INTEXT QUESTIONS 25.1
1. What is meant by the terms (1) Heredity and (2) Variation.
Answer: (1) Heredity is the process by which characteristics or traits are passed from parents to their offspring through genes. For example, eye colour, height, and hair type are inherited traits.
(2) Variation means the differences in characteristics found among individuals of the same species. These differences make each individual unique. For example, differences in height, skin colour, or shape of leaves in plants.
2. Why is Mendel considered as the founder or father of genetics?
Answer: Mendel is considered the founder or father of genetics because he carried out experiments on pea plants and explained how traits are passed from parents to offspring. He proposed the basic laws of inheritance and showed that characteristics are inherited through factors (now called genes). His work became the foundation of genetics.
3. Formulate a sentence to demonstrate your understanding of the terms ‘dominant’ and ‘recessive.’
Answer: A dominant trait is expressed in an organism even when only one copy of its gene is present, whereas a recessive trait is expressed only when both copies of the gene are recessive.
Example: In pea plants, tallness is a dominant trait, while dwarfness is a recessive trait.
4. Name the scientist who discovered that Mendelian factors are present on chromosomes.
Answer: The scientist who discovered that Mendelian factors (genes) are present on chromosomes was Walter Sutton.
5. Give the synonym for Mendelian factor.
Answer: The synonym for Mendelian factor is Gene.
INTEXT QUESTIONS 25.2
1. Name the sex chromosomes.
Answer: The sex chromosomes are X chromosome and Y chromosome.
2. How many autosomes do humans have?
Answer: Humans have 44 autosomes, arranged in 22 pairs.
3. Why does an organism (except bacteria) have diploid number of chromosomes?
Answer: An organism (except bacteria) has a diploid number of chromosomes (2n) because it receives one set of chromosomes from the mother and one set from the father during fertilisation. Thus, chromosomes occur in pairs and maintain the fixed chromosome number of the species.
4. State any two typical features of chromosomes.
Answer: Any two typical features of chromosomes are:
(i) present in pairs, (ii) seen only during cell division, (iii) present in fixed number etc.
5. Define a gene with respect to its chemical nature.
Answer: Genes are segments of a DNA molecule. So it is made of Deoxyribonucleic acid or DNA.
6. State the three major steps in DNA replication.
Answer: The three major steps in DNA replication are:
(a) The double stranded DNA molecule unwinds with the help of certain enzymes to expose two strands of DNA.
(b) A DNA polymerase enzyme catalyzes the formation of a new daughter strand which can form a double helix with one strand of parental DNA molecule. So two DNA molecules, each with a parental strand and a new strand get generated.
(c) The two identical DNA molecules then become two chromatids which remain attached by a centromere.
7. Why is DNA fingerprinting a fool proof test?
Answer: DNA fingerprinting is considered a foolproof test because the DNA pattern of every individual is unique (except identical twins). Therefore, it helps in accurate identification of a person and establishing biological relationships.
INTEXT QUESTIONS 25.3
1. What is a gene made of ?
Answer: A gene is made of DNA (Deoxyribonucleic Acid).
2. To which blood group would a person having genes IAi belong?
Answer: A person having genes Iᴬi belongs to Blood Group A.
3. If a Y bearing sperm fuses with an egg, what will be the sex of the individual developing from the zygote?
Answer: If a Y-bearing sperm fuses with an egg (which always carries an X chromosome), the resulting zygote will have XY chromosomes. Therefore, the developing individual will be male (boy).
4. How many X chromosomes can be found in the cells of the body of (i) a boy, and (ii) a girl.
Answer: (i) For boy: Autosomes and one X and one Y chromosomes
(ii) For girl: 44 autosomes and 2 X chromosomes
5. How many molecules of DNA are present in one chromosome?
Answer: One chromosome contains one DNA molecule.
INTEXT QUESTIONS 25.4
1. What will be the blood group of an individual with genetic combination IA IB?
Answer: An individual with the genetic combination IᴬIᴮ will have Blood Group AB.
2. How can a person be normal for a trait even when carrying one defective gene for that trait?
Answer: A person can be normal for a trait even when carrying one defective gene because the normal gene is dominant and masks the effect of the defective recessive gene. Such a person is called a carrier of the defective gene.
3. Which is the safest blood group for donation if an accident victim of an unknown blood group has to be given immediate blood transfusion?
Answer: The safest blood group for donation in an emergency when the recipient's blood group is unknown is O negative (O⁻).
4. On which kind of chromosome, the autosomes or the sex chromosomes, are defective genes causing, Thallasemia, colour blindness and Haemophilia located?
Answer: (a) Thalassemia: defective genes are located on autosomes.
(b) Colour blindness: defective genes are located on sex chromosomes (X chromosome).
(c) Haemophilia: defective genes are located on sex chromosomes (X chromosome).
5. Name the therapy in which a defective gene is substituted by a normal gene.
Answer: The therapy in which a defective gene is substituted by a normal gene is called Gene replacement therapy .
6. The given box diagram represents the ratio of females to males or the sex ratio in our country for 10 decades (1901 to 2001). Answer the following questions in the light of your knowledge of sex determination and the data presented in the box diagram.
• What does the bar diagram show? _________________
• As per scientific knowledge regarding sex determination, what should be the sex ratio or the male to female ratio at a given point of time. _____________
• Assign one reason to the trend showing deviation from the expected sex ratio.
___________________________________________________________
• In what ways is such a trend unfavourable? ________________________
• Suggest a way by which such a trend can be stopped. ___________________
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Do you notice any reversal in the trend? What would you attribute it to?
Answer: Bar diagram shows the proportion of females in the population over a decade
• 1:1
• Female foeticide;
• Male to female ratio becomes lopsided
• Banning sex tests of unborn baby; increasing awareness
• Yes, awareness and education/ No- with reasons.
TERMINAL EXERCISES
1. Which statement is true for ‘genes’? Select the correct answer
(a) Genes are imaginary factors.
(b) Genes are fragments of DNA.
(c) Genes are present in the cytoplasm.
(d) Genes are not inherited.
Answer: (b) Genes are fragments of DNA.
[ Genes are specific segments (fragments) of DNA (Deoxyribonucleic Acid) that carry hereditary information from parents to offspring. They control inherited characteristics such as eye colour, height, and many body functions.]
2. What are “factors” named by Mendel called today?
Answer: Mendel’s “factors” are now known as genes, which are units of heredity that control inherited characteristics.
3. What is the chemical nature of a gene? Name the three components of this chemical.
Answer: The chemical nature of a gene is DNA (Deoxyribonucleic Acid).
DNA is made up of units called nucleotides and each nucleotide has three components:
(i) A nitrogenous base
(ii) A deoxyribose sugar
(iii) A phosphate group .
4. Where are genes located?
Answer: Genes are located on chromosomes inside the nucleus of the cell. They are arranged in a linear order on chromosomes and carry hereditary information from parents to offspring.
5. State two differences between autosomes and sex chromosomes.
Answer: The differences between autosomes and sex chromosomes:
|
Autosomes |
Sex Chromosomes |
|
Autosomes control body (somatic) characters. |
Sex chromosomes determine the sex of an individual and related traits. |
|
They are same in both males and females. |
They are different in males and females (for example, XX in females and XY in males). |
6. Define heredity, variation, genetic disorder and sex chromosomes.
Answer: 1. Heredity: Heredity is the process by which characters or traits are passed from parents to offspring through genes.
2. Variation: Variation refers to the differences in characteristics among individuals of the same species.
3. Genetic Disorder: A genetic disorder is a disease or abnormal condition caused by changes or defects in genes or chromosomes.
4. Sex Chromosomes: Sex chromosomes are the chromosomes that determine the sex of an individual (for example, X and Y chromosomes in humans).
7. Why does DNA have to be duplicated before cell division?
Answer: DNA has to be duplicated before cell division so that each daughter cell receives a complete and identical set of genetic information. This helps in maintaining continuity of hereditary characters from one cell generation to the next.
8. Mention the main steps in DNA replication.
Answer: The major steps of DNA duplication are :
(i) The double stranded DNA molecule unwinds with the help of certain enzymes to expose two strands of DNA.
(ii) A DNA polymerase enzyme catalyzes the formation of a new daughter strand which can form a double helix with one strand of parental DNA molecule. So two DNA molecules, each with a parental strand and a new strand get generated.
(iii) The two identical DNA molecules then become two chromatids which remain attached by a centromere.
9. What will be the blood group of the following which contain the genes IAi.
Answer: The genotype IAi produces Blood Group A.
[ The gene IA is dominant over i. Therefore, a person having the genotype IAi will show Blood Group A.]
10. Why is haemophilia found mostly in boys?
Answer: Haemophilia is found mostly in boys because it is an X-linked recessive genetic disorder. Boys have only one X chromosome (XY), so if they inherit the defective gene on the X chromosome, they develop haemophilia. Girls have two X chromosomes (XX), so a normal gene on one X chromosome can usually mask the defective gene. Therefore, girls are more often carriers than affected.
11. With the help of a line diagram explain the chromosomal basis of a zygote developing into a male child.
Answer: The sex of a child is determined by the combination of sex chromosomes during fertilisation. The mother always contributes an X chromosome, while the father contributes either X or Y. When the father contributes a Y chromosome, a male child (XY) is formed.
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12. What is the basis of sex determination in humans?
Answer: The basis of sex determination in humans is the sex chromosomes (X and Y chromosomes). Females have XX chromosomes and males have XY chromosomes. During fertilisation, the mother always contributes an X chromosome, while the father contributes either X or Y. If the zygote receives XX, a female child develops; if it receives XY, a male child develops.
13. Write notes on any one genetic disorder.
Answer: Haemophilia is a genetic disorder in which a person has either a defective gene or lacks the genes responsible for producing the substance needed for blood clotting. Due to the absence of this substance, blood does not coagulate properly. Therefore, once bleeding starts, it does not clot easily and may continue for a long time.
14. What is meant by “gene replacement therapy”?
Answer: Gene replacement therapy is a method of treating genetic disorders by replacing a defective or faulty gene with a normal and healthy gene in a person’s cells. This helps the cells produce the required substances and reduces the effects of the disorder.
15. Rahul’s maternal grandfather (mother’s father) was colour-blind. What are the chances of Rahul being colour-blind if his father has normal colour vision?
Answer: Colour blindness is an X-linked recessive trait.
If Rahul's maternal grandfather was colour-blind, Rahul's mother would most likely be a carrier (having one normal X chromosome and one colour-blindness X chromosome). Rahul receives his Y chromosome from his father and one X chromosome from his mother.
Therefore, Rahul has a 50% (1 in 2) chance of being colour-blind .
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