[B6] Inheritance, variation and evolution

What is sexual reproduction?

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

What cells are involved in sexual reproduction in animals and in plants?

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

What is asexual reproduction?

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).

What are some advantages of sexual reproduction?

• 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

What are some advantages of asexual reproduction?

• 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.

What is meiosis?

When cells divide to form gametes (which have half the number of chromosomes as body cells)

What happens during meiosis?

• The cell makes copies of 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.

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

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.

What are gametes?

An organism's reproductive (sex) cells, which have half the usual genetic information (each cell carries only one copy of each chromosome).

What are the female and male gametes called in animals?

Female gametes are ova / egg cells, and male gametes are sperm.

What are the advantages of sexual reproduction?

Produces variation in the offspring and can provide survival advantage,

What are the advantages of asexual reproduction?

• only one parent needed • more time and energy efficient as do not need to find a mate • faster than sexual reproduction • many identical offspring can be produced when conditions are favourable

When do malarial parasites reproduce sexually?

When interacting with other mosquitos

When do malarial parasites reproduce asexually?

In the human host

When will fungi reproduce sexually?

To produce variation

When will fungi reproduce asexually?

To produce spores

What can plants produce through asexual production?

Runners such as strawberries and bulbs for daffodils

What is DNA?

DNA is the chemical that a cell's genetic information is composed from. DNA is a polymer, made up of two strands forming a double helix.

Where is DNA found?

DNA is found organised into chromosomes in the nucleus

What is a gene?

A small section of DNA on a chromosome. Each gene codes for a particular sequence of amino acids, to make a specific protein.

What is a genome?

All of the genetic material of an organism

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

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.

What is DNA made from?

A polymer made up from repeating nucleotide units

What are the four bases of DNA?

A (adenine), C (cytosine), G (guanine) and T (thymine)

What does a sequence of three bases provide the code for?

How amino acids are assembled

What is protein synthesis?

The production of proteins from amino acids, which happens in the ribosomes of the cell

In strands of DNA, what is the complementary base pair to T molecules?

A

In strands of DNA, what is the complementary base pair to C molecules?

G

What are some of the functions of folder protein chains?

They form enzymes, hormones or other structures in the body such as collagen

What is the function of non-coding DNA?

Switching genes on and off

What is an allele?

One of two or more versions of a gene. There are two alleles for each gene.

What are dominant and recessive alleles?

The combination of a pair 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 (dd)

Define heterozygous and homozygous.

Homozygous = a gene that has two identical alleles (e.g. DD or dd) Heterozygous = a gene that has two different alleles (e.g. Dd)

What do genotype and phenotype mean?

Genotype = the alleles present. Phenotype = the physical expression, or characteristics, of that trait (e.g. hair colour)

How does an organism's genotype relate to its phenotype?

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

Give two examples of characteristics that are controlled by a single gene (MOST characteristics are a result of multiple genes interacting)

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

Give two examples of disorders caused by the inheritance of certain alleles

• Polydactyly (having extra fingers or toes) • Cystic fibrosis (a disorder of cell membranes)

Is polydactyly caused by a recessive or dominant allele?

Dominant

Is cystic fibrosis caused by a recessive or dominant allele?

Recessive

What could be some advantages of embryo screening?

• Alleviate suffering • Reduce treatment costs for disorders • There are laws which prevent boundaries being crossed (e.g. people choosing desired characteristics, like sex or eye colour).

What could be some disadvantages of embryo screening?

• It could imply that 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.

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

23 pairs

How many chromosomes do human gametes have?

23 (23 only, NOT 23 pairs)

What are the sex chromosomes in males and females?

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

What is a species?

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

What is variation?

Differences in the characteristics of individuals in a population.

What can cause variation within a population?

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

Why is there usually extensive genetic variation within a population of a species?

All variants of genes arise from continuously occurring mutations

What are mutations?

Random alterations in DNA which occur continuously.

When do mutations lead to changes in a species?

• Most mutations have no effect on the phenotype of an organism • Very rarely, a mutation will lead to a new phenotype • If the new phenotype is suited to an environmental change, it can lead to a relatively rapid change in the species.

How can cloning be used to preserve rare plant species?

Tissue cultures (a small group of cells) can be used to grow new plants

How can many identical plants be made from one parent plant?

By taking and planting cuttings

How do embryo transplants work?

By splitting cell from a developing animal embryo before they become specialised, then transplanting the identical embryos into host mothers

Why was Darwin's work on the origin of species not originally published?

• There was insufficient evidence at the time • It challenged the idea that God created all plants and animals

What does survival of the fittest mean?

Organisms with characteristics most suited to the environment are more likely to survive to breed successfully and pass this trait onto their offspring

What is speciation?

The formation of new and distinct species in the course of evolution

What did Mendel observe in his study of plants?

The inheritance of each characteristic is determined by 'units' that are passed on to offspring

What is evolution?

the inherited characteristics of populations over successive generations, which occurs through natural selection.

What does the theory of evolution state?

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

Who proposed the theory of evolution by natural selection?

Charles Darwin

What are the key points in Darwin's theory of evolution by natural selection?

• Organisms within a particular species show a wide range of 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

Why was Darwin's hypothesis on evolution eventually accepted over others?

• Other scientist's hypotheses were rejected because experiments didn't support them • Darwin's hypothesis was supported because it's been shown that characteristics are passed on to offspring in genes • There is further evidence for Darwin's hypothesis in the fossil record and in antibiotic resistance

What is selective breeding?

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

How is selective breeding done?

• Parents 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.

What has selective breeding been used to produce?

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

What is the main drawback of selective breeding?

Leads to inbreeding, where organisms are particularly prone to inherited defects and disease (a disease that can kill one can likely kill them all).

What is genetic engineering?

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

What has genetic engineering been used to produce?

• 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.

What are some benefits of genetic engineering?

• Genetically modified (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.

What are some concerns that some people have about genetic engineering?

• 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

[𝐇𝐢𝐠𝐡𝐞𝐫] Describe the process of genetic engineering.

• 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.

What are fossils?

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

How are fossils formed?

• From organisms that have not decayed because one or more of the conditions needed for decay are absent (e.g. in peat bogs or glaciers) • From parts of an organism that are replaced by minerals as they decay • From impression from organisms, such as footprints, burrows and rootlet traces

Why is the fossil record incomplete?

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.

What can be learned from fossils?

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

What is extinction?

when there are no remaining individuals of a species still alive

What factors may contribute to the extinction of a species?

• 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.

Why can bacteria evolve rapidly?

Because they reproduce at a high rate

How can a strain of bacteria become resistant to an antibiotic?

• 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

Give an example of an antibiotic-resistant bacterium.

MRSA

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

• 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.

Why is antibiotic resistance such a big problem?

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

How have living things traditionally been classified?

Traditionally, living things have been classified into groups depending on their structure and characteristics in a system developed by Carl Linnaeus. Now, they are classified using the three-domain system.

How did Linnaeus classify living things?

Kingdom Phylum Class Order Family Genus Species

How are organisms named?

By the binomial system: genus and species. e.g. Homo sapiens. Always capitalise genus, but never species.

How have our classifications systems for living organisms has developed over time?

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

What are the three domains in the 'three-domain system' of classification of living organisms?

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

Who developed the 'three-domain system' of classification of living organisms?

Carl Woese