49 Questions with Answers and Explanations on Human Genetics & Genetic Disorders for Biology Students.
1. Both husband and wife have normal vision though their fathers were colour blind. The probability of their daughter becoming colour blind is
(a) 0%
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(b) 25%
(c) 50%
(d) 75%.
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Answer and Explanation:
1. (a): Both husband and wife have normal vision through their fathers were colour blind, the probability of their daughter becoming colour blind is 0%. The chances of daughter becoming colour blind arises only when the father is also colour blind.
2. Which one is a hereditary disease?
(a) cataract
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(b) leprosy
(c) blindness
(d) phenylketonuria.
Answer and Explanation:
2. (d): Phenylketonuria is a hereditary disease. Phenylketonuria is an inherited error of metabolism caused by a deficiency in the enzyme phenylalanine hydroxylase. It results in mental retardation and is inherited as an autosomal recessive trait. It is a hereditary human condition resulting from the inability to convert phenylaline into tyrosine. This change can be traced to a tiny mutation in a single gene on chromosome 12.
3. Haemophilia is more common in males because it is a
(a) recessive character carried by Y-chromosome
(b) dominant character carried by Y-chromosome
(c) dominant trait carried by X-chromosome
(d) recessive trait carried by X-chromosome.
Answer and Explanation:
3. (d): Haemophilia is more common in males because it is a recessive trait carried by X-chromosome. Haemophilia A is the most common X-linked genetic disease that prevents normal blood clotting when blood vessels are ruptured. It appears in about in 7000 males. In most of the severe cases, an affected person can bleed to death from a bruise or cut. Haemophilia A is caused by the absence of a protein called antihaemophilic factor of factor VIII that is essential for blood clotting.
4. In Down’s syndrome of a male child, the sex complement is
(a) XO
(b) XY
(c) XX
(d) XXY.
Answer and Explanation:
4. (b): In Down’s syndrome of a male child, the sex complement is XY. Down syndrome is a relatively common birth defect caused by the presence of an extra chromosome number 21 (three instead of two number 21 chromosomes, or trisomy 21). This chromosome abnormality adversely affects both the physical and intellectual development of the individual. It is also the first syndrome to be described in humans.
5. A colour blind girl is rare because she will be born only when
(a) her mother and maternal grandfather were colour blind
(b) her father and maternal grandfather were colour blind
(c) her mother is colour blind and father has normal vision
(d) parents have normal vision but grandparents were colour blind.
Answer and Explanation:
5. (b): A colour blind girl is rare because she will be born only when her father and maternal grandfather were colour blind. The genotype of the mother was to be either XCXC or XCX and that of father XCY so that the daughter becomes colour blind.
6. In human beings 45 chromosomes/ single X/XO abnormality causes
(a) Down’s syndrome
(b) Kinefelter’s syndrome
(c) Turner’s syndrome
(d) Edward’s syndrome.
Answer and Explanation:
6. (c): In human beings, 45 chromosomes/single X/ XO abnormality causes Turner’s syndrome. Individuals having a single X chromosome 2A + XO (45) have female sexual differentiation but ovaries are rudimentary. Other associated phenotypes of this condition are short stature, webbed neck, broad chest, lack of secondary sexual characteristics and sterility. Thus, any unbalance in the copies of the sex chromosomes may disrupt the genetic information necessary for normal sexual development.
7. Down’s syndrome is due to
(a) crossing over
(b) linkage
(c) sex-linked inheritance
(d) nondisjunction of chromosomes.
Answer and Explanation:
7. (d): In Down’s syndrome of a male child, the sex complement is XY. Down syndrome is a relatively common birth defect caused by the presence of an extra chromosome number 21 (three instead of two number 21 chromosomes, or trisomy 21). This chromosome abnormality adversely affects both the physical and intellectual development of the individual. It is also the first syndrome to be described in humans.
8. A colour blind mother and normal father would have
(a) colour blind sons and normal/carrier daughters
(b) colour blind sons and daughters
(c) all colour blind
(d) all normal.
Answer and Explanation:
8. (a): A colour blind mother and normal father would have colour blind sons and normal/carrier daughter. When a colour blind woman marries a normal man the recessive homozygous gene is expressed in sons whereas the daughters become carriers.
9. The colour blindness is more likely to occur in males than in females because
(a) the Y-chromosome of males have the genes for distinguishing colours
(b) genes for characters are located on the sex- chromosomes
(c) the trait is dominant in males and recessive in females
(d) none of the above.
Answer and Explanation:
9. (b): Cololur blindness is a recessive sex-linked trait in which the eye fails to distinguish red and green colours. The gene for the normal vision is dominant. The normal gene and its recessive allele are carried by X- chromosomes. In females colour blindness appears only when both the sex chromosomes carry the recessive gene (XCXC). The females have normal vision but function as carrier if a single recessive gene for colour blindness is present (XXC). However, in human males the defect appears in the presence of a single recessive gene (XCY) because Y-chromosome of male does not carry any gene for colour vision.
10. Albinism is a congenital disorder resulting from the lack of which enzyme?
(a) tyrosinase
(b) xanthine oxidase
(c) catalase
(d) fructokinase.
Answer and Explanation:
10. (a): Albinism is caused by the absence of the enzyme tyrosinase which is essential for the synthesis of the pigment from dihydroxyphenylalanine. The gene for albinism does not produce the enzyme tyrosinase but its normal allele does. Thus, only homozygous individual is affected by this disease. Albinos (individuals with albinism) lack dark pigment melanin in the skin, hair and iris. Although albinos have poor vision yet they lead normal life.
11. A child’s blood group is ‘O’. The parent’s blood groups cannot be
(a) A and B
(b) A and A
(c) AB and O
(d) B and O.
Answer and Explanation:
11. (c): O blood group of a child cannot be obtained from the parents having blood group O x AB. The parents blood groups may be A x O, A x B, B x O, B x A, O x A and O x B.
12. The genes, which remain confined to differential region of Y-chromosome, are
(a) autosomal genes
(b) holandric genes
(c) completely sex-linked genes
(d) mutant genes.
Answer and Explanation:
12. (b): Despite differences in morphology, the XY chromosomes are homologous and synapse during zygotene. It is because they have two parts, homologous and differential. Homologous regions of the two help in pairing. They carry same genes which may have different alleles. The differential region of Y-chromosome carries
only Y-linked or holandric genes, e.g, testis determining factor (TDF). It is perhaps the smallest gene occupying only 14 base pairs. Other holandric genes are hypertrichosis (excessive hairiness) on pinna, porcupine skin, keratoderma dissipatum (thickened skin of hands and feet) and webbed toes. Holandric genes are directly inherited by a son from his father. Chromosomes which control most of the morpho-physiological characters other than sex are called autosomes. Sex linked genes are those which are found on the sex chromosomes. Mutant genes are formed by a change in the nucleotide type and sequence of a DNA segment representing a gene or a cistron.
13. An abnormal human male phenotype involving an extra X-chromosome (XXY) is a case of
(a) Edward’s syndrome
(b) Klinefelter’s syndrome
(c) intersex
(d) Down’s syndrome.
13. (b): Klinefelter’s syndrome is formed by the union of an XX egg and a normal Y sperm or normal X egg and abnormal XY sperm. The individual has 47 chromosomes (44 + XXY). Such persons are sterile males with undeveloped testes, mental retardation, sparse body hair, and long limbs and with some female characteristics such as enlarged breasts. It is considered that the more the X chromosomes, the greater is the mental defect. Down’s syndrome is caused by the presence of an extra chromosome number 21. Intersex is an individual who shows anatomical characteristics of both sexes.
14. The most striking example of point mutation is found in a disease called
(a) Down’s syndrome
(b) sickle cell anaemia
(c) thalassaemia
(d) night blindness.
Answer and Explanation:
14. (b): Point mutation involves only the replacement of one nucleotide with another. One type of point mutation is missense mutation. These are base changes that alter the codon for an amino acid resulting in its substitution with a different amino acid.
For example, mutation of the codon CTT to ATT would result in the replacement of the hydrophobic amino acid leucine with isoleucine, another hydrophobic amino acid. Many other missense mutations have been described which do affect the encoded protein and result in genetic diseases.
These include an A to T mutation in the gene for p-globin, one of the polypeptides of hemoglobin. This mutation changes codon six of the gene from GAG which encodes glutamic acid to GTG which encodes valine.
The mutation results in a condition called sickle cell anaemia in which the red blood cells adopt an abnormal sickle shape due to aggregation of the hemoglobin molecules. The abnormal cells are short-lived, which causes anaemia and become lodged in capillaries, which reduce the blood supply to organs.
15. A genetically diseased father (male) marries with a normal female and gives birth to 3 carrier girls and 5 normal sons. It may be which type of genetic disease?
(a) sex-influenced disease
(b) blood group inheritance disease
(c) sex-linked disease
(d) sex-recessive disease.
Answer:
(c) sex-linked disease
16. A person whose father is colour blind marries a lady whose mother is daughter of a colour blind man. Their children will be
(a) all sons colour blind
(b) some sons normal and some colour blind
(c) all colour blind
(d) all daughters normal.
Answer:
(d) all daughters normal
17. In which of the following diseases, the man has an extra X-chromosome?
(a) Turner’s syndrome
(b) Klinefelter’s syndrome
(c) Down’s syndrome
(d) haemophilia.
Answer and Explanation:
17. (b): Klinefelter’s syndrome is formed by the union of an XX egg and a normal Y sperm or normal X egg and abnormal XY sperm. The individual has 47 chromosomes (44 + XXY). Such persons are sterile males with undeveloped testes, mental retardation, sparse body hair, and long limbs and with some female characteristics such as enlarged breasts.
It is considered that the more the X chromosomes, the greater is the mental defect. Turner’s syndrome is formed by the union of an abnormal O egg and a normal X sperm or a normal egg and an abnormal O sperm.
The individual has 45 chromosomes (44 + X) instead of 46. Down’s syndrome is caused due to presence of an extra chromosome number 21. Haemophitia is sex-linked disease which is also known as bleeder’s disease as the patient will continue to bleed even from a minor cut since he or she does not possess the natural phenomenon of blood clotting due to absence of antihaemophiliac globulin (haemophilia – A) or plasma thromboplastin (haemophilia-B) essential for it.
18. A person with the sex chromosomes XXY suffers from
(a) gynandromorphism
(b) Klinefelter’s syndrome
(c) Down’s syndrome
(d) Turner’s syndrome.
Answer and Explanation:
18. (b): Klinefelter’s syndrome is an aneuploid condition with 3 sex chromosomes (trisomy). Klinefelter’s syndrome is caused by XXY genotype. This genotype results from the union of a nondisjunct XX egg and a normal Y sperm or normal X egg and abnormal XY sperm. The individual has 47 chromosomes (2n + 1).
The person having Klinefelter’s syndrome is a sterile male with small testes, unusually long legs, obesity, and sparse body hair, and with many female characteristics such as breasts. The victim usually has normal intelligence. The more the X chromosomes, the greater is the mental defect. Klinefelter’s syndrome occurs about once in 2000 live births.
Turner’s syndrome is formed by the union of an abnormal O egg and a normal X sperm or a normal egg and an abnormal O sperm. The individual has 45 chromosomes (44 + X). Down’s syndrome is caused by the presence of an extra chromosome number 21. Gynandromorphism is a phenomomenon by which one part of the body of an animal is male and the other part is female.
19. Albinism is known to be due to an autosomal recessive mutation. The first child of a couple with normal skin pigmentation was an albino. What is the probability that their second child will also be an albino?
(a) 50%
(b) 75%
(c) 100%
(d) 25%.
Answer and Explanation:
19. (d): Albinism is caused by the absence of the enzyme tyrosinase which is essential for the synthesis of the pigment from dihydroxyphenyalanine. The gene for albinism (a) does not produce the enzyme tyrosinase but its normal allele (A) does. Thus, only homozygous individual (aa) is affected by this disease. Albinos (individuals with albinism) lack dark pigment melanin in the skin, hair and iris. Although albinos have poor vision yet they lead normal life. On the basis of principles of simple recessive inheritance, the probability of albinic child from a normally pigmented parent, will be 1/4 or 25 %.
20. Mental retardation in man, associated with sex chromosomal abnormality is usually due to
(a) moderate increase in Y complement
(b) large increase in Y complement
(c) reduction in X complement
(d) increase in X complement.
Answer and Explanation:
20. (d): In humans, sex chromosomal abnormality is due to gene carried on X-chromosome. Increase in X- complement leads to, Klinefelter’s syndrome. Klinefelter’s syndrome, 47, XXY or XXY syndrome is a condition caused by a chromosome aneuploidy. Affected males have an extra X sex chromosome. It is formed by the union of an XX egg and normal Y sperm or normal X egg and abnormal XY sperm.
Affected males are almost always effectively sterile, although advanced reproductive assistance is sometimes possible and some degree of language learning impairment and mental retardation may be present.
In adults, possible characteristics vary widely and include little to no signs of affectedness, a lafiky, youthful build and facial appearance, or a rounded body type with some degree of gynecomastia (increased breast tissue). It is the second most common extra chromosome condition, and is named after Dr. Harry Klinefelter, an endocrinologist at Massachusetts General Hospital, Boston, Massachusetts, who first described it in 1942 the condition, exists in roughly 1 out of every 500 to 1,000 males.
21. A woman with two genes for haemophilia and one gene for colour blindness on one of the ‘X’ chromosomes marries a normal man. How will the progeny be?
(a) 50% haemophilic colour-blind sons and 50% normal sons
(b) 50% haemophilic daughters (carrier) and 50% colour blind daughters (carrier)
(c) all sons and daughters haemophilic and colourblind
(d) haemophilic and colour-blind daughters.
Answer:
(b) 50% haemophilic daughters (carrier) and 50% colour blind daughters (carrier)
22. In human beings, multiple genes are involved in the inheritance of
(a) sickle-cell anaemia
(b) skin colour
(c) colourblindness
(d) phenylketonuria.
Answer and Explanation:
22. (b): In human beings, inheritance of skin colour is due to multiple genes. This type of inheritance is called quantitative inheritance. Quantitative inheritance is a type of inheritance controlled by one or more genes in which the dominant alleles have cumulative effect with each codominant allele expressing a part or unit of the trait, the full trait being shown only when all the dominant alleles are present.
A few instances of such inheritance are skin colour in human beings, human intelligence, height in human beings etc. Sickle-cell anaemia and phenylketonuria are due to recessive genes (autosomal) in homologous condition. Colourblindness is produced by a recessive gene lying on X-chromosome.
23. Haemophilic man marries a normal woman. Their off springs will be
(a) all haemophilic
(b) all boys haemophilic
(c) all girls haemophilic
(d) all normal.
Answer:
(d) all normal.
24. The problem, due to Rh” factor arises when the blood of two (Rh+ and Rh”) mix up
(a) during pregnancy
(b) in a test tube
(c) through transfusion
(d) both (a) and (c).
Answer and Explanation:
24. (d): A protein named as rhesus antigen, is present on the surface of red blood corpuscles in many persons. Depending on the race, 85 to 99 percent of the white population have this rhesus antigen (also called Rh factor) and are called Rh positive (Rh+). Others who do not have this factor are known as Rh negative (Rh–). Both Rh+ and Rh+ individuals are phenotypically normal. The problem arises during blood transfusion and pregnancy. Incompatibility during blood transfusion: The first blood transfusion of Rh+ blood to the person with R+ blood causes no harm because the R+ person develops anti Rh factors or antibodies in his/her blood. In second blood transfusion of Rh+ blood to the Rh+ person, the latter’s anti Rh factors attack and destroy the red blood corpuscles of the donor.
If father’s blood is Rh+ and mother’s blood is R+, the foetus’ blood is Rh+. This is a serious problem. If the Rb+ blood of mother has not earlier come in contact with Rh+ blood through transfusion, her first child does not suffer (although the Rh+ blood of the foetus stimulates the formation of anti Rh factors or antibodies in the mother’s blood yet enough anti Rh factors are not produced in the mother’s blood to harm the foetus). But in second pregnancy (with Rh+ foetus), the anti Rh factors of the mother’s blood destroy the foetal red blood corpuscles. This is called erythroblastosis foetalis. New born may survive but it is often anaemic.
25. A marriage between normal visioned man and colour blind woman will produce offspring
(a) colour blind sons and 50% carrier daughter
(b) 50% colourblind sons and 50% carrier daughter
(c) normal males and carrier daughters
(d) colour blind sons and carrier daughters.
Answer:
(d) colour blind sons and carrier daughters.
26. Erythroblastosis foetalis is caused when fertilization takes place between gametes of
(a) Rh female and Rh+ male
(b) Rh+ female and Rh– male
(c) Rh+ female and Rh+ male
(d) Rh~ female and Rh– male.
Answer and Explanation:
26. (a): If fertilization takes place between gametes of R+ female and Rh+ male then the resulting foetus’ blood is Rh+. This is a serious problem. If the R+ child does not suffer (although the Rh+ blood of the foetus stimulates the formation of anti Rh factors are not produced in the mother’s blood to harm the foetus). But in second pregnancy (with Rh+ foetus), the anti Rh factors of the mother’s blood destroy the foetal red blood corpuscles. This is called erythroblastosis foetalis. New born may survive but it is often anaemic.
27. Mongolian Idiocy due to trisomy in 21st chromsorne is called
(a) Down’s syndrome
(b) Turner’s syndrome
(c) Klinefelter’s syndrome
(d) Triple X syndrome.
Answer and Explanation:
27. (a): Down’s syndrome (Mongolian Idioc), Mongolism) is caused by the presence of an extra chromosome number 21. It is characterised by rounded face, broad fore-nead, permanently open mouth protruding tongue, projecting lower lip, short neck, Hat hands and stubby (small) lingers, mongolian type eye lid fold. The victim has little intelligence. Heart and organs may be defective. Gonads and genita are undeveloped.
28. Probability of four sons to a couple is
(a) 1/4
(b) 1/8
(c) 1/16
(d) 1/32.
Answer:
28. (c): 1/16.
29. Male XX and female XY sometime occur due to
(a) deletion
(b) transfer of segments in X and Y chromosome
(c) aneuploidy
(d) hormonal imbalance.
Answer and Explanation:
29. (b): Male XX and female XY sometimes occur due to transfer of segments in X and Y chromosomes. Deletion is the loss of an intercalary segment of a chromosome which is produced by a double break in the chromosomes followed by the union of remaining parts. Aneuploidy is a condition of having fewer or extra chromosomes than the normal genome number of the species.
30. Number of Barr bodies in XXXX female is
(a) 1
(b) 2
(c) 3
(d) 4.
Answer and Explanation:
30. (c): Barr body is a mass of condensed sex chromatin in the nuclei of normal female somatic cells due to inactive X chromosome. Thus Barr body represents the inactivated X-chromosome. Whenever the number of X- chromosomes is two or more than two, the number of barr bodies is one less than the number of X- chromosomes. Therefore, the number of barr bodies in XXXX female is three.
31. Sickle cell anaemia induce to
(a) change of amino acid in a- chain of haemoglobin
(b) change of amino acid in b- chain of haemoglobin
(c) change of amino acid in both a and b chains of haemoglobin
(d) change of amino acid either a or b chains of haemoglobin.
Answer and Explanation:
31. (b): Sickle cell anaemia is an autosomal hereditary disorder in which the erythrocytes become sickle shaped. The disorder or disease is caused by the formation of abnormal haemoglobin called haemoglobin-S.
As found out by Ingram (1958), haemoglobin-S differs from normal haemoglobin-A in only one amino acid – 6th amino acid of P-chain, glutamic acid, is replaced by valine. This is the major effect of the allele. During conditions of oxygen deficiency 6-valine forms hydrophobic bonds with complementary sites of other globin molecules. It distorts their configuration. As a result, erythrocytes having haemoglobin-S become sickle-shaped.
32. Which of the following is a correct match?
(a) Down’s syndrome – 21st chromosome
(b) Sickle cell anaemia – X-chromosome
(c) Haemophilia – Y-chromosome
(d) Parkinson’s disease – X and Y chromosome.
Answer and Explanation:
32. (a): Down’s syndrome (Mongolian Idiocy, Mongolism) is caused by the presence of an extra chromosome number 21. The infant has a characteristic face with a flat nasal bridge, brushfield spots, protuding tongue, small ears and cardiac lesions. All affected children have mental retardation (moderate), short stature and autoimmune abnormalities. All males are infertile and females have reduced fertility.
Sickle cell anaemia is not a sex linked (i.e., X linked) disease but an autosomally inherited recessive trait. Hemophilia is X-linked but not holandric/Y-Iinked. Parkinson’s disease is a degenerative disease due to absence of neurotransmitter substance “dopamine” from brain. It is not at all hereditary.
33. A diseased man marries a normal woman. They get three daughters and five sons. All the daughters were diseased and sons were normal. The gene of this disease is
(a) sex linked dominant
(b) sex linked recessive
(c) sex limited character
(d) autosomal dominant.
Answer and Explanation:
33. (a): In the inheritance pattern of sex chromosomes X-chromosome of father always passes to daughter and X-chromosome of mother passes to son. As the father is diseased and all the girls inherit it, it is obvious the disease is sex-linked. The mother is not a carrier (as evident from the fact that no son is diseased). Thus the gene is dominant and expresses even in heterozygous condition.
34. Down’s syndrome is caused by an extra copy of chromosome number 21. What percentage of offspring produced by an affected mother and a normal father would be affected by this disorder?
(a) 100%
(b) 75%
(c) 50%
(d) 25%
Answer and Explanation:
34. (c): Down’s syndrome is the example of autosomal aneuploidy. Here, an extra copy of chromosome 21 occurs. As it is an autosomal disease, the offsprings produced from affected mother and normal father should be 50%.
35. Which one of the following conditions though harmful in itself, is also potential saviour from a mosquito borne infectious disease?
(a) thalassaemia
(b) sickle cell anaemia
(c) perniciofls anaemia
(d) leukaemia
Answer and Explanation:
35. (b): In sickle cell anaemia, the shape of the RBC change (sickle shaped) in comparison to normal one. The sickle cells (RBCs) cannot pass through capillaries and thus clog them.
On the other hand, the mosquito borne disease is malaria. Here, the main phase of sexual cycle, in affected mosquito occurs in normal RBC. But as in sicke cell anaemia, RBCs become deformed, the mosquito cycle cannot be continued here. Thus sickle cell anaemia is a potential saviour from malaria.
36. Pattern baldness, moustaches and beard in human males are examples of
(a) sex-linked traits
(b) sex limited traits
(c) sex influenced traits
(d) sex determining traits
Answer and Explanation:
36. (c): Sex influenced traits are autosomal traits that are influenced by sex. If a male has one recessive allele, he will show that trait, but it will take two recessive alleles for the female to show that same trait e.g. pattern baldness, moustaches and beard in males. Sex linked traits are those traits the determining genes of which are found on the sex chromosomes. Sex limited traits are the traits which are expressed in a particular sex though their genes also occur in the other sex e.g. milk secretion in mammalian females.
37. The recessive genes located on X-chromosome humans are always
(a) lethal
(b) sub-lethal
(c) expressed in males
(d) expressed in females.
Answer and Explanation:
37. (c): The recessive genes located on X-chromosome of humans are always expressed in males e.g. colour blindness is a recessive sex-linked trait in which the eye fails to distinguish red and green colours. The gene for the normal vision is dominant. The normal gene and its recessive allele are carried by X-chromosomes. In females colour blindness appears only when both the sex chromosomes carry the recessive gene (XCXC).
The females have normal vision but function as carrier if a single recessive gene for colour blindness is present (XXC).~ However, in human males the defect appears in the presence of a single recessive gene (XCY) because Y- chromosome of male does not carry any gene for colour vision.
38. A normal woman, whose father was colour-blind is married to a normal man. The sons would be
(a) 75% colour-blind
(b) 50% colour-blind
(c) all normal
(d) all colour-blind.
Answer:
(b) 50% colour-blind
39. A woman with normal vision, but whose father was colour blind, marries a colour blind man. Suppose that the fourth child of this couple was a boy. This boy
(a) may be colourblind or may be of normal vision
(b) must be colour blind
(c) must have normal colour vision
(d) will be partially colour blind since he is heterozygous for the colourblind mutant allele.
Answer:
(a) may be colourblind or may be of normal vision
40. Which of the following is not a hereditary disease?
(a) cystic fibrosis
(b) thalassaemia
(c) haemophilia
(d) cretinism.
Answer and Explanation:
40. (d): Cretinism occurs due to hyposecretion of thyroid hormones. Haemophilia is a sex linked recessive trait. Cystic fibrosis is also a recessive autosomal disorder resulting in mucus clogging in lungs. Thalassemia involves a gene mutation in the polypeptide chains of haemoglobin.
41. Haemophilia is more commonly seen in human males than in human females because
(a) a greater proportion of girls die in infancy
(b) this disease is due to a Y-linked recessive mutation
(c) this disease is due to an X-linked recessive mutation
(d) this disease is due to an X-linked dominant mutation.
Answer and Explanation:
41. (c): Haemophilia is a sex linked disease in which the patient continues to bleed even from a minor cut since he or she does not possess the natural phenomenon of blood clotting. Haemophilia (= hemophilia) is genetically due to the presence of a recessive gene h, carried by X- chromosome. A female becomes haemophiliac only when both its X-chromosomes carry the gene (XhXh). However, such females generally die before birth because the combination of these two recessive alleles is lethal.
A female having only one allele for haemophilia (XXh) appears normal because the allele for normal blood clotting present on the other X-chromosome is dominant. Such females are known as carriers. In case of males, a single gene for the defect is able to express itself as the Y-chromosome is devoid of any corresponding allele (XhY) in which an organism has three times (3n) the haploid number (n) of chromosomes.
42. A woman with 47 chromosomes due to three copies of chromosome 21 is characterized by
(a) super femaleness
(b) triploidy
(c) Turner’s syndrome
(d) Down’s syndrome.
Answer and Explanation:
42. (d): Down’s syndrome is caused by the presenc of an extra chromosome number 21. Both the chromosomes of the pair 21 pass into a single egg. Thus, the egg possesses 24 chromosomes instead of 23 and offspring has 47 chromosomes (45 + XY in males, 45 + XX in males) instead of 46. Turner’s syndrome is formed by the union of an abnormal O egg and a normal X sperm or a normal egg and an abnormal O sperm.
The individual has 45 chromosomes (44 + X) instead of 46. Female with more than two X chromosomes is called superfemale. Triploidy is a condition in which an organism has three times (3n) the haploid number (n) of chromosomes.
43. A man and a woman, who do not show any apparent signs of a certain inherited disease, have seven children (2 daughters and 5 sons). Three of the sons suffer from the given disease but none of the daughters affected. Which of the following mode of inheritance do you suggest for this disease?
(a) sex-linked dominant
(b) sex-linked recessive
(c) sex-limited recessive
(d) autosomal dominant.
Answer:
(b) sex-linked recessive
44. G-6-P dehydrogenase deficiency is associated with haemolysis of
(a) leucocytes
(b) lymphocytes
(c) platelets
(d) RBCs.
Answer and Explanation:
44. (d): Glucose-6-phosphate dehydrogenase (G-6-PD)deficiency is a group of hereditary abnormalities (X linked disorder) in which the activity of the erythrocyte enzyme G-6-PD is markedly diminished leading to haemolysis.
45. Sickle cell anaemia has not been eliminated from the African population because
(a) it is controlled by dominant genes
(b) it is controlled by recessive genes
(c) it is not a fatal disease
(d) it provides immunity against malaria.
Answer and Explanation:
45. (d): Sickle cell anaemia is an autosomal hereditary disorder in which the erythrocytes become sickle shaped. The disorder or disease is caused by the formation of an abnormal haemoglobin called haemoglobin-S. As found out by Ingram (1958), haemoglobin-S differs from normal haemoglobin-A in only one amino acid – 6th amino acid of P-chain, glutamic acid, is replaced by valine.
This is the major effect of the allele. During conditions of oxygen deficiency 6-valine forms hydrophobic bonds with complementary sites of other globin molecules. It distorts their configuration. As a result, erythrocytes having haemoglobin-S become sickle-shaped. Carriers of the sickle cell anaemia gene are protected against malaria because of their particular haemoglobin mutation; this explains why sickle cell anaemia is particularly common among people of African origin.
The material parasite has a complex life cycle and spends part of it in red blood cells and feeds on haemoglobin. Both sickle-cell anaemia and thalassemia are more common in malaria areas, because these mutations convey some protection against the parasite. In a carrier, the presence of the malaria parasite causes the red blood cell to rupture, making the Plasmodium unable to reproduce. Further, the polymerization of Hb affects the ability of the parasite to digest Hb in the first place. Therefore, in areas where malaria is a problem, people’s chances of survival actually increase if they carry sickle cell anaemia.
46. Cri-du-chat syndrome in humans is caused by the
(a) trisomy of 21st chromosome
(b) fertilization of an XX egg by a normal Y-bearing sperm
(c) loss of half of the short arm of chromosome 5
(d) loss of half of the long arm of chromosome 5
Answer and Explanation:
46. (c): Cri du chat syndrome, also called deletion 5p syndrome, (or 5p minus), is a rare genetic disorder. Cri du chat syndrome is due to a partial deletion of the short arm of chromosome number 5. The name of this syndrome is French for “cry of the cat,” referring to the distinctive cry of children with this disorder.
The cry is caused by abnormal larynx development, which becomes normal within a few weeks of birth. Infants with cri du chat have low birth weight and may have respiratory problems. Some people with this disorder have a shortened lifespan, but most have a normal life expectancy.
47. If a colourblind woman marries a normal visioned man, their sons will be
(a) all colourblind
(b) all normal visioned
(c) one-half colourblind and one-half normal
(d) three-fourths colourblind and one-fourth normal (2006)
Answer:
(a) all colourblind
48. Both sickle cell anaemia and Huntington’s chorea are
(a) virus-related diseases
(b) bacteria-related diseases
(c) congenital disorders
(d) pollutant-induced disorders
Answer and Explanation:
48. (c): A congenital disorder is a medical condition that is present at birth. Congenital disorders can be a result of gem-‘ic abnormalities, the intrauterine environment, or rnknown factors. Sickle cell disease [a group of genetic disorders caused by sickle haemoglobin (Hgb S) HbS molecules tend to clump together, making red blood cells sticky, stiff, and more fragile, and causing them to form into a curved, sickle shape.] and Huntington’s chorea (an inherited disorder characterized by degenerative changes in the basal ganglia structures, which ultimately result in a severely shrunken brain and enlarged ventricles, abnormal body movements called chorea, and loss of memory) are congenital disorders.
49. A human male produces sperms with the genotypes AB, Ab, aB, and ab pertaining to two diallelic characters in equal proportions. What is the corresponding genotype of this person?
(a) AaBB
(b) AABb
(c) AABB
(d) AaBb.
Answer and Explanation:
49. (d): As sperms produced are with genotypes AB, Ab, aB, ab (two diallelic character) the person must be heterozygous for both genes. So his genotype will be AaBb.