Cambridge IGCSE0610

Chromosomes, genes and proteins

Biology 0610 Chapter Notes

What this chapter covers

Chromosomes, genes and proteinsMitosisMeiosisMonohybrid inheritance

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1. Chromosomes and genes

Chromosomes are made of DNA, which contains genetic information in the form of genes.

A gene is a length of DNA that codes for a protein. Chromosomes are found in the nucleus.

Examiner insight

“A common error was to make reference to a “strand” of DNA rather than a “piece” of DNA.” “Many responses stated that a gene codes for a characteristic or a trait, rather than stating that a gene codes for a protein.”

Worked example 11 mark

What is defined as a length of DNA that codes for a protein?

A amino acid · B chromosome · C gene · D nucleus

1
C: a gene is a length of DNA that codes for a protein. [1]

2. Alleles

An allele is an alternative form of a gene.

A gene and its alleles are not the same thing. The gene is the length of DNA. An allele is one of its alternative forms.

Examiner insight

“Common errors were definitions of genes or to state that an allele is part of a gene.” “While many candidates understood that an allele is a version of a gene, some candidates incorrectly believed that an allele is a version of a chromosome.”

Worked example 11 mark

Define the term allele.

1
an alternative form of a gene [1]

3. Inheritance of sex in humans

Sex is decided by one pair of chromosomes, the sex chromosomes. A female has two X chromosomes: XX. A male has one X and one Y: XY.

Every egg cell carries an X. Half of the sperm carry X and half carry Y. So each child has an equal chance of being a girl or a boy.

Examiner insight

“Many candidates understood that all egg cells have an X chromosome, and with sperm cells, half have an X chromosome and half have a Y chromosome.” “A small minority seemed unfamiliar with the existence of the Y chromosome and hence did not distinguish between the two sex chromosomes, using X throughout.”

Worked example 11 mark

Which statement about the human sex chromosomes is correct?

A Females have an X chromosome and a Y chromosome.

B Females have two Y chromosomes.

C Males and females have at least one X chromosome.

D Males and females have at least one Y chromosome.

1
C: females are XX and males are XY, so everyone has at least one X. [1]

4. Base sequence, amino acids and shape

The sequence of bases in a gene determines the sequence of amino acids used to make a specific protein.

Different sequences of amino acids give different shapes to protein molecules. Shape matters: a receptor molecule must have a shape complementary to its neurotransmitter for the neurotransmitter to bind.

Examiner insight

“There were also many vague answers such as DNA, RNA, and gene, with no mention of the sequence or order of bases.” “A common misconception was that the mutation changed the amino acid sequence of DNA or the base sequence of proteins.”

Worked example 11 mark

Which row describes the control of the production of proteins in a cell?

	what forms the genetic code	what DNA codes for
A	sequence of amino acids	sequence of bases
B	sequence of amino acids	sequence of proteins
C	sequence of bases	sequence of amino acids
D	sequence of bases	sequence of proteins

1
C: the sequence of bases forms the genetic code, and DNA codes for a sequence of amino acids. [1]

5. DNA controls the cell through proteins

DNA controls cell function by controlling the production of proteins, including enzymes, membrane carriers and receptors for neurotransmitters.

Enzymes are proteins, so DNA controls which enzymes a cell has.

Examiner insight

“There was some confusion evident as some did not seem to know that enzymes are proteins.” “A common misconception was to refer to the proteins, rather than the ions, moving across the membrane.”

Worked example 11 mark

A scientist investigates the action of a drug that affects the production of proteins by cells.

The scientist grows two groups of cells. The scientist adds the drug to one group of the cells. After one day, the scientist determines the mass of proteins in each group of cells. The table shows the results.

Which statements could explain the results in the table?

1 The drug stops messenger RNA passing through ribosomes.

2 The drug stops production of messenger RNA.

3 The drug stops the copying of DNA before mitosis.

A 1, 2 and 3 · B 1 and 2 only · C 1 and 3 only · D 2 and 3 only

group of cells	mass of proteins produced per million cells / μg
with no drug	25
with drug	5

1
B: statements 1 and 2 only. [1]
2
Statement 1 fits: mRNA must pass through ribosomes for amino acids to be assembled into proteins.
3
Statement 2 fits: mRNA is the copy of a gene, so with no mRNA nothing reaches the ribosomes.
4
Statement 3 does not fit: it is about copying DNA before mitosis, which is not a step in making a protein.

6. How a protein is made

The gene coding for the protein remains in the nucleus. Messenger RNA (mRNA) is a copy of the gene. It is made in the nucleus and moves to the cytoplasm.

The mRNA passes through a ribosome. The ribosome assembles amino acids into protein molecules. The process requires energy from respiration.

The specific sequence of amino acids is determined by the sequence of bases in the mRNA.

Examiner insight

“Many candidates also suggested that mRNA enters the nucleus to copy the gene instead of mRNA being synthesised from DNA in the nucleus.” “Some candidates incorrectly believed that the gene coding for the protein moves from the nucleus to the cytoplasm.”

Worked example 16 marks

DNA controls cell function by controlling the production of proteins.

Explain how a protein is made.

1
a gene is copied [1]
2
to make messenger RNA / mRNA [1]
3
mRNA is moved to the cytoplasm / to a ribosome [1]
4
mRNA passes through ribosomes [1]
5
ribosomes assemble amino acids into protein molecules [1]
6
the sequence of amino acids is determined by the sequence of bases of the mRNA [1]

7. Same genes, different proteins

Most body cells in an organism contain the same genes. But many genes in a particular cell are not expressed.

The cell only makes the specific proteins it needs. Two cells can hold the same genes and still make different proteins.

Examiner insight

“Few candidates realised that cells with different functions use only some of the genes in their nuclei.” “Some thought that genes are not expressed because they have mutated.”

Worked example 13 marks

A cell in the pancreas of a person makes insulin. A skin cell from the same person does not. Explain why the skin cell does not make insulin.

1
both cells contain the same genes, including the gene for insulin [1]
2
in the skin cell the insulin gene is not expressed [1]
3
the skin cell only makes the specific proteins it needs [1]

8. Haploid and diploid nuclei

A haploid nucleus contains a single set of chromosomes. A diploid nucleus contains two sets of chromosomes.

In a diploid cell there is a pair of each type of chromosome. In a human diploid cell there are 23 pairs, which is 46 chromosomes.

Examiner insight

“Many knew that the haploid nucleus contains chromosomes but did not qualify their statement further to distinguish it from a diploid nucleus.” “Almost all candidates assumed that the species in the question, in fact all species, have a diploid number of 46.”

Worked example 12 marks

The nucleus of a human skin cell contains 46 chromosomes. State the number of pairs of chromosomes in the nucleus and name this type of nucleus.

1
23 pairs [1]
2
diploid: it has two sets of chromosomes [1]

Recap

Chromosomes are made of DNA, which contains genetic information in the form of genes.
A gene is a length of DNA that codes for a protein. An allele is an alternative form of a gene.
Females are XX and males are XY. Eggs carry X. Sperm carry X or Y.
Extended: the sequence of bases in a gene determines the sequence of amino acids. Different sequences give different protein shapes.
Extended: DNA controls the cell by controlling which proteins are made, such as enzymes, membrane carriers and receptors.
Extended: the gene stays in the nucleus. mRNA is a copy of the gene, moves to the cytoplasm and passes through a ribosome, which assembles amino acids.
Extended: body cells hold the same genes, but a cell only expresses the genes for the proteins it needs.
Extended: a haploid nucleus has one set of chromosomes. A diploid nucleus has two sets, so a pair of each type. Human diploid cells have 23 pairs.

9. Mitosis as nuclear division

Mitosis is nuclear division giving rise to genetically identical cells.

The details of the stages are not required. What matters is the result: daughter cells that are genetically identical to each other and to the parent cell.

Mitosis does not increase genetic variation.

Examiner insight

“Few candidates referred to the separation of chromosomes or nuclear division.” “Some candidates incorrectly believed that mitosis can increase genetic variation in a species.”

Worked example 11 mark

A cell divides by mitosis once every 3 hours and 24 minutes.

How long will it take to produce 8 new cells from this original cell?

A 583 minutes · B 612 minutes · C 1555 minutes · D 1632 minutes

1
B: one cell becomes 2, then 4, then 8, which is 3 divisions. 3 × 204 = 612 minutes. [1]

10. The roles of mitosis

Mitosis has four roles: growth, repair of damaged tissues, replacement of cells and asexual reproduction.

In asexual reproduction one parent produces offspring by mitosis. The offspring are genetically identical to the parent.

Examiner insight

“Few candidates could state that mitosis is required for growth. Meiosis was frequently seen.” “Most candidates incorrectly believed that meiosis occurs when a reptile regrows its tail. Mitosis is used for growth and tissue repair.”

Worked example 13 marks

Describe the roles of mitosis in a root.

1
mitosis produces new cells [1]
2
for growth, an increase in length of the root [1]
3
repair of damaged tissues, or replacement of cells [1]

11. Chromosomes before and during mitosis

The exact replication of chromosomes occurs before mitosis. Each chromosome is copied, so the mass of DNA in the nucleus doubles.

During mitosis the copies of chromosomes separate, so the chromosome number is maintained in each daughter cell. The DNA is shared, so each daughter cell has half the mass present at the start of mitosis.

Examiner insight

“Candidates often stated that chromosomes thickened or split in two instead of saying that they are copied, duplicated or replicated.” “Few used their knowledge of mitosis to explain that chromosomes must duplicate and separate before a cell can divide.”

Worked example 11 mark

At the start of mitosis, a cell contained 26 chromosomes. The mass of DNA within the cell was 50 arbitrary units.

Which row shows the correct number of chromosomes and mass of DNA in a daughter cell produced at the end of mitosis?

	number of chromosomes	mass of DNA / arbitrary units
A	13	25
B	13	50
C	26	25
D	26	50

1
C: the chromosome number is maintained at 26. The DNA is shared: 50 ÷ 2 = 25 units. [1]

12. Stem cells

Stem cells are unspecialised cells that divide by mitosis to produce daughter cells that can become specialised for specific functions. The specialised cells replace damaged cells.

Examiner insight

“Stem cells do not repair damaged cells as some candidates thought.” “A small proportion of candidates incorrectly thought that stem cells are specialised cells.”

Worked example 11 mark

Roots contain stem cells.

Suggest the role of stem cells in a root.

1
to produce unspecialised cells, or daughter cells that can then become specialised [1]

Recap

Extended: mitosis is nuclear division giving rise to genetically identical cells.
Extended: the roles of mitosis are growth, repair of damaged tissues, replacement of cells and asexual reproduction.
Extended: the exact replication of chromosomes occurs before mitosis.
Extended: during mitosis the copies of chromosomes separate, so the chromosome number is maintained in each daughter cell.
Extended: stem cells are unspecialised cells that divide by mitosis to produce daughter cells that can become specialised for specific functions.

13. Meiosis makes gametes

Meiosis is involved in the production of gametes. Sperm cells are male gametes. Egg cells are female gametes.

In males, meiosis occurs in the testes. It is a division of nuclei, not a division of the sperm. Gametes are the products of meiosis.

Gametes are needed for sexual reproduction. Fertilisation is a separate event that comes after meiosis.

Examiner insight

“A few stated that meiosis is a division of the sperm instead of a division of nuclei that occurs in the production of sperm.” “Gametes are the products of meiosis, they do not divide by meiosis.”

Worked example 12 marks

Name the type of nuclear division involved in the production of sperm cells and state why sperm cells are needed.

1
meiosis [1]
2
sperm cells are male gametes, needed for sexual reproduction [1]

14. A reduction division

Meiosis is a reduction division. The chromosome number is halved, from diploid to haploid.

A diploid nucleus has two sets of chromosomes. A haploid nucleus has one set. Each nucleus made by meiosis is haploid.

In humans the diploid number is 46, so a human gamete has 23. Other species have other numbers, so use the number the question gives.

haploid number = (diploid number)/2

Examiner insight

“A few candidates confused the terms haploid and diploid and therefore thought that during meiosis a haploid cell divides to form diploid cells.” “Many knew that the haploid nucleus contains chromosomes but did not qualify their statement further to distinguish it from a diploid nucleus.”

Worked example 12 marks

A diploid nucleus in an animal has 12 chromosomes. It divides by meiosis.

State the number of chromosomes in each nucleus produced and whether each is haploid or diploid.

1
chromosome number is halved: 12 ÷ 2 = 6 [1]
2
the nuclei are haploid [1]

15. Genetically different cells

Meiosis results in genetically different cells. It does not result in genetically identical cells.

It is the gametes that are genetically different. Meiosis allows genetic variation between sperm, and so in offspring.

Fertilisation then fuses two haploid gametes. The zygote is diploid again.

Examiner insight

“Another common error was to describe the ‘offspring’ rather than the gametes as being genetically different, suggesting that the sequence of meiosis in gamete production followed by fertilisation had become confused.” “Most candidates did not know that during meiosis a diploid cell produces haploid cells that are genetically different.”

Worked example 14 marks

Meiosis occurs in the testes.

Explain the importance of meiosis occurring in the testes.

1
to produce sperm / male gametes [1]
2
to halve the number of chromosomes / produce haploid sperm cells [1]
3
so that after a sperm fertilises an egg cell the zygote is diploid [1]
4
allows genetic variation between sperm / in offspring [1]

Recap

Extended: meiosis is involved in the production of gametes.
Extended: meiosis is a reduction division. The chromosome number is halved, from diploid to haploid.
Extended: meiosis results in genetically different cells.
Fertilisation is separate: two haploid gametes fuse to make a diploid zygote.

16. Inheritance, genotype and phenotype

Inheritance is the transmission of genetic information from generation to generation.

Genotype is the genetic make-up of an organism, described by the alleles present. Phenotype is the observable features of an organism.

Phenotype is what you can see. Genotype is the alleles that are there. Do not swap them.

Examiner insight

“Common responses discussed passing of traits and characteristics but made no reference to genetic information.” “Common incorrect responses seen were characteristics or dominance.”

Worked example 11 mark

What is the phenotype of an organism?

A the combination of alleles · B the family pedigree · C the genetic make-up · D the observable features

1
D: the observable features [1]

17. Homozygous, heterozygous and pure-breeding

Homozygous means having two identical alleles of a particular gene. TT and tt are homozygous.

Two identical homozygous individuals that breed together will be pure-breeding.

Heterozygous means having two different alleles of a particular gene. Tt is heterozygous. A heterozygous individual will not be pure-breeding.

Examiner insight

“There was a lack of understanding by most candidates of what the term pure-breeding means.” “A few candidates gave the term ‘homogenous’.”

Worked example 13 marks

The shape of pea seeds is controlled by a single gene. The allele for round is dominant and is represented by the letter R. The allele for wrinkled is recessive and is represented by the letter r.

A farmer wants pure-breeding pea plants with round seeds. The farmer says: All pea plants with round seeds are pure-breeding.

Explain why the farmer's statement is not correct.

1
plants with round seeds are homozygous or heterozygous [1]
2
round seeds could be RR or Rr [1]
3
pure-breeding must be homozygous or RR [1]

18. Dominant and recessive alleles

A dominant allele is an allele that is expressed if it is present in the genotype.

A recessive allele is an allele that is only expressed when there is no dominant allele of the gene present in the genotype.

The papers use a capital letter for the dominant allele and a small letter for the recessive one, as in R and r. The recessive phenotype needs rr. The dominant phenotype can be RR or Rr.

Examiner insight

“The terms homozygous, heterozygous, dominant and recessive were not well recalled.” “It is often helpful to write out the genotypes suggested by the options.”

Worked example 11 mark

A species of flower can have white or red petals. White flowers must have two alleles for white petals. Red flowers only need one allele for red petals.

Which statement describes the genotype of a white flower?

A heterozygous dominant for petal colour · B heterozygous recessive for petal colour · C homozygous dominant for petal colour · D homozygous recessive for petal colour

1
white needs two alleles for white, so that allele is recessive and both alleles are the same
2
D: homozygous recessive for petal colour [1]

19. Genetic diagrams and Punnett squares

A genetic diagram predicts the offspring of a monohybrid cross, for a characteristic controlled by a single gene. Write the parents' genotypes, then their gametes, then the offspring.

In a Punnett square one parent's gametes go along the top and the other's go down the side. Each gamete carries one allele. Each box shows the two alleles that meet.

Count the phenotypes to get the ratio. Two heterozygous parents give 3 : 1. A heterozygous parent crossed with a homozygous recessive gives 1 : 1.

Bb × Bb → 3 dominant : 1 recessive

Rr × rr → 1 dominant : 1 recessive

Examiner insight

“A number of candidates need further practice in using a Punnett square as, fairly frequently, the gametes were written as the full genotype: for example, Rr and rr.” “Those who used the space to construct a Punnett square usually determined the correct answer.”

Worked example 13 marks

The shape of pea seeds is controlled by a single gene. The allele for round is dominant and is represented by the letter R. The allele for wrinkled is recessive and is represented by the letter r.

Two pea plants with the genotypes Rr and rr for seed shape are bred together. Complete the Punnett square by writing in the genotypes of the gametes and the genotypes of the offspring for this cross. Calculate the phenotypic ratio.

1
one set of gametes is R and r, the other set is r and r [1]
2
offspring are Rr, Rr, rr, rr [1]
3
defarraystretch1.5beginarray|c|c|c| hline & R & r hline r & Rr & rr hline r & Rr & rr hline endarray
4
1 (round seed) : 1 (wrinkled seed) [1]

20. Interpreting pedigree diagrams

A pedigree diagram shows how a characteristic is inherited in a family.

Work through it one person at a time. Write the known genotypes on the diagram first, then fill in the rest.

Two parents who do not show a characteristic but have a child who does must both carry the recessive allele. The parents are heterozygous and the child is homozygous recessive.

Examiner insight

“It is often helpful to annotate a pedigree diagram with the known genotypes and then work the rest of the individuals.” “Candidates should work through pedigree diagrams carefully.”

Worked example 13 marks

A characteristic is caused by a recessive allele, d. Two parents who do not show the characteristic have a child who does.

Deduce the genotypes of the child and of both parents. Explain your answer.

1
the child is homozygous recessive, dd [1]
2
the child gets one d from each parent [1]
3
the parents do not show the characteristic, so each also has D: both are heterozygous, Dd [1]

21. Using a test cross

An individual with the dominant phenotype could be homozygous or heterozygous. You cannot tell by looking.

A test cross crosses it with a homozygous recessive individual. The phenotypes of the offspring show the unknown genotype.

If any offspring show the recessive phenotype, the unknown parent is heterozygous. If all the offspring show the dominant phenotype, the unknown parent is homozygous dominant.

BB × bb → all dominant

Bb × bb → 1 dominant : 1 recessive

Examiner insight

“Many candidates were unable to identify the test cross that identifies a heterozygous parent, that is, tt × Tt.” “Some candidates incorrectly believed that the expected result would be 100% black. Maybe they did not recall that in a test cross, one parent is always homozygous recessive.”

Worked example 13 marks

In mice, the allele for black coat colour, B, is dominant to the allele for brown coat colour, b. A black male mouse of unknown genotype is crossed with a brown female. They have eight offspring: four black and four brown.

State the genotype of the male mouse and explain your answer.

1
the brown female is homozygous recessive, bb [1]
2
brown offspring, bb, must get a b from the male [1]
3
the male is black, so he is heterozygous, Bb [1]

22. Codominance and ABO blood groups

Codominance is a situation in which both alleles in heterozygous organisms contribute to the phenotype.

In the ABO system the phenotypes are A, B, AB and O blood groups. The alleles are I^(A), I^(B) and I^(o). I^(A) and I^(B) are codominant, so a person with both is group AB. I^(o) is recessive.

Write blood group genotypes with the letter I and a superscript. Use the same Punnett square method to predict the offspring.

A = I^(A)I^(A) or I^(A)I^(o) B = I^(B)I^(B) or I^(B)I^(o)

AB = I^(A)I^(B) O = I^(o)I^(o)

Examiner insight

“Many discussed genes or genotypes instead of alleles or failed to state that they are expressed in the phenotype.” “Some candidates knew the two possible genotypes for blood group A, but many were unfamiliar with the correct symbols, with many stating the letters AA and AO, without the use of superscripts.”

Worked example 14 marks

A chicken that is pure-breeding for white feathers is bred with a chicken that is pure-breeding for black feathers. All of the offspring have black and white feathers that are described as speckled.

The allele for white feathers is represented by F^(W). The allele for black feathers is represented by F^(B). The genotype of all of the speckled chicken offspring is F^(W)F^(B).

(i) State the term that is used to describe the situation where both alleles in heterozygous organisms contribute to the phenotype.

(ii) Two of the speckled chicken offspring were bred together. Complete the Punnett square to determine the genotypes and phenotypic ratio of the offspring from this cross.

1
codominance [1]
2
gametes from each parent: F^(W) and F^(B) [1]
3
offspring: F^(W)F^(W), F^(W)F^(B), F^(W)F^(B), F^(B)F^(B) [1]
4
defarraystretch1.5beginarray|c|c|c| hline & F^(W) & F^(B) hline F^(W) & F^(W)F^(W) & F^(W)F^(B) hline F^(B) & F^(W)F^(B) & F^(B)F^(B) hline endarray
5
1 white : 2 speckled : 1 black [1]

23. Sex linkage and colour blindness

A sex-linked characteristic is a feature in which the gene responsible is located on a sex chromosome. This makes the characteristic more common in one sex than in the other.

The human sex chromosomes are X and Y. Females are XX and males are XY. Write a sex-linked allele as a superscript on the X, such as X^(R). Do not put an allele on the Y: write X^(r)Y.

Red-green colour blindness is an example of sex linkage. The gene is on the X chromosome. A colour-blind male has the recessive allele on his X.

Examiner insight

“Most candidates omitted the sex when giving the phenotypic ratio.” “Whilst most candidates could identify the genotype of a red-green colour-blind male, some candidates incorrectly opted for the dominant allele on the X chromosome.”

Worked example 13 marks

Eye colour in fruit flies is controlled by a single gene. The alleles for this gene are found on the X chromosome. The allele for red eye colour is dominant and is represented by the letter R. The allele for white eye colour is recessive and is represented by the letter r.

A male fruit fly with the genotype X^(R)Y and a female fruit fly with the genotype X^(r)X^(r) are bred together. Complete the Punnett square to determine the genotypes and phenotypic ratio of the offspring.

1
gametes: male X^(R) and Y, female X^(r) and X^(r) [1]
2
offspring: X^(R)X^(r), X^(R)X^(r), X^(r)Y, X^(r)Y [1]
3
defarraystretch1.5beginarray|c|c|c| hline & X^(R) & Y hline X^(r) & X^(R)X^(r) & X^(r)Y hline X^(r) & X^(R)X^(r) & X^(r)Y hline endarray
4
1 red-eyed female : 1 white-eyed male [1]

Recap

Inheritance is the transmission of genetic information from generation to generation.
Genotype is the genetic make-up, described by the alleles present. Phenotype is the observable features.
Homozygous: two identical alleles. Heterozygous: two different alleles. Only homozygous parents breed pure.
A dominant allele is expressed if it is present. A recessive allele is only expressed when no dominant allele is present.
Gametes carry one allele. Two heterozygous parents give 3 : 1, and heterozygous with homozygous recessive gives 1 : 1.
In a pedigree diagram, write in the known genotypes first.
Extended: a test cross uses a homozygous recessive partner.
Extended: codominance means both alleles contribute to the phenotype. Blood group alleles are I^(A), I^(B) and I^(o).
Extended: sex-linked genes are on a sex chromosome, so the characteristic is more common in one sex. The gene for red-green colour blindness is on the X chromosome.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

Complete the sentence. Chromosomes are made of ......, which contains genetic information in the form of ......2 marks
Explain the difference between a gene and an allele.2 marks
State the sex chromosomes in a human female and in a human male.2 marks
A couple already have three daughters. Their fourth child is born. State the chance that it is a boy and explain your answer.3 marks
A student writes: "Chromosomes are made of genes, which contain DNA. A gene is a strand of DNA that codes for a characteristic." The student has made three mistakes. Write a correct version of each sentence.3 marks
State what determines the sequence of amino acids in a protein molecule.1 mark
The sequence of bases in the gene for an enzyme is changed. The enzyme made from the changed gene does not work. Explain why.3 marks
Describe the part played by mRNA in making a protein.4 marks
Petal cells of a plant make a protein called enzyme P. Leaf cells of the same plant do not. A scientist counts the mRNA molecules that are copies of the gene for enzyme P in each type of cell. The table below gives the results. (a) A student says the leaf cells make no enzyme P because they do not have the gene. Use the results to explain why the student is wrong, and give the correct explanation. [3] (b) The scientist treats petal cells with a substance that stops mRNA moving out of the nucleus. The cells still make mRNA. Predict the effect on the production of enzyme P and explain your answer. [3] | type of cell | gene for enzyme P | mRNA copies of the gene per cell | | --- | --- | --- | | petal | present | 600 | | leaf | present | 0 |6 marks
An egg cell of a cat has a haploid nucleus containing 19 chromosomes. (a) State the number of chromosomes in the diploid nucleus of a skin cell of the cat. Explain your answer. [2] (b) State the number of pairs of chromosomes in the skin cell nucleus. [1] (c) Calculate how many more chromosomes there are in a human diploid nucleus than in the diploid nucleus of the cat. [2] (d) A student writes: "The haploid nucleus of the cat contains 19 pairs of chromosomes." Explain what is wrong with this. [2]7 marks
Describe mitosis.2 marks
A lizard loses its tail and grows a new one. State the type of nuclear division involved and its role.2 marks
State what happens to the chromosomes in a cell before mitosis begins.1 mark
State what is meant by the term stem cell.2 marks
A plant produces plantlets by asexual reproduction. State the type of nuclear division involved and how a plantlet compares genetically with the parent plant.2 marks
A cell with 16 chromosomes divides by mitosis. Each daughter cell then divides by mitosis. State the number of chromosomes in each of the four cells now present.2 marks
A cell in a root tip has 24 chromosomes and 6.0 arbitrary units of DNA after it has divided. It then prepares to divide again by mitosis. (a) State the number of chromosomes and mass of DNA in the nucleus just before it divides. (b) State the number of chromosomes and mass of DNA in each new daughter nucleus.4 marks
A student says: ‘Stem cells in the lining of the stomach repair the cells damaged by acid.’ Explain, using correct terms, the role of stem cells in replacing damaged cells.4 marks
The table gives the mass of DNA in the nucleus of a cell, and then in one daughter nucleus after the cell divides by mitosis. The nucleus has 18 chromosomes at 0 hours. (a) State the time taken for the mass of DNA to double. (b) Explain the change in the mass of DNA between 4 hours and 12 hours. (c) State the number of chromosomes in the daughter nucleus at 16 hours. Give a reason. | time / hours | mass of DNA in the nucleus / arbitrary units | | --- | --- | | 0 | 6.0 | | 4 | 6.0 | | 8 | 9.0 | | 12 | 12.0 | | 14 | 12.0 | | 16 | 6.0 |5 marks
A flatworm is cut in two and each piece regrows its missing part. Stem cells at the wound have 8 chromosomes and divide by mitosis once every 6 hours. After each division, one daughter cell remains a stem cell. The other becomes specialised and does not divide again. (a) Calculate the number of specialised cells produced from one stem cell in 24 hours. (b) State the number of chromosomes in each specialised cell. Give a reason. (c) A student writes: ‘The stem cells repair the damaged cells by meiosis.’ Correct the two mistakes in this sentence.6 marks
State the type of nuclear division involved in the production of gametes.1 mark
The nucleus of a diploid cell contains 18 chromosomes. State the number of chromosomes in a nucleus produced from it by meiosis.1 mark
Complete the sentence. Meiosis is a ........... division in which the chromosome number is halved from ........... to ...........3 marks
A student says: ‘Meiosis makes cells that are identical to the parent cell.’ Give two reasons why the student is wrong.2 marks
A student writes: ‘Sperm divide by meiosis to make more sperm.’ Correct this sentence.2 marks
A student describes the nucleus of a sperm cell: ‘It contains chromosomes.’ Explain why this does not distinguish it from the nucleus of a diploid cell, and improve the description.2 marks
The nucleus of a human sperm cell contains 23 chromosomes. A sperm cell fertilises an egg cell. State the number of chromosomes in the nucleus of the zygote and explain your answer.2 marks
Explain why gametes must be haploid.3 marks
The nucleus of a sperm cell from a cat contains 19 chromosomes. (a) State the number of chromosomes in the nucleus of the cell in the testes that divided by meiosis to produce this sperm cell. (b) State the number of chromosomes in the nucleus of an egg cell from a cat. (c) State the number of chromosomes in the nucleus of a zygote of a cat. Explain your answer.4 marks
The nucleus of a skin cell from a zebrafish contains 50 chromosomes. A student writes this account. ‘In the testes, haploid cells divide by meiosis to make diploid sperm. The sperm are genetically identical. After fertilisation the zygote divides by meiosis, and this is what makes the offspring different from each other.’ (a) Correct the first sentence of the account. Include the number of chromosomes in each type of nucleus. (b) Correct the second sentence. (c) Explain what is wrong with the third sentence and state what does cause the variation in the offspring. (d) State the number of chromosomes in the nucleus of a zebrafish zygote.6 marks
State what is meant by the term inheritance.1 mark
A pea plant has round seeds. Its genetic make-up is Rr. (a) Name the term for ‘round seeds’. (b) Name the term for ‘Rr’. (c) Name the term for an individual with two different alleles of a gene.3 marks
Tall is dominant in pea plants. A breeder has two tall plants, TT and Tt. (a) Name the term for the genotype TT. (b) Explain why a plant with the genotype Tt is not pure-breeding.3 marks
In guinea pigs the allele for a smooth coat, S, is dominant to the allele for a rough coat, s. State the phenotype of a guinea pig with the genotype Ss and of one with the genotype ss. Explain each answer.3 marks
In mice, the allele for black coat colour, B, is dominant to the allele for brown coat colour, b. Two heterozygous black mice are bred together. Use a Punnett square to give the offspring genotypes and the phenotypic ratio.3 marks
In the mice from question 5, a heterozygous black mouse is bred with a brown mouse. They have 24 offspring. Calculate the number of offspring expected to be brown.3 marks
A characteristic is caused by a recessive allele, a. A man and a woman do not show the characteristic. Their first child does. (a) Give the genotype of the child and the genotype of both parents. (b) Calculate the probability that their next child shows the characteristic.4 marks
In dogs, the allele for black coat colour, B, is dominant to the allele for brown coat colour, b. A breeder wants to find out whether a black dog is BB or Bb. Describe the cross the breeder should use and explain the result expected for each genotype.4 marks
Two babies are born in a hospital on the same day, one to each of two couples. Baby 1 has blood group O. Baby 2 has blood group B. In couple P, the woman has blood group AB and the man has blood group O. In couple Q, the woman has blood group A and the man has blood group B, and their first child has blood group O. (a) Give the genotypes of the woman and the man in couple Q. Explain your answer. (b) Use a Punnett square to give the phenotypic ratio of the children that couple P can have. (c) Deduce which baby belongs to couple P. Explain your answer.6 marks
Red-green colour blindness is caused by a recessive allele on the X chromosome. X^(N) is the allele for normal colour vision and Xⁿ is the allele for colour blindness. A woman has normal colour vision. Her father is colour-blind. Her partner is also colour-blind. (a) Deduce the genotype of the woman. Explain your answer. (b) Use a Punnett square to give the genotypes of their children and the phenotypic ratio. (c) A student says that a colour-blind son of this couple inherited colour blindness from his father. Explain why the student is not correct.7 marks

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1. Chromosomes and genes#

2. Alleles#

3. Inheritance of sex in humans#

4. Base sequence, amino acids and shape#

5. DNA controls the cell through proteins#

6. How a protein is made#

7. Same genes, different proteins#

8. Haploid and diploid nuclei#

9. Mitosis as nuclear division#

10. The roles of mitosis#

11. Chromosomes before and during mitosis#

12. Stem cells#

13. Meiosis makes gametes#

14. A reduction division#

15. Genetically different cells#

16. Inheritance, genotype and phenotype#

17. Homozygous, heterozygous and pure-breeding#

18. Dominant and recessive alleles#

19. Genetic diagrams and Punnett squares#

20. Interpreting pedigree diagrams#

21. Using a test cross#

22. Codominance and ABO blood groups#

23. Sex linkage and colour blindness#

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Workbook

Chapter at a Glance

Free notes are just the start.

Related chapters

1. Chromosomes and genes

2. Alleles

3. Inheritance of sex in humans

4. Base sequence, amino acids and shape

5. DNA controls the cell through proteins

6. How a protein is made

7. Same genes, different proteins

8. Haploid and diploid nuclei

9. Mitosis as nuclear division

10. The roles of mitosis

11. Chromosomes before and during mitosis

12. Stem cells

13. Meiosis makes gametes

14. A reduction division

15. Genetically different cells

16. Inheritance, genotype and phenotype

17. Homozygous, heterozygous and pure-breeding

18. Dominant and recessive alleles

19. Genetic diagrams and Punnett squares

20. Interpreting pedigree diagrams

21. Using a test cross

22. Codominance and ABO blood groups

23. Sex linkage and colour blindness