Class 10 Science Heredity Exam Notes

Class 10 Science Heredity and Evolution Exam Notes. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

Class 10 Science: Heredity and Evolution Exam Notes

INTRODUCTION

Variation is the degree of difference found in morphological, physiological and other traits among individuals belonging to the same family, race and species.

Most variations are due to mutations caused by errors in DNA replication.

ACCUMULATION OF VARIATIONS DURING REPRODUCTION

Variations appear during reproduction, whether organisms are multiplying asexually or sexually. Each generation provides the next generation with a common basic body design and some subtle changes or variations.

The variations accumulate and are passed on to more and more individuals with each generation. The diversity is small in asexually reproducing organisms, as it is caused only by errors of the DNA copying mechanism.

SIGNIFICANCE OF VARIATIONS

Preadaptations: Some of the neutral variations function as preadaptations to a changing environment. For example, a heat wave may kill most bacteria except a few which have a preadaptation or variation to tolerate high temperature.

Evolution: Variations are the raw material for evolutionary processes. Picking up suitable variants by nature or the environment forms the basis of evolution.

Struggle for existence: Useful variations provide an advantage to the individuals in the struggle for existence and hence survival in nature.

Individuality: Variations provide a distinct identity to each and every individual.

Artificial selection: Picking up of certain variations by breeders has resulted in the development of a number of breeds and varieties of domesticated plants and animals.

HEREDITY

Heredity (L. hereditas – heirship or inheritance) is the transmission of genetic characters from parents to offspring, or from one generation to the next.

Inherited traits: These are the traits which are controlled by the genetic material of the individual and are obtained from the parents in inheritance.

MENDELISM

• Mendel was a monk in Austria.
• Mendel performed a number of experiments on the garden pea plant (Pisum sativum).

Mendel's laws: The theoretical explanation of Mendel's results is now firmly established as Mendel's laws of inheritance. These are as follows:

1. Law of Dominance: According to this law, if a cross is made between plants with a contrasting pair of characters, the character that appears in the first generation is dominant and the other is recessive.

Parents	Red flowered (RR) — dominant × White flowered (rr) — recessive
F1 generation	All Red (Rr)

Mendel's monohybrid cross between a homozygous red-flowered (RR) and a homozygous white-flowered (rr) plant shows that red colour is dominant over white (recessive).

2. Law of Segregation (or law of purity of gametes): Both parental alleles (i.e. dominant as well as recessive) segregate or separate and are expressed phenotypically in the F₂ generation. This is called Mendel's law of segregation.

Explanatory hypothesis: RR (red) × rr (white). All gametes of the red parent carry R and all gametes of the white parent carry r, so the F₁ generation is 100% red (Rr). On self-pollination of F₁, the alleles segregate and the gametes (egg cells and pollen grains) are ½ R and ½ r:

Egg cells ↓ / Pollen grains →	R (½)	r (½)
R (½)	RR	Rr
r (½)	rR	rr

F₂ generation: 75% red and 25% white (phenotypic ratio 3 : 1; genotypic ratio 1 RR : 2 Rr : 1 rr).

3. Law of Independent Assortment: This law states that most of the characters of the parents can appear in any combination in their offspring.

P generation	YYRR (yellow, round) × yyrr (green, wrinkled)
Gametes	YR and yr
F1 generation	All round yellow (YyRr)
F1 gametes	YR, Yr, yR, yr

F₂ generation (self-pollination of F₁):

Gametes	YR	Yr	yR	yr
YR	YYRR Round yellow	YYRr Round yellow	YyRR Round yellow	YyRr Round yellow
Yr	YYRr Round yellow	YYrr Wrinkled yellow	YyRr Round yellow	Yyrr Wrinkled yellow
yR	YyRR Round yellow	YyRr Round yellow	yyRR Round green	yyRr Round green
yr	YyRr Round yellow	Yyrr Wrinkled yellow	yyRr Round green	yyrr Wrinkled green

Result of the dihybrid cross between pea plants having round yellow seeds and wrinkled green seeds. Phenotypic ratio: 9 round yellow : 3 wrinkled yellow : 3 round green : 1 wrinkled green.

SEX DETERMINATION

It is the process by which the sex of a person is determined. Genetics is involved in the determination of the sex of a person, which is explained as follows:

• A male has one X chromosome and one Y chromosome, i.e., half of the male gametes (sperms) will have the X chromosome and the other half will have the Y chromosome.
• A female has two X chromosomes, i.e., all the female gametes (ova) will have only the X chromosome.
• The sex of a child depends on what happens at fertilisation.
• If a sperm carrying an X chromosome fertilises an ovum (which carries an X chromosome), the child born will be a girl.
• If a sperm carrying a Y chromosome fertilises an ovum (which carries an X chromosome), the child born will be a boy.
• Thus, the sperm determines the sex of the child.
• Sex determination is also controlled by environmental factors in some animals. For example, in some reptiles like the turtle, a high incubation temperature leads to the development of female offspring, while in the lizard a high incubation temperature results in male offspring.

Mother's ova ↓ / Father's sperms →	X	Y
X	XX (female child, girl)	XY (male child, boy)

Determination of sex in humans.

EVOLUTION

Evolution (L. evolvere – to unfold) or organic evolution (Spencer, 1852) is the unfolding of nature wherein newer types of organisms develop from pre-existing ones through modification.

Acquired traits: Acquired traits are those variations which an individual develops during its lifetime due to the effect of environmental factors, use and disuse of organs and conscious efforts.

Theory of Inheritance of Acquired Characters (Lamarckism): It is the first theory of evolution, proposed by the French biologist Jean Baptiste de Lamarck. It is based on the following:

1. Internal vital force: There is a vital force in organisms which tends to change them, generally making them larger and more complex.
2. Environment: A change in environment brings about direct changes in plants. It produces new needs in animals.
3. New needs (doctrine of desires): New needs bring about new desires that result in changes of older organs and formation of new organs.
4. Use and disuse of organs: Repeated use of an organ makes it more complex and efficient. Non-use of an organ brings about its degeneration.
5. Inheritance of acquired characters: The traits acquired by an individual during its lifetime are passed on to the next generation. After several generations the accumulation of changes results in the formation of new species.

Inherited traits: Inherited traits are those characteristics which are passed from parents to their offspring, generation after generation, because they are controlled by genes. There is also reshuffling of inherited variations during gametogenesis and fertilisation.

SPECIATION

It is the formation of new species from an existing one due to reproductive isolation of a section of its population. Development of reproductive isolation is basic to the formation of new species. This can occur by the following methods:

• Physical barrier: A physical barrier like a valley, mountain, water body, etc. develops between two populations of a species.
• Two ends of a long range: Subpopulations at the two ends of a long range seldom interbreed.
• Mutations: A large mutation can make some members reproductively isolated from the rest.
• Genetic drift (Wright effect): It is a random change in gene frequency that occurs in a small population due to fixation of certain alleles and elimination of others.

EVIDENCE OF EVOLUTION

Homologous organs: These organs are similar in structure but dissimilar in function. Homologous organs are found in forms showing adaptive radiation from a common ancestor, so they give evidence of ‘divergent evolution’. Examples: forelimbs of mammals, a bat's wing, a cat's paw. In plants, homologous organs may be a thorn or a tendril, as they arise in the axillary position. If homology is seen at the molecular level, it is called molecular homology.

Homology in Endoskeleton of Forelimbs of Some Vertebrates

Labelled bones: Humerus, Radius, Ulna, Carpals, Metacarpals, Phalanges. Forelimbs shown: Seal flipper, Bird wing, Bat wing, Horse forelimb, Human arm.

Bone	Seal flipper	Bird wing	Bat wing	Horse forelimb	Human arm
Humerus	present	present	present	present	present
Radius and Ulna	present	present	present	present	present
Carpals	present	present	present	present	present
Metacarpals	present	present	present	present	present
Phalanges	present	present	present	present	present

Analogous organs: These organs perform similar functions but are quite different in origin and development. Analogous organs are also called homoplastic organs. Analogous organs like the wings of insects, birds and bats illustrate ‘convergent evolution’. Example: the wings of birds and the wings of insects show the same function but their origin and development are different.

Vestigial organs: Vestigial organs are useless organs found in the body. They are present in reduced form. Examples: wings of kiwi, limbs of snakes, the nictitating membrane of the eye, body hair and the muscles that move the ears. During the period of evolution, vestigial organs have become functionless.

Fossil evidences: Fossils are the remains of organisms that lived in the ancient period. We can determine a fossil's age from the age of the rock by using radioactive material. The histological time scale is a chronological order of evolutionary history based upon the study of fossils. The study of fossils is known as palaeontology.

Petrification: Hard parts are preserved in this type of fossil.

Archaeopteryx: A fossil bird which shows both reptilian and avian features. It had wings like birds, thecodont dentition and a tail like reptiles. Hence, Archaeopteryx is a connecting link between reptiles and birds.

Connecting links:

Organism	Connects
Peripatus	Arthropods and annelids
Neopilina	Annelids and molluscs
Protopterus	Amphibians and fishes
Viruses	Living and non-living
Euglena	Plants and animals
Platypus	Reptiles and mammals

Embryological evidences: The study of embryos of various organisms reveals similarity in the early stages of embryo development, and this suggests that these organisms have evolved from common ancestors. Example: embryos of fish, turtle, bird, pig and man show similarity during embryo development.

Developmental stages of five vertebrates

Stage	Fish	Turtle	Bird	Pig	Man
Early stage	embryo with curved body, large head, tail	similar curved embryo	similar curved embryo	similar curved embryo	similar curved embryo
Middle stage	elongated, tadpole-like embryo	embryo with head, eye and tail bud	similar	similar	similar
Late stage	fish form	turtle form	bird form	pig form	human form

The early stages of all five vertebrates look very similar.

Signification Of  Variations