lq - 23 genetics

Certificate Biology New Mastering Basic Concepts Chapter 23Question Bank V2.0 Structured Questions

Ch 23 Genetics

[231001]

The eye colours of the individuals of three families are shown in the following

diagram. The allele for brown eye is dominant over the allele for blue eye. Let B

be the allele for brown eye and b for blue eye.

a State the genotypes of individuals 1, 4 and 8. Give reasons. (6 marks)

b If individual 7 and 8 plan to have another child, what is the chance that the

child is blue-eyed? Explain. (4 marks)

-- ans --

a As individual 7 is blue-eyed, her genotype must be bb. She got one of the b alleles

from individual 1. 1m

As individual 1 is brown-eyed, his genotype must be Bb. 1m

For individual 4, she is brown-eyed. Her genotype may be BB or Bb. 2m

Individual 3 must have genotype bb as he is blue-eyed. He passed one b allele to

individual 8. 1m

As individual 8 is brown-eyed, his genotype must be Bb. 1m

b Parent genotype bb Bb

Oxford University Press 2005 / 21


Gametes b B b 1m

Offspring genotype Bb bb 1m

Offspring phenotype brown-eyed blue-eyed 1m

The chance of getting a blue-eyed child is 50%. 1m

-- ans end --

[231002]

[E] The graph below shows the change in the relative amount of chromosomal

materials in a mammalian cell.

a What brings about the changes in the amount of chromosomal materials at

A, B and C? (3 marks)

b Name this type of cell division. What is the name of the cells produced?

(2 marks)

c What is the significance of this type of cell division? (2 marks)

d Does this type of cell division occur in all kinds of organisms? Explain.

(2 marks)

e What is a chromosome made of? (1 mark)

-- ans --

a A -- Each chromosome replicates its DNA. 1m

B -- The two members of each homologous pair of chromosomes separate from



each other. 1m

C -- The chromatids split apart and separate. 1m

b Meiotic cell division 1m

Gamete 1m

c Meiotic cell division leads to the chromosome number of each gamete being

reduced to half the normal body cell number. 1m

The number of the chromosomes is then restored in the zygote when two gametes

fuse at fertilization. 1m

d No. Meiotic cell division only occurs in organisms which reproduce sexually. It does

not occur in organisms which reproduce asexually. 2m

e DNA molecules and special proteins 1m

-- ans end --

[231003]

a Cystic fibrosis is a genetic disease. People with cystic fibrosis have two

recessive alleles. A couple who show no symptoms of the disease and plan to

have a baby found that both of them are carriers after a medical examination.

Using suitable symbols, draw a genetic diagram to deduce the probability of

having a child with cystic fibrosis. (5 marks)

b In ancient China, it was very important that a married man had a son to

inherit the family business and fortune. A man had 12 daughters but no sons.

Do you think the chance of having a son would increase for the 13th child?

Explain. (4 marks)

c Can mutations in body cells be passed on to the next generation? Why?

(1 mark)

-- ans --

a Let F be the allele for the normal character and f be the allele for cystic fibrosis. 1m

Genetic diagram:



Parental phenotype normal normal 0.5m

Parental genotype Ff Ff 0.5m

Gametes F f F f 0.5m

Offspring genotypes FF Ff Ff ff 1m

Offspring phenotypes normal normal normal cystic fibrosis

Ratio 3 : 1 0.5m

The probability of having a child with cystic fibrosis is 25%. 1m

b During meiosis in sperm production, two kinds of sperms are produced, one

carrying chromosome X and one carrying chromosome Y. 1m

All egg cells carry chromosome X. 1m

As the number of X-bearing sperms is equal to the number of Y-bearing sperms,

and which type of sperms meets the egg is random, the chance of having a baby

boy or baby girl is 50%. 1m

Since the chance of each birth is not related to the previous one, the chance of

having a boy for the 13th child is still 50%. 1m

c No. Only mutations that occur in gametes can be passed on to the next generation.

1m

-- ans end --



[231004]

The family tree below shows the inheritance of blood groups in a family.

(Genetic diagrams are not accepted for the following questions.)

a Based on the given information, explain which blood group, B or O, is the

recessive character. (4 marks)

b What may the genotype of individual D be? Explain. (4 marks)

c Which is usually more easily affected by environmental factors, continuous

or discontinuous variations? Give one example of your answer. (2 marks)

-- ans --

a Individual F is the son of A and B. F belongs to the blood group O. He must have

received one allele for blood group O from each parent. 2m

But neither parent is of blood group O. Blood group O is the recessive character

because the gene for blood group O does not express itself in the parents. 2m

b L is homozygous recessive. He must have got one recessive allele from each

parent. H must be heterozygous. 2m

The recessive allele of H must have come from C and the dominant allele must

have come from D. 1m



D may be homozygous or heterozygous for the blood group B. 1m

c Continuous variation 1m

Height, weight OR any other correct examples 1m

-- ans end --



[231005]

A group of students took an intelligence quotient (IQ) test. The table below

shows the number of students in each IQ interval.

Intelligence quotient (IQ) Number of students

91 - 95 1

96 - 100 5

101 - 105 7

106 - 110 15

111 - 115 18

116 -120 10

121 - 125 4

a Construct a histogram to show the variation in IQ of the group of students.

(3 marks)

b Which type of variation is it? Explain your justification. (2 marks)

c Give two more examples of this type of variation in humans. (1 mark)

d List the causes of variations. (4 marks)

-- ans --

a



Correct title 0.5m

Correct scale and labelling of axes 0.5m, 0.5m

Correct plotting of data 1.5m

b Continuous variation 1m

Because there is a continuous range of intermediate phenotypes between the

extremes of the character. 1m

c Height / pulse rate / heartbeat rate / hand span (any 2) 0.5m, 0.5m

d Independent assortment of chromosomes at meiosis 1m

Random fertilization of gametes 1m

Mutation 1m

Environmental effect 1m

-- ans end --



[231006]

In rabbits, the dominant gene G is for grey fur and the recessive gene g is for

white fur. A rabbit with grey fur is mated with one with white fur. When their

offspring are reared at 25C, some of them are white and some of them are grey.

However, all the offspring are white if they are reared below 10C.

a What are the genotypes of the parents? (2 marks)

b Draw a genetic diagram of the cross of the parents. In the diagram, state the

genotypic and phenotypic ratios of the offspring. (5 marks)

c How is the fur colour affected by the temperature at which the offspring are

reared? (2 marks)

d If the parents mate again, what is the probability that their offspring are grey

if they are reared at 25C? (1 mark)

-- ans --

a Grey rabbit: Gg 1m

White rabbit: gg 1m

b

Correct genetic diagram 3m

Correct genotypic and phenotypic ratios 1m, 1m



c Grey fur colour fails to develop at a temperature below 10℃. 2m

d 50% 1m

-- ans end --

[231007]

The pedigree below shows the inheritance of tongue rolling ability in a family.

a Deduce from the pedigree whether tongue rolling is a dominant or recessive

character. (5 marks)

b State the genotype of member W. Define the symbols you use. (2 marks)

c Draw a genetic diagram of the cross of members R and S. (3 marks)

-- ans --

a Both members R and S are tongue rollers, therefore each of them must possess at

least one allele for tongue rolling. 1m

One of their children (member W) is a non-tongue roller, that means at least one of

the parents must also possess one allele for non-tongue rolling. 1m

Al least one of the parents is heterozygous. 1m

In a heterozygous condition, only the dominant allele will be expressed. 1m



Therefore, tongue rolling is a dominant character. 1m

b Let G = allele for tongue rolling 0.5m

g = allele for non-tongue rolling 0.5m

Genotype of W is gg. 1m

c

-- ans end --



[231101]

People with albinism cannot produce the pigment melanin, so they have very

pale skin, hair and eyes. The ability to produce melanin is controlled by a pair of

alleles. The pedigree below shows the inheritance of albinism in a family.

a What is the function of melanin in the skin? (2 marks)

b Deduce, with reasons, whether the allele for albinism is dominant or

recessive. (Marks will not be given for genetic diagrams.) (4 marks)

c State the possible genotype(s) of individual 8. Define the symbols you

use.

(3 marks)

d What is the probability of inheriting albinism in the next child of

individuals 10 and 11? Explain with a genetic diagram. (3 marks)

-- ans --

a Melanin protects the skin 1m

by absorbing harmful ultraviolet light in the sunlight. 1m



b Individual 13 has albinism, therefore she must have at least one allele for

albinism. 1m

The allele(s) for albinism must come from individual 10 and/or individual 11. 1m

Since individuals 10 and 11 are normal for this character, 1m

the allele for albinism must be recessive or it will be expressed in individual 10

and/or individual 11. 1m

OR

Individual 5/7 has albinism, therefore he/she must have at least one allele for

albinism. 1m

The allele(s) for albinism must come from individual 1 and/or individual 2. 1m

Since individuals 1 and 2 are normal for this character, 1m

the allele for albinism must be recessive or it will be expressed in individual 1

and/or individual 2. 1m

c A = allele for normal condition 1m

a = allele for albinism 1m

Genotype of individual 8: Aa 1m

d

Individual 10 Individual 11

Parents Aa x Aa 0.5m

Gametes A a A a 0.5m

Offspring AA Aa Aa aa 0.5m

Phenotypes I---------------- Normal ----------------I Albino 0.5m

Ratio 3 : 1

The probability of inheriting albinism in their next child is 25 %. 1m



-- ans end --



[231102]

Cystic fibrosis is a genetic disorder that causes the lungs and the pancreas to

produce excessive thick mucus. Whether a person has cystic fibrosis is

controlled by a pair of alleles. The allele for the normal condition (N) is

dominant to the allele for the disease (n). The pedigrees below show the

inheritance of cystic fibrosis in two families.

a State how the following organs would be affected if they produce

excessive thick mucus.

i The lungs (2 marks)

ii The pancreas (2 marks)

b Individuals 4 and 7 are going to get married. Using Punnett square, give

the possible cross(es) between individuals 4 and 7 that involve(s) no risk

of producing a child with cystic fibrosis. (6 marks)

c Using Punnett square, give the cross between individuals 4 and 7 that

involves the risk of producing a child with cystic fibrosis. (2 marks)

d From the results of b and c, what is the probability of producing a child

with cystic fibrosis from individuals 4 and 7? (1 mark)

-- ans --

a i The thick mucus would block the air passage in the lungs, 1m

causing difficulty in breathing. 1m



OR

The thick mucus with trapped bacteria has trouble moving out of the lungs,

1m

so bacteria would remain and cause infections. 1m

ii The thick mucus would block the passage of digestive juice from the

pancreas to the small intestine, 1m

causing indigestion. 1m

b

2m x 3

c

2m

d 1/4 x 1/4 = 1/16 1m

-- ans end --



[231103]

The diagram below shows the full set of chromosomes of a foetus suffering

from a genetic disorder.

a i Give one similarity and one difference between A and B in relation

to the genes they carry. (2 marks)

ii What is the term for the structure making up of A and B? (1 mark)

b What is the sex of this foetus? (1 mark)

[E] c i From the abnormality of chromosomes shown in the diagram, state

the name of the genetic disorder of this foetus. (1 mark)

ii Suggest how this abnormality of chromosomes occurs. (4 marks)

-- ans --

a i Similarity:

They carry the same number of genes. /

The genes they carry are in the same sequence. (any 1) 1m

Difference:

They carry different forms of the genes (alleles). /



The genes they carry come from different parents. (any 1) 1m

ii Homologous chromosomes 1m

b Male 1m

c i Down syndrome 1m

ii The two members of chromosome pair 21 fail to separate in the production

of gametes / sperm / ova in the father / mother 1m

and both of them go to the same gamete / sperm / ovum. 1m

When the gamete / sperm / ovum carrying an extra chromosome 21 fuses

with a normal gamete during fertilization, 1m

an extra chromosome 21 exists in the zygote and hence the cells coming

from it. 1m

-- ans end --



[231104]

[E] Glucose-6-phosphate dehydrogenase (G6PD) deficiency is caused by an

abnormality in the gene for the enzyme.

a Explain how an abnormality in the gene causes a reduction in G6PD.

(3 marks)

b Explain why G6PD deficiency is more commonly found in males than in

females. (3 marks)

c What is the probability of a normal father and a heterozygous mother

giving birth to a son with G6PD deficiency? Explain with a genetic

diagram and define the symbols you use. (4 marks)

-- ans --

a The gene is a string of codes for the sequence of amino acids that make up

G6PD. 1m

An abnormality in the gene changes the codes, 1m

thus functional G6PD cannot be produced. 1m

b G6PD deficiency is caused by a recessive allele located on the X chromosome.

1m

Females have two X chromosomes, thus females must inherit one allele for

G6PD deficiency from each of the parents for the disease to develop. 1m

Males have only one X chromosome, thus a single allele for G6PD deficiency will

cause the disease in males. 1m

c XN = allele for normal condition 0.5m

Xn = allele for G6PD deficiency 0.5m

Father Mother

Parents XN Y x XN Xn 0.5m

Gametes XN Y XN Xn 0.5m

Offspring XN XN XN Xn XN Y Xn Y 0.5m

Phenotypes Normal daughters Normal

son

Diseased

son

0.5m



Ratio 2 : 1 : 1

The probability of giving birth to a son with G6PD deficiency is 25 %. 1m

-- ans end --


lq - 23 genetics

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