CONCEPTS
1. The similarities between parents and their offspring are accounted for by ‘heredity’, and the dissimilarities by ‘variation’.
2. Both heredity and variation are accounted for when we study ‘genetics’.
3. Asexual reproduction tends to produce ‘clones’, i.e., organisms which are genetically identical.
4. Chances of variations are higher in organisms showing sexual reproduction.
5. Inheritance from the previous generation provides a common basic body design, as well as subtle changes in it, for the next generation.
6. Depending on the nature of variations, different individuals would have different kinds of advantages, like bacteria that can withstand heat will survive better in a heat wave.
7. The rules of heredity determine the process by which traits and characteristics are reliably inherited.
8. The differences or dissimilarities between parents and children as individuals of a species are called variations.
9. Hereditary variations refer to the differences which are inherited by the progeny from their parents.
10. The inheritance of such heritable variations is determined by the genetic constitution of an individual.
11. Examples of inheritable variations are blood type, skin colour, hair colour, height, etc.
12. Environmental variations are caused due to differences in light, water, soil, nutrients and other factors of environment.
13. Causes of variation are —
(i) reshuffling of genes during meiosis,
(ii) errors in DNA copying,
(iii) environment.
14. Gregor Johann Mendel is considered the ‘father of genetics’. Mendel attributed contrasting ‘factors’ coming from the parents and their random combinations, as the cause of ‘variation’.
15. Mendel's ‘factors’ were termed ‘genes’ by Johannsen.
16. It was observed that the paired condition of the ‘factors’ is present in the diploid state. But when haploid gametes are formed, the factors ‘segregate’. Since chromosomes are paired in its diploid state but separate during gametic meiosis, it is fair enough to judge that the ‘factors’ are placed on the ‘chromosomes’. Today it is known that ‘genes’ are parts of a chromosome molecules.
17. Chromosomes are made up of DNA. DNA molecule consists of two polynucleotide strands forming a double helix. DNA is the genetic material throughout the living world except in a few viruses where RNA serves as the genetic material (e.g., HIV).
18. Chromosomes in each of the 22 pairs are similar in size and shape in both man and woman. The chromosomes of the 23rd pair in man are different from the 23rd pair in woman. These are the sex chromosomes. The larger chromosomes are called the X-chromosomes, the smaller chromosome is called the Y-chromosome.
19. Mendel represented the inheritance of characters as follows :
20. Mendel's Dihybrid Cross
22. Eukaryotic chromosomes have basic proteins mixed with DNA and are usually more complex in structure than prokaryotic chromosomes. A gene is the functional unit of DNA that serves as hereditary unit.
23. Each human nucleus has 23 pairs of chromosomes out of which 22 are same in male and female and are known as ‘autosomes’. The 23rd pair is different in male and female and are known as ‘sex chromosomes’. Females have ‘XX’ sex chromosomes and males have sex chromosomes ‘XY’.
24. Evolution is a set of changes brought about by the forces of nature selection on the variation within a population, so that resultant organisms are better adapted to their surroundings.
25. Evidences of evolution can be summarised as follows :
A. Paleontological or fossil evidence Fossils are the remains or impressions of organisms of the past. Archaeopteryx is a fossil that had both reptilian and avian features, proving that birds have evolved from the reptiles. B. Morphological and anatomical evidences
(i) Homologous organs : Organs which are similar in structure and origin but different in function and appearance are called homologous organs, e.g., forearm of man, wing of a bat, flipper of a seal, etc.
(ii) Analogous organs : Organs which are similar in appearance and function but different in structure and origin are known as ‘analogous organs’, e.g., wings of a bird and the wings of a butterfly.
26. The most accepted theory of evolution comes from Charles Darwin, whose theory is known as ‘Origin of Species by Natural Selection’. Its tenets are as follows :
(a) Organisms have a far higher capacity to reproduce that can be sustained by nature, thereby creating ‘competition’ or ‘struggle for existence’.
(b) Offspring vary from one another and from the parents.
(c) Those with ‘favourable variations’ are fit for the ‘Struggle for Existence’.
(d) The favourable variations are accumulated over a long time period creating a new species. The weak point of Darwinism is that it could not explain how variations arise.
27. The latest theory of evolution is the ‘synthetic theory of evolution’ which is a mixture of genetic variation and natural selection.
28. Certain fossils show features which are intermediate between two groups. Such fossils form the connecting links and clearly demonstate stages during the evolutionary process.
29. A very well known example of an extinct animal is dinosaur that existed long ago in large numbers.
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