B6: Inheritance, variation and evolution

Question 1

What is DNA and where is it found?

Answer 1

DNA (deoxyribonucleic acid), found within chromosomes in the nucleus, is the chemical a cell’s genetic information is composed from.

DNA is a polymer, made up of two strands forming a double helix, and has a sugar phosphate backbone.

Question 2

What is a gene?

Answer 2

A small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids, which are arranged to make a specific protein. (The type of proteins a cell produces determines which type of cell it is.)

Question 3

What are chromosomes?

Answer 3

Folded up strands of DNA.

Question 4

Describe how genetic information is arranged and where it is found.

Answer 4

An organism is composed of many cells, each containing a nucleus. Within this are 23 homologous pairs of chromosomes (in humans, excluding gametes), made of coiled DNA. Arranged along chromosomes are small sections of DNA called genes.

Question 5

Define “genome”.

Answer 5

The entire set of genetic material in an organism.

Question 6

Why is it important that scientists now understand the human genome?

Answer 6

It allows us to:

• search for genes linked to different types of disease.
• understand and treat inherited disorders.
• trace human migration patterns from the past.

Question 7

Describe the structure of DNA and how this is related to its function.

Answer 7

• DNA is a polymer made from four different repeating nucleotide units.
• Strands of DNA consist of alternating sugar and phosphate sections.
• Each nucleotide consists of a common sugar and phosphate group (one sugar molecule and one phosphate molecule, forming the backbone). Attached to each sugar is one of the four bases: A, C, G and T.
• Bases are arranged in pairs, in the combinations A+T, C+G.

Question 8

What is complementary base pairing?

Answer 8

In the DNA polymer chain, a C is always linked to a G on the opposite strand, and a T to an A.

Question 9

Describe the process of protein synthesis. (6)

Answer 9

• A sequence of three bases is the code for a particular amino acid.
• The order of bases controls the order in which amino acids are assembled to produce a particular protein.
• Proteins are synthesised on ribosomes, according to a template.
• A molecule called mRNA, a copy of the DNA’s code, acts as a messenger by carrying the code to the ribosomes.
• Carrier molecules bring specific amino acids to add to the growing protein chain in the correct order.
• When the protein chain is complete, it folds up to form a unique shape.
• The shape enables the protein to do its job as an enzyme, hormone, or structural protein, e.g. collagen.

Question 10

Not all parts of DNA code for proteins. What is the function of non-coding parts?

Answer 10

Non-coding parts of DNA can switch genes on and off, so variations in these areas of DNA may affect how genes are expressed.

Question 11

What are mutations?

Answer 11

Random alterations in DNA which occur continuously. They change the sequence of bases in a gene, producing a genetic variant of a protein.

Question 12

Explain the effects of mutations on the function of proteins.

Answer 12

• Most mutations don’t alter the protein, or only alter it slightly so that its appearance or function is unaffected.
• Some mutations code for an alteration in a protein’s shape, which affect its ability to function:
• If an enzyme’s active site is deformed, its substrate might not be able to bind to it.
• A structural protein may lose its strength.
• A mutation in non-coding DNA may affect how genes are expressed.

Question 13

Name 3 possible consequences of mutations.

Answer 13

• If an enzyme’s active site is deformed, its substrate might not be able to bind to it.
• A structural protein may lose its strength.
• A mutation in non-coding DNA may affect how genes are expressed.

Question 14

Name 3 types of mutations.

Answer 14

Insertions, deletions and substitutions.

Question 15

What is an insertion mutation and what are its effects?

Answer 15

• Where an extra base is inserted into the DNA base sequence, changing the way groups of 3 bases are read and therefore the amino acids produced.
• They have a knock-on effect on bases further on, changing more than one amino acid.

Question 16

What is a deletion mutation and what are its effects?

Answer 16

• Where a random base is deleted from the DNA base sequence, changing the way groups of 3 bases are read and therefore the amino acids produced.
• They have a knock-on effect on bases further on, changing more than one amino acid.

Question 17

What is a substitution mutation and what are its effects?

Answer 17

• Where a random base in the DNA base sequence is replaced with another, changing the way its group of 3 bases is read, and therefore the amino acid produced.
• These do not have a knock-on effect on the sequence.

Question 18

What are mutations caused by?

Answer 18

They can occur naturally over time, or be caused by background radiation or chemicals.

Question 19

What is sexual reproduction?

Answer 19

A process which involves the fusion of male and female gametes. The mixing of the 2 parents’ genes means offspring are genetically different to their parents, causing variation.

Question 20

Give 2 types of sexual reproduction.

Answer 20

• Sperm and egg cells in animals.
• Pollen and egg cells in flowering plants.

Question 21

What is asexual reproduction?

Answer 21

A process which involves only one parent and no fusion of gametes; only mitosis is involved. There is no mixing of genetic information, so offspring are genetically identical, with no variation (clones).

Question 22

Some organisms reproduce by both sexual and asexual reproduction, depending on the circumstances. Give 3 examples of organisms which do this.

Answer 22

• Malarial parasites reproduce asexually in the human host, but sexually in the mosquito.
• Many fungi reproduce asexually by spores but also reproduce sexually to give variation.
• Many plants produce seeds sexually, but also reproduce asexually by runners such as strawberry plants, or bulb division such as daffodils.

Question 23

Give 2 advantages of sexual reproduction.

Answer 23

• Produces variation in the offspring which, if the environment changes, gives a survival advantage by natural selection
• It means natural selection can be sped up by humans in selective breeding to increase food production.

Question 24

Give 3 advantages of asexual reproduction.

Answer 24

• Only one parent needed, making it more time and energy efficient as it doesn’t need to find a mate.
• Faster than sexual reproduction.
• Many identical offspring can be produced when conditions are favourable.

Question 25

What is meiosis and where does it occur?

Answer 25

The process by which cells in reproductive organs divide to form gametes, which have half the number of chromosomes as somatic (body) cells.

Question 26

How many chromosomes do human gametes have and why?

Answer 26

In a human, 23, so that, when they fuse with the other sex’s gamete during fertilisation, the product has the full number of 46 chromosomes (like regular somatic cells).

Question 27

Describe what happens during meiosis.

Answer 27

• The cell duplicates its genetic information.
• The cell divides twice to form 4 gametes, each with a single set of chromosomes.
• All the gametes are genetically different from each other.

Question 28

What happens during and after the fusion of gametes in fertilisation?

Answer 28

Gametes join at fertilisation to restore the normal number of chromosomes. The new cell divides by mitosis and the number of cells increases. As the embryo develops, cells differentiate.

Question 29

What are gametes? Name the female and male gametes.

Answer 29

An organism’s reproductive (sex) cells, which are haploid (each cell carries only one copy of each chromosome). Female gametes are ova / egg cells, and male gametes are sperm.

Question 30

How many pairs of chromosomes do ordinary human body cells contain?

Answer 30

23.

Question 31

Ordinary human body cells contain 23 pairs of chromosomes. What role do they play?

Answer 31

22 pairs control characteristics only, but the 23rd pair carries the genes that determine sex.

Question 32

The 23rd pair of chromosomes in human somatic cells carries the genes that determine sex. What are the sex chromosomes in males and females?

Answer 32

• In females, the sex chromosomes are XX.
• In males, the chromosomes are XY.

Question 33

What is an allele?

Answer 33

A variant form of a gene. There are two alleles for each gene, each on one of a homologous pair of chromosomes.

Question 34

Compare dominant and recessive alleles.

Answer 34

Each trait is controlled by two alleles, one on each of a homologous pair of chromosomes. The combination of alleles determines the trait expressed:

A dominant allele is always expressed, even if only one copy is present (DD or Dd).

A recessive allele is only expressed if two copies are present (and therefore no dominant allele is present - so only dd).

Question 35

Define heterozygous and homozygous.

Answer 35

Homozygous = a particular gene that has identical alleles on both homologous chromosomes (both alleles present in an organism for one trait are the same).

Heterozygous = a particular gene that has a different allele on each homologous chromosome (the alleles present in an organism for one trait are the different).

Question 36

Define “genotype” and “phenotype”.

Answer 36

Genotype = the set of genes in an organism’s DNA which is responsible for a particular trait. Phenotype = the physical expression, or characteristics, of that trait.

Question 37

Although most phenotypic features are the result of multiple genes interacting, some characteristics are controlled by a single gene. Give 2 examples of this.

Answer 37

Fur colour in mice and red-green colour blindness in humans.

Question 38

Explain how an organism’s genotype is related to its phenotype.

Answer 38

The alleles present in an organism, or genotype, operate at a molecular level to determine its characteristics that are expressed as a phenotype.

Question 39

Long cat hair is caused by a dominant allele (H) and short by a recessive one (h).

What would the ratio of phenotype probability be if a heterozygous long-haired cat were bred with a short-haired one?

Answer 39

You need to work out the genotype of both cats, then construct a genetic cross diagram to work out the probabilities:

•​ Heterozygous long-haired cat: long-haired allele = dominant (H), and this cat is heterozygous, so its genotype must be Hh.

• Short-haired cat: short-haired allele = recessive (h), so must have 2 recessive alleles (homozygous) to show this trait, so its genotype = hh.

Using the diagram, the probability of long-short hair is in a 1:1 ratio.

Question 40

Give 2 examples of disorders caused by the inheritance of certain alleles, and state whether each is caused by a dominant or recessive allele.

Answer 40

• Polydactyly (having extra fingers or toes) is caused by a dominant allele.
• Cystic fibrosis (a disorder of cell membranes) is caused by a recessive allele.

Question 41

Embryo screening is used to analyse the genes of a cell removed from an embryo, in order to detect genetic disorders.

Gene therapy (still being researched) could be used to treat these disorders, or embryos may be terminated if a serious disorder is detected.

However, these proceses are controversial because of ethical issues which arise.

Name 3 advantages of these practices.

Answer 41

• Alleviate suffering.
• Reduce treatment costs for disorders, paid by the government and taxpayers.
• There are laws which prevent boundaries being crossed (people choosing desired characteristics, like sex or eye colour).

Question 42

Embryo screening is used to analyse the genes of a cell removed from an embryo, in order to detect genetic disorders.

Gene therapy (still being researched) could be used to treat these disorders, or embryos may be terminated if a serious disorder is detected.

However, these proceses are controversial because of ethical issues which arise.

Name 3 disadvantages of these practices.

Answer 42

• You could argue that it implies people with genetic problems are “undesirable” - leading to prejudice.
• Screening is expensive.
• There is a risk that people will start to seek screening in order to select desirable traits (sex, eye colour) for their babies.

Question 43

What did Gregor Mendel discover and when?

Answer 43

In the mid-19th century, Mendel carried out breeding experiments on plants. One of his observations was that the inheritance of each characteristic is determined by hereditary ‘units’ that are passed on unchanged down generations (one unit per parent, and units could be dominant/recessive).

^{For example, he crossed a tall and dwarf pea plant, which resulted in all the offspring being tall. He bred 2 of these together; the result was a tall : dwarf ratio of 3:1. This showed that the dwarf ‘unit’ was carried by the 2nd generation, even though these plants didn’t show this trait.}

Question 44

Why was the importance of Mendel’s discoveries not recognised until after his death?

Answer 44

The concepts of Mendel’s work was new to the scientists of his day. They didn’t have knowledge of genetics (DNA, genes, chromosomes) which allowed them to understand his findings.

It was only after these things were discovered that the significance of his work was recognised.

Question 45

Using Mendel’s work as a starting point, the observations of many scientists have contributed to modern understanding of genetics. Give 3 examples of important discoveries made.

Answer 45

• In the late 19th century, behaviour of chromosomes during cell division was observed.
• In the early 20th century, great similarity was observed between chromosomes and Mendel’s ‘units’, leading to the idea that the ‘units’, now called genes, were located on chromosomes.
• In the mid-20th century, the structure of DNA was determined, allowing the mechanism of gene function to be worked out.

Question 46

Define species.

Answer 46

A group of similar organisms which can interbreed to produce fertile offspring.

Question 47

Define variation.

Answer 47

Differences in the characteristics of individuals in a population.

Question 48

Give the 3 things which cause variation within a population.

Answer 48

• The genes organisms inherit (genetic causes).
• The conditions in which they develop (environmental causes).
• A combination of genes and the environment.

Question 49

There is usually extensive genetic variation within a population of a species. Explain what causes this variation.

Answer 49

• All variants arise from continuously occurring mutations.
• Most mutations have no effect on the phenotype and some influence phenotype.
• Very rarely will a mutation will lead to a new phenotype but, if it is suited to an environmental change, it can lead to a relatively rapid change in the species.

Question 50

What is evolution? What does the theory of evolution state?

Answer 50

Evolution is change in the inherited characteristics of populations over successive generations, which occurs through natural selection.

The theory of evolution states that all species of living things have evolved from simple life forms that first developed more than three billion years ago.

Question 51

Who proposed the theory of evolution by natural selection?

Answer 51

Charles Darwin.

Question 52

How did Darwin come up with his theory of evolution?

Answer 52

He made observations on a round the world expedition, backed by years of experimentation, discussion and new knowledge of geology and fossils.

Question 53

How and when did Darwin publish his ideas on evolution?

Answer 53

In “On the Origin of Species” in 1859.

Question 54

Explain Darwin’s theory of evolution by natural selection. (6)

Answer 54

• Organisms within a particular species show a wide range of (phenotypic) variation for a given characteristic.
• Individuals with characteristics (phenotypes) most suited to the environment are more likely to survive to breed successfully.
• The characteristics that have enabled these individuals to survive are then passed on to the next generation.
• Over time, beneficial characteristics become more common in the population, and it evolves.

Question 55

Why was there controversy surrounding Darwin’s revolutionary ideas about evolution, causing them to be only gradually accepted?

Answer 55

• The theory challenged creationism (the idea that God made all the animals and plants that live on Earth).
• There was little scientific evidence for Darwin to convince other scientists at the time the theory was published. The mechanism of inheritance and variation was not known until 50 years after.

Question 56

Name one scientist who proposed a different hypothesis to Darwin on evolution.

Answer 56

Jean-Baptiste Lamarck.

Question 57

What did Lamarck hypothesise on evolution?

Answer 57

That changes that an organism aquires during its lifetime can be inherited by its offspring.

E.g. if a rabbit uses its legs often to escape predators, its legs get longer, and its offspring are born with long legs.

Question 58

Why was Darwin’s hypothesis on evolution eventually accepted over Lamarck’s?

Answer 58

• Lamarck’s hypothesis was rejected because experiments didn’t support it.
• Darwin’s hypothesis was supported because it’s been shown that characteristics are passed on to offspring in genes.
• There is further evidence in the fossil record and in antibiotic resistance.

Question 59

What is speciation?

Answer 59

The process by which two populations of a species become so different in phenotype that they can no longer interbreed to produce fertile offspring.

Question 60

Explain how speciation occurs. (6)

Answer 60

• Two populations of a species become geographically isolated; conditions in their environments will be slightly different.
• There is genetic variation within each of the populations.
• Individuals with characteristics better suited to their environments are more likely to survive and reproduce, passing these on to the next generation.
• Therefore, over time, different characteristics become more common in each of the populations, and they evolve separately.
• Eventually, it gets to the point where an individual from one population cannot breed with an individual from the other population to produce fertile offspring: this is speciation.

Question 61

Describe the work of Alfred Russel Wallace in the development of the theory of evolution by natural selection.

Answer 61

• Wallace independently proposed the theory of evolution by natural selection, and gathered evidence worldwide.
• He published joint writings with Darwin in 1858, which prompted Darwin to publish “On the Origin of Species” in 1859.
• He is best known for his work on warning colouration in animals and his theory of speciation.
• More evidence over time has led to our current understanding of the theory of speciation.

Question 62

Give 5 factors which may cause the extinction of a species.

Answer 62

• The environment changes too quickly for a species to adapt (e.g. a destruction of habitat).
• A new predator.
• A new disease.
• A catastrophic event, like a natural disaster.
• A species is unable to compete with another for resources.

Question 63

What is selective breeding / artificial selection?

Answer 63

The process by which humans breed plants and animals for particular genetic characteristics.

Question 64

Explain the process of selective breeding.

Answer 64

• Parents, from a mixed population, with the desired characteristic are chosen and bred together.
• From their offspring, those with the desired characteristic are bred together.
• This continues over many generations, until all the offspring show the desired characteristic.

Question 65

In artificial selection, characteristics can be chosen for usefulness or appearance. Give 4 examples.

Answer 65

• Disease resistance in food crops.
• Animals which produce more meat or milk.
• Domestic dogs with a gentle nature.
• Large or unusual flowers.

Question 66

What is the main drawback of selective breeding?

Answer 66

• The process causes a reduction in the gene pool (the number of alleles in a population).
• The population becomes closely related.
• Leads to inbreeding, where organisms are particularly prone to inherited defects and disease (little variation means a disease that can kill one can likely kill them all).

Question 67

Explain the similarities and differences between natural and artificial selection. (6)

Answer 67

Similarities:

• Species become adapted to become more suited to a purpose.
• This relies on variation, caused by mutations in the genome.
• Both processes happen over many generations.

Differences:

• Natural selection happens naturally, whereas artificial selection relies on human intervention.
• Natural selection results in phenotypes suited to an environment, whereas selective breeding results in a phenotype beneficial to humans.
• Selective breeding is quicker - only selected individuals of a population are interbred, meaning the gene pool is more rapidly reduced.

Question 68

What is genetic engineering?

Answer 68

A process which involves modifying the genome of an organism, by introducing a gene from another organism, to give a desired characteristic.

Question 69

Describe the process of genetic engineering.

Answer 69

• Enzymes are used to isolate the desired gene.
• This gene is inserted into a vector, usually a bacterial plasmid or a virus.
• The vector is used to insert the gene into the cells of the other organism (animal/plant/microorganism).
• Genes are transferred to cells at an early stage in their development, so that they develop with the desired characteristics.

Question 70

Give 2 examples of uses of genetic engineering.

Answer 70

• Plant crops have been genetically engineered to be resistant to diseases or to produce more and better yield.
• Bacterial cells have been genetically engineered to produce useful substances, such as human insulin to treat diabetes.

Question 71

Name 4 benefits of genetic engineering.

Answer 71

• GM crops often have increased yields.
• GM crops can be resistant to insect attack or herbicides.
• Crops in developing countries can be modified to contain a nutrient that people there lack in their diets.
• Research is exploring the possibility of GE to overcome some inherited disorders.

Question 72

Name 3 concerns that some people have about genetic engineering.

Answer 72

• Some say GM crops will have a destructive effect on populations of wild flowers and therefore of insects.
• Some are concerned that the effects of eating GM crops on human health have not been fully explored.
• Some are concerned that transplanted genes may escape to the natural environment, e.g. weeds picking up herbicide resistance.

Question 73

Name 4 types of cloning.

Answer 73

• Tissue cultures.
• Cuttings.
• Embryo transplants.
• Adult cell cloning.

Question 74

Explain tissue culture cloning.

Answer 74

Small groups of cells from part of a plant are used to grow identical new plants.

This is important for preserving rare plant species, or commercially, in nurseries.

Question 75

Explain cuttings (a cloning method).

Answer 75

Gardeners take cuttings from a good plant, and plant them to produce many genetically identical new plants.

Question 76

Explain how embryo transplant cloning is carried out.

Answer 76

Cells from a developing animal embryo are split apart before they become specialised. The identical embryos are then transplanted into host mothers.

Question 77

Explain how adult cell cloning is carried out.

Answer 77

• The nucleus is removed from an unfertilised egg cell.
• The nucleus from an adult body cell (e.g. skin) is inserted into the egg cell.
• An electric shock stimulates the egg cell to divide to form an embryo.
• These embryo cells contain the same genetic information as the adult skin cell.
• When the embryo has developed into a ball of cells, it is inserted into the womb of an adult female to continue its development.

Question 78

Give 2 potential benefits of cloning.

Answer 78

• Studying animal clones could lead to greater understanding of the development of the embryo and aging.
• Cloning could be used to preserve endangered species.

Question 79

Explain 3 risks of cloning.

Answer 79

• Cloning means a reduced gene pool, meaning weakened disease resistance.
• It’s possible that cloned animals might not be as healthy as normal ones.
• Some worry that people will be cloned in the future, and that clones could be born severely disabled.

Question 80

What are fossils?

Answer 80

The remains of organisms from millions of years ago, embedded in rocks.

Question 81

In what 3 ways are fossils formed?

Answer 81

• Organisms can be preserved, because one or more of the conditions needed for decay are absent (peat bogs, glaciers).
• Parts of an organism are replaced by minerals as they decay.
• Impression from organisms, such as footprints, burrows and rootlet traces, can be preserved, forming casts.

Question 82

Why is the fossil record incomplete?

Answer 82

Many early forms of life were soft-bodied, so left few traces behind. What traces there were have been mainly destroyed by geological activity.

This is why scientists cannot be certain about how life began on Earth.

Question 83

What can be learned from fossils?

Answer 83

How much different organisms have changed as life developed on Earth.

Question 84

Why can bacteria evolve rapidly?

Answer 84

Because they reproduce at a high rate.

Question 85

Explain how a strain of bacteria may become resistant to an antibiotic. (6)

Answer 85

• Mutations of bacterial pathogens produce new strains.
• Some strains may be resistant to antibiotics, and so are not killed.
• These ones survive and reproduce, so the population of the resistant strain rises.
• The resistant strain will then spread, because people are not immune to it, and there is no effective treatment.

Question 86

Give an example of an antibiotic-resistant bacterium.

Answer 86

MRSA.

Question 87

What can and should be done to reduce the rate of development of antibiotic resistant strains?

Answer 87

• Doctors should not prescribe antibiotics inappropriately (treating non-serious or viral infections).
• Patients should complete their course of antibiotics, so that all bacteria are killed and none survive to mutate and form resistant strains.
• Agricultural use of antibiotics should be restricted.

Question 88

Why is antibiotic resistance such a big problem?

Answer 88

The development of new antibiotics is costly and slow. It is unlikely to keep up with the emergence of new resistant strains.

Question 89

How have living things traditionally been classified?

Answer 89

Traditionally, living things have been classified into groups depending on their structure and characteristics in a system developed by Carl Linnaeus.

Question 90

How did Linnaeus classify living things?

Answer 90

Kingdom

Phylum

Class

Order

Family

Genus

Species

Question 91

How are organisms named?

Answer 91

By the binomial system: genus and species.

E.g. Homo sapiens. Always capitalise genus, but never species.

Question 92

Describe the impact of biological developments on classification systems.

Answer 92

As evidence of internal structures became more developed, due to improvements in microscopes, and the understanding of biochemical processes progressed, new models of classification were proposed.

Question 93

Due to evidence available from chemical analysis, there is now a three domain system of classification, developed by Carl Woese. What are the 3 domains?

Answer 93

• Archaea (primitive bacteria, usually living in extreme environments)
• Bacteria (true bacteria)
• Eukaryota (includes protists, fungi, plants and animals).

Question 94

Due to evidence available from chemical analysis, there is now a three domain system of classification: archaea, bacteria and eukaryota. Who proposed this system?

Answer 94

Carl Woese.

Question 95

This is an evolutionary tree. What are these used for and how do they use data?

Answer 95

Evolutionary trees are a method used by scientists to show how they believe organisms are related.

They use current classification data for living species, and fossil data for extinct species.

Question 96

This is a section of an evolutionary tree.

1. Which species is the most recent common ancestor of species F and G? (1)
2. Would you expect species D to look similar to species E? Give a reason for your answer. (1)

Answer 96

1. Species C.
2. Yes, because they share a recent common ancestor, so are closely related / have similar genes.