Ch. 9 Heredity and Evolution

Q. How does the mechanism of heredity work?

A. Cellular DNA is the information source for making proteins in the cell. A section of DNA that provides information for one protein is called the gene for that protein. Genes are the segments of DNA that are carried on the chromosomes and they determine specific characteristics.

Heredity is the passing of genes from one generation to the next. Genes hold the instructions for making protein products. As our cell duplicates, they pass this genetic information to the next cell. Genes, serve an instruction book for making functional molecules such as  RNA and proteins, which perform the chemical reactions in our bodies. It is the proteins that carry out the actual functions and controlling various characteristics. So genes produce proteins which in turn controls the characteristics.

Q. Explain the mechanism of sex determination in humans.

A. In human beings, the sex of the individual is genetically determined. Most human chromosomes have a maternal and a paternal copy, and we have 22 such pairs. But one pair, called the sex chromosomes, is odd in not always being a perfect pair. Women have a perfect pair of sex chromosomes, both called X, whereas men have a mismatched pair in which one is a normal-sized X while the other is a short one called Y. So women are XX, while men are XY. 

Q.  List a few factors that can affect the sex of an organism apart from genetic determination.

A. Different species use very different strategies for this. Some rely entirely on environmental conditions. Thus in some animals, the temperature at which fertilised eggs are kept determines whether the animals developing in the eggs will be male or female.

In snails, individuals can change sex, indicating that sex is not genetically determined.

Q. Differentiate between Homologous organs and Analogous organs.

A. 

Homologous Organs	Analogous Organs
Organs that have some basic structural design but different functions	Organs that have the different structural design but same function
Eg Forelimbs of a frog and bird	Eg Wings of a bird and bat

Q. What are fossils? What is their significance in the study of evolution?

A. Preserved traces of living organisms are called fossils

They help us know as to how features have evolved in the different group of organisms. Also, the organism that has evolved earlier can be traced.

Q. How can the estimation of the age of fossils be done?

A. It can be done in two ways:

i)The fossils found in upper layers are recent and the ones that are deeper are older

ii)  By radioactive carbon dating: detecting the ratios of different isotopes of the same element in the fossil material.

Q. What is artificial selection?

A. It is the process by which man selects traits useful to him for improving the qualities of domesticated plants and animals, thereby resulting in the formation of new species.

Q. What are the different ways in which individuals with a particular trait may increase in a population?

A. Different ways in which individuals with a particular trait may increase in a population are:

i) Genetic Drift: It is the change in frequency which occurs by chance in a small population

ii) Natural Selection:  The phenomenon in which nature selects the fittest organisms, species which would survive.

iii) Gene flow: It is the movement of alleles or genes between populations of same species by migration of individuals which can interbreed among themselves.

Q. Will geographical isolation be a major factor in the speciation of a self-pollinating plant species? Why or Why not?

A. No, geographical isolation will not be a major factor in the speciation of self-pollinating plant species. 

Geographical isolation is mainly due to:

・Natural selection

・ Gene flow

・ Genetic drift

In a self pollinating plant species, the plant receives pollen grains from the same flower, or another flower on the same plant and its distance from other plants hardly affect its reproduction. Moreover, self pollinated flowers rarely show variations in characters.

Q. Can the wings of a butterfly and wings of a bat be considered homologous organs? Why or Why not?

A. Homologous organs are the organs which have similar structure or origin but different function. Wings of a butterfly and wings of a bat both serve the same function in that they help in the flight of the animal. But, butterfly belongs to phylum Arthropoda whereas, bat belongs to class Mammalia, hence their origin is different. 

Thus they are analogous organs.

Q. In evolutionary terms, can we say which among bacteria, spiders, fish and chimpanzees have a 'better' body design?

A. ・Evolution is generation of diversity due to environmental selection. More and more complex body designs have emerged over time.

・Among bacteria, spiders, fish and chimpanzees we can say that the chimpanzees have a better body design.

・Chimpanzees are best adapted to survive the present day conditions and have a proper division of labour in their body i.e. they have different organs for performing different vital functions inside their body.

Q. Why are human beings who look so different from each other in terms of size, colour and looks said to belong to same species?

A. Human beings who look so different in terms of size, colour and looks said to belong to same species because they have similar DNA sequences and have descended from same ancestors. They belong to species homo sapiens. Also they are capable of reproducing among themselves. Their basic body design and structure are same.

Variations arise due to: 

・Natural selection

・Gene flow

・Genetic drift

・Mutations

・Environmental factors