Inheritance, Variation and Evolution- Topic 6 Paper 2

Topic 6

What is sexual reproduction?

* A type of reproduction
* Involves the production of gametes by meiosis
* A Gamete from each parent fuses to form a zygote
* Genetic information from each gamete is mixed so the resulting zygote is unique

What are gametes?

* Sex cells (sperm and egg cells in animals, pollen and egg cells in flowering plants)
* Haploid (half the number of chromosomes)

What is meiosis?

* Form of cell division involved in the formation of gametes (non-identical haploid cells) in reproductive organs
* Chromosome number is halved
* Involves two divisions

What must occur prior to meiosis?

Interphase- copies of genetic material information are made during this process

What happens during the first phase of meiosis?

* The chromosome pairs line up along the cell equator
* The pair of chromosomes are separated and move to opposite poles of the cell (the side to which each chromosome is pulled is random, creating variation)
* Chromosome number is halved

What happens during the second phase of meiosis?

* Chromosomes line up along the cell equator
* Chromatids are separated and move to opposite poles of the cell
* Four unique haploid gametes are formed

Why is meiosis important for sexual reproduction?

* It increases genetic variation
* Ensures that the zygote formed at fertilisation is diploid

Describe fertilisation and its resulting outcome?

Gametes join together to restore the normal number of chromosomes and the new cell then divides by mitosis (increases the number of cells)

As the embryo develops, cells differentiate

What advantages of sexual reproduction?

It increases genetic variation in offspring, increasing the probability of a species adapting to and surviving environmental changes

Natural Selection can be speeded up by humans by selective breeding to increase food production

What disadvantages of sexual reproduction?

Two parents required- This makes production difficult in endangered populations or in species that exhibit solitary lifestyles

More time and energy is required so fewer offspring are produced

What is asexual reproduction?

* A type of reproduction
* Involves mitosis only
* Producer genetically identical to offspring, known as daughter cells

What advantages of asexual reproduction?

* Only one parent is required
* Lots of offspring can be produced in a short period of time., enabling the reduction of competition and colonising of an area
* Requires less energy and time as they do not need to mate

What disadvantages of asexual reproduction?

No genetic variation( except spontaneous mutations) reduces the probability of a species being able to adapt to an environmental change.

Circumstances for malarial parasites that can reproduce asexually and sexually?

* Sexual reproduction in mosquito
* Asexual reproduction in the human host

Circumstances for fungi that can reproduce asexually and sexually?

* Sexual reproduction gives variation
* Asexual reproduction by the spores

Circumstances for plants that can reproduce asexually and sexually?

* Sexual reproduction to produce seeds
* Asexual reproduction by runners (strawberry plant) or bulb division (daffodils)

What is DNA?

* A double-stranded polymer of nucleotides wound to form a double helix
* genetic material in the cell found in the nucleus

Define genome?

The entire genetic material of an organism.

Why is understanding the human genome important?

* Important for the development of medicine
* Searching for genes that link to different types of disease
* Understanding and treating inherited disorders
* Tracing human migration patterns from the past

What is a chromosome?

A Long coiled molecule of DNA that carries genetic information in the genes

How many chromosomes does a human body cell have?

46 (23 pairs)

How many chromosomes does a human gamete have?

23

Define gene?

A small section of DNA that codes for a specific sequence of amino acids which undergo polymerisation to form a protein

What are the monomers of DNA?

Nucleotides

What are DNA Nucleotides made up of?

A Base (A, T, C or G)

A common Sugar

A phosphate group

State the full names of the four bases found in nucleotides?

A- Adenine

G- Guanine

T- Thymine

C- Cytosine

How do nucleotides interact to form a molecule of DNA?

* Sugar and phosphate molecules join together to form a sugar-phosphate backbone in each DNA strand
* Base connected to each sugar
* Complementary base pairs ( A-T, C-G) joined by weak hydrogen bonds

How does a gene code for protein?

* A Sequence of three bases in a gene forms a triplet
* Each triplet codes for an amino acid
* The order of amino acids will be determined by the structure (how it folds) and function of the protein formed

Why is the folding/structure important in proteins such as enzymes?

The folding of amino acids determines the shape of the active site which must be highly specific to the shape of its substrate

What is protein synthesis?

The formation of protein from a gene

What are the two stages of protein synthesis?

* Transcription
* Translation

What is transcription?

The formation of mRNA from a DNA template

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* DNA double helix unfolds
* RNA polymerase binds to a specific base sequence of non-coding DNA in front of a gene and moves along the DNA strand
* RNA polymerase joins free RNA nucleotides to complementary bases on the coding DNA strand
* mRNA formation complete. mRNA detaches and leaves the nucleus

What is translation?

A Ribosome joins amino acids in a specific order dictated by mRNA to form a protein

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* mRNA attaches to a ribosome
* The ribosome reads mRNA bases in triplets. Each triplet codes for one amino acid which is brought to the ribosome by a tRNA molecule (carrier molecule)
* A polypeptide chain is formed from the sequence of amino acids which join together

What is a mutation?

* A Random change in the base sequence of DNA which results mostly in no change to the protein coded for, or genetic variations of the protein
* Mutations occur continuously

Describe the effect of a gene mutation in coding DNA?

If a mutation changes the amino acid sequence,

Protein structure, and function may change ( an enzyme may no longer fit in its substrate binding site or structural protein may lose its strength)

If a mutation does not change the amino acid sequence, there is no effect on the protein structure or function

What is non-coding DNA?

DNA which does not code for a protein but instead controls gene expression

Describe the effect of a gene mutation in non-coding DNA?

Gene expression may be altered, affecting protein production and the resulting phenotype

What are alleles?

Different versions of the same gene

What is a dominant allele?

A version of a gene where only one copy is needed for it to be expressed

What is a recessive allele?

A version of a gene where two copies are needed for it to be expressed

What is meant by when an organism is homozygous?

When an organism has two copies of the same allele

(two recessive or dominant)

What is meant by when an organism is heterozygous?

When an organism has two different versions of the same gene (one dominant, one recessive)

What is a genotype?

The genes present for a trait

What is a phenotype?

The visible characteristic

How are dominant alleles shown in punnet squares?

Uppercase letters

How are recessive alleles shown in punnet squares?

Lowercase letters (same letter as dominant)

What is the problem with single gene crosses?

Most characteristics are controlled by multiple alleles rather than just one

What is an inherited disorder?

A disorder caused by the inheritance of certain alleles

Give 2 examples of inherited disorders?

Cystic Fibrosis (a disorder of cell membranes)- is caused by a recessive allele.

Polydactyly (Having extra fingers or toes)- is caused by a dominant allele.

How are embryos screened for inherited disorders?

During IVF, one cell is removed (from an 8-cell embryo) and tested for disorder-causing alleles. If the cell doesn’t have any indicator alleles then the originating embryo is implanted into the uterus

What are the ethical issues concerning embryo screening?

* It could lead to beliefs in society that being disabled or having a disorder is less human or associated with inferiority
* The destruction of embryos with inherited disorders is seen by some as murder as these would go on to become human beings
* It could be viewed as part of the concept of designer babies as it may be for the parents’ convenience or wishes rather than the children’s wellbeing

What are the economical issues regarding embryo screening?

* Costs of hospital treatment and medication will need to be considered if it is known that a child will have an inherited disorder and financial support explored if necessary

What are the social issues regarding embryo screening?

Social care for children with inherited disorders may need to be considered if parents are unable to care for

If an embryo is found to have an inherited disorder and is terminated this can prevent a child and its parents from potentially suffering in the future due to the disorder

What is gene therapy?

The insertion of a normal allele into the cells of a person with an inherited disorder to functionally replace the faulty allele

What are the ethical issues concerning gene therapy?

Some people believe it is going against God

The introduced genes could enter sex cells and be passed on to future generations

What are sex chromosomes?

A pair of chromosomes that determine the sex

males have X and Y chromosomes

Females have two X chromosomes

Why does the inheritance of a Y chromosome mean the embryo develops into a male?

Testes development is stimulated by a gene in Y chromosome

What is a sex-linked characteristic?

A characteristic that is coded by an allele found on a sex chromosome

Why are the majority of genes carried on the X chromosome rather than the Y chromosome?

The X chromosome is bigger therefore there are more genes

What is Variation?

Differences in the characteristics of individuals in a population are called variation

What causes variation in species?

* Genetics
* Environment
* A mixture of both

What is genetic variation?

* Variations in the genotypes of organisms of the same species due to the presence of different alleles
* Creates differences in phenotypes

What creates genetic variation in species?

* Spontaneous mutations
* Sexual reproduction

What is a mutation?

A random change to the base sequence in DNA which results in genetic variants. They occur spontaneously

State the 3 types of gene mutation?

* Insertion
* Deletion
* Substitution

How may a gene mutation affect an organism’s phenotype?

A Mutation may:

* Cause no change, the structure and function of the protein remain the same
* Cause a minor change i.e. change of eye colour
* Cause a severe change that could completely change the sequence of amino acids, making the protein non-functional

What is the consequence of a new phenotype caused by a mutation being suited to an environmental change?

There will be a rapid change in the species

What is evolution?

* A Gradual change in the inherited traits within a population over time
* Occurs due to natural selection which may result in the formation of a new species

What is the theory of natural selection?

All species of living things have evolved from simple life forms that first developed more than 3 billion years ago

* Genetic variation exists due to spontaneous mutations
* Selection pressures (competition, disease) exist
* Random mutations give an organism a selective advantage
* The organism is better adapted to the environment and survives
* Organism reproduces, passing on beneficial alleles
* The frequency of advantageous alleles increase

How do two populations become different species?

When their phenotypes become different to the extent that they cannot no longer interbreed to produce fertile offspring

What is selective breeding?

The process by which humans artificially select organisms with desirable characteristics to breed and produce offspring with similar phenotypes

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1. Identify a desired characteristic e.g. disease resistance
2. Select parent organisms that show desired traits and breed them together
3. Select offspring with the desired traits and breed them together
4. The process is repeated until all offspring have the desired trait

Give examples of characteristics selected for selective breeding?

* Disease resistance in crops
* Higher milk or meat production in animals
* Gentle nature in domestic dogs
* Large flowers

The main advantage of selective breeding?

Creates organisms with desired characteristics

Where else can selective breeding be useful apart from agriculture?

* In medical research
* In sports e.g. Horse races

Disadvantages of selective breeding?

* Reduction in the gene pool (harmful if an environmental change occurs)
* Inbreeding results in genetic disorders
* Development of other physical problems e.g. respiratory problems in bulldogs
* Potentially selecting harmful recessive alleles

What is Genetic Engineering?

* The modification of the genome of an organism by the insertion of a desired gene from another organism- genes from chromosomes of humans and other organisms can be “cut out” and transferred to cells of other organisms
* Enables the formation of an organism with beneficial characteristics

Give examples of uses for genetically modified plants?

* Disease Resistance
* Produce larger fruit

What is a use for genetically modified bacteria cells?

To produce human insulin to treat type 1 diabetes

Describe the benefits of Genetic Engineering?

* Increases crop yields for growing populations e.g. herbicide- resistance, disease resistance
* Useful in medicine e.g. insulin-producing bacteria, possibility to overcome inherited disorders
* GM Crops can produce scarce resources e.g. golden rice has vitamin A

Describe the risks of genetic engineering?

* the Long-term effects of consumption of GM crops are unknown
* Negative environmental impacts e.g. reduction in biodiversity , impact on the food chain, contamination of non-GM crops forming “superweed”
* Late-onset health problems in GM Animals
* GM seeds may be expensive, less economically developed countries may be unable to afford them or may be dependent on businesses to sell them

What is the name for crops that have their genes modified

Genetically modified (GM) crops

What is Bacillus thuringiensis?

Insect larvae are harmful to crops

Bt is a bacterium that secretes a toxin that kills insect larvae

How is genetic engineering used to protect crops against insects?

* The gene for toxin production in Bt can be isolated and inserted into the DNA of crops
* Bt crops now secrete the toxin which kills any insect larvae that feed on it

What are the benefits of Bt crops?

* Increase in crop yield (fewer crops damaged)
* Lessens the need for artificial insecticides
* Bt toxin is specific to certain insect larvae so it is not harmful to other organisms that ingest it

What are the risks of Bt crops?

* The long-term effects of Bt crops consumption are unknown
* Insect larvae may become resistant to the Bt toxin
* Killing insect larvae reduces biodiversity

Describe the process of Genetic Engineering?

1. DNA is cut at a specific bases sequence by restriction enzymes to create sticky ends
2. Vector DNA cut using the same restriction enzymes create complementary sticky ends
3. Ligase enzymes join the sticky ends of DNA and Vector DNA forming recombinant DNA
4. Recombinant DNA mixed with and taken up by target cells

What is a Vector?

A Structure that delivers the desired gene into the recipient cell e.g. plasmids, viruses

How can plants be cloned?

* Taking plant cuttings
* Tissue culture

What is tissue culture?

Using small groups of cells from a part of a plant to grow identical new plants

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1. Select a plant that shows desired characteristics
2. Cut multiple small sample pieces from meristem tissue
3. Grow in a petri dish containing a growth medium
4. Transfer into compost for further growth

What must be ensured when preparing tissue cultures?

Ensure aseptic conditions to prevent contamination by microorganisms

What does a growth medium contain?

Nutrients and Growth Hormones

What advantages of growing plants by tissue culture?

* Fast and simple process
* Requires little space
* Enables the growth of many plant clones with the same desired characteristics
* Useful in the preservation of endangered plant species

What disadvantages of growing plants by tissue culture?

* Reduction in Gene Pool
* Plant clones often have a low survival rate
* Could unknowingly increase the presence of harmful recessive alleles

Describe the method of Plant cuttings?

Older but simpler method than tissue culture

Gardeners use this method to produce many identical new plants from a parent plant


1. A branch is cut off from the parent plant
2. The lower leaves of the branch are removed and the stem is planted
3. Plant hormones are used to encourage new root development
4. A plastic bag is used to cover the new plant and keep it warm and moist
5. New roots and a new plant is formed after a few weeks

What does embryo transplanting involve?

* Pre-specialised cells from a developing animal embryo being split apart
* The resulting separate but identical embryos are transplanted into host mothers

Describe how adult cell cloning is performed?

1. The nucleus is removed from an unfertilised egg cell
2. The nucleus from an adult body cell is inserted into the empty egg cell
3. An electric shock stimulates the nucleated egg cell to divide and it forms an embryo
4. The embryo cell contains the same genetic information as the adult body cell
5. When the embryo is a ball of cells, it is inserted into the uterus of an adult female to continue developing (surrogate mother)

Outline the theory of evolution by natural selection?

* Individuals of a species show a wide range of variation for a characteristic
* Those with the characteristics most suited for the environment will survive and breed most successfully
* The desirable characteristic that has enabled the individuals to survive are passed on to their offspring

Why was Darwin’s theory of evolution not accepted initially?

* Most people believed in creationism
* Insufficient evidence to prove the theory
* The mechanism of variation and inheritance was not known at the time