Higher: B6 - Inheritance, variation and evolution

Question 1

What is DNA and where is it found?

Answer 1

DNA 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.

Question 2

What is a gene?

Answer 2

A small section of DNA on a chromosome. Each gene codes for a particular sequence of which codes for a protein.

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

What are mutations?

Answer 7

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

Question 8

What is sexual reproduction?

Answer 8

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 9

Give 2 types of sexual reproduction.

Answer 9

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

Question 10

What is asexual reproduction?

Answer 10

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 11

Give 2 advantages of sexual reproduction.

Answer 11

• 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 12

Give 3 advantages of asexual reproduction.

Answer 12

• 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 13

What is meiosis and where does it occur?

Answer 13

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

Question 14

How many chromosomes do human gametes have and why?

Answer 14

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 15

Describe what happens during meiosis.

Answer 15

• 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 16

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

Answer 16

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 17

What are gametes? Name the female and male gametes.

Answer 17

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 18

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

Answer 18

23.

Question 19

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

Answer 19

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

Question 20

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 20

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

Question 21

What is an allele?

Answer 21

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

Question 22

Compare dominant and recessive alleles.

Answer 22

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 23

Define heterozygous and homozygous.

Answer 23

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 24

Define “genotype” and “phenotype”.

Answer 24

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 25

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 25

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

Question 26

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

Answer 26

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

Question 27

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 27

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 28

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 28

• 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 29

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 29

• 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 30

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 30

• 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 31

Define species.

Answer 31

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

Question 32

Define variation.

Answer 32

Differences in the characteristics of individuals in a population.

Question 33

Give the 3 things which cause variation within a population.

Answer 33

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

Question 34

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

Answer 34

• 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 35

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

Answer 35

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 36

Who proposed the theory of evolution by natural selection?

Answer 36

Charles Darwin.

Question 37

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

Answer 37

• 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 38

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

Answer 38

Jean-Baptiste Lamarck.

Question 39

What did Lamarck hypothesise on evolution?

Answer 39

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 40

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

Answer 40

• 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 41

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

Answer 41

• 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 42

What is selective breeding / artificial selection?

Answer 42

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

Question 43

Explain the process of selective breeding.

Answer 43

• 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 44

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

Answer 44

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

Question 45

What is the main drawback of selective breeding?

Answer 45

• 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 46

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

Answer 46

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 47

What is genetic engineering?

Answer 47

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

Question 48

Describe the process of genetic engineering.

Answer 48

• 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 49

Give 2 examples of uses of genetic engineering.

Answer 49

• 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 50

Name 4 benefits of genetic engineering.

Answer 50

• 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 51

Name 3 concerns that some people have about genetic engineering.

Answer 51

• 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 52

What are fossils?

Answer 52

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

Question 53

In what 3 ways are fossils formed?

Answer 53

• 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 54

Why is the fossil record incomplete?

Answer 54

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 55

What can be learned from fossils?

Answer 55

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

Question 56

Why can bacteria evolve rapidly?

Answer 56

Because they reproduce at a high rate.

Question 57

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

Answer 57

• 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 58

Give an example of an antibiotic-resistant bacterium.

Answer 58

MRSA.

Question 59

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

Answer 59

• 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 60

Why is antibiotic resistance such a big problem?

Answer 60

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

Question 61

How have living things traditionally been classified?

Answer 61

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

Question 62

How did Linnaeus classify living things?

Answer 62

Kingdom

Phylum

Class

Order

Family

Genus

Species

Question 63

How are organisms named?

Answer 63

By the binomial system: genus and species.

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

Question 64

Describe the impact of biological developments on classification systems.

Answer 64

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 65

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 65

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

Question 66

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 66

Carl Woese.