DNA & Inheritance (WJEC GCSE Science (Double Award): Biology): Flashcards

• Adenine pairs with thymine

• Cytosine pairs with guanine

The base adenine always pairs with the base thymine.

False.

Guanine pairs with cytosine. Thymine pairs with adenine.

It holds the two strands together in the double helix, and it allows DNA to be copied accurately for protein synthesis and cell division.

A sequence of nucleotide bases in DNA that codes for a specific sequence of amino acids, which in turn makes up a specific protein.

The three-base code in DNA, in which each sequence of three bases codes for one amino acid.

Three. This is why it is called the triplet code.

20.

Almost every organism uses the same code, so the same triplets code for the same amino acids in all living things.

It means genetic information is transferable between species, which is what makes techniques such as gene transfer possible.

In the cytoplasm.

DNA is too large to leave the nucleus.

A smaller molecule carries a copy of the code to the cytoplasm, where the protein is made.

Each sequence of three bases in DNA codes for one amino acid.

Protein synthesis takes place in the cytoplasm of the cell.

False.

DNA is too large to leave the nucleus. A smaller molecule carries a copy of the code instead.

True.

Each amino acid is coded for by three bases, so 12 ÷ 3 = 4 amino acids.

Each triplet of bases codes for one amino acid.

The order of triplets therefore sets the order of amino acids in the chain, and that order determines which protein is made.

A technique that cuts DNA into short pieces and separates them into bands, producing a pattern that is unique to an individual.

Obtain the DNA from a sample such as hair root, blood, semen or saliva.

Use enzymes to cut the DNA into fragments of different sizes.

Separate the fragments using gel electrophoresis.

Treat the gel so the fragments show up as bands, then analyse the pattern.

Enzymes.

Because every person's DNA sequence is unique, so the enzymes cut it in different places.

The DNA is suspended in a gel and an electric current is passed through it.

DNA is negatively charged, so it moves towards the positive end.

Smaller fragments travel faster and further than larger ones.

DNA carries a negative charge, and opposite charges attract.

The smallest fragments.

They move more easily through the gel, so they travel faster and further than larger fragments.

• The root of a hair

• A spot of blood

• Semen

• Saliva

DNA fragments are separated by size using gel electrophoresis.

Enzymes are used to cut DNA into fragments of different sizes.

True, with one exception.

Identical twins share the same DNA and so have the same genetic profile.

False.

Smaller fragments travel faster and further. Larger fragments stay nearer the start.

A pattern of bands, each band representing DNA fragments of a particular size.

They share more of their DNA sequence, so the enzymes cut their DNA in similar places and produce similar band patterns.

The patterns of bands produced by each sample are compared.

The more bands they have in common, the more similar the two samples are.

• Criminal cases, to identify suspects

• Paternity cases, to identify a child's father

• Classification, to compare how closely related two species are

All three of the mother, the child and the alleged father.

Every band in the child's profile must appear in either the mother's or the father's profile.

If a band appears in neither, that man is not the father.

The child must have inherited that band from the father, so it must be present in the true father's profile.

Samples of body cells or fluids are taken from the crime scene or victim.

The DNA is extracted and profiled.

The profile is compared with samples from suspects, the victim, and control samples from people with no connection to the crime.

To avoid contamination of the samples, which would make the profile unreliable.

• Identifying bodies or body parts that are otherwise unidentifiable

• Eliminating innocent people whose DNA happens to be at the scene

The more similar the genetic profiles of two species, the more closely related they are and the more recent their common ancestor.

The more similar the genetic profiles of two species, the more closely related they are.

In a paternity test, any band in the child's profile must appear in the mother's or the father's profile.

True.

The more bands in common, the more likely the crime scene DNA came from that person.

True.

Mammals share a more recent common ancestor with each other than with other vertebrate groups, so their DNA is more similar.

So they can be aware of any inherited illnesses that might affect them in the future.

It can identify the presence of certain genes that may be associated with a particular disease.

The use of an individual's DNA profile to determine their risk of developing a particular disease.

On an embryo, a fetus, or an adult.

Any three of:

• Take preventative measures

• Begin effective treatment early

• Plan for the future

• Make informed decisions about pregnancy

• Take part in research or clinical trials

If there is a family history of a genetic disease, so they can find out the risk to the child.

Genetic data is very sensitive, so its testing and storage must be tightly controlled to prevent misuse. It raises the question of who should have access to it.

The results could be used against the patient, for example by insurance companies or employers.

It can trigger emotional stress and anxiety for the patient and their family.

Counselling may be needed but is not always readily available.

Results affect relatives as well as the patient, so difficult decisions arise about who should be tested and who should be told.

Genetic screening uses a DNA profile to determine the risk of developing a particular disease.

One concern about genetic screening is that results could be used by insurance companies to discriminate against a patient.

False.

Because genes are inherited, results also carry information about relatives, which raises difficult decisions about sharing them.

True.

Early diagnosis allows preventative measures and effective treatment to begin sooner.

So they fully understand the ethical implications for themselves and their family before deciding whether to be tested.

A section of a DNA molecule that determines an inherited characteristic.

A different version of the same gene. Alleles are inherited in pairs, one from each parent.

A sex cell, e.g. an egg or sperm cell in animals, or pollen and an ovule in plants.

An allele that is always expressed, even if only one copy is present.

An allele that is only expressed if two copies are present, with no dominant allele.

A genotype with two of the same alleles for a particular gene.

A genotype with two different alleles for a particular gene.

The combination of alleles that control a particular characteristic.

The observable characteristics of an organism, resulting from its genotype and environmental influences.

Self-fertilisation, when the male and female gametes of the same organism fuse to make a zygote, e.g. in plants.

• F₁: the first generation of offspring from a cross between two parents

• F₂: the offspring produced when two individuals from the F₁ generation are crossed

The dominant allele is given a capital letter and the recessive allele the same letter in lower case, e.g. T and t.

One allele is inherited from the mother and one from the father, so they do not have to be identical.

A genotype with two identical alleles for a gene is described as homozygous.

The observable characteristics of an organism make up its phenotype.

False.

A dominant phenotype is shown by both the homozygous dominant and heterozygous genotypes, so they cannot be told apart by appearance.

False.

Only homozygous individuals are pure breeding. A heterozygous individual can pass on different alleles.

The inheritance of a characteristic controlled by a single gene.

A genetic diagram showing the possible combinations of alleles in the offspring of a cross.

Determine the parental genotypes.

Split the alleles of each parent and write them around the outside of the grid.

Fill in the four middle squares to give the possible offspring genotypes.

All offspring are Tt, heterozygous.

There is a 100% chance that all offspring will be tall.

3 : 1 tall to short.

Each offspring has a 75% chance of being tall and a 25% chance of being short.

1 : 1 tall to short.

There is a 50% chance of tall and a 50% chance of short.

That it is homozygous for that characteristic, i.e. it has two identical alleles.

Homozygous recessive, e.g. tt.

A recessive characteristic only shows when there is no dominant allele present.

The capital and lower case forms must look clearly different, so the examiner is in no doubt which allele is dominant and which is recessive.

A cross between two heterozygous individuals gives a phenotype ratio of 3 : 1.

A genetic diagram used to predict the outcome of a monohybrid cross is called a Punnett square.

True.

Every offspring receives one dominant allele from one parent and one recessive allele from the other.

False.

A ratio gives the probability of each outcome, not a guaranteed result in any small group of offspring.

The dominant allele first, followed by the recessive allele, e.g. Tt not tT.

The inheritance of a characteristic that is controlled by more than one gene.

Several. Most characteristics are the result of multiple genes interacting, not single gene inheritance.

Eye colour. Although brown is dominant to blue, several genes are involved.

It is controlled by several genes, which produce a wide range of phenotypes such as green and hazel.

There is a wide range of possible allele combinations across several genes, which a simple Punnett square cannot represent.

Many, or more than one, referring to the number of genes involved.

A characteristic controlled by more than one gene is described as polygenic.

Eye colour is a common example of a polygenic characteristic.

False.

Most characteristics are polygenic, controlled by several genes working together.

True.

Because several genes interact, there are many possible combinations and therefore many possible phenotypes.

• Single gene: one gene controls the characteristic, giving few clear-cut phenotypes

• Polygenic: several genes interact, giving a wide range of phenotypes

Eye colour is polygenic, so several genes contribute.

This is why intermediate colours such as green and hazel exist, which a simple dominant/recessive model cannot explain.

The sex chromosomes, which are either XX or XY.

• Female: XX

• Male: XY

Sex is determined by an entire chromosome pair, whereas most other characteristics are determined by one or a few genes.

The father.

Only he can pass on a Y chromosome, since the mother has only X chromosomes to give.

The pair separates, so each gamete receives one chromosome.

They then combine randomly at fertilisation.

About half. The other half carry a Y chromosome.

50%, or 1 in 2.

Half the father's sperm carry an X chromosome and half carry a Y, and fertilisation is random.

The X and Y chromosomes are written in place of the usual alleles.

The mother contributes X and X, the father X and Y, giving two XX and two XY boxes.

A human female has the sex chromosomes XX.

If a sperm carrying a Y chromosome fertilises the egg, the fetus will be male.

False.

The father determines it, because only he can pass on a Y chromosome. All the mother's eggs carry an X.

True.

Which sperm fertilises the egg is a matter of chance, giving a roughly 50:50 ratio of males to females.

50%.

Each fertilisation is independent, so previous children have no effect on the outcome.

The mother's sex chromosomes are XX, so every egg she produces must carry an X.

A form of artificial gene transfer in which the genetic material of one organism is introduced into another.

An organism that has received genetic material from another organism. It is described as genetically modified.

The DNA of an organism that now contains DNA from another organism.

They have been given a gene from a bacterium that produces a poison which kills insects, making the crop resistant to pests such as caterpillars.

So that when the herbicide is sprayed it kills only the weeds and leaves the crop plant unaffected.

Rice genetically modified with genes from another plant and a bacterium so the grains make a chemical the body turns into vitamin A.

It could help prevent vitamin A deficiency in parts of the world.

• Reduced use of pesticides and herbicides, which is better for the environment and cheaper for farmers

• Increased yields, as crops are not damaged by pests or competing with weeds

• Increased seed costs, which smaller and poorer farmers may not be able to afford

• Risk of inserted genes being transferred to wild plants by pollination

If weeds gain the herbicide-resistance gene, the herbicide will no longer kill them and the GM crop loses its usefulness.

Heavy herbicide use leaves fewer plant species in the fields, which in turn affects insects and the birds that feed on them.

They have a rapid reproduction rate, so they can produce large quantities of a product in a short time.

There are also few ethical concerns about growing modified bacteria.

An organism that has received genetic material from another organism is described as transgenic.

Golden rice has been modified so its grains produce a chemical the body converts into vitamin A.

False.

Many types of organism can be modified, including animals, microorganisms, insects and fish.

False.

Some research has shown that plants with inserted genes do not grow as well as non-GM plants.