questions with their answers, specifically designed for Class 12 CHSE Biology students, covering the chapter "Heredity and Variation."
Chapter: Heredity and Variation
Multiple Choice Questions (MCQ)
1. The "Father of Genetics" is:
a) T.H. Morgan
b) Gregor Mendel
c) Hugo de Vries
d) W. Bateson
Answer: b) Gregor Mendel
2. In a dihybrid cross, the phenotypic ratio for the F2 generation is typically:
a) 3:1
b) 1:2:1
c) 9:3:3:1
d) 1:1:1:1
Answer: c) 9:3:3:1
3. Which of the following is an example of incomplete dominance?
a) ABO blood groups in humans
b) Flower colour in Mirabilis jalapa (Four o'clock plant)
c) Sickle cell anemia
d) Height in humans
Answer: b) Flower colour in Mirabilis jalapa (Four o'clock plant)
4. If a red cow (RR) is crossed with a white bull (WW), and the offspring are roan (RW), this is an example of:
a) Incomplete dominance
b) Co-dominance
c) Pleiotropy
d) Polygenic inheritance
Answer: b) Co-dominance
5. How many alleles control the ABO blood groups in humans?
a) One
b) Two
c) Three
d) Four
Answer: c) Three
6. A single gene affecting multiple phenotypic traits is called:
a) Multiple alleles
b) Co-dominance
c) Pleiotropy
d) Polygenic inheritance
Answer: c) Pleiotropy
7. Who proposed the Chromosomal Theory of Inheritance?
a) Gregor Mendel
b) T.H. Morgan
c) Sutton and Boveri
d) Watson and Crick
Answer: c) Sutton and Boveri
8. Genes located on the same chromosome are said to be:
a) Alleles
b) Linked
c) Dominant
d) Recessive
Answer: b) Linked
9. The phenomenon of exchange of genetic material between non-sister chromatids of homologous chromosomes is called:
a) Linkage
b) Crossing over
c) Segregation
d) Independent assortment
Answer: b) Crossing over
10. In humans, sex determination is based on:
a) XY type
b) ZW type
c) XO type
d) ZO type
Answer: a) XY type
11. Which of the following is a sex-linked recessive disorder?
a) Down's syndrome
b) Turner's syndrome
c) Colour blindness
d) Thalassemia
Answer: c) Colour blindness
12. Haemophilia is a genetic disorder due to a defect in:
a) Blood clotting proteins
b) Haemoglobin synthesis
c) Chromosome number
d) Melanin production
Answer: a) Blood clotting proteins
13. A person with Klinefelter's syndrome has the chromosomal complement:
a) XO
b) XXY
c) XXX
d) XYY
Answer: b) XXY
14. Thalassemia is a Mendelian disorder primarily affecting:
a) Nervous system
b) Skeletal system
c) Blood (hemoglobin synthesis)
d) Reproductive system
Answer: c) Blood (hemoglobin synthesis)
15. Down's syndrome is caused by:
a) Monosomy of X chromosome
b) Trisomy of chromosome 21
c) Absence of Y chromosome
d) Deletion in chromosome 5
Answer: b) Trisomy of chromosome 21
Short Answer Questions (2-3 marks)
1-State Mendel's Law of Segregation.
Answer: Mendel's Law of Segregation states that during gamete formation, the two alleles for a heritable character separate (segregate) from each other such that each gamete receives only one allele.
2-Differentiate between dominant and recessive alleles.
Answer: * Dominant Allele: An allele that expresses its phenotype even when present with a recessive allele (in a heterozygous condition). * Recessive Allele: An allele that only expresses its phenotype when present in a homozygous condition (i.e., when two copies of the recessive allele are present). Its expression is masked by a dominant allele in heterozygotes.
3-Explain incomplete dominance with an example.
Answer: Incomplete dominance is a pattern of inheritance where the heterozygous genotype results in a phenotype that is intermediate between the two homozygous phenotypes. Neither allele is completely dominant over the other. Example: In Mirabilis jalapa (Four o'clock plant), a cross between true-breeding red (RR) and white (rr) flowers produces pink (Rr) flowers in the F1 generation.
4-What is co-dominance? Give an example.
Answer: Co-dominance is a pattern of inheritance where both alleles in a heterozygote are fully and equally expressed, resulting in a phenotype that shows characteristics of both alleles. Example: ABO blood groups in humans, where alleles I^A and I^B are co-dominant, resulting in AB blood type with both A and B antigens expressed.
5-Briefly explain multiple alleles with reference to human blood groups.
Answer: Multiple alleles refer to the presence of more than two alternative forms of a gene controlling a particular trait. In human ABO blood groups, the gene 'I' (isoagglutinogen) has three alleles: I^A, I^B, and i. I^A and I^B are co-dominant, while 'i' is recessive. This combination of three alleles allows for four different blood types (A, B, AB, O).
6-What is pleiotropy? Give an example.
Answer: Pleiotropy is a phenomenon where a single gene affects multiple, seemingly unrelated phenotypic traits. Example: Phenylketonuria (PKU) in humans is caused by a defect in a single gene (for the enzyme phenylalanine hydroxylase), leading to mental retardation, reduced hair and skin pigmentation, and other symptoms.
7. State the Chromosomal Theory of Inheritance.
Answer: The Chromosomal Theory of Inheritance, proposed by Sutton and Boveri, states that Mendelian genes (factors) are located on specific loci on chromosomes, and it is the chromosomes that undergo segregation and independent assortment during meiosis, thus accounting for the patterns of inheritance.
8. Differentiate between gene and allele.
Answer: * Gene: A basic unit of heredity; a specific sequence of DNA (or RNA in some viruses) that codes for a specific protein or RNA molecule, influencing a trait. * Allele: An alternative form or variant of a gene, located at a specific locus on a chromosome. For example, the gene for height in peas has two alleles: T (tall) and t (dwarf).
9. What is linkage? How does it affect Mendelian ratios?
Answer: Linkage is the phenomenon where genes located closely on the same chromosome tend to be inherited together during meiosis, rather than assorting independently. This leads to deviations from the Mendelian dihybrid ratio (9:3:3:1) because the linked genes do not segregate independently.
10. What is crossing over and what is its significance?
Answer: Crossing over is the process of exchange of genetic material (DNA segments) between non-sister chromatids of homologous chromosomes during prophase I of meiosis. Its significance is that it leads to genetic recombination, producing new combinations of alleles on chromosomes, thereby increasing genetic variation in offspring.
11. Explain sex determination in humans.
Answer: In humans, sex determination is of the XY type. Females are homogametic (XX) and produce only one type of gamete (ova) with an X chromosome. Males are heterogametic (XY) and produce two types of gametes (sperm): 50% carry an X chromosome and 50% carry a Y chromosome. The sex of the offspring is determined by the sperm that fertilizes the egg: if an X-sperm fertilizes the egg, the offspring is female (XX); if a Y-sperm fertilizes the egg, the offspring is male (XY).
12. What are sex-linked genes? Give one example of a sex-linked disorder.
Answer: Sex-linked genes are genes located on the sex chromosomes (X or Y chromosomes). Their inheritance pattern is different from autosomal genes. Example of a sex-linked disorder: Haemophilia or Colour blindness.
13. What is thalassemia? Is it a Mendelian or chromosomal disorder?
Answer: Thalassemia is a group of inherited blood disorders characterized by abnormal hemoglobin production, leading to insufficient oxygen-carrying capacity and anemia. It is a Mendelian disorder (autosomal recessive) caused by mutations in genes responsible for globin chain synthesis.
14. What is Down's syndrome? What is its chromosomal basis? Answer: Down's syndrome (also known as Trisomy 21) is a genetic disorder caused by the presence of an extra copy of chromosome 21 (i.e., three copies instead of the usual two). It results in characteristic facial features, intellectual disability, and developmental delays.
15. Differentiate between Turner's syndrome and Klinefelter's syndrome based on their chromosomal complement.
Answer: * Turner's Syndrome: Caused by the absence of one X chromosome in females, resulting in a chromosomal complement of 45, XO. Individuals are typically sterile females with underdeveloped secondary sexual characteristics. * Klinefelter's Syndrome: Caused by the presence of an extra X chromosome in males, resulting in a chromosomal complement of 47, XXY. Individuals are typically sterile males with underdeveloped testes, gynaecomastia (development of breasts), and often tall stature.
Long Answer Questions (5-6 marks)
1. Explain Mendelian inheritance with the help of a monohybrid and a dihybrid cross. State Mendel's Laws of Inheritance derived from these crosses.
Answer:
Mendelian Inheritance refers to the principles of heredity discovered by Gregor Mendel through his experiments on pea plants. He proposed that traits are passed from parents to offspring as discrete units called "factors" (now known as genes).
A. Monohybrid Cross: A monohybrid cross involves studying the inheritance of a single pair of contrasting characters.
Experiment: Mendel crossed true-breeding tall pea plants (TT) with true-breeding dwarf pea plants (tt).
F1 Generation: All the offspring in the F1 generation were tall (Tt). This showed that tallness was dominant over dwarfness.
F2 Generation: When the F1 tall plants (Tt) were self-pollinated, the F2 generation showed a phenotypic ratio of 3 Tall : 1 Dwarf and a genotypic ratio of 1 TT : 2 Tt : 1 tt.
Diagrammatic Representation (Punnett Square):
| Parent P1 | TT (Tall) | x | tt (Dwarf) | | :-------- | :-------- | : | :--------- | | Gametes | T | | t | | F1 | Tt (Tall) | | |
| Selfing F1 | Tt (Tall) | x | Tt (Tall) | | :--------- | :-------- | : | :-------- | | Gametes | T, t | | T, t |
|
F2 Offspring |
T |
t |
|---|---|---|
|
T |
TT |
Tt |
|
t |
Tt |
tt |
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Phenotypic Ratio: 3 Tall : 1 Dwarf
Genotypic Ratio: 1 TT : 2 Tt : 1 tt
Mendel's Laws derived from Monohybrid Cross:
Law of Dominance:
In a heterozygous condition, one allele (dominant) expresses itself phenotypically, masking the expression of the other allele (recessive).
Law of Segregation (or Law of Purity of Gametes):
Alleles for a trait separate or segregate from each other during gamete formation, so that each gamete receives only one allele. These alleles then unite randomly during fertilization.
B. Dihybrid Cross: A dihybrid cross involves studying the inheritance of two pairs of contrasting characters simultaneously.
Experiment: Mendel crossed pea plants that were true-breeding for yellow and round seeds (YYRR) with plants that were true-breeding for green and wrinkled seeds (yyrr).
F1 Generation: All F1 offspring produced yellow and round seeds (YyRr), indicating that yellow colour and round shape were dominant.
F2 Generation: When the F1 plants (YyRr) were self-pollinated, the F2 generation showed a phenotypic ratio of 9 (Yellow Round) : 3 (Yellow Wrinkled) : 3 (Green Round) : 1 (Green Wrinkled). This ratio indicated that the two pairs of traits segregated independently.
Mendel's Law derived from Dihybrid Cross:
Law of Independent Assortment:
When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters. In other words, alleles for different genes assort independently of each other during gamete formation, provided they are located on different chromosomes.
2. Explain sex-linked inheritance with reference to haemophilia and colour blindness in humans. Describe any two Mendelian disorders and two chromosomal disorders in humans.
Answer:
Sex-Linked Inheritance: Sex-linked inheritance refers to the pattern of inheritance of genes located on the sex chromosomes (specifically the X chromosome in humans, as the Y chromosome carries very few genes). Since males have only one X chromosome (XY), they express all X-linked traits whether dominant or recessive. Females (XX) are usually carriers for recessive X-linked traits and express them only if they are homozygous recessive.
a) Haemophilia:
Description: Haemophilia is a sex-linked recessive disorder characterized by a defect in the blood clotting mechanism, leading to prolonged bleeding from even minor injuries.
Inheritance: The gene for haemophilia is located on the X chromosome.
A female can be a carrier (X^H X^h) or affected (X^h X^h, rare as it often leads to death in utero or early in life).
A male with one recessive allele (X^h Y) will be affected, as there is no corresponding allele on the Y chromosome to mask its effect.
Example Cross: If a carrier female (X^H X^h) marries a normal male (X^H Y):
Daughter: 50% normal (X^H X^H), 50% carrier (X^H X^h)
Son: 50% normal (X^H Y), 50% haemophilic (X^h Y)
b) Colour Blindness:
Description: Colour blindness (most commonly red-green colour blindness) is a sex-linked recessive disorder where an individual is unable to distinguish between certain colours.
Inheritance: The genes for red and green colour perception are located on the X chromosome.
Similar to haemophilia, a female can be a carrier (X^C X^c) or affected (X^c X^c, rare).
A male with the recessive allele (X^c Y) will be colour blind.
Example Cross: If a carrier female (X^C X^c) marries a normal male (X^C Y):
Daughter: 50% normal (X^C X^C), 50% carrier (X^C X^c)
Son: 50% normal (X^C Y), 50% colour blind (X^c Y)
Mendelian Disorders in Humans: These are genetic disorders caused by alterations or mutations in a single gene (Mendelian genes) and follow Mendelian patterns of inheritance.
Thalassemia:
Description: An autosomal recessive blood disorder affecting the synthesis of globin chains of hemoglobin, leading to severe anemia. It is classified as alpha (α) or beta (β) thalassemia depending on which globin chain is affected.
Inheritance: If both parents are carriers (heterozygous), there is a 25% chance of the child being affected.
Symptoms: Anemia, fatigue, bone deformities, enlarged spleen, iron overload.
Phenylketonuria (PKU):
Description: An autosomal recessive metabolic disorder where the body cannot effectively metabolize the amino acid phenylalanine due to a deficiency of the enzyme phenylalanine hydroxylase.
Inheritance: Follows autosomal recessive pattern.
Symptoms: Accumulation of phenylalanine leads to mental retardation, developmental delay, skin problems (reduced pigmentation), and a characteristic mousy odor in urine and sweat. Early dietary management can prevent severe symptoms.
Chromosomal Disorders in Humans: These disorders are caused by an abnormal number or structure of chromosomes.
Down's Syndrome (Trisomy 21):
Description: Caused by the presence of an extra copy of chromosome 21 (i.e., three copies instead of the usual two). This usually results from non-disjunction during meiosis in either parent.
Symptoms: Characteristic facial features (flat face, epicanthic folds, open mouth), short stature, intellectual disability (varying degrees), congenital heart defects, and increased susceptibility to infections.
Klinefelter's Syndrome:
Description: A chromosomal disorder in males caused by the presence of an extra X chromosome, resulting in a chromosomal complement of 47, XXY. It results from non-disjunction during meiosis.
Symptoms: Overall masculine development but with some feminized features such as gynaecomastia (development of breasts), tall stature, sparse body hair, underdeveloped testes, and sterility. Individuals may also have mild intellectual impairment.
