17.1 Inheritance, Genes & Cell Division

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The four o’clock plant, Mirabilis jalapa, can have flowers of three different colours as shown in Fig. 1.

Fig .1

A student crossed some red-flowered plants with some yellow-flowered plants (cross 1).

She collected the seeds and grew them. All of the plants that grew from these seeds had orange flowers.

Complete the genetic diagram to explain the result of cross 1.

The student then carried out three further crosses as shown in Table 1.

Table 1

		genotypes of offspring
cross 2	offspring of cross 1 × offspring of cross 1
cross 3	offspring of cross 1 × red-flowered plant
cross 4	offspring of cross 1 × yellow-flowered plant

Complete the table by writing in the genotypes of the offspring of crosses 2, 3 and 4, using the same symbols as in the genetic diagram in (a).

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Flower colour in M. jalapa is not an example of the inheritance of dominant and recessive alleles.

Explain how the results of the crosses show that these alleles for flower colour are not dominant or recessive. 

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Some people inherit colour blindness and cannot identify certain colours, even in bright light.

The gene responsible for colour vision is located on the X chromosome.

There are two alleles for this gene on the X chromosome:

• X^B – normal colour vision
• X^b – colour blindness.

People that are heterozygous for colour blindness are called carriers.

State the genotype of a heterozygous female carrier.

[1]

There is no gene for colour vision on the male sex chromosome.

State the genotype of a colour-blind male.

Fig. 1 shows a pedigree diagram for colour blindness.

Fig. 1

Person 13 in Fig. 1 is male. His parents are person 7 and person 8.

Use the key to complete Fig. 1 by drawing the correct symbol for person 13.

[1]

Colour blindness is a sex-linked characteristic.

Explain why females 4 and 5 are carriers even though their mother is not a carrier.

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Fig. 1 shows the sequence of events that occur in sexual reproduction.

Fig 1

State the name of cell T in Fig. 1.

[1]

(ii)

State the name of the process that takes place at S to form cell T in Fig. 1.

[1]

(iii)

State where in the human body process S takes place.

[1]

Some humans have the ability to roll their tongues and some cannot roll their tongues.

This characteristic is controlled by genes.

Fig. 2 shows two boys: boy A cannot roll his tongue and boy B can roll his tongue.

Fig. 2

The allele for tongue rolling (T) is dominant to the allele for non-tongue rolling (t).

Fig. 3 shows a family tree for this characteristic. Individual 1 and his partner are both heterozygous for tongue rolling.

Fig. 3

Complete Table 1 by inserting the genotypes of the numbered individuals in Fig. 3.

Table 1

Individual	Genotype
1
3
4

[3]

Individual 2 in Fig. 3 is heterozygous for tongue rolling. He marries a woman who cannot roll her tongue.

State all of the possible genotypes of their children and predict the ratio of phenotypes for their children.

Possible offspring genotypes: .............................................

Ratio of phenotypes: ..............................................................

[2]

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Haemophilia is a sex-linked blood disorder in which blood takes a long time to clot.

Fig. 1 is a pedigree diagram showing the inheritance of haemophilia.

Fig. 1

The normal allele is represented by X^H and the mutant allele is represented by X^h.

State the genotypes of the people identified as P, Q and R in Fig. 1.

[3]

The couple S and T are expecting another child.

What is the probability that the child will have haemophilia?

Space for working is provided for this question.

Define the term sex-linked characteristic.

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Haemophilia is a sex-linked blood disorder. The blood of people with haemophilia takes longer to clot.

Fig. 1 is a pedigree diagram showing the inheritance of haemophilia.

Fig. 1

• The allele for normal clotting time is represented by X^H.
• The allele for haemophilia is represented by X^h.

State the genotypes of the people identified as P, Q and R in Fig. 1.

[3]

The couple S and T are expecting another child.

State the probability that the child will have haemophilia.

[1]

Describe what is meant by the term sex-linked characteristic.

[2]

Fig. 1 shows a dwarf sunflower and a tall sunflower, Helianthus annuus.

The height of the dwarf sunflower is 0.45 m and the height of the tall sunflower is 4.5 m.

Fig. 1

Dwarf plants like the one in Fig. 1 have mutant alleles.

Define the term allele.

[1]

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Shoot growth in plants is controlled by auxins. An enzyme in shoot tips converts molecules of an amino acid into auxins as shown in Fig. 2.

Fig. 2

Explain how a mutation in DNA results in an abnormal enzyme which does not catalyse the reaction shown in Fig. 2.

[3]

Two tall sunflower plants were crossed. 25 % of the offspring produced were dwarf.

Explain how it is possible for two tall parent plants to have this percentage of dwarf offspring.

[2]

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Fig. 3 shows how several strawberry plants can be formed from one parent plant.

Fig. 3

Explain the type of reproduction that produces plants by the method shown in Fig. 3.

[3]

Explain the disadvantages of the type of reproduction shown in Fig. 3.

Organisms pass on their genetic information in their gametes.

State the name of the type of cell division that produces gametes.

[1]

State the name of the cell formed when the nuclei of two gametes join together.

A rabbit that was homozygous for black fur was crossed with a rabbit that was homozygous for brown fur.

All of their offspring had black fur.

This is shown in Fig. 1.

Fig. 1

Define the term homozygous.

[1]

State the dominant allele for the rabbits' fur colour and give a reason for your answer.

The F₁ offspring all have the same phenotype as the male parent but their genotype is not the same as the male parent.

State how the phenotype of an organism is different to its genotype.

A rabbit with brown fur is mated with one of the F₁ rabbits with black fur.

Complete the genetic diagram to show the possible fur colours that could occur from this mating.

New breeds of rabbits can be produced by selective breeding.

Describe the stages in the process of selective breeding.

The Indian muntjac deer, Muntiacus muntjak, is recorded as the mammal with the lowest number of chromosomes.

Fig. 1 is an image of the chromosomes in the nucleus of a diploid cell of a female muntjac deer.

Fig 1

State the diploid number of chromosomes for the female muntjac deer.

Fig. 2 is an image of the chromosomes in the nucleus of a diploid cell of a male muntjac deer.

Fig 2

Describe how the sex chromosomes of the male muntjac deer shown in Fig. 2 differ from those of the female shown in Fig. 1.

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Explain how meiosis can result in variation in a species.

Use the words chromosome and gametes in your answer.

[4]

Another cause of variation is the formation of new alleles.

Describe how new alleles can be formed.

[3]