Here is a compilation of term papers on ‘Inheritance’ for class 9, 10, 11 and 12. Find paragraphs, long and short term papers on ‘Inheritance’ especially written for school and college students.
Term Paper on Inheritance
Term Paper Contents:
- Term Paper on Variation in Offspring
- Term Paper on the Chemical Structure of Chromosomes
- Term Paper on the Unit of Inheritance
- Term Paper on Genetic Inheritance
- Term Paper on Variation as a Result of Mutation
- Term Paper on Monohybrid Inheritance
- Term Paper on the Inheritance of Sex
- Term Paper on the Selection of Environment
- Term Paper on Genetic Engineering
1. Term Paper on Variation in Offspring:
Sexual reproduction leads to variation in the offspring, that is, each individual has different characteristics. No two offspring from the same parents, produced by sexual reproduction, are genetically identical. An exception occurs when the offspring develop from the same ovum and sperm, in which case they are ‘identical twins’.
Two types of variation are seen:
A. Continuous, and
A. Continuous Variation:
Continuous variation is the result of the interaction of two factors:
(i) The genes that are inherited by an individual
(ii) The effect of the environment on the individual.
The environmental factors involved might include:
(i) The availability and type of food (in animals)
(iii) The Climate:
a. Amount of sunlight
c. Amount of available water.
(iv) The ions present in the soil (in plants)
(v) Competition from other organisms in the environment.
In continuous variation, individuals show a range between the two extremes. Every possible form between the two extremes will exist.
Examples of continuous variation are:
(i) Body mass
(iii) Foot size.
B. Discontinuous Variation:
This is the result of inheritance only. There are few types, with no intermediates.
Examples of discontinuous variation are:
(i) Blood groups
(ii) The ability to roll the tongue into a ‘U’ shape (either you can or you cannot!).
Chromosomes, situated in the nuclei of all living cells (except bacteria, which have no true nucleus) are made of the chemical substance DNA. The DNA molecule, looking rather like a very long, twisted rope ladder, is made up of two strands (of alternating sugar and phosphate units) held together by pairs of chemical units called bases (the rungs of the ladder – see the upper section of the diagram below). The molecule is described as a double helix (Greek ‘helix’ = a spiral).
There are four bases only:
These bases link with one another in the following ways (‘The Rule of Base Pairing’)
3. Term Paper on the Unit of Inheritance:
All living organisms manufacture proteins in their cells. These are used either for structural purposes within the cell or as enzymes to control chemical processes in the cell. All proteins are made up of linked amino acids.
The sequence of bases (e.g. CATGCTAGCCTA) on one of the two strands is a code. When protein molecules are made in the cytoplasm of a cell, a copy of the bases on a section of a DNA strand is made and passed out into the cytoplasm of the cell. The sequence of bases is first split into triplets (CAT, GCT, AGC, and CTA). Each triplet is then responsible for lining up one particular amino acid in the sequence of amino acids that will link to form a protein. Each of the 22 amino acids has its own triplet.
Since the sequence of bases on DNA molecules is different for each (sexually produced) individual, it follows that no two individuals will make protein molecules with exactly the same sequence of amino acids. The length of chromosome which contains the bases necessary to make one protein molecule is otherwise known as a gene.
Definition of Gene:
A Gene is defined as a unit of inheritance.
For the purpose of understanding the mechanism of simple inheritance, it is convenient to imagine a chromosome as a string of beads, like that shown below, each bead represents one gene.
During cell division, genes are copied and these copies are passed on from parent to offspring via chromosomes in the nuclei of the parents’ gametes.
4. Term Paper on Genetic Inheritance:
Every member of the same species has the same number of chromosomes in each (healthy) cell of their body. These chromosomes exist in matching pairs. For example, human beings have 23 matching or homologous pairs of chromosomes, a total number of 46. Of each pair of matching chromosomes, one is inherited from a person’s mother and one is inherited from their father.
The genes of homologous chromosomes also match. In other words, if we look at two strings of beads, like those shown in Fig. 79, the order of the different shapes of beads is the same on both strings.
Matching genes on homologous chromosomes are called alleles.
You can see in Fig. 79 that a pair of beads (like the two ■ shown at position 1) always match in shape, but do not always match in colour. This is a way of showing that one pair of allele’s controls one character, but each allele may exist in two forms- they may be dominant or recessive.
In Fig. 79, the alleles in position 1 are both dominant, in position 2 they are both recessive and in position 3, there is one of each.
For a particular character, an offspring may therefore inherit either:
i. Two dominant alleles, one from each parent. The offspring is described as homozygous dominant.
ii. Two recessive alleles, one from each parent. The offspring is described as homozygous recessive.
iii. One dominant and one recessive allele. The offspring is described as heterozygous.
These are the three possible genotypes of the individual.
If at least one dominant allele is present in the genotype, the individual will show the dominant feature in their appearance (or phenotype). Thus the homozygous dominant and heterozygous genotypes will give the same phenotype. The homozygous recessive individual will have the alternative (or ‘contrasting’) phenotype.
5. Term Paper on Variation as a Result of Mutation:
Genes and chromosomes are always subject to change (or mutation) as a result of environmental forces acting upon them. These forces are known as mutagens, and include X-rays, atomic radiation, ultraviolet light and some chemicals. Exposure to higher doses of any of these mutagens will lead to a greater rate of mutation.
Definition of Mutation:
A mutation is a spontaneous change in the structure of a gene or chromosome.
Sickle-cell anaemia is an example of a condition caused by a gene mutation.
Both parents pass on a mutated (and recessive) allele for making haemoglobin in red blood cells. The homozygous recessive offspring cannot make effective haemoglobin, and cannot carry sufficient oxygen in their blood. Their red blood cells also take on a distorted shape. A person with this condition is likely to die at an early age.
Down’s syndrome is an example of a condition caused by a chromosome mutation.
As described above, there are 46 (23 pairs of) chromosomes in every normal cell of the human body; there are 23 unpaired chromosomes in each gamete. Forty-six is known as the diploid number and 23 as the haploid number.
If, in the production of gametes by one of the parents, one extra chromosome enters one of the gametes, then there will be 24 (instead of 23) chromosomes in that gamete. If this gamete is involved in the process of fertilisation, there will be 47 (instead of 46) chromosomes in the zygote. In older parents, there is a greater tendency for chromosome number 21 not to separate properly as gametes are being made.
A child who inherits the extra chromosome will suffer from Down’s syndrome. Their physical and mental development will be slow, and they will have a distinctive facial appearance.
6. Term Paper on Monohybrid Inheritance:
Organisms inherit alleles for thousands of different contrasting characters, for example, human hair is either curly or straight, and we either can or cannot smell the scent of certain flowers. Monohybrid inheritance refers to only one pair of contrasting characters, such as curly or straight hair, controlled in the individual by one pair of alleles.
There are two types of monohybrid inheritance:
A. With complete dominance, and
B. With co-dominance.
A. With Complete Dominance:
This is where the presence of only one dominant allele will decide the appearance (or phenotype) of the individual.
Example: Coat colour in mice.
In mice, brown coat colour is dominant over grey coat colour. In an experiment, a homozygous dominant (or ‘pure-breeding’) brown male mouse mated with a homozygous recessive (also pure-breeding) grey female mouse. All their offspring (that is, the F1 or first filial generation) were found to be brown.
The offspring of the F1 generation were then allowed to freely interbreed. It was found that their offspring (the F2 generation) were brown to grey in a 3: 1 ratio.
This can be explained in a genetic diagram, set out below:
Genetic diagrams are a way of looking at the combinations of alleles produced by two parents. In constructing genetic diagrams, the letters of the alphabet (rather than beads) are used to represent alleles. A dominant allele is represented by a capital letter (like A, B, C) and its recessive allele is represented by a small (or lower case) version of the same letter (like a, b, c).
Genetic diagram: cross between homozygous brown-coated mouse and homozygous grey-coated mouse:
Key to alleles:
i. ‘B’ represents the dominant allele for brown coat colour in mice,
ii. ‘b’ represents the recessive allele for grey coat colour in mice.
(Note: Statistically, there is an exactly equal chance of either of the alleles from the male combining with either of the alleles from female at the time of fertilisation.)
The results are given as a statistical ratio in a large sample. The smaller the sample, the less likely that the ratios will be the same as shown.
In humans, where only one offspring is likely to be produced at a time, the probability of that offspring inheriting a particular feature is often given. Probability is usually expressed as a percentage.
Example: Cystic fibrosis in humans
Cystic fibrosis is an inherited condition that affects the type of mucus found in people’s lungs. Most people produce normal protein in the mucus of their lungs. They possess at least one dominant allele, which may be called ‘F’. The homozygous recessive person, suffering from cystic fibrosis, has the genotype ‘ff’. Their lungs contain particularly thick and sticky mucus, which makes gaseous exchange difficult.
Genetic Diagram: Both Parents Heterozygous for Cystic Fibrosis:
In the diagram below, there are two parents who are both heterozygous for cystic fibrosis (their genotype is ‘Ff’). If they have a child, the probability of this child having the genotype ff, and therefore suffering from cystic fibrosis, is 25%.
The Test (or Back) Cross:
Of the brown mice in the F2 generation in the example given below, approximately one-third of them will be homozygous dominant (BB), and two- thirds will be heterozygous (Bb). There is no way of telling from their phenotype which type they are. Therefore, a test (or ‘back’) cross is performed.
In a test cross, the individual is mated with a homozygous recessive (bb) partner.
If the individual is heterozygous (Bb):
B. With Co-Dominance:
Sometimes both alleles have an equal effect on the phenotype of an individual. When this happens, the alleles are said to be co-dominant. The individual which is heterozygous will therefore show a third phenotype – different from the two homozygous possibilities.
Example: the inheritance of human blood groups:
This is also an example of monohybrid inheritance, but this time, there exist three possible alleles, only two of which are possessed by any one person.
The alleles are described as: IA, IB and Io.
Both IA and IB are dominant over Io, but are co-dominant to one another.
The following combinations of alleles are possible, resulting in the blood groups (phenotypes) shown:
An example of the inheritance of blood groups in humans can be shown in the following genetic diagram:
7. Term Paper on the Inheritance of Sex:
Whether a child is born male or female is determined at the moment of fertilisation.
Of the 23 pairs of chromosomes in a human nucleus, one pair is known as the sex chromosomes. In the female, the sex chromosomes are identical and are called ‘X’ chromosomes.
In the male, they are not identical. One of them is an ‘X’ chromosome, exactly like those in the female, but the other is a (shorter) ‘Y’ chromosome.
The sex chromosomes are:
XX for a female
XY for a male
The gametes contain 23 single chromosomes, and therefore only one of the two sex chromosomes that exist in normal body cells.
In females, all gametes contain an ‘X’ chromosome (she has no other type to give).
In males, 50% of the gametes contain an ‘X’ chromosome and 50% contain a ‘Y’ chromosome.
There is an exactly equal chance of the ‘X’ chromosomes in the ovum:
(i) Fusing with an ‘X’-carrying sperm to produce a daughter, or
(ii) Fusing with a ‘Y’-carrying sperm to produce a son.
8. Term Paper on the Selection of Environment:
Examples of the variation shown by members of a population in a given habitat include:
(i) The shade of colour of a leaf-eating insect
(ii) The sharpness of vision in a bird of prey
(iii) The speed at which a gazelle can run.
In all of these examples, the variation can have some effect on the success or even on the chances of survival of that organism in its environment:
(i) The leaf insect with better camouflage may escape the notice of a hungry predator.
(ii) The bird of prey with sharper vision is more likely to find a meal – particularly important when food is scarce.
(iii) The faster the gazelle can run, the more chance it has of escaping from a hungry lion.
All organisms are therefore in competition with other members of their species in that particular environment. The winners in that competition survive to reproduce. The losers provide food for predators, or fail to obtain enough food to remain alive.
It is the environment which ‘decides’ which organisms survive. The process is called natural selection.
Variation is controlled by the genes possessed by the organisms. Surviving individuals are able to hand on their advantages, via their genes, to the next generation.
The offspring of the survivors also show variation, so the process is repeated over many generations. For example, through natural selection, lions may improve their stealth and camouflage over several generations. As they do so, more species become threatened by the lion. Thus, ecosystems are constantly changing – and organisms may have to adapt to climatic change as well.
This gradual change by natural selection is known as evolution. During the process, a population of organisms may become separated and form two isolated branches. Each of the populations will adapt to different environmental changes, and new species may evolve.
In the process of artificial selection, humans – not the environment – perform the selecting. That is, people deliberately choose to breed organisms with particular characteristics.
Many crop plants and farm animals are the result of selective breeding programmes.
Some examples are:
i. Increased milk production in cows
ii. Increased meat production in farm animals
iii. Increased yield from cereals
iv. Increased disease resistance in many crops.
As a result, greater profits are made from greater quantities of better quality produce.
Selective breeding follows this procedure:
1. The individuals showing the quality required are selected.
2. Those individuals are used as breeding stock.
3. Only those offspring showing the desired quality to the greatest extent are selected.
4. These selected individuals are used for breeding.
5. This process is continued over many generations.
There is a danger, however, that this form of ‘inbreeding’ will increase the chances of two recessive alleles coming together. This may give rise to a genetically-controlled deformity (e.g. a heart defect).
9. Term Paper on Genetic Engineering:
Since we are now able to identify specific genes (i.e. lengths of DNA which encode for the production of a particular protein), that gene can then be isolated and inserted into another organism.
In this way, genes for disease resistance existing in a crop plant with low yield can be introduced into a crop plant with a high yield but low disease resistance. Where a person inherits a genetically-controlled condition (e.g. cystic fibrosis), it may be possible to improve their condition by the introduction of genes from a healthy person. In both these examples, gene transfer is between organisms of the same species.
Gene transfer between organisms of different species is commonly used in the production of the hormone insulin. In this example, a bacterium is used as the ‘host’ organism. The gene for insulin production is taken from a healthy person and (using enzymes) attached to the bacterium’s chromosome.
Every time the bacterium divides, it makes a further copy of the inserted gene. A culture of the bacteria will then not only be manufacturing all the proteins it needs for its own survival, it will also produce an ample supply of insulin molecules. Insulin is now commercially produced using such a process.
Public Concern over Genetic Engineering:
Genetic engineering can increase food production and help to control disease, but there is much concern over the possibility that bacteria with ‘introduced’ genes may be accidentally released.
Those which make a plant resistant to disease or to pesticides may find their way into pests. There is worry over the safety of foods which have been genetically engineered to improve their flavour and texture. ‘Watchdog’ committees have been set up to monitor gene transfer experiments.