Heredity and Evolution – CBSE Notes for Class 10 Science
1. Heredity : It refers to the transmission of characters or traits from the parents to their offspring. Heredity is the continuity of features from one generation to another which are present in fertilised egg or zygote. The zygote develops into an organism of a particular type only.
2. Genetics : It is the branch of biology which deals with heredity and variation. Genetics is to help our understanding of heredity by knowing how offspring inherit characteristics from their parents.
3. Variation : It means the differences in the characters or traits among the individuals of a species. Variations occur during reproduction both because of error in DNA copying and as a result of sexual reproduction. Variations contribute to evolution.
Causes of variations:
- Different combinations of genetic material.
- Some positive gene mutations.
- Interaction of genes with environmental changes (adaptations).
Importance of variations:
- It forms, the. basis of heredity.
- It causes adaptations due to which organism can easily adjust to its changing environment.
- Accumulation of variations forms the basis of evolution.
Variations are produced both in sexual and asexual reproduction but amount of variations produced in asexual reproduction are subtle (so little) that they are hardly noticeable as compared to variations caused due to sexual reproduction.
4. Genotype : The genetic constitution of an organism e.g., Genotype of human male is 44 + XY and
genotype of human female is 44 + XX
5. Phenotype : The appearance of the organism, i.e., the way in which genotype is expressed. Phenotype is the result of interaction of genes with the environment.
e.g., Red colour may be controlled by a pair of genes RR. Now if genotype is RR phenotype will be red only but if genotype is Rr then also phenotype will be red since R is a dominant gene.
6. Gene : It is the basic unit of inheritance by which characters are transferred from parents to their offspring. Gene consists of a specific length of DNA on a chromosome. A specific Segment of DNA that provides the information for one protein is called gene for that protein.
According to Mendel, both parents must contribute equally to the DNA of the progeny during sexual reproduction. As both parents determine the trait in the progeny, so both parents must be contributing a copy of the same gene.
7. Chromosomes : These are the long threads present in the nucleus of every cell. Chromosomes are made- up of DNA and protein. Each chromosome contains very long molecule of DNA.
Each gene set is present as separate independent pieces each called a chromosome. Each cell have two copies of each chromosome, one each from male and female parents. Every germ cell will take one chromosome from each pair and these may be of either maternal or paternal origin. When two germ cells combine, they will restore the normal number of chromosomes in the progeny, ensuring the stability of the DNA of the species. Such mechanism of inheritance is used by all sexually and asexually reproducing organisms.
8. Allele : It is a alternative form of a gene occupying the same position on a chromosome and affecting the same characteristic but in two alternative ways, e.g., the free and attached ear lobe are the alleles of ear lobe character.
Expressing allele of a gene :
- Homozygous dominant in capital letters, e.g., tallness(TT)
- Homozygous recessive in smalMetters, e.g., shortness or dwarfness (tt)
- Heterozygous (Tt)-lt will be called hybrid tall.
9. Dominant allele: An allele that affects the phenotype of an organism both in heterozygous and homozygous condition. It is denoted by a capital letter, e.g., tallness in pea plant is denoted by ‘T.
10. Recessive allele: An allele that affects the phenotype of the organism in absence of a dominant allele, i.e., in homozygous recessive individuals. It is denoted by a small alphabet, e.g., dwarfness in pea plant is denoted by’t’.
11. Homozygous: When both alleles of a particular gene are the same, e.g., TT
12. Heterozygous : When both alleles of a particular gene are different, e.g., Tt
13. Diploid : Cells or organism containing two sets of genes, e.g., human body cells. Diploid cells have genetic constitution of 2n.
14. Haploid : Cells or organism containing one set of genes, e.g., human reproductive cells (sperms and ova). Haploid cells have genetic constitution of n.
15. Monohybrid cross : A cross between two parents taking the alternative traits of one single character, e.g., A cross between tall and dwarf pea plants.
Monohybrid Ratio :
- In F1 generation : 100% hybrid
- In F2 generation : phenotypic ratio is 3 : 1 and genotypic ratio is 1 : 2 : 1
16. Dihybrid cross: A cross between two parents taking into consideration alternative traits of two different characters, e.g., A cross between two pea plants one having round, green seeds and the other having wrinkled, yellow seeds.
Dihybrid Ratio :
- F1 ratio is 100% Hybrid type.
- F2 ratio : Phenotypic is 9 : 3 : 3 : 1 and Genotypic . ratio is very complex.
17. Human Blood Groups: There are four types of blood groups A, B, AB or O. These are controlled by a gene which is denoted by symbols IA, IB and IO (sometimes also denoted as i). The genes IA and IB show no dominance over each other (they are codominant, i.e., both expresses themselves independently). But these both genes are dominant over the gene IO. Therefore, blood group of a person depends on the type of genes present, e.g., (i) Blood group A has the following gene types :
18. Determining sex of a newborn individual genetically:
(a) In human beings the sex of the individual is ” determined genetically.
(b) There are 23 pairs of chromosomes of which 22 are similar in male and female and are known as autosomes.
(c) The remaining one is sex chromosome which is XY in males and XX in females.
(d) Males produce two types of sperms X and Y, while female produces one type of egg X.
(e) If a X type of sperm fertilizers the egg then the sex of baby will be female (XX).
(f) If Y type of sperm fertilizers the egg then the sex of the baby will be male (XY).
19. Mendel’s experiment to show that traits may be dominant or recessive:
(a) Mendel conducted breeding experiments in garden pea.
(b) selected pure plant of a tall/short plant.
(c) produced first generation plants by crossing them.
(d) found that all plants were tall.
(e) produced second generation by self fertilization of hybrids.
(f) found that three quarter of the plants were tall and one quarter was short.
20. Homologous chromosomes: A pair of corresponding chromosomes of the same shape and size, one from each parent.
21. Autosomes and Sex chromosomes : The identical » chromosome pairs are called autosomes. The
chromosome pair which is different are called sex chromosomes. Humans have 23 pairs of chromosomes. 1-22 pairs are autosomes while 23rd pair (XX in females and XY in males) which are designated as X and Y are sex chromosomes.
22. Molecular Phylogeny: It is the study of evolutionary relationships by comparing DNA of different species.
23. Natural selection : Natural selection is one of the basic mechanisms of evolution, along with mutation, migration and genetic drift. Natural selection means the environmental conditions prevailing around an organism against which organism adapts itself, grows – and reproduces further. This leads to a change in the composition of genes within a population further causing evolution. Thus, it can be said that,
Natural selection results in adaptation in population to fit their environment better. Thus, natural selection direct evolution in the population of a particular species.
24. Fossils of the information which they provide regarding evolution: Fossils are the remains of ancient life forms, which got preserved somehow in the layers of earth, snow or oil.
Information given by fossils:
- They reveal that the life forms which existed earlier do not exist today
which indicate that the living forms are ever changing (evolving).
- They are used to guess the time when a particular organism existed on earth. It is done through carbon dating.
25. Genetic drift: The change in the frequency of some genes in a population which provides diversity without any survival advantage is called genetic drift.
26. The various ways in which individuals with a particular trait may increase in a population : Differences in population are responsible for the diversity such as, colour of eyes, hair, shape of ear lobes. This occurs due to : (i) Sexual reproduction (ii) Inaccuracies during DNA replication (iii) Due to environmental changes. This diversity will increase with time as these variations can be passed on only through DNA/genes during reproduction through reproductive tissue (germ cells or gametes).
(i) If these variations give survival advantage, then such traits are selected in nature and such traits increase in a population.
(ii) Due to genetic drift. This occurs due to geographical or reproductive isolation. It results in the change in gene frequency in a particular : population.
(iii) Migration which leads to gene flow in and out of the population.
(v) Mutation caused due to particular type of environment. ,
(v) Acquired traits due to particular type of environment.
27.Evidence of evolution : Errors in DNA copying (mutation) and sexual reproduction lead to variations which form the basis of evolution. Characteristics that
are common in different kinds of living organisms provide evidence in favour of evolution.
28.Evolution : Evolution can be defined as a naturally occurring slow, continuous and irreversible process of change. The gradual change of living organisms from pre-existing organisms since the beginning of life is called organic evolution. Whereas, gradual change in elements from one form to another with time is termed as inorganic evolution, i.
29.Inherited traits : are those traits which are passed from one generation to another through specific genes. Any change in DNA of the germ cells will be passed.
30. Acquired traits : are those traits which are acquired by the organism in its lifetime, e.g., removal of tail cannot change the genes of the germ cells of the mice thus cannot be passed to next generation.
31. Speciation : It means the origin of new species from the existing ones. It happens when different populations of the same species evolve along different lines.
How speciation occurs ?
- It occurs when two populations are isolated (both geographically and reproductively) leading to almost no gene flow between the two populations.
- Over generations, genetic drift will accumulate different changes in each sub-population.
- Natural selection may also operate differently in these different locations.
- Together natural selection and genetic drift will cause such changes (severe changes in the DNA) that these two groups will not be able to reproduce with each other even if they happen to meet.
- When DNA changes occur to larger extent, it may lead to change in the number of chromosomes or gene expression, eventually the germ cells of the two groups cannot fuse with each other. This leads to emergence of new species.
32. Estimating Age of Fossil: There are 2 methods :
- Relative method : On digging, the fossils which are closer to the surface are more recent than the fossils found in deeper layers.
- Dating fossils (carbon dating method): It is done by detecting the ratios of different isotopes of the same element (i.e., isotope of C-14 which is radioactive) in the fossil material.
33. Evolution by stages :
- Complex organs like eye has evolved from rudimentary organs, (e.g., rudimentary eye in flatworm might be useful enough to give only a fitness advantage and the structure of eye in different organisms is different indicating them to have different evolutionary origins) not by a single DNA change but created bit-by-bit over generations.
- A change that is useful for one property to start with can become useful later for quite a different
function (e.g., Feathers might start as providing insulation in cold weather. But later, they might be useful for flight. Some heavy birds and reptiles also have feathers but they do not fly.
- Some very dissimilar looking structures evolve from a common ancestral design, e.g., wild cabbage was cultivated as a food plant and many different vegetables were generated by selection over last two thousand years, (a) Selection of very small distances between the leaves gave rise to cabbage we eat. (b) Selection for arrested flower development gave rise to broccoli, (c) Selection for sterile flowers gave rise to cauliflower (d) Selection for swollen parts gave rise to kohlrabi.(e) Selection for larger leaves gave rise to leafy vegetable kale. It suggests that, if these selections were not done then there would have been only wild cabbage.
34. Homologous organs are organs having same origin and basic structure but they appear different and perform different functions in various organisms, e.g., (a) Fore limbs of horse and arms of man.
(b) Wings of birds and flippers of whale.
Similarities in basic structure of (homologous) organs in different organisms, indifferent groups indicate common ancestry.
35. Analogous organs are organs, which look similar because they perform same function, but they do not have same origin and basic structure.
e.g.,(a) Wings of birds and wings of insects. (b) Fins of fish and flippers of whale.