Genetics: Mendel’s Laws of Inheritance and Sex-linked Inheritance of Diseases

True/False Questions

1. Mendel’s laws of inheritance were based on pea plants.
   Answer: True
2. Alleles are different forms of the same gene.
   Answer: True
3. Homozygous organisms have two different alleles for a particular trait.
   Answer: False (Homozygous organisms have two identical alleles for a trait.)
4. The phenotype of an organism refers to its genetic makeup.
   Answer: False (Phenotype refers to the physical characteristics, while genotype refers to genetic makeup.)
5. Colour blindness is a Y-linked disease.
   Answer: False (Colour blindness is an X-linked disease.)

Fill in the Blanks

6. Mendel’s law of segregation states that during the formation of gametes, the two alleles for a trait separate ________.
   Answer: independently
7. An organism with two identical alleles for a trait is called ________.
   Answer: homozygous
8. The genetic makeup of an organism is known as its ________.
   Answer: genotype
9. If a person has one allele for haemophilia and one normal allele, they are considered a ________.
   Answer: carrier
10. A mutation is a permanent change in the ________ of a gene.
    Answer: DNA sequence

Short Answer Questions

11. What is the difference between genotype and phenotype?
    Answer: Genotype refers to the genetic composition of an organism, while phenotype refers to the physical appearance or traits resulting from that genetic makeup.
12. What is a dihybrid cross?
    Answer: A dihybrid cross is a genetic cross that involves two traits. It demonstrates how the inheritance of one trait is independent of the other.
13. Define the term ‘mutation’.
    Answer: A mutation is a change in the DNA sequence of a gene, which can lead to variations in traits or cause genetic disorders.
14. What does it mean if an individual is heterozygous for a trait?
    Answer: A heterozygous individual has two different alleles for a particular trait, one dominant and one recessive.
15. What is the role of the Y chromosome in human sex determination?
    Answer: The Y chromosome contains genes responsible for male sex determination. The presence of a Y chromosome results in male development, while its absence leads to female development.

Long Answer Questions

16. Explain Mendel’s Law of Segregation with an example.
    Answer: Mendel’s Law of Segregation states that during the formation of gametes, the two alleles for a trait separate from each other, so that each gamete carries only one allele. For example, in pea plants, a plant with genotype Tt (tall) will pass either the T or t allele to its offspring. This explains why offspring may inherit either a tall or dwarf trait depending on the combination of alleles they receive.
17. Describe the process of sex determination in humans.
    Answer: Sex determination in humans is controlled by the combination of X and Y chromosomes. Males have one X and one Y chromosome (XY), while females have two X chromosomes (XX). During reproduction, the male’s sperm may carry either an X or a Y chromosome, while the female’s egg always carries an X chromosome. If the sperm carrying a Y chromosome fertilizes the egg, the offspring will be male (XY); if an X-carrying sperm fertilizes the egg, the offspring will be female (XX).
18. What is a monohybrid cross, and how does it differ from a dihybrid cross?
    Answer: A monohybrid cross is a genetic cross that involves only one trait, such as flower color in pea plants. A dihybrid cross, on the other hand, involves two traits, such as seed shape and seed color. The main difference is that monohybrid crosses track the inheritance of one trait, while dihybrid crosses track two traits simultaneously.
19. Explain how haemophilia is inherited and why it is more common in males than females.
    Answer: Haemophilia is an X-linked recessive disorder, meaning that the defective gene is located on the X chromosome. Males, having only one X chromosome (XY), will exhibit the disorder if they inherit the defective X chromosome. Females have two X chromosomes (XX), so they must inherit two defective X chromosomes to exhibit the disorder. If a female inherits only one defective X chromosome, she becomes a carrier but does not show symptoms. Therefore, haemophilia is more common in males since they only need one defective X chromosome to express the disease.
20. Why are X-linked diseases like colour blindness less common in females?
    Answer: X-linked diseases, such as colour blindness, are caused by mutations on the X chromosome. Since females have two X chromosomes, they are less likely to exhibit the disease because they would need two copies of the defective gene (one on each X chromosome) to express the condition. Males, with only one X chromosome, will express the disease if they inherit a defective X chromosome, as there is no second X chromosome to compensate.

Multiple Choice Questions (MCQs)

21. In a monohybrid cross, the ratio of dominant to recessive phenotypes in the F2 generation is:

• (a) 1:1
• (b) 3:1
• (c) 9:3:3:1
• (d) 1:2:1
  Answer: (b) 3:1

22. Which of the following terms refers to a permanent change in the DNA sequence of a gene?

• (a) Variation
• (b) Mutation
• (c) Phenotype
• (d) Genotype
  Answer: (b) Mutation

23. Which genotype represents a heterozygous individual?

• (a) TT
• (b) tt
• (c) Tt
• (d) Both TT and tt
  Answer: (c) Tt

24. Which of the following is an example of a recessive X-linked disorder?

• (a) Haemophilia
• (b) Down syndrome
• (c) Sickle cell anemia
• (d) Huntington’s disease
  Answer: (a) Haemophilia

25. In a dihybrid cross, the F2 phenotypic ratio is:

• (a) 3:1
• (b) 1:1
• (c) 9:3:3:1
• (d) 1:2:1
  Answer: (c) 9:3:3:1

Conceptual Questions

26. How does Mendel’s Law of Independent Assortment explain the inheritance of traits in a dihybrid cross?
    Answer: Mendel’s Law of Independent Assortment states that the alleles for different traits are distributed to gametes independently of one another. In a dihybrid cross, this means that the inheritance of one trait, such as seed shape, does not influence the inheritance of another trait, such as seed color. For example, in pea plants, the inheritance of round seeds is independent of the inheritance of yellow or green seeds.
27. Explain how carriers of X-linked diseases can pass on the disease to their offspring.
    Answer: Carriers of X-linked diseases have one normal allele and one mutated allele on their X chromosome. Female carriers (XX) can pass the mutated X chromosome to their offspring. If a male offspring inherits the mutated X chromosome, he will express the disease because he has no second X chromosome to compensate. If a female offspring inherits the mutated X chromosome, she will be a carrier like her mother, but will not express the disease unless she inherits another mutated X chromosome from her father.
28. What is the significance of Mendel’s Law of Dominance in inheritance?
    Answer: Mendel’s Law of Dominance states that in a pair of alleles, one is dominant and the other is recessive. The dominant allele will mask the effect of the recessive allele when both are present in a heterozygous individual. This explains why certain traits, like tallness in pea plants, are expressed in the presence of a dominant allele, even if a recessive allele is also present.
29. What role does mutation play in genetic variation?
    Answer: Mutations introduce new genetic variations by altering the DNA sequence of a gene. These variations can lead to new traits or characteristics, which may be passed on to future generations. Mutations are a key factor in evolution, as they contribute to the diversity of traits within a population.
30. How is the sex of a child determined genetically in humans?
    Answer: In humans, the sex of a child is determined by the combination of sex chromosomes inherited from the parents. Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY). During fertilization, the mother contributes an X chromosome, and the father contributes either an X or a Y chromosome. If the father’s sperm carries an X chromosome, the child will be female (XX); if it carries a Y chromosome, the child will be male (XY).

Numerical and Challenging Questions

31. A couple has a son with colour blindness. The mother has normal vision, but the father is also colour blind. What are the chances that their next son will also be colour blind?
    Answer: Since colour blindness is X-linked, the mother must be a carrier (XʙX), and the father is colour blind (XʙY). The possible combinations for a son (XY) will give a 50% chance of him being colour blind.
32. In a monohybrid cross between two heterozygous tall pea plants (Tt), what is the probability that their offspring will be homozygous recessive (tt)?
    Answer: In a monohybrid cross (Tt x Tt), the offspring genotypic ratio is 1 TT : 2 Tt : 1 tt. The probability of a homozygous recessive (tt) offspring is 1/4 or 25%.
33. In a dihybrid cross involving two traits (seed shape and seed color), both heterozygous (RrYy), what is the probability of producing offspring with round seeds and green color (R_Yy)?
    Answer: The phenotypic ratio in a dihybrid cross is 9:3:3:1. The probability of having round seeds and green color (R_Yy) is 3/16.
34. If a woman is a carrier for haemophilia and her husband is not affected, what is the probability that their daughter will have haemophilia?
    Answer: Since haemophilia is X-linked recessive, the daughter would have to inherit two affected X chromosomes (XʰXʰ) to express the disease. The probability of this happening in this case is 0%, but there is a 50% chance she will be a carrier.

 

Fill in the Blanks (Continued)

35. The physical appearance or observable characteristics of an organism is its ________.
    Answer: phenotype
36. An organism’s ________ contains the genetic information that determines traits.
    Answer: genotype
37. The law of ________ states that each pair of alleles segregates independently of the other pairs during gamete formation.
    Answer: independent assortment
38. ________ is the process by which the sex of an individual is determined.
    Answer: Sex determination
39. A ________ individual has two identical alleles for a given gene.
    Answer: homozygous

True/False (Continued)

40. Sex-linked traits are only passed on through the Y chromosome.
    Answer: False (They can also be passed on through the X chromosome.)
41. All mutations lead to genetic diseases.
    Answer: False (Not all mutations cause diseases; some are harmless or beneficial.)
42. Mendel’s experiments showed that the inheritance of one trait does not affect the inheritance of another.
    Answer: True
43. Colour blindness is more common in females than in males.
    Answer: False (It is more common in males because they have only one X chromosome.)
44. An individual with the genotype Tt is heterozygous.
    Answer: True

Short Answer (Continued)

45. What is meant by dominant and recessive traits?
    Answer: A dominant trait is expressed when at least one dominant allele is present, while a recessive trait is expressed only when two recessive alleles are present.
46. What does the term “carrier” mean in the context of sex-linked diseases?
    Answer: A carrier is an individual, usually female, who has one copy of a recessive gene for a disease but does not express the disease themselves.
47. Why are Mendel’s laws still important in modern genetics?
    Answer: Mendel’s laws form the foundation of classical genetics, helping scientists understand how traits are inherited and predicting inheritance patterns.
48. Explain why haemophilia is rare in females.
    Answer: Haemophilia is X-linked, so females need two copies of the defective gene (one on each X chromosome) to express the disorder, which is less likely to occur.
49. How does the environment influence phenotype?
    Answer: The environment can affect the expression of certain traits. For example, temperature can influence the color of a flower or the height of a plant, even though these traits are genetically determined.

Multiple Choice (Continued)

50. Which of the following is a type of genetic variation caused by a sudden change in the DNA sequence?

• (a) Mutation
• (b) Crossing over
• (c) Independent assortment
• (d) Fertilization
  Answer: (a) Mutation

51. A pea plant with genotype Tt is crossed with another plant with genotype Tt. What proportion of their offspring will have the genotype TT?

• (a) 1/4
• (b) 1/2
• (c) 3/4
• (d) 0
  Answer: (a) 1/4

52. In human beings, the male determines the sex of the child because:

• (a) Males have two X chromosomes
• (b) Males have one X and one Y chromosome
• (c) Males have two Y chromosomes
• (d) Females have one X chromosome
  Answer: (b) Males have one X and one Y chromosome

53. Which of the following is not a Mendelian trait?

• (a) Attached earlobes
• (b) Skin color
• (c) Flower color in pea plants
• (d) Seed shape in pea plants
  Answer: (b) Skin color

54. If a woman with normal vision marries a man with color blindness, what is the probability that their son will be color blind?

• (a) 0%
• (b) 50%
• (c) 25%
• (d) 100%
  Answer: (b) 50%

Matching Questions (Continued)

55. Match the following terms to their correct definitions:

• (a) Phenotype – (i) The genetic makeup of an organism
• (b) Genotype – (ii) A visible trait
• (c) Homozygous – (iii) Two identical alleles for a gene
• (d) Heterozygous – (iv) Two different alleles for a gene
  Answer: (a) – ii, (b) – i, (c) – iii, (d) – iv

56. Match the genetic disorder with its inheritance pattern:

• (a) Haemophilia – (i) Autosomal dominant
• (b) Cystic fibrosis – (ii) Autosomal recessive
• (c) Huntington’s disease – (iii) X-linked recessive
  Answer: (a) – iii, (b) – ii, (c) – i

Conceptual Questions (Continued)

57. Why did Mendel choose pea plants for his experiments?
    Answer: Mendel chose pea plants because they had distinct traits, could be easily cross-pollinated, had a short generation time, and produced a large number of offspring.
58. Explain the concept of a test cross and its significance.
    Answer: A test cross is used to determine the genotype of an organism expressing a dominant trait. It involves crossing the organism with a homozygous recessive individual to observe the phenotypic ratio of the offspring.
59. Why are X-linked disorders more frequently expressed in males than females?
    Answer: Males have only one X chromosome, so if they inherit an X-linked disorder, they express it, whereas females need two copies of the defective gene to express the disorder.
60. Describe the difference between autosomal and sex-linked inheritance.
    Answer: Autosomal inheritance involves genes located on the autosomes (non-sex chromosomes), while sex-linked inheritance involves genes located on the sex chromosomes (X or Y).

Numerical and Problem-solving Questions (Continued)

61. In a dihybrid cross between two heterozygous individuals (RrYy), what proportion of offspring will show the dominant phenotype for both traits?
    Answer: 9/16 of the offspring will show the dominant phenotype for both traits (R_Y_).
62. A man with blood group A (genotype IᴬIᴬ or Iᴬi) marries a woman with blood group O (genotype ii). What are the possible blood types of their children?
    Answer: The possible blood types of the children are either A (Iᴬi) or O (ii).
63. In a pedigree, if both parents are unaffected carriers of an autosomal recessive trait, what is the probability that their child will inherit the disorder?
    Answer: The probability is 1/4 or 25%.
64. If a woman who is heterozygous for colour blindness (XʙX) has children with a man who has normal vision (XY), what proportion of their sons will be colour blind?
    Answer: 50% of their sons will be colour blind.
65. In a monohybrid cross between two heterozygous parents, how many of the offspring will be homozygous recessive?
    Answer: 1/4 of the offspring will be homozygous recessive.

Long Answer Questions (Continued)

66. How does the concept of linked genes challenge Mendel’s Law of Independent Assortment?
    Answer: Linked genes are located close to each other on the same chromosome and tend to be inherited together, which means they do not assort independently. This challenges Mendel’s law, which assumes that all genes are independently assorted during gamete formation.
67. Describe the concept of co-dominance and give an example.
    Answer: Co-dominance occurs when both alleles in a heterozygous individual are fully expressed. An example is blood type AB, where both A and B alleles are expressed equally.
68. Explain how sex-linked inheritance works and give an example of an X-linked recessive disorder.
    Answer: In sex-linked inheritance, genes are located on the sex chromosomes. X-linked recessive disorders, such as haemophilia, are expressed in males who inherit the defective gene on their single X chromosome, whereas females must inherit two copies of the defective gene to express the disorder.
69. What role does mutation play in evolution and genetic diversity?
    Answer: Mutation introduces new alleles into a population, contributing to genetic diversity. Beneficial mutations can be acted upon by natural selection, driving evolution and adaptation to changing environments.
70. Why is it possible for two carriers of a recessive genetic disorder to have a child without the disorder?
    Answer: Carriers of a recessive genetic disorder have one normal allele and one defective allele. For a child to inherit the disorder, they must inherit the defective allele from both parents. If they inherit a normal allele from either parent, they will not express the disorder.

Challenging Questions (Continued)

71. A woman with a rare X-linked dominant disorder marries a healthy man. What are the chances that their daughters will inherit the disorder?
    Answer: 100% of their daughters will inherit the disorder because they receive one X chromosome from their mother.
72. How do epistatic interactions differ from Mendelian inheritance?
    Answer: Epistasis occurs when one gene masks or modifies the expression of another gene. This differs from Mendelian inheritance, where traits are controlled by alleles at a single locus without interference from other genes.
73. Explain how gene mutations can be both harmful and beneficial.
    Answer: Gene mutations can be harmful if they result in non-functional proteins or diseases. However, mutations can also be beneficial if they lead to adaptations that enhance survival in changing environments.
74. In a dihybrid cross, what would the F2 phenotypic ratio be if the two genes show linkage rather than independent assortment?
    Answer: The phenotypic ratio would deviate from the expected 9:3:3:1 ratio, depending on the degree of linkage and crossover frequency.

Matching Questions (Continued)

75. Match the following terms with their correct descriptions:

• (a) Monohybrid cross – (i) Inheritance of two traits
• (b) Dihybrid cross – (ii) Inheritance of one trait
• (c) Genotype – (iii) Genetic composition
• (d) Phenotype – (iv) Physical traits
  Answer: (a) – ii, (b) – i, (c) – iii, (d) – iv

Case Study Questions

76. Case Study: A family has a history of haemophilia. The father is unaffected, but the mother is a carrier. They have two sons, one of whom has haemophilia. What are the chances that their next child will have haemophilia if it’s a boy?
    Answer: There is a 50% chance that their next son will have haemophilia because he can inherit the X chromosome with the haemophilia gene from the carrier mother.
77. Case Study: A man with blood type AB marries a woman with blood type O. What are the possible blood types of their children?
    Answer: The possible blood types of their children are A (Iᴬi) or B (Iᴮi).
78. Case Study: In a family, a father is colorblind and the mother has normal vision but is a carrier. What are the chances their daughter will be colorblind?
    Answer: There is a 50% chance that the daughter will be colorblind because she can inherit the defective X chromosome from both parents.

True/False (Continued)

79. Linked genes always assort independently.
    Answer: False (Linked genes tend to be inherited together.)
80. Co-dominance results in both alleles being expressed equally in the phenotype.
    Answer: True
81. In an X-linked dominant disorder, males and females are equally likely to inherit the disorder.
    Answer: False (Males are more likely to be affected by X-linked disorders.)

Numerical Questions (Continued)

82. In a test cross between a heterozygous tall pea plant (Tt) and a homozygous recessive dwarf plant (tt), what proportion of the offspring will be tall?
    Answer: 50% of the offspring will be tall.
83. In a dihybrid cross between two heterozygous parents (RrYy), what is the probability of getting an offspring with the genotype RRYy?
    Answer: The probability is 1/8.

Long Answer Questions (Continued)

84. Describe the process of genetic recombination and its importance in evolution.
    Answer: Genetic recombination occurs during meiosis when homologous chromosomes exchange segments of DNA. This process creates new combinations of alleles in offspring, increasing genetic diversity and contributing to the evolution of populations.
85. How do genetic mutations contribute to the process of natural selection?
    Answer: Genetic mutations introduce new variations into a population. If a mutation provides a survival advantage, it may be passed on to future generations, promoting adaptation through natural selection.
86. Explain the concept of incomplete dominance with an example.
    Answer: In incomplete dominance, neither allele is completely dominant, and the phenotype is a blend of both alleles. An example is the snapdragon flower, where a cross between a red-flowered plant (RR) and a white-flowered plant (WW) produces pink flowers (RW).
87. How does the principle of segregation ensure genetic diversity in offspring?
    Answer: The principle of segregation ensures that each gamete receives only one allele for a trait. This leads to a variety of allele combinations in offspring, contributing to genetic diversity.
88. What are the implications of sex-linked inheritance for genetic counseling?
    Answer: Genetic counseling helps families understand the risks of inheriting sex-linked disorders, such as haemophilia and color blindness. It allows for informed reproductive decisions and early interventions if necessary.

True/False (Continued)

89. Dominant alleles always appear more frequently in a population than recessive alleles.
    Answer: False (Dominant alleles may not necessarily be more frequent than recessive alleles.)
90. A test cross is used to determine the genotype of an organism with a dominant phenotype.
    Answer: True
91. Mutation always leads to a harmful outcome.
    Answer: False (Some mutations are neutral or even beneficial.)
92. Dihybrid crosses involve the inheritance of two traits simultaneously.
    Answer: True

Matching Questions (Continued)

93. Match the genetic terms to their descriptions:

• (a) Allele – (i) Physical expression of a trait
• (b) Homozygous – (ii) Alternative form of a gene
• (c) Phenotype – (iii) Identical alleles for a trait
• (d) Heterozygous – (iv) Different alleles for a trait
  Answer: (a) – ii, (b) – iii, (c) – i, (d) – iv

94. Match the following modes of inheritance with the traits they represent:

• (a) X-linked recessive – (i) Huntington’s disease
• (b) Autosomal dominant – (ii) Colour blindness
• (c) Autosomal recessive – (iii) Cystic fibrosis
  Answer: (a) – ii, (b) – i, (c) – iii

Multiple Choice (Continued)

95. Which of the following is not an X-linked disorder?

• (a) Haemophilia
• (b) Duchenne muscular dystrophy
• (c) Down syndrome
• (d) Colour blindness
  Answer: (c) Down syndrome

96. Which of the following can increase genetic variation in a population?

• (a) Cloning
• (b) Independent assortment
• (c) Mitosis
• (d) Identical twins
  Answer: (b) Independent assortment

Case Study Questions (Continued)

97. Case Study: A woman who is a carrier for haemophilia marries a man who is unaffected. What is the probability of their daughter being a carrier?
    Answer: There is a 50% chance that their daughter will be a carrier.
98. Case Study: A family pedigree shows a trait that skips generations and affects only males. What is the most likely mode of inheritance for this trait?
    Answer: The most likely mode of inheritance is X-linked recessive.

Challenging Questions (Continued)

99. Explain the difference between epigenetics and traditional Mendelian genetics.
    Answer: Traditional Mendelian genetics focuses on the inheritance of traits through DNA and alleles, while epigenetics involves changes in gene expression without altering the DNA sequence, often due to environmental factors.
100. Why are X-linked dominant disorders rare, and how are they inherited?
     Answer: X-linked dominant disorders are rare because they often result in severe effects that reduce survival or reproductive success. They are inherited when a parent (usually the mother) carries a dominant allele on the X chromosome.