The Class 9 is an important year in a student’s life and Maharashtra State Board Science is one of the subjects that require dedication, hard work, and practice. It’s a subject where you can score well if you are well-versed with the concepts, remember the important formulas and solving methods, and have done an ample amount of practice. Worry not! Home Revise is here to make your Class 9 journey even easier. It’s essential for students to have the right study material and notes to prepare for their board examinations, and through Home Revise, you can cover all the fundamental topics in the subject and the complete Maharashtra State Board Class 9 Science Book syllabus.
1. Explain Mendel’s monohybrid progeny with the help of any on cross.
Answer: Mendel’s monohybrid progeny is explained with the help of the cross between two pea plants with only one pair of contrasting characters. This type of cross is called a monohybrid cross. Now, to study the monohybrid ratio, consider the characteristic ‘plant height’ with a pair of contrasting characteristics: tall plant and dwarf plant. Get the overview of monohybrid cross from this.
2. Explain Mendel’s dihybrid ratio with the help of any one cross.
Answer: To explain Medel’s dihybrid ratio, two pairs of contrasting characteristics are under consideration. Mendel conducted additional experiments on hybridization, in which he considered more than one pair of contrasting characteristics. He brought about a cross between a pea plant that produces round and yellow coloured seeds and a pea plant with wrinkled and green coloured seeds. In this cross, since two pairs of contrasting characteristics were considered colour of seeds and shape of seeds, it is called a dihybrid cross. Mendel’s dihybrid ratio is explained here.
3. Distinguish between monohybrid and dihybrid cross.
Answer: These are two types of breeding methods used-monohybrid and dihybrid. In the first experiment, only a single character, that is the plant height was considered and was known as monohybrid inheritance. Another experiment was based on two characters (seed shape and colour), thus it was called dihybrid inheritance. Know more with Mendel’s law of inheritance or check out the differences with differences between monohybrid and dihybrid .
4. Is it right to avoid living with a person suffering from a genetic disorder?
Answer: This is not a communicable disease and does not spread to anyone who comes in contact with the person who has genetic disorder. Hence, there is no need to stay away from a person with genetic disorder. This disease transmits through reproduction only.
5. What is meant by ‘chromosome’? Explain its types.
Answer: The structure made up of nucleic acids and proteins in the nucleus of cells that carries the hereditary characteristics is the chromosome. The genes that contain information about the hereditary characteristics in coded forms can be found on chromosomes. Every species has a specific number of chromosomes and they are made of DNA. These dumb bell shaped chromosomes appear midst of cell division and there is a constricted region on each chromosome. This is called the ‘Primary constriction’ or ‘Centromere,’ which divides the chromosome into two parts. Each part is called an ‘arm’. The centromere has a specific position in each chromosome. Based on this, there are four types of chromosomes.
1. Metacentric : The centromere is exactly at the midpoint in this chromosome, thus making it look like the English letter ‘V’ with arms of equal length.
2. Sub-metacentric : The centromere is somewhere near the midpoint in this chromosome, so it looks like English letter ‘L’ with one arm slightly shorter than the other.
3. Acrocentric : The centromere is near one end of this chromosome and so it looks like the English letter ‘j’ with one arm much smaller than the other.
4. Telocentric: The controller is right at the end of this chromosome, making it look like the English letter ‘i’. This chromosome consists of only one arm.
6. Describe the structure of the DNA molecule.
Answer: Chromosomes are mainly made up of the acid DNA, discovered in 1869 by the Swiss biochemist, Frederick Miescher while studying white blood cells. The structure of the DNA molecule is the same in all organisms. As per the DNA molecule model, made in 1953 by Watson and Crick, two parallel threads of nucleotides are coiled around each other in an arrangement called a ‘double helix’. This structure can be compared with a coiled and flexible ladder. Find more about DNA structure
7. Explain the structure, function and types of RNA.
Answer: RNA, the second important nucleic acid of the cell is made up of ribose sugar, phosphate molecules and four types of nitrogenous bases adenine, guanine, cytosine and uracil. Meanwhile,the nucleotide i.e. the smallest unit of the chain of the RN molecule is formed by combining a ribose sugar, phosphate molecule and one of the nitrogenous bases. Large numbers of nucleotides are bonded together to form the macromolecule of RNA. Know more about the structure, function and types of RNA
8. Why is it necessary for people to have their blood examined before marriage?
Answer: Sickle-cell anaemia is a hereditary disease that occurs due to the alterations in genes during conception. In case, the father and mother are both affected by sickle-cell anaemia or if they are carriers of this disorder, their offspring are likely to suffer from this disease. Hence, people before marriage should have their blood examined before marriage to confirm if they are carriers or are suffering from sickle-cell anaemia or not.
9. Write a short note on down syndrome.
Answer : A disorder caused by chromosomal abnormality is called Down syndrome. Described at first as a chromosomal disorder in human beings, this is characterised by the presence of the 47 chromosomes. This is also called the trisomy of the 21st chromosome. Infants suffering from this disorder have one extra chromosome along with the 21st pair in every cell of their body. So, they have 47 chromosomes instead of 46. Children with Down’s syndrome are mentally retarted and have a short lifespan. One of the most prominent characteristics is the mental retardation. Added symptoms include the short wide neck, short fingers, flat nose, short height, scanty hair, horizontal crease on the palm and so on. Their life expectancy is short of about 16 to 20 years.
10. Write a very short note on Monogenic disorders.
Answer: Monogenic disorders are disorders or diseases caused as a result of mutation in any single gene into a defective on. Currently, we know of approximately 4000 different disorders of this type. As a result of the abnormal genes, their products are either manufactured in insufficient quantities or not at all, leading to abnormal metabolism that may cause death at a tender age. Examples of such disorders are galactosaemia, Hutchinson’s disease, phenylketonuria, Tay-Sachs disease, albinism, cystic fibrosis, haemophilia, night blindness, sickle cell anaemia and more.
11. Write a note on Sickle cell anaemia: symptoms and treatment.
Answer: Diseases or disorders can be caused by even a slight change in molecular structure of proteins and DNA. Regular haemoglobin has glutamic acid as the 6th amino acid in its molecular structure, which if replaced by valine, leads to a change in the shape/ structure of the haemoglobin molecule. This results in the erythrocytes or red blood corpuscles (RBC), modifying its shape from the usual biconcave to sickle shape. This condition is known as ‘sickle-cell anaemia’. In individuals suffering from sickle cell anaemia, the oxygen carrying capacity of haemoglobin is very low. This condition also causes clumping often leading to destruction of erythrocytes. This can result in obstruction of the blood vessels, thus damaging the circulatory system, brain, lungs, kidneys and so on. Sickle cell anaemia is a hereditary disease, caused by the changes in genes during conception. It is often seen that offspring of parents affected by sickle-cell anaemia or carriers of the disorder are likely to get the disease. For this reason, marriages between people who are carriers or having the illness are to be avoided. We have also mentioned here some symptoms of the disease. Symptoms of sickle-cell anaemia are pain in joints, frequent colds and cough, swelling of hands and legs, severe general body aches, constant low grade fever, exhaustion, pale face, low haemoglobin content and so on. This disease is transmitted via reproduction. Hence, the best way to treat it is to avoid marriage with another carrier or sufferer. Both the bride and groom are expected to take a blood test prior to marriage so as to avoid the spread of sickle cell anaemia. Another remedy or treatment is that sufferer or the career of this disease should take a tablet of folic acid on a daily basis.
12. How are the items given in the table under Column A, B and C related?
| A | B | C |
| Leber hereditary optic neuropathy | 44+XXY | Pale skin, white hairs |
| Diabetes | 45+X | Men are sterile |
| Albinism | Mitochondrial disorder | Women are sterile |
| Turner syndrome | Polygenic Syndrome | This disorder arises during development of zygote |
| Klinefelter syndrome | Monogenic disorder | Effect on blood-glucose level |
Answer:
| A | B | C |
| Leber hereditary optic neuropathy | Mitochondrial disorder | This disorder arises during development of zygote |
| Diabetes | Polygenic Syndrome | Effect on blood-glucose level |
| Albinism | Monogenic disorder | Pale skin, white hair |
| Turner syndrome | 45+X | Women are sterile |
| Klinefelter syndrome | 44+XXY | Men are sterile |
13. What is ‘genetics’?
Answer: Genetics is the branch of biology that studies the transfer of characteristics, and genes in particular of organisms from one generation to the next.
14. What is heredity?
Answer: Heredity is the transfer of characteristics from parents to offspring. Puppies are similar to dogs, squabs are similar to pigeons and infants are similar to humans because of heredity. Even if there are many similarities between parents and their offsprings there are some differences, as well. These similarities and differences are all the effect of heredity.
15. Illustrate the organization of chromosome and label it.
Answer:
16. What is the number of chromosomes found in the following organisms;
a) Crab
b) Maize
c) Frog
d) Roundworm
e) Potato
f) Human
Answer: The number of chromosomes found in each of these organisms are listed below:
a) Crab-200 chromosomes
b) Maize-20 chromosomes
c) Frog-26 chromosomes
d) Roundworm-04 chromosomes
e) Potato-48 chromosomes
f) Human-46 chromosomes
17. What are ‘homologous chromosomes’ or ‘heterologous chromosomes’?
Answer: In somatic cells, the chromosomes are usually found in pairs. If the pair contains chromosomes that are similar by shape and organization, they are termed ‘homologous chromosomes’ or in case they are dissimilar then they are known as ‘heterologous chromosomes’.
18. What are allosomes and autosomes?
Answer: One of the chromosome pairs found in the organisms that reproduce sexually, is different from all others. Chromosomes of different pair is known as ‘sex chromosomes’ or allosomes, whereas all other chromosomes are ‘autosomes’.
19. For what is DNA fingerprinting useful?
Answer: DNA fingerprinting helps to identify the sequence of the genes in the DNA of a person i.e. the genome of the person. The process is also useful to determine the lineage and identify criminals, because it is unique to every person.
20. Draw the structure of DNA.
Answer:
21. Explain the ‘Human Genome Project.’
Answer: The ‘Human Genome Project’ was undertaken together by all the geneticists of the world, in 1990. Later, in June 2000, a private industry in the USA, Celera Genomics Corporation and scientists of this project collectively announced the discovery of the complete DNA sequence of the human genome. According to the findings of this project, scientists have confirmed that the number of genes in the human genome is about 20,000 to 30,000. They also discovered the genomic sequence of many microorganisms. As a result of this research in genomics, disease causing genes were also identified, which helped the genetic diseases to be diagnosed and treated properly.
22. According to function, there are three types of RNA. Which are they?
Answer: According to function, there are three types of RNA.
1. Ribosomal RNA (rRNA) is the molecule of RNA that is a component of the ribosome organelle. Ribosomes take over the function of protein synthesis.
2. Messenger RNA (mRNA) is the RNA molecule that carries the information of protein synthesis from genes, that is DNA chain in the cell nucleus to ribosomes in the cytoplasm that produce the proteins.
3. Transfer RNA (tRNA) is the RNA molecule that, according to the message of the mRNA, carries the amino acid up to the ribosomes.
23. Name the four types of nitrogenous bases found in a DNA?
Answer: There are four types of nitrogenous bases in a DNA.They are adenine, guanine, cytosine and thymine
24. If both the parents make equal contribution to inheritance of characteristics, which characteristics will appear in the progeny?
Answer: According to Mendel, the factors responsible for inheritance of characteristics are present in pairs, also referred to as genes. If both the parents make equal contribution to inheritance of characteristics, the dominant characteristics make their presence known in the next generation, while recessive characteristics do not appear in the next generation at all.
25. What is Klinefelter syndrome (44+XXY)?
Answer: Abnormalities in sex chromosomes causes this disorder in men. Men suffering from this disorder have one extra X chromosome and so their chromosomal condition becomes 44+XXY. Such men are expected to be sexually sterile as their reproductive organs are not well developed.
26. What is the main objective of the National Health Mission?
Answer: The main objectives of the National Health Mission is to strengthen the health facilities of rural and urban areas, control various diseases and illnesses, increase public awareness about health, and offer financial assistance to patients via several schemes.
27. What is albinism?
Answer: Albinism is a genetic disorder, in which our eyes, skin and hair do not have colour, as a result of the absence of brown pigment, melanin. In this disease the body is unable to generate melanin. Hence, the skin becomes pale, hair is white and eyes are usually pink, due to the absence of melanin pigment in the retina and sclera.
28. Which are the two types of persons affected by sickle-cell anaemia?
Answer: There are two types of persons affected by sickle-cell anaemia. They are:
1. Sickle-cell anaemia carrier (AS)
2. Sufferer from sickle-cell anaemia (SS)
29. How is sickle cell anaemia diagnosed?
Answer: As part of the National Health Mission scheme, the ‘Solubility Test’ that is used to diagnose sickle-cell anaemia is available at all district hospitals. Similarly, the confirmatory diagnostic test- ‘Electrophoresis’ is performed at rural and sub-district hospitals.
30. What are the harmful effects of smoking and tobacco?
Answer: Tobacco, that is consumed by smoking or by chewing can cause cancer. Also, smoking of cigarettes and bidi adversely affects the process of digestion and results in a burning sensation in the throat and a cough. Excessive smoking also causes instability and trembling of fingers. The nicotine found in the tobacco has harmful effects. It affects the central and peripheral nervous system and the arteries become hard, causing arteriosclerosis and hypertension. Meanwhile, tobacco smoke also consists of harmful chemicals such as ammonia, carbon monoxide, pyridine, aldehyde furfural, nicotine, sulphur dioxide and more. This results in uncontrolled cell division. Tobacco smoke is full of minute carbon particles that causes the normal tissues of the lung to transform into thickened black tissue, which causes cancer. Also, while chewing tobacco or other tobacco products, a large section of the extract is absorbed into the body. Excessive tobacco consumption could also result in cancer of lips or tongue, visual disorders or tremors and so on.