NCERT Class 10 Science Chapter 9 – Hereditary and Evolution

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NCERT Class 10 Science Chapter 9: Hereditary and Evolution (Dr. Manishika) | English | CBSE

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Brain Teasers

Zygote with X Chromosome inherited from father would turn to

What is homologous and analogous

What is monohybrid and dihybrid

Recessive and dominant gene

P, F1 and F2 Generation

Why did Mendel choose pea plant for his experiments?

Tendril of a pea plant and phylloclade of Opuntia are homologous (radish and carrot)

According to the evolutionary theory, formation of a new species is generally due to accumulation of variations over several generations

Some dinosaurs had feathers although they could not fly but birds have feathers that help them to fly. In the context of evolution this means that birds have evolved from repitles

Accumulation of Variations

Reproduction – gives rise to new individuals (variations occur) both in sexual (more variations) and asexual reproduction

If one bacterium divides, and then the resultant two bacteria divide again, the four individual bacteria generated would be very similar. There would be only very minor differences between them, generated due to small inaccuracies in DNA copying

Sexual – more diversity with 1st generation, 2nd generation and so on

Bacteria that can withstand heat will survive better in a heat wave – selection of variants by environmental factors forms basis for evolutionary process


Inherited Traits – Free or Attached Earlobe

Mendel’s Contribution – sc and maths at Vienna University but failed and went to monastery and started growing peas - was the first one to keep count of individuals exhibiting a particular trait in each generation and come with laws of inheritance

Inheritance – both by paternal and maternal DNA

Peas criteria - round/wrinkled seeds, tall/short plants, white/violet flowers

Why pea was choosen? Easy to grow; Short life span; Easily distinguishable characters; Larger size of flower and Self pollinated

F1 (tall and short) – All Tall here (TTxtt gave tt) – only tallness is expressed

F2 (self pollination) – (TtxTt gave TT, Tt, Tt and tt) – 1/4th are short – ratio 1:2:1 (genotype) and 3:1 (phenotype)

Traits like ‘T’ are called dominant traits, while those that behave like ‘t’ are called recessive traits

Dihybrid Cross: Tallness and round seeds are thus dominant traits- 9:3:3:1 – expression of genes (phenotype)

1:2:1:2:4:2:1:2:1 (genotype cross)

Monhybrid and Dihybrid cross – Punnett Square

F1 generation: The first filial generation; offspring of the P generation.

F2 generation: The second filial generation; offspring of the F1 generation.

law of segregation: Mendel’s first law of inheritance; states that the two factors controlling a characteristics separate during gamete formation. These factors separate and go to different gametes when a parent reproduces.

P generation: The parent generation; parent plants in genetic crosses.

Mechanism of Inheritance

(i) Characters are controlled by genes.

(ii) Each gene controls one character

(iii) There may be two or more forms of the gene

(iv) One form may be dominant over the other

(v) Genes are present on chromosomes

(vi) An individual has two forms of the gene whether similar or dissimilar

(vii) The two forms separate at the time of gamete formation

(viii) The two forms are brought together in the zygote

Expression of Traits

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.

Plant height can thus depend on the amount of a particular plant hormone

If the gene for that enzyme has an alteration that makes the enzyme less efficient, the amount of hormone will be less, and the plant will be short. Thus, genes control characteristics, or traits.

If both parents can help determine the trait in the progeny, both parents must be contributing a copy of the same gene. This means that each pea plant must have two sets of all genes, one inherited from each parent. For this mechanism to work, each germ cell must have only one gene set.

Sex Determination

XX – Female

XY – Male

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

In other animals, such as snails, individuals can change sex, indicating that sex is not genetically determined.

Humans – sex is genetically determined (23 pairs of chromosomes – 22 pairs have both material and paternal copy but 1 pair is different). All children will inherit an X chromosome from their mother regardless of whether they are boys or girls. Thus, the sex of the children will be determined by what they inherit from their father.


Image of Evoluation

Image of Evoluation

Image of Evoluation

inbuilt tendency to variation during reproduction, both because of errors in DNA copying, and as a result of sexual reproduction

It is obvious that in both situations, what started out as a rare variation came to be a common characteristic in the population. In other words, the frequency of an inherited trait changed over generations. Since genes control traits, we can say that the frequency of certain genes in a population changed over generations.

Case 1 : Survival Advantage (natural selection) – color variations

Case 2: Accidental Survival (accidents in small populations can change the frequency of some genes in a population, even if they give no survival advantage) – lead to genetic drift (diversity without adaptation

Case 3: Food Availability – Adaptation (low weight is inherited trait)

Acquired or Inherited Traits

Change in non-reproductive tissues cannot be passed on to the DNA of the germ cells. Therefore the experiences of an individual during its lifetime cannot be passed on to its progeny, and cannot direct evolution

Tails of mouse removed by surgery – not tailless mouse in next generation due to no change in germ cells

Charles Darwin, who came up with the idea of evolution of species by natural selection in the nineteenth century

Darwin – 22 yrs old went to voyage (5 yr voyage to South America) - after he got back to England, he never left its shores again - evolution took place due to natural selection. We often associate Darwin solely with the theory of evolution. But he was an accomplished naturalist, and one of the studies he conducted was to do with the role of earthworms in soil fertility.

Characters that are passed on from parents to offspring are inherited characters e.g., colour of seeds, colour of eyes.

Characters appearing in an individual’s life time but cannot be transmitted to next generation are acquired characters e.g.,

obese body, loss of a finger in an accident.

Origin of Life

How Organic Molecules Arose?

Darwin’s theory of evolution tells us how life evolved from simple to more complex forms and Mendel’s experiments give us the mechanism for the inheritance of traits from one generation to the next. But neither tells us anything about how life began on earth in the first place.

J.B.S. Haldane, a British scientist (who became a citizen of India later), suggested in 1929 that life must have developed from the simple inorganic molecules which were present on earth soon after it was formed. The first primitive organisms would arise from further chemical synthesis.

Stanley L. Miller and Harold C. Urey in 1953. They assembled an atmosphere similar to that thought to exist on early earth (this had molecules like ammonia, methane and hydrogen sulphide, but no oxygen) over water. This was maintained at a temperature just below 100°C and sparks were passed through the mixture of gases to simulate lightning. At the end of a week, 15% of the carbon (from methane) had been converted to simple compounds of carbon including amino acids which make up protein molecules


Micro-evolution – small changes, simple changes are common characteristics of particular species

New Species – beetle split in 2 population that cannot interbred across mountain range

in this huge population of beetles, there will be sub-populations in neighbourhoods. Since male and female beetles have to meet for reproduction to happen, most reproduction will be within these sub-populations

beetle is picked up by a crow from one site and dropped in the other site without being eaten (genes migrate to large distance)

Level of gene flow decreases as distance increases

Over generations, genetic drift will accumulate different changes in each sub-population. Geographical isolation gradually leads to genetic drift.

One population crow is eliminated by eagle while in other many crow (beetle population would vary) -green variation will not be selected at the first site, while it will be strongly selected at the second.

Processes of genetic drift and natural selection will result in these two isolated sub-populations of beetles becoming more and more different from each other

New variation emerges in which green females will not mate with red males, but only with green males. This allows very strong natural selection for greenness

Speciation may take place when variation is combined with geographical isolation.

Evolution and Classification

Characteristics are details of appearance or behaviour; in other words, a particular form or a particular function

We have 4 limbs; plant do photosynthesis is characteristics

Cell has nucleus (bacterial cell do not have)

Common characteristics relate to common ancestry – brother and sister are closely related

Trace evolutionary relationship – mammals have 4 limbs similar to birds, reptiles and amphibians (basic structure is modified based on functions)- homologous characteristic helps to identify an evolutionary relationship between apparently different species

Design, structure and component differ - wings of bats are skin folds stretched mainly between elongated fingers. But the wings of birds are a feathery covering all along the arm. (both to fly) - analogous characteristics

Structures which have a common basic structure but perform different functions are called homologous structures

Fossils & Their Age

Usually, when organisms die, their bodies will decompose and be lost. But every once in a while, the body or at least some parts may be in an environment that does not let it decompose completely. If a dead insect gets caught in hot mud, for example, it will not decompose quickly, and the mud will eventually harden and retain the impression of the body parts of the insect. All such preserved traces of living organisms are called fossils

One is relative. Fossils we find closer to the surface are more recent than the fossils we find in deeper layers. The second way of dating fossils is by detecting the ratios of different isotopes of the same element in the fossil material.

Fossils represent mode of preservation, evolutionary trait and establish time period

Evolution by Stages

the eye – like the wing – seems to be a very popular adaptation. Insects have them, so does an octopus, and so do vertebrates. And the structure of the eye in each of these organisms is different

Feather as insulation for cold weather but later for flight

Dinosaurs – some had feathers

very dissimilar looking structures evolve from a common ancestral design. It is true that analysis of the organ structure

in fossils allows us to make estimates of how far back evolutionary relationships go.

Cabbage - Humans have for more than 2000 years, cultivated wild cabbage as a food plant, and generated different

vegetables from it by selection. This is, of course, artificial selection rather than natural selection. So some farmers have wanted to select for very short distances between leaves, and have bred the cabbage we eat. Some have wanted to select for arrested flower development, and have bred broccoli, or for sterile flowers, and have made the cauliflower. Some have selected for swollen parts, and come up with kohlrabi. Some have simply looked for slightly larger leaves, and come up with a leafy vegetable called kale. - changes in DNA during reproduction are the basic events in evolution. Comparing the DNA of different species should give us a direct estimate of how much the DNA has changed during the formation of these species

Molecular phylogeny: This approach is based on the idea that organisms which are more distantly related will accumulate a greater number of differences in their DNA

Evolution Not Equated by Progress

Multiple branches at every stage- So it is not as if one species is eliminated to give rise to a new one. A new species has emerged (but does not mean that old species will disappear)

It is just that natural selection and genetic drift have together led to the formation of a population that cannot reproduce with

the original one. So, for example, it is not true that human beings have evolved from chimpanzees. Rather, both human beings and chimpanzees have a common ancestor a long time ago. That common ancestor is likely to have been neither human or chimpanzee

Evolution is simply the generation of diversity and the shaping of the diversity by environmental selection

Evolution cannot be said to ‘progress’ from ‘lower’ forms to ‘higher’ forms. Rather, evolution seems to have given rise to more complex body designs even while the simpler body designs continue to flourish

Human Evolution

The same tools for tracing evolutionary relationships – excavating, time-dating and studying fossils, as well as determining DNA sequences – have been used for studying human evolution.


Skin Color

The earliest members of the human species, Homo sapiens can be traced back to our African roots

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