Selina Concise Solutions for ICSE Class 10 Biology Chapter 2 Genetics Some Basic Fundamentals

Multiple Choice Type

Question 1. Which one of the following has the smallest number of chromosomes?
(a) Onion
(b) Mouse
(c) Monkey
(d) Ascaris
Answer: (d) Ascaris
In simple words: Ascaris is a parasitic worm that has only 2 chromosomes, which is much fewer than other organisms like onions, mice, or monkeys.

📝 Teacher's Note: Use this as an example to show students that chromosome number doesn't determine complexity - a simple worm has fewer chromosomes than a plant. Create a simple chart comparing chromosome numbers across different organisms.

🎯 Exam Tip: Remember the pattern: simpler organisms often have fewer chromosomes. Ascaris (roundworm) is the classic example with just 2 chromosomes.

Question 2. Which one of the following is the phenotypic monohybrid ratio in F2 generation?
(a) 3:1
(b) 1:2:1
(c) 2:2
(d) 1:3
Answer: (a) 3:1
In simple words: In Mendel's experiments, when you cross two hybrid plants, you get 3 plants with the dominant trait and 1 with the recessive trait - like 3 purple flowers and 1 white flower.

📝 Teacher's Note: Use Punnett squares with actual pea plant examples. Show students that 1:2:1 is the genotypic ratio, while 3:1 is phenotypic. This distinction confuses many students.

🎯 Exam Tip: Don't confuse phenotypic (3:1) with genotypic (1:2:1) ratios. Phenotypic means what you can see, so recessive traits are hidden in heterozygotes.

Very Short Answer Type

Question 1. Match the terms in column I with their explanations in column II.

Answer: (a) - (iii) Study of laws of inheritance of characters
(b) - (v) Chromosomes other than the pair of sex chromosomes
(c) - (iv) A gene that can express when only in a similar pair
(d) - (ii) The alternative forms of a gene
(e) - (i) Chromosomes similar in size and shape

📝 Teacher's Note: Create mnemonics for each term. For example, "Genetics = Gene + Ethics" to remember it's about studying gene behavior. Use visual diagrams to show homologous chromosomes.

🎯 Exam Tip: Learn the definitions word-for-word as they often appear in matching questions. Focus on keywords like "alternative forms" for alleles and "similar pair" for recessive genes.

Question 2. Name two animals which have nineteen pairs of chromosomes.
Answer: Lion, tiger, domestic cat (Any two)

📝 Teacher's Note: Emphasize that cats, lions, and tigers are all in the same family (Felidae) which explains their identical chromosome number. This helps students understand evolutionary relationships.

🎯 Exam Tip: Remember the cat family pattern - all have 19 pairs (38 total). This is a common fact-based question in genetics.

Question 3. Name any two genetic disorders in humans.
Answer: Colour-blindness, Thalassemia, Sickle cell anemia and Hemophilia (Any two)

📝 Teacher's Note: Connect each disorder to its inheritance pattern - color blindness is X-linked, sickle cell is autosomal recessive. Use real-world examples to make it relatable.

🎯 Exam Tip: Learn at least 4-5 genetic disorders with their inheritance patterns. This knowledge helps in multiple question types.

Question 4. Which one of the following genotypes is homozygous dominant and which one homozygous recessive in regards to tongue rolling: Rr, rr, RR
Answer: Homozygous dominant - RR
Homozygous recessive - rr

📝 Teacher's Note: Have students practice tongue rolling to connect the concept with a real trait. Explain that "homo" means same, so same alleles make it homozygous.

🎯 Exam Tip: Remember: Same letters = homozygous, different letters = heterozygous. Capital letters = dominant, small letters = recessive.

Short Answer Type

Question 1. Differentiate between: (a) genotype and phenotype. (b) character and trait. (c) monohybrid and dihybrid cross (phenotypic ratio)

Answer: [Table content as shown above]
In simple words: Genotype is like a recipe (the genes), phenotype is like the cake you see (the appearance). Character is a feature like height, trait is the specific form like tall or short.

📝 Teacher's Note: Use analogies like recipe vs. cake for genotype vs. phenotype. For monohybrid vs. dihybrid, compare to studying one subject vs. two subjects at the same time.

🎯 Exam Tip: Always present differences in tabular format for clarity. Remember the key ratios: monohybrid F2 = 3:1, dihybrid F2 = 9:3:3:1.

Question 2. Among lion, tiger and domestic cat, all the three have the same number of 38 chromosomes, yet they have different appearances. How do you account for such differences?
Answer: The characteristics of a species such as physical appearance, body functions and behavior are not only the outcome of chromosome number, but these depend on the genotype of every organism. That means the set of genes present in the organisms may vary and therefore lion, tiger and domestic cat have the same number of 38 chromosomes, their characteristics (like different appearances) are the result of the genes located on the chromosomes.
In simple words: It's not how many chromosomes you have, but what genes are on them that matters - like having the same number of books but different stories inside.

📝 Teacher's Note: Use the analogy of different books having the same number of pages but different content. This helps students understand that chromosome number ≠ organism complexity or appearance.

🎯 Exam Tip: Emphasize that genes on chromosomes (genotype) determine appearance, not just chromosome number. This concept appears frequently in exam questions.

Question 3. List any three features of garden pea with their dominant and recessive traits.

Answer: [Table as shown above]
In simple words: Mendel chose pea plants because they had clear either-or traits like purple vs. white flowers, making it easy to track inheritance patterns.

📝 Teacher's Note: Show actual pea plant pictures or bring samples if possible. Explain why Mendel chose pea plants - clear contrasting traits, easy to cross-pollinate, and quick generation time.

🎯 Exam Tip: Memorize at least 5 pea plant traits for Mendel's experiments. This data is frequently asked in various question formats.

Question 4. Explain why generally only the male child suffers from colour blindness and not the female?
Answer: Colour-blindness is caused due to recessive genes which occur on the X chromosome. Males have only one X chromosome. If there is recessive gene present on X chromosome, then the male will suffer from colour-blindness. Females have two X chromosomes. It is highly impossible that both the X chromosomes carry abnormal gene. Hence, if one gene is abnormal and since it is recessive, its expression will be masked by the normal gene present on the other X chromosome. Females are unlikely to suffer from colour-blindness.
In simple words: Males have only one X chromosome, so if it has the color blindness gene, they get the condition. Females have two X chromosomes, so they need the gene on both to be color blind, which is very rare.

📝 Teacher's Note: Use the analogy of having one vs. two backup copies of important information. Draw simple XY vs. XX diagrams to visualize the concept.

🎯 Exam Tip: Remember the key phrase "X-linked recessive inheritance." Males need only one copy, females need two copies of the recessive allele to express the trait.

Question 5. In a certain species of animals, black fur (B) is dominant over brown fur (b) Show the possible ratio of genotypes and phenotypes of the offspring of the pure breeding different coloured parents.
Answer: Phenotypic Ratio - 3 (Black Fur) :1 (Brown Fur)
Genotypic Ratio - 1(Homozygous Black Fur):2 (Heterozygous Black Fur): 1 (Homozygous Brown Fur)
In simple words: When you cross a pure black-furred animal with a pure brown-furred animal, their children will be mixed (Bb), and when these mixed animals breed, you get the classic 3:1 ratio.

📝 Teacher's Note: Always work through the cross step by step: P generation (BB × bb) → F1 (all Bb) → F2 cross (Bb × Bb) → 3:1 ratio. Use Punnett squares for clarity.

🎯 Exam Tip: Clearly distinguish between genotypic (1:2:1) and phenotypic (3:1) ratios. Draw the Punnett square to show your working for full marks.

Long Answer Type

Question 1. Explain the following terms: (a) heterozygous (b) Homozygous (c) pedigree chart.
Answer:
(a) Heterozygous: The condition in which a pair of homologous chromosomes carries dissimilar alleles for a particular character.
For example -
(i) A daughter (XX^o) from a normal homozygous mother for colour vision (XX) and a colour blind father has one normal and one defective allele (X^oY).
(ii) Certain tongue rollers are heterozygous with Rr genotype.

(b) Homozygous: The condition in which a pair of homologous chromosomes carries similar alleles for a particular character.
For example -
(i) A colorblind daughter (X^oX^o) will have both the X chromosomes with defective alleles.
(ii) A non-roller will have rr (homozygous) genotype.

(c) Pedigree Chart: A pedigree chart is a diagram that shows the occurrence and appearance or phenotypes of a particular gene or organism and its ancestors from one generation to the next. In the pedigree chart, males are shown by squares and females by circles.
In simple words: Heterozygous means having two different versions of a gene, homozygous means having two identical versions, and a pedigree chart is like a family tree that tracks genetic traits.

📝 Teacher's Note: Use family examples for pedigree charts - track simple traits like dimples or ear lobes through student families. Practice drawing pedigree symbols (squares for males, circles for females).

🎯 Exam Tip: Remember the prefixes: "hetero" = different, "homo" = same. For pedigree charts, always mention that squares = males, circles = females, and filled shapes = affected individuals.

Question 2. State the three mendel's laws of inheritance.
Answer: Mendel's laws of inheritance are:
(i) Law of Dominance: Out of a pair of contrasting characters present together, only one is able to express itself while the other remains suppressed. The one that expresses is the dominant character and the one that is unexpressed is the recessive one.
(ii) Law of Segregation: The two members of a pair of factors separate during the formation of gametes. The gametes combine together by random fusion at the time of zygote formation. This law is also known as 'law of purity of gametes'.
(iii) Law of Independent Assortment: When there are two pairs of contrasting characters, the distribution of the members of one pair into the gametes is independent of the distribution of the other pair.
In simple words: Mendel's laws explain how traits pass from parents to children: one trait masks another (dominance), traits separate during reproduction (segregation), and different traits are inherited independently (independent assortment).

📝 Teacher's Note: Use simple analogies for each law: dominance = louder voice drowns quieter one, segregation = splitting a deck of cards, independent assortment = shuffling multiple decks separately.

🎯 Exam Tip: Learn the alternative names too: Law of Segregation = Law of Purity of Gametes. State each law clearly with examples for maximum marks.

Question 3. Does the sex of the child depend on the father or it is just a matter of chance? Discuss
Answer: The sex of the child depends on the father. The egg contains only one X chromosome, but half of the sperms contain X-chromosome whereas the other half contains Y-chromosome. It is simply a matter of chance as to which category of sperm fuses with the ovum and this determines whether the child will be male or female.

If the egg fuses with X-bearing sperm, the resulting combination is XX and the resulting child is female.
If the egg fuses with Y-bearing sperm, the resulting combination is XY and the resulting child is male.
In simple words: The father determines the baby's sex because mothers always give an X chromosome, but fathers can give either X (for a girl) or Y (for a boy) chromosome - it's like flipping a coin.

📝 Teacher's Note: Use the analogy of the father being like someone who can give either a red or blue marble, while the mother always gives a red marble. This makes the concept of sex determination very clear.

🎯 Exam Tip: Always mention that it's the father's contribution that determines sex, but it's random chance (50:50) which sperm fertilizes the egg. Include the XX vs XY combinations.

Structured / Application And Skill Type

Question 1. In a certain species of animals, black fur (B) is dominant over brown fur (b) Predict the genotype and phenotype of the offspring when both parents are 'Bb' or have heterozygous black fur.

Answer:
Genotype - 1(Homozygous Black Fur) :2 (Heterozygous Black Fur):1 (Homozygous Brown Fur)
Phenotype - 3 (Black Fur) :1(Brown Fur)
In simple words: When two black-furred animals that carry the brown gene (Bb) mate, their babies will be mostly black (3 out of 4) with some brown (1 out of 4).

📝 Teacher's Note: Always draw the Punnett square first, then count the different genotypes and phenotypes. Emphasize that phenotype groups together BB and Bb since both appear black.

🎯 Exam Tip: Draw neat Punnett squares and clearly separate genotypic ratios (gene combinations) from phenotypic ratios (visible appearances). This distinction often carries separate marks.

Question 2. Two pairs (A & B) of rabbits were crossed as given below:
(a) Can you tell which coat colour (black or white) is dominant?
(b) Is the coat colour sex-linked?
Answer:
(a) Black
(b) No
In simple words: Black is dominant because when different colored parents mate, all offspring are black. It's not sex-linked because both male and female offspring show the same pattern regardless of their gender.

📝 Teacher's Note: Show students the crossing diagram visually and explain that dominant traits mask recessive ones. Point out that sex-linked traits would show different patterns in males vs females, which doesn't happen here.

🎯 Exam Tip: Always look at the offspring pattern - if all F1 generation shows one trait, that trait is dominant. For sex-linkage, check if males and females show different inheritance patterns.

Question 3. Make a punnett square for finding out the proportion of different genotypes in the progeny of a genetic cross between.
(a) A pure tall (TT) pea plant with a pure dwarf (tt) pea plant.
(b) red flower variety of pea (RR) with white flower variety of pea (rr)
Answer:
(a)
F1 generation: Tt (Hybrid tall)
F2 generation:

Genotype - 1(Homozygous tall) : 2 (Heterozygous tall) : 1 (Homozygous dwarf)
Phenotype - 3 (Tall) : 1(Dwarf)

(b)
F1 generation: Rr (Hybrid red flower)
F2 generation:

Genotype - 1(Homozygous red) : 2 (Heterozygous red) : 1 (Homozygous white)
Phenotype - 3 (Red) : 1(White)

In simple words: Punnett squares help us predict what traits offspring will have. When you cross pure parents, F1 generation all look the same, but F2 generation shows a 3:1 ratio of dominant to recessive traits.

📝 Teacher's Note: Emphasize that students must first cross to get F1, then cross F1 × F1 to get F2. Many students skip the F1 step and directly make F2 crosses, which is incorrect.

🎯 Exam Tip: Always write both genotype and phenotype ratios clearly. Draw the Punnett square neatly with proper labels for gametes. The 3:1 phenotype ratio is a classic Mendelian pattern.

Question 4. A family consists of two parents and their five children and the pedigree chart below shows the inheritance of the trait colour blindness in them.
(a) Who is colour blind in the parents – the Father or the Mother?
Answer:
(a) Father
(b) Two sons and three daughters
(c) The child 1 (daughter) is colour blind
(d) X chromosome
(e) Haemophilia
In simple words: Color blindness is carried on the X chromosome. Fathers pass X chromosomes only to daughters, so if a daughter is color blind, she got the gene from her father.

📝 Teacher's Note: Use the pedigree chart to explain X-linked inheritance. Emphasize that males have only one X chromosome, so they express X-linked recessive traits more frequently than females.

🎯 Exam Tip: In pedigree analysis, look for patterns - X-linked traits often skip generations and affect more males than females. Always identify who is affected by looking at the filled symbols.