Inheritance

Question 1

Describe what is Meant by Sexual Reproduction

Answer 1

Sexual Reproduction is where genetic information from two organisms is combined to produce offspring which are genetically different to either parents.

Question 2

Describe how Sexual Reproduction Leads to Variation

Answer 2

-Sperm, eggs and pollen are all examples of gametes. When gametes fuse together, this is called fertilisation.

-In sexual reproduction the genetic information from both gametes is mixed.

-This leads to variation in the offspring.

Question 3

Describe what is Meant by Asexual Reproduction

Answer 3

-In asexual reproduction, there is only one parent.

-There is no fusion of gametes and no mixing of chromosomes so there is no genetic variation between parent and offspring.

-The offspring are genetically identical to the parent- they are clones.

Question 4

Describe how Asexual Reproduction Leads to Cloning

Answer 4

-In asexual reproduction the offspring are produced by mitosis.

-There is only one parent and no gametes are involved.

-There is no mixing of genetic information.

-This means there is no genetic variation between offspring and parent so the offspring are clones.

Question 5

Compare Asexual Reproduction to Sexual Reproduction

Answer 5

Sexual reproduction:
-Involves meiosis (which produces gametes)
-Involves fertilisation (ie gametes joining together)
-Involves gametes
-Two parents needed
-Offspring are not genetically identical

Asexual reproduction
-Only one parent needed
-Only involves mitosis (because it does not involve gametes)
-Offspring are genetically identical
-Does not involve fertilisation
-No gametes used

Question 6

Describe the Differences Between a Gamete and a Normal Body Cell

Answer 6

-Chromosomes are found in the nucleus of cells. In a normal human cell, there are 23 pairs of chromosomes.

-Normal human cells are produced by a type of cell division called mitosis. Mitosis produces two identical copies of a cell.

-However, cells such as sperm and egg are different. These
are called gametes.

-In these, the chromosomes are not paired. Human gametes contain 23 single chromosomes.

-Gametes are formed by a type of cell division called meiosis. Gametes are not identical.

Question 7

Describe the Stages of Meiosis

Answer 7

-Before the cells starts to divide, it duplicates its genetic information, forming two armed chromosomes- one arm of each chromosome is an exact copy of the other arm.

-After replication, the chromosomes arrange themselves into pairs. In the first division in meiosis, the chromosome pairs line up in the centre of the cell.

-The pairs then pull apart so each new cell only has one copy of each chromosome. Chromosomes from each parent go into each new cell.

-In the second division, the chromosomes line up again in the centre of the cell. The arms of the chromosomes are pulled apart. Four gametes are produced, each with only a single sets of chromosomes in it (23).

Question 8

Explain why the Gametes Produced from Meiosis are Genetically Different.

Answer 8

-Each of the gametes is genetically different from the others because the chromosomes all get shuffled up during meiosis and each gamete only gets half of them, at random.

-This means the gametes have different alleles.

Question 9

Explain why it is Important that Meiosis Halves the Number of Chromosomes when Producing Gametes

Answer 9

-In fertilisation, the sperm and egg cells fuse (join together).

-In humans, the fertilised egg should have 23 chromosome pairs (ie 23 chromosomes from the sperm cell and 23 chromosomes from the egg cell).

-This is why meiosis halves the number of chromosomes when gametes are produced.

-If this did not happen, then the fertilised egg would contain 46 chromosome pairs (23 pairs from the sperm cell and 23 pairs from the egg cell).

Question 10

State the Parts of Normal Fertilisation

Answer 10

Sperm (haploid) + Egg (haploid) → Zygote (diploid)

-A haploid cell has half the normal amount of chromosomes whereas a diploid cell has the full amount.

Question 11

Describe what happens to a Cell After Fertilisation

Answer 11

-After two gametes have fused during fertilisation, the resulting new cell divides by mitosis to make a copy of itself.

-Mitosis repeats many times to produce lots of new cells in an embryo.

-As the embryo develops, these cells then start to differentiate into the different types of specialised cells that make up a whole organism.

Question 12

Describe what is Meant by Differentiation

Answer 12

-When cells differentiate, they change to form specialised cells. In the early embryo, all of the cells are identical.

-However, at a certain point they begin to form specialised cells such as nerve cells and muscle cells. This is called differentiation.

Question 13

Describe the Stages in the Cell Cycle

Answer 13

-The DNA replicates to form two copies of each chromosome. The cell grows and copies its internal structures such as mitochondria and ribosomes

-This is mitosis. One set of chromosomes is pulled to each end of the cell. The nucleus also divides.

-The cytoplasm and cell membrane divide to form two identical cells.

Question 14

Compare Mitosis and Meiosis

Answer 14

Mitosis:
-Only one cell division occurs
-Chromosomes replicate
-Produces genetically identical cells
-Occurs during growth and asexual reproduction
-A chromatid goes to each new cell.

Meiosis:
-Four daughter cells produced
-Chromosomes replicate
-Only half the chromosomes go to each new daughter cell
-Occurs in the formation of gametes
-Daughter cells are genetically different
-Two cell divisions occur

Question 15

Describe the Advantages of Sexual Reproduction

Answer 15

-The offspring all show genetic variation

-In a rapidly changing environment, this can increase the chances of survival due to natural selection

-Allows humans to use selective breeding to increase food production in crops

Question 16

Describe the Disadvantages of Sexual Reproduction

Answer 16

-Two parents are needed which takes time and energy

-Relies on two gametes meeting and fusing which can be risky

Question 17

Describe the Advantages of Asexual Reproduction

Answer 17

-Because no mate is needed, neither time nor energy is wasted looking for a mate

-Only requires one parent so an organism can reproduce even if it is by itself

-This is a very rapid way to reproduce

-Ideal when conditions are favourable as many identical offspring can be produced quickly

-Offspring are certain to keep favourable characteristics

Question 18

Describe the Disadvantages of Asexual Reproduction

Answer 18

-Asexual reproduction produces clones. This means all the offspring are genetically identical to the parent. This is no problem under favourable conditions.

-However, if the conditions become unfavourable then because the offspring are all genetically identical, they could all die.

-This makes asexual reproduction very risky when the environment changes rapidly.

Question 19

Describe how Malaria Reproduces by Both Sexual and Asexual Reproduction

Answer 19

-Malarial parasites spend part of their lifecycle in a mosquito (the vector) and part of it in a human body.

-The female mosquito drinks human blood. When it feeds, it releases the malarial parasite into the human blood stream.

-The parasites reproduce asexually in human blood and liver cells. When the mosquito feeds, it takes in red blood cells containing the parasite.

-The drop in temperature between the human and mosquito causes sexual forms to burst out of the red blood cells and fuse together.

-These then undergo meiosis to produce new asexual parasites that will infect a new human when the mosquito feeds.

Question 20

Describe how Fungi Reproduce by Both Sexual and Asexual Reproduction

Answer 20

-Some species of fungi release spores that grow into new fungi when they land in an appropriate place. Spores can be produced sexually and asexually.

-Asexually produced spores form fungi that are genetically identical to the parent fungus.

-Sexually produced spores introduce variation and are often produced in response to an unfavourable change in the environment, increasing the chance that the population will survive the change.

Question 21

Describe how Strawberries and Daffodils Reproduce by Asexual Reproduction

Answer 21

-When a strawberry plant reproduces asexually, a long runner grows out of the side of the plant.

-When the runner touches the ground, a new (offspring) plant develops. These are all produced by mitosis and are all clones of the parent plant.

-Daffodils grow from bulbs in the soil. These bulbs split to form new offspring plants which are genetically identical to the parent. This is called bulb division.

Question 22

Describe what is Meant by DNA

Answer 22

-DNA is a polymer that holds the instructions for growth and development in every living thing.

-Its structure is described as a double stranded helix held together by complementary base pairs.

-The base units of DNA are amino acids.

Question 23

Describe what is Meant by a Gene

Answer 23

A gene is a small section of DNA found on a chromosome. Each gene codes for a particular sequence of amino acids which are put together to make a specific protein.

Question 24

Describe the Benefits of Studying the Human Genome

Answer 24

-The genome is the entire genetic material of an organism.

-Studying the genome allows us to identify the genes which increase the risk of conditions such as cancer or Alzheimer’s disease.

-Studying the genome helps us to learn about human migration patterns. This is useful for determining our ancestry.

-Studying the genome helps us to develop better treatments for inherited conditions such as Cystic Fibrosis.

Question 25

Describe the Structure of DNA

Answer 25

-DNA strands are polymers made op of lots of repeating units called nucleotides. Each nucleotide consists of a sugar, a phosphate group and one ‘base’.

-The sugar and phosphate groups in the nucleotides form a ‘backbone’ to the DNA strands. The sugar and phosphate groups alternate. One of four bases - A.T,C or g- joins to each sugar.

-Each base links to a base on the opposite strand in the helix. A always pairs up with T and C always pairs up with G. This is called complementary base pairing.

-It is the order of bases in a gene that decides the order of amino acids in a protein. Each amino acid is coded for by a sequence of three bases in the gene.

Question 26

Explain why DNA is a Polymer

Answer 26

-A polymer is a made up of many similar units bonded together.

-DNA is a polymer because it is made up of many nucleotide monomers joined together.

Question 27

Describe how the Different Nucleotide Molecules are Similar and Different in Their Structures

Answer 27

-The difference between the nucleotide molecules is the base. This is different for the four different nucleotides that we see in DNA.

-All of the nucleotides are similar in that they all contain the same sugar and phosphate.

Question 28

Explain why the Order of Amino Acids is Critical for the Function of a Protein

Answer 28

The order of amino acids determines how the protein folds into its final shape. The shape of a protein is critical for its function.

Question 29

Describe Different Types of Protein and Theirs Functions

Answer 29

-Enzymes act as biological catalysts to speed up chemical reactions in the body e.g. amylase.

-Hormones used to carry messages around the body e.g. insulin is a hormone released into the blood by the pancreas to regulate the blood sugar level.

-Structural proteins are physically strong e.g. collagen is a structural protein that strengthens connective tissues (such as ligaments and cartilage).

Question 30

Describe the Stages in Protein Synthesis

Answer 30

Stage 1- Transcription:

-This takes place in the nucleus. The base sequence of the gene is copied into a complementary template molecule.

-The template molecule is called messenger RNA (mRNA). The single stranded mRNA moves from the nucleus to the cytoplasm.

Stage 2- Translation:

-The mRNA attaches to a ribosome. Amino acids are brought on carrier molecules called tRNA.

-The ribosome uses the triplets of bases on the mRNA to join the correct amino acids in the correct order.

-Finally the protein folds into the correct shape that will enable it to carry out its function.

Question 31

Describe what is Meant by a Mutation

Answer 31

-Mutations are random changes to DNA taking place all the time.

-Each amino acid is encoded by three bases. A mutation is when a base changes. Mutations happen all the time.

Question 32

Explain how Mutations Often Have no Effect on the Function of the Protein

Answer 32

-A mutation may not cause an amino acid to change at all.

-Other mutations may only have a very minor effect on the overall shape of the protein.

-In both of these cases, there could be no effect on the function of the protein.

Question 33

Describe how a Mutation Could Affect the Function of a Structural Protein

Answer 33

-A mutation could change the shape of a structural protein such as collagen.

-Structural proteins play a number of key roles for example in bone or muscle.

-If the shape of a structural protein changes, the protein may lose its strength.

-This would mean that the protein would not be able to carry out its function effectively.

Question 34

Describe how a Mutation Could Affect the Function of an Enzyme

Answer 34

-If the shape of an enzyme’s active site is changed, its substrate may no longer be able to bind to it.

Question 35

Describe the Function of Non-coding DNA

Answer 35

-Non-coding DNA determines when genes are switched on (ie a protein is produced) or switched off (no protein produced) in different cells at different times.

-A good example is insulin. Every cell in the body contains the gene for insulin but only cells in the pancreas produce the protein insulin.

-That is because in every other cell in the body, the insulin gene is turned off. In pancreatic cells, the insulin gene is turned on.

Question 36

Describe how a Mutation in Non-coding DNA Could Lead to a Disease Such as Cancer

Answer 36

-A mutation in non-coding DNA could cause a gene to be turned on when it should be turned off or vice-versa.

-This could lead to a range of different conditions including cancer.

-Cancer cells undergo uncontrolled mitosis and in some cases, this is because they are making a protein that they should not make (in other words a gene is turned on when it should not be).

-This can be due to a mutation in non-coding DNA.

Question 37

Explain how Mutations are Caused by Insertion

Answer 37

-Insertions are when a new base is inserted into the DNA base sequence where it shouldn’t be.

-Every three bases in a DNA base sequence codes for a particular amino acid.

-An insertion changes the way the groups of three bases are ‘read’ which can change the amino acids that they code for.

-Insertions can change more than one amino acid as they have a knock-on effect on the bases further on in the sequence.

Question 38

Explain how Mutations are Caused by Deletions

Answer 38

-Deletions are when a random base is deleted from the DNA base sequence.

-They change the way that the base sequence is ‘read’ and have a knock-on effect further down the sequence.

Question 39

Explain how Mutations are Caused by Substitutions

Answer 39

-Substitution mutations are when a random base in the DNA base sequence is change to a different base.

Question 40

Give the Definitions of the Different Terms Linked with Inheritance

Answer 40

Allele - A version of a gene at a specific location on a chromosome

Genotype - This tells us the alleles that a person has for a given gene

Homozygous - Two copies of the same allele e.g. bb

Phenotype - This tells us the physical characteristics caused by a person’s alleles

Heterozygous - Copies of different alleles e.g. Bb

Dominant allele - This allele determines the phenotype even if only one copy is present

Recessive allele - This allele only determines the phenotype if two copies are present

Question 41

Describe the Condition of Cystic Fibrosis

Answer 41

-Cystic Fibrosis is a genetic disorder of the cell membranes. It is caused by a single cell.

-It results in the body producing a lot of thick, sticky mucus in the air passages and in the pancreas.

-Because it is recessive, people with only one copy of the allele won’t have the disorder- they are carriers.

-For a child to have the disorder, both parents must be carriers or have the disorder themselves.

Question 42

Explain why the Actual Number of Offspring with Each Genotype May not match the Ratios Predicted From a Punnett Square

Answer 42

-The ratios of offspring that we get from a genetic cross are only probabilities.

-A couple could have a number of offspring and none of them might have cystic fibrosis or all of them might have cystic fibrosis (or any number if between).

-Another point which is important to consider is that cystic fibrosis is controlled by a single gene so the genetic crosses are straightforward.

-However, most characteristics are controlled by multiple
genes. In these cases, predicting the numbers of offspring from genetic crosses is extremely difficult.

Question 43

Describe the Condition of Polydactyl

Answer 43

-Polydactyl is a genetic disorder where a baby is born with extra fingers or toes. It usually doesn’t cause any other problems so isn’t life threatening.

-The disorder is caused by a dominant allele and so can be inherited if just one parent carries the defective allele.

-The parent that has the defective allele will have the condition too since the allele is dominant.

Question 44

Explain why the Actual Ratio of Offspring with Polydactyl may Not Match the Ratio Predicted by the Punnett Square

Answer 44

-Genetic crosses only predict the probability of the offspring having the genotype / phenotypes.

-Even if it is predicted that half of the offspring will have polydactyly and half will be unaffected. In reality, all of the offspring may have polydactyly or all may be unaffected (or any number between).

-However, although a cross only shows probabilities, it also shows the only possible genotypes / phenotypes from that cross.

-We could never get any offspring at all which are carriers of polydactyly. That genotype is impossible as polydactyly is caused by a dominant allele.

Question 45

Describe Arguments For Embryo Screening

Answer 45

-It will help to stop suffering.

-Treating disorders costs the Government (and the taxpayers) a lot of money.

-There are laws to stop it going too far. At the moment, parents cannot even select the sex of their baby (unless it is for health reasons).

Question 46

Describe Arguments Against Embryo Screening

Answer 46

-It implies that some people with genetic problems are ‘undesirable’ -this could increase prejudice.

-There may come a point where everyone wants to screen their embryos so they can pick the most ‘desirable’ one.

-Some people feel that the benefits do not justify the cost. as screening is expensive.

Question 47

Explain how Family Trees Prove that Cystic Fibrosis is Caused by a Recessive Gene

Answer 47

-Looking at the parents, we can see that neither of them have the phenotype cystic fibrosis.

-However, they have produced an offspring who does have
cystic fibrosis. This means that the cystic fibrosis allele must be recessive and the parents are both carriers.

-If the cystic fibrosis allele was dominant then it would not be possible to be a carrier of cystic fibrosis (as anyone with even one copy of the cystic fibrosis allele would have cystic fibrosis).

-In this case, to produce an offspring with cystic fibrosis, at least one of the parents would have to have cystic fibrosis and the family tree shows us that this is not the case.

Question 48

State which Chromosomes Determine Sex

Answer 48

-In humans, 22 of our chromosome pairs determine our inherited features e.g. eye colour.

-The remaining chromosome pair determines our sex. In males this is XY and in females it is XX.

Question 49

Explain why the Ratio in a Punnett Square Cannot be Used to Predict the Number of Male or Female Children in a Family

Answer 49

-With all genetic crosses, the cross only shows us the probability that the offspring will have the genotype / phenotype shown.

-A couple could have a large number of children and all of them could be male or all of them could be female (or any number in between).

-However, if they have another child, there is still a 50%
probability of the child being male and a 50% probability of the child being female.