Heredity And Evolution

Sexual reproduction, unlike asexual reproduction, provides room for a larger number of variations. In it, the intensity of variations maximises due to increased inaccuracies in the DNA copying mechanism because of involvement of two individuals in the reproductive process. Hence, greater diversity is observed in sexual reproduction, which favours evolution, and thus, it is considered superior to asexual reproduction.

The experiences of an individual during its lifetime are acquired traits. These are changes in the non-reproductive tissue of the individual, and do not affect the DNA of the germ cell of that individual. As in reproduction, only the germ cells participate, the changes in non-reproductive tissues are not passed on to the next generation.

The source of information for making proteins is the DNA in the cells. The part of DNA which codes for a particular protein is the gene for that protein.

The part of cellular DNA, which provides information regarding a particular protein, i.e. codes for that protein, is called the gene for that protein.

Genetic mutations and natural selection for the basis of evolution.

The DNA copying mechanism of sexual reproduction are precise, but they are not completely accurate. This allows for a good extent of variations to take place. But, the variations are not so large to change the basic structure of the human being. Thus, the child bears all basic features but doesn’t look exactly like its parents.

The frequency of inherited traits, or the genes carrying those traits increases over generations. The variations that provide better adaptive advantage over others are promoted, or say, naturally selected over others. Thus, creation of variations in a species promotes their survival.

The genes present in the DNA control characteristics or traits. The phenotypic or physical expression of a character depends upon the genotypic constitution. The expressed traits are a result of the specific genotypic combination of the alleles, which are a pair of genes themselves. Also, particular genes code for specific proteins that determine the physical characteristics of an individual. This is how genes control the traits.

Considering, B to stand for black hair, and b to stand for brown hair, Homozygous dominant black hair = BB

Homozygous recessive brown hair= bb.

The essence of the idea of evolution is that the frequency of certain genes that control traits, changes in a population over generations. This, in turn, changes the frequency of inherited traits or characters and some rare variations, become a common character in population.

Mendel used pea plants to conduct experiments demonstrating the inheritance of characters like height, colour, etc. When a tall plant was crossed with a dwarf plant, the progeny or first generation showed all tall plants. When two of these plants were crossed, the progeny showed three tall and one dwarf plant.

The above crosses show that only one T is enough to make the plant tall, that is, it dominates the inheritance of height. Thus, it is called the dominant trait. To make a plant dwarf, both the alleles must be t. Thus, it is the recessive trait.

Genetic drift is a phenomenon that occurs in small populations. Here, there is a change in frequency of a gene or genes, due to a random event or chance when there is disappearance of certain part of population, or when reproduction does not take place.

The notion of genetic drift is that certain accidents can alter the frequency of some genes on a small population, even if they don’t provide any survival advantage.

The organs of different species, which are similar in structure, but are modified to perform different functions are called homologous organs.

For example Limbs of a human, frog, lizard and a bird.

The sex of a child in human beings in genetically determined. The humans have 23 pairs of chromosomes, out of which 22 are a perfect pair of one paternal and one maternal copy. The last pair of chromosomes also called the sex chromosomes, is different for males and females. Females have a pair of X chromosomes, whereas males have XY chromosomal pair. The inheritance of these sex chromosomes that determine the gender of an individual is as follows :

The organs of different species, which are similar in function, but have different structural framework are called analogous organs.

For example: wings of a bat, and wings of a bird.

In Mendel’s experiments, when pea plants with two characters such as colour of seed and shape of seed were bred together, plants with Yellow coloured Round seeds were crossed with plants having Green coloured wrinkled seeds. The F₁ progeny showed all plants with Yellow coloured Round seeds, showing that these were the dominant characters. However, when self-pollinated for second generation, certain progeny plants also exhibited new combinations like Yellow coloured Wrinkled seeds, or Green coloured Round seeds. This shows that both the characters were inherited independently.

There are two ways in which individuals with a particular trait can increase in a population.
They are :

1. Natural selection: Here the organisms which have a particular trait that enhances the survival rate or provides a survival benefit are selected naturally over others.
2. Genetic drift: This occurs in a small population where an accident or a chance event can change the frequency of genes, thus increasing a particular trait in a population, even if it provides no survival advantage.

No, geographical isolation will not be a factor in speciation of self-pollinated plants. Self – pollinating plants are those which have both the male and the female reproductive organs in one [d1] plant themselves. Thus, they don’t require any other plant for reproduction. Hence, geographical isolation of such species will not change its genetic constitution.

The relative closeness of two species can be determined using characters called the homologous characteristics. These are similar in different species, because of their inheritance from a common ancestor. For example, the limbs of human, frog, lizard and bird are similar in structure, though they perform different functions, suggesting their evolutionary relationship.

No. Organisms which reproduce asexually, do not require mating or a partner for reproductive process. The reproduction process requires a single parent, that is the organism itself. Thus, geographical isolation does not affect the speciation process.

The factors that can lead to rise of new species are:

1. Variations inherited from one generation to the other

2. Natural selection

3. Genetic drift

4. Genetic mutations

5. Geographical isolation

6. Environmental factors acting on geographically isolated species.

7. Reproductive isolation for a longer time.

The four ways in which individuals with a particular trait may increase in population are:

1. Natural selection

2. Genetic drift

3. Geographical isolation

4. Reproductive isolation or migration.

Sometimes when an organism dies, it’s body, or a part of it ends up in an environment which does not let it decompose. Thus, the impressions of body structures of these organisms are preserved. These preserved traces of organisms are called fossils.

Fossil dating helps in determining the order of appearance of organisms in the evolutionary process. There are two ways of fossil dating. They are:

1. Digging of fossils from Earth

2. By detecting the ratio of different isotopes of same element in fossil material.

The changes in germ cells of reproductive tissue of a sexually reproducing organism, which can be passed on to the next generation are called inherited trait. For example colour of the eye.

The changes in non -reproductive tissue of an organism, which can not be passed on to the progeny, because they do not involve any change in the DNA of germ cells, are called acquired traits. For example reduction in weight due to starvation. These are the traits acquired during lifetime of an individual and are not inherited because of lack of change in germ cells that participate in reproduction process.

Human beings though different in size, colour and looks, belong to the same species because:

1. They have similarities in the DNA sequences

2. They have descended from the same ancestor

3. They can reproduce among themselves.

(i) RRyy, rrYY, RrYy,

RRYY, RrYy, RrYy, RRyy, rrYY, rrYy, rryy.

(ii) Mendel carried out an experiment with two traits to know if the inheritance of one trait depended on or affected the inheritance of another trait.

(iii) In F₁ generation, the round and yellow traits appear, signifying that they are the dominant traits. In the second generation, new combinations of pea seeds, like round with green, and wrinkled with yellow appear, signifying that the two traits are inherited independently.

In human males, the 23^rd pair of chromosomes, or the sex chromosomal pair, is not perfectly paired, and contains one X and one Y chromosome. This combination determines the gender of the individual.

The Y chromosome is contributed by the father or the male partner during reproduction. It is a sex chromosome which pairs up with the X chromosome from mother, to provide maleness or male characters to the child.

In humans, the sex of the child is determined by the sex chromosomes inherited from parents. As females are XX, the mother can contribute only an X chromosome to the child. The males are XY and thus the father can contribute either an X or a Y chromosome. If an X chromosome is inherited from father, the child is a female.

Reduction in weight of organisms due to starvation is a change in non-reproductive tissues of the organism, and does not change the genes of the germ cells. Thus, it is not genetically controlled.

new species are formed due to change in genetic material of the reproductive tissues of the organism. Thus, changes in the DNA or chromosome number can lead to formation of new species.

The size of an individual’s body depends upon the lifestyle and not much on the genetic constitution. Hence, it is an acquired character.

Copies of same chromosome from the male and female gametes pair up to form a chromosomal pair or gene which combines the two versions of a trait from both the parents, so as to create a new version of the character in the child.

Genes are specific sequences of bases on a DNA molecule, located on a particular chromosome, and code for proteins. A chromosome has more than one genes.

In the F2 progeny, there is one pure tall, one pure dwarf and two mixed tall plants.

Radish and carrot are both roots. That is they are structurally similar. Thus they are homologous in nature.

The tendril of a pea plant and phylloclade of Opuntia are both stem joints. They have similar basic structure and thus are homologous.

The fossil digging is a direct method of determining the order of appearance and extinction of organisms on earth. The fossils found deeper are older than those found in the superficial layers of earth.

All the variations occurring in species don’t provide survival advantage or are due to natural selection. Some variations are lethal or may occur due to accident, not necessarily providing any survival benefit.

Any trait in an individual, which can be genetically inherited is contributed equally by the maternal and paternal DNA during reproduction, as a version of a trait is received from both, then forming a pair.

If such results are obtained from a cross, it is self-pollinated plant.

In heterozygous conditions such those of first progeny, only the dominant trait expresses itself in the phenotype. Thus, tallness is a dominant trait.

Fat molecules are not formed under genetic monitoring that is, they are not coded by genes.

Accumulation of variations during DNA copying mechanism over several generations leads to formation of new species. Sudden creation by nature, asexual reproduction or movement of individuals, do not affect the evolutionary process much.

The presence of feathers in dinosaurs and their development as an appendage in birds shows the evolutionary process.

False

There is only one pair of sex chromosome in an individual, either XX or XY type.

True

Sex is not genetically determined in snails and can be changed.

False

A child who inherits an X chromosome from father will be a girl.

False

A child who inherits a Y chromosome from father will be a boy.

True

The two factors that cause variations are errors in DNA copying and sexual reproduction

True

The change in frequency of certain genes brings about changes in a population. This is the essence of evolution.

True

Natural selection permits the adoption of certain changes that benefit the survival of organisms and thus directs evolution.

False

The experiences of an individual over lifetime are acquired characters and thus cannot be passed on to the progeny.

True

The preserved traces of all living organisms buried in an environment that does not allow decomposition are called fossils.

True

Relative method and carbon dating help in fossil dating, which helps to know about evolution.