of a person is determine is called sex determination full stop genetics is involved in the determination of the sex of a person. This can be explained as follow: the chromosomes which determine the sex of a person are called sex chromosomes. There are two types of sex chromosomes 1 is called X chromosome and the other is called y chromosome. A mail has won X chromosome and one chromosome full stop this means that half the male gametes or half the sperms will have X chromosomes and the other half will have y chromosomes full stop female has two X chromosomes full stop this means that all the female gametes called over will have only X chromosomes.

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

If a sperm carrying X chromosome fertilized and ovum which carries X chromosome then, the child born will be a girl. This is because the child will have XX combination of chromosome.

If a sperm carrying why chromosomes fertilized and ovum which carries ex chromosome than the child born will be a boy or male. It is the sperm which determines the sex of the child. This is because half of the sperms have X chromosomes and the other half have Y chromosomes full stop that's there is a 50% chance of a boy and 50% chance of a girl being born to the parents full stop this is why the human population is ruffle half male and half female. In some of the animals, sex determination is also control by the environmental factors. For example in some reptiles the temperature at which the fertilized egg is incubated before hatching place a role in determination the sex of the offspring. It has been found that in a turtle high incubation temperature leads to the development of female of spring. On the other hand in the low temperature in cubation the of spring will be a female.

Acquired and inherited traits

A trait of an organisms which is not inherited but develops in response to the environment is called an acquired trait. For example if a beetle does not get sufficient food for a considerable time it's weight will be reduced due to starvation full stop the low weight of this beetle is an acquired trades of the beetle which have been acquired in response to the environment which contains in sufficient food. The acquired traits of organisms cannot be passed on to their future generations. Only those trades can be transmitted to future generation in which changes have orchid in the DNA present in the reproductive cell of parent organisms.

A characteristic of an organism which is caused by a change in its DNA is called an inherited traits full stop inherited traits can be passed on to the progeny of the organism because they have produced changes in the DNA of the organism. Suppose there is a population of red beetles in the green bushes. Again suppose that a colour variation arises during reproduction so that there is one beetle which is green in colour instead of red full stop this change of green colour in the beetle has been brought about by a change in the DNA of the reproductive cells. The green colour of this beetle is an inherited traits which can be passed on to the next generations. The change from red beetle to green beetle can be considered to be an example of evolution because it helps in IT Survival by mixing with green bushes. 

Evolution

There is an enormous number and types of living organisms on this earth full stop in addition to this wide variety of living organisms the remains of the dead organisms which lived in the remote past are also known. an important question now arises: how and from where has such a great variety of living organisms come to exist on this earth? Also have the human beings have evolved on this earth? All this things are studied in the branch of biological evolution. The word evolution has been derived from the Latin word evolver which means to unroll or unfold. Evolution is a kind of gradual unfolding of the new organism from the pre existing primitive organism through slow and steady changes. Evolution is the sequence of gradual changes which takes place in the primitive organisms over millions of air in which new species are produced full stop since the evolution is of the living organisms so it is also called organic evolution. All the plants and animals which we see today around us have the other and sisters that lived on this earth long long ago.

1. Homologous organs provide evidence for evolution

If you look at the way in which living organisms are made we can often she quite striking similarities in their construction full stop one of this is the presence of homologous organs. Those organs which have the same basic structure or basic design but different function are called homologous organs. This organs of different animals provide evidence for evolution. There are many organs in different groups of animals which all sim to be built from the same basic design but are used for many different purposes. This are called homologous organs. For example the four limbs of a man a lizard a frog a bird and a bat same to be built from the same basic design of bones but they perform different functions. The four limbs of a man are used for grasping; the four lines of a lizard are used for running; the forelimbs of a frog are used to prop up the front and of its body when  rest, and also act as shock absorbers when the frog lands back on the ground after a leap; where is the four limbs of a bird and a bat are modified for flying full stop since the four limbs of a woman a lizard a frog a bird and a bad have similar structure but perform different function they are the homologous organs. 

2. Analogous organs provide evidence for evolution

Those organs which have different basic construction but have similar appearance and perform similar functions are called analogous organs full stop the analogous organs provide the evidence for evolution. There are many organs in different groups of animals with seems to be built from different basic structure but appear to be similar in safe and perform similar functions. These are called analogous organs full stop for example the wings of an insect and a bird have different structure insect have a fold of membrane as wing which are associated with a few muscles where as skeleton, flash and feather support birds wings. Birthday perform the same function of flying full stop since the wings of insects and birds have different structures but perform similar functions they are analogous organs.

Darwin's theory of evolution

Charles Robert Darwin gave the theory of evolution in his famous book the origin of species full stop the theory of evolution proposed by Darwin is known as the theory of natural selection full stop this theory is called the theory of natural selection because its suggest that the best adapted organisms are selected by nature to pass on there characteristics to the next generation. Darwin's theory of evolution applies to plants as well as animals.

Balance theory of evolution can be described as follows:

•within any population there is natural variation. Some individuals have more favourable variations than others.

•even though all species produce a large number of springs population remain fairly constant naturally.

•this is due to the struggle between members of the same species and different species for food space and mate.

•the struggle for survival within populations eliminate the unfit individuals. The fit individuals possessing favourable variation survive and reproduce full stop this is called natural selection or survival of the fittest.

•the individuals having favourable variations pass on this variations to their progeny from generation to generation.

•this variations when accumulated over a long period of time lead to the origin of a new species.