Topic 2: Genes and Health

Question 1

2.1 i) What are the properties of gas exchange surfaces in living organisms?

Answer 1

1. A large surface area to volume ratio (larger objects have smaller surface area to volume ratios than smaller objects).
2. A thin surface ensures a short diffusion pathway.
3. A steep concentration gradient ensures rapid diffusion.

Question 2

2.1 ii) What is Fick’s Law?

Answer 2

Fick’s Law of diffusion: the rate of diffusion is proportional to the surface area multiplied by the difference in concentration, divided by the thickness of the gas exchange surface.

Question 3

2.1 ii) Explain how the rate of diffusion is dependent on certain properties?

Answer 3

1. Surface are - the rate of diffusion is directly proportional to the surface area. As the surface area increases, the rate of diffusion increases.
2. Concentration gradient - the rate of diffusion is directly proportional to the difference in concentration. The greater the concentration gradient, the faster the rate of diffusion.
3. Thickness of gas exchange surface - the rate of diffusion is inversely proportional to the thickness of the gas exchange surface. The thicker the surface, the slower the rate of diffusion.

Question 4

2.1 iii) How is the structure of the mammalian lung adapted for rapid gaseous exchange?

Answer 4

1. Large surface area of the alveoli and the large network of capillaries surrounding the alveoli ensures a large surface area for more diffusion to occur at once.
2. The alveolar endothelium and capillary endothelium are each one cell thick, and this thin walls ensures a short diffusion pathway.
3. The steep concentration gradient between the alveolar air and the blood is maintained by ventilation of the alveoli and the continuous flow of blood through the lungs.

Question 5

2.2 i) Explain the basic structure of a cell membrane.

Answer 5

The fluid mosaic model proposes a phospholipid bilayer. Each phospholipid molecule has a hydrophilic phosphate head and a hydrophobic tail made of two fatty acids. The bilayer has the hydrophilic heads on the outside and the hydrophobic tails on the inside.

Question 6

2.2 i) Why is the cell membrane a bilayer?

Answer 6

Cells are filled with an aqueous cytoplasm and are surrounded by aqueous tissue fluid. A bilayer, however, ensures that the hydrophobic fatty acid tails have no contact with the water on either side of the membrane, and the hydrophilic phosphate heads remain in contact with the aqueous environment.

Question 7

2.2 i) What other components are found within the cell membrane?

Answer 7

The cell surface membrane also contains:

• glycoproteins: protein molecules with polysaccharides attached
• glycolipids: lipid molecules with polysaccharides attached
• proteins: peripheral proteins, or extrinsic proteins, are loosely attached on the outside surface of the membrane and partially embedded, while integral proteins, or intrinsic proteins, are fully embedded within the phospholipids; the proteins can be enzymes, carrier proteins or channel proteins
• cholesterol: it forms a bond with phospholipids, maintaining the fluidity by ensuring that the bilayer doesn’t become too rigid or too fluid.

Question 8

2.2 i) Why is the cell membrane more fluid with unsaturated rather than saturated phospholipids making up the bilayer?

Answer 8

The cell membrane is described as fluid because some parts can move around freely (if they are not attached to other parts of the cell). The greater the ratio of phospholipids that contain unsaturated fatty acids to those containing saturated fatty acids, the more fluid the membrane will be. The kinks in the hydrocarbon tails of the unsaturated phospholipids prevent them from packing closely together, so more movement is possible.

Question 9

2.2 ii) How has scientific data developed the fluid mosaic model?

Answer 9

Before the 1970s, the most widely accepted model was a three-layer membrane, composed of a phospholipid layer between two continuous layers of proteins.

1. This was a result of electron microscope images showing three layers, two darker around one lighter layer. However, this model does not allow the hydrophilic phosphate heads to be in contact with water. Improved EM techniques and interpretations supported the bilayer model, with the more electron dense phosphate heads being the two darker edges and the lipid being the lighter inner part.
2. Additional experiments showed that the proteins were randomly distributed across the cell membrane rather than in a continuous layer, with some peripheral and others integral.
3. Scientists also fused a mouse cell with a human cell, marking a specific membrane protein green in mice and red in humans. After some time, the proteins were completely intermixed - this could only happen through fluid cell membranes.

Question 10

2.3 Explain osmosis.

Answer 10

Osmosis the net movement of water molecules, across a partially permeable membrane, from a higher concentration of water molecules (and lower concentration of solutes) to a lower concentration of water molecule (and higher concentration of solutes). It is a form of diffusion, and is a passive process.

Question 11

2.4 i) Explain diffusion.

Answer 11

Diffusion is the net movement of particles from an area of high concentration to an area of lower concentration. While molecules will diffuse both ways, the net movement will be to the area of lower concentration. This will continue until equilibrium is reached, where the particles are evenly distributed. Diffusion is a passive process, so no energy is required. Hydrophobic (lipid-soluble) particles or small uncharged molecules (such as oxygen and carbon dioxide) can diffuse across a membrane. Although carbon dioxide is polar, it’s small enough to diffuse rapidly across.

Question 12

2.4 i) Explain facilitated diffusion.

Answer 12

Hydrophilic molecules (such as polar ones), charged particles (such as ions bigger than carbon dioxide) and larger molecules (such as amino acids and glucose) are all insoluble in lipids and can’t diffuse through the phospholipid bilayer. Instead they diffuse through carrier proteins or channel proteins through the process of facilitated diffusion. The particles move down a concentration gradient, and this passive process doesn’t require any energy.

Question 13

2.4 i) Explain active transport.

Answer 13

Active transport allows substances to be moved against a concentration gradient, using energy from ATP (produced during respiration). When the ATP is hydrolysed, a phosphate group is removed and ADP is formed. The phosphate group then becomes hydrated, and a lot of energy is released when it bonds with water. This energy changes the shape of a carrier protein, causing a substance to be released on the other side of the membrane and moving it against the concentration gradient.

Question 14

2.4 i) Explain endocytosis.

Answer 14

Substances are taken into a cell by the creation of a vesicle from the cell surface membrane. Part of the cell engulfs the substance that is to be transported, and the membrane then pinches off to form a vesicle inside the cell, containing the substance. This process requires energy from ATP, and is used as bulk transport for large molecules or large quantities.

Question 15

2.4 i) Explain exocytosis.

Answer 15

Exocytosis is the release of substances from the cell. Vesicles (small membrane bound sacs containing the substance) pinch off from the sacs of the Golgi apparatus, and fuse with the cell membrane to release the contents outside the cell. This process requires energy from ATP, and is used as bulk transport for large molecules or large quantities.

Question 16

2.4 ii) Explain how channel proteins work.

Answer 16

Channel proteins form water filled pores in the membrane, allowing polar molecules and ions to diffuse through. Different channel proteins facilitate the diffusion of difference particles. Each channel protein has a specific shape that permits the passage of only one type of particle. Some channels can be opened or closed depending on the absence or presence of a signal - these are gated channels.

Question 17

2.4 ii) Explain how carrier proteins work.

Answer 17

The ion or molecule binds to a specific site on the carrier protein embedded in the membrane. The protein changes shape and as a result the particle crosses the membrane. The movement can occur in either direction, with the net movement being dependent on the concentration gradient across the membrane. In facilitated diffusion, down the concentration gradient, this is a passive process. In active transport, against a concentration gradient, this requires energy from ATP.

Question 18

2.5 i) What is the basic structure of mononucleotides?

Answer 18

A mononucleotide is made from a pentose sugar, a nitrogen-containing organic base and a phosphate group. In DNA the pentose sugar is deoxyribose, and the 4 bases are adenine, thymine, cytosine and guanine. In RNA the pentose sugar is ribose, and the 4 bases are adenine, uracil, cytosine and guanine.

Question 19

2.5 i) How are polynucleotides composed?

Answer 19

A polynucleotide is a polymer of mononucleotides. The mononucleotides are joined through a condensation reaction between the phosphate of one mononucleotide and the sugar group of another. Water is a by-product. The bond produced between each mononucleotide is a phosophodiester bond.

Question 20

2.5 ii) Explain DNA’s structure.

Answer 20

Whereas RNA (ribonucleic acid) is a single polynucleotide, DNA (deoxyribonucleic acid) is made from two polynucleotides. The two strands are joined together with hydrogen bonding between the bases: complementary base pairing between A and T and between C and G means there is always an equal amount of A and T in a DNA molecule, and an equal amount of C and G. While A and G have a two-ring structure, T and C have a one-ring structure. The bases pair so there are three rings across two mononucleotides, ensuring the DNA has a uniform width along its whole length. Two hydrogen bonds form between A and T, and three hydrogen bonds form between C and G. The polynucleotides are antiparallel, running in opposite directions, and so twist to form the DNA double helix structure.

Question 21

2.6 i) Explain transcription.

Answer 21

1. At the beginning of transcription, the enzyme RNA polymerase attaches to a start codon on the DNA and breaks the hydrogen bonds between the bases, causing it to unwind. One of the strands in then used as a template strand (antisense strand) to make an mRNA copy.
2. The RNA polymerase lines up free RNA mononucleotides alongside the template strand, and complementary base pairing results in the mRNA strand being a complementary copy of the DNA template strand. Once the free RNA mononucleotides have lined up, they’re joined by RNA polymerase in a condensation reaction, forming phosophodiester bonds between each mononucleotide. The mRNA strand has the same base sequence as the DNA coding strand (sense strand), with U instead of T.
3. The hydrogen bonds reform between the DNA strands and it winds back up into a double helix as the RNA polymerase moves along. When the RNA polymerase reaches a stop codon, the mRNA is finished being transcribed and detaches from the DNA, moving out of the nucleus through nuclear pores.

Question 22

2.6 i) Explain translation.

Answer 22

1. Once in the cytoplasm, the mRNA attaches to a ribosome where it begins translation. A ribosome is composed of two subunits, with the larger subunit containing two tRNA binding sites, with the mRNA attaching to the smaller subunit.
2. The tRNA molecules carry amino acids to the ribosome, where a tRNA molecule, with a complementary anticodon to the start codon on the mRNA, attaches itself to the mRNA by complementary base pairing. A second tRNA molecules attaches itself to the next codon in the same way.
3. The two amino acids attached to the tRNA molecules are then joined together by a peptide bond. The first tRNA molecule moves away, leaving the amino acid, and the ribosome moves along to the next codon. The process continues, producing a polypeptide chain, until there’s a stop codon on the mRNA molecule and the polypeptide chain moves away.

Question 23

2.6 i) Explain how stop codons work.

Answer 23

There are no tRNA molecules with complementary anticodons to stop codons, so no amino acids can be transferrd, the polypeptide chain stops growing and detaches from the ribosome.

Question 24

2.6 ii) Explain the differences between mRNA and tRNA.

Answer 24

Messenger RNA:
1. Made in the nucleus during transcription.
2. Many codons (one codon is three adjacent bases) along a single, unfolded, polynucleotide strand.
Transfer RNA:
1. Found in the cytoplasm
2. A sequences of three bases called an anticodon on one end, with an amino acid binding site on the other - it is a single polynucleotide strand, but folded with complementary base pairing and hydrogen bond giving it a clover shape.

Question 25

2.7 What is the nature of the genetic code?

Answer 25

The genetic code is the sequence of base triplets (codons) in DNA and mRNA, which codes for specific amino acids.
The code is non-overlapping, so each triplet code is adjacent.
The code is also degenerate, so there are more possible combinations of triplets than there are amino acids. This means that some amino acids are coded for by more than one base triplet. This reduces the effect of possible mutations.

Question 26

2.8 What is a gene?

Answer 26

A gene is a sequence of bases on a DNA molecule that codes for the sequence of amino acids in a polypeptide chain. It’s the order of bases in a gene that determines the order of amino acids in a particular protein, as different proteins have difference numbers and order of amino acids.

Question 27

2.9 i) What is the basic structure of an amino acid?

Answer 27

Amino acids have a common structure: a central carbon atom is bonded to an amine group (-NH2), a carboxylic acid group (-COOH), a hydrogen group (-H) and a residual group (-R). This’ R’ group is variable, and each different amino acid differs by a an ‘R’ group.

Question 28

2.9 ii) How is the primary structure of a protein formed?

Answer 28

Two amino acids join in a condensation reaction to form a dipeptide molecule. A peptide bond is formed between each amino acid, releasing a water molecule each time (The OH is removed from -COOH, and the H from -NH2). This process can be repeated to form polypeptide chains. The sequence of amino acids in the polypeptide chain is known as the primary structure of a protein. A protein is made up of one or more of theses polypeptide chains.

Question 29

2.9 iii) How is the secondary structure of a protein formed?

Answer 29

Hydrogen bonds form between amino acids, causing it to change shape:

1. The chain of amino acids may twist into an alpha-helix structure, where hydrogen bonds form between every fourth amino acid. The slightly negative oxygen of the carboxylic acid ( C=O) on one amino acid bonds with the slighting positive hydrogen of the amine group (H-N) on another amino acid.
2. Amino acids may fold back on themselves, or several lengths of the chain may link together with hydrogen bonds holding the parallel chains in a beta-pleated sheet.

Question 30

2.9 iii) How is the tertiary structure of a protein formed?

Answer 30

A polypeptide chain often bends and folds further, producing a precise 3D shape. Chemical bonds form between different parts of the polypeptide chain:

1. Ionic bonds form between oppositely charged R groups.
2. A disulphide bond forms between two cysteine amino acids ( each R group contains a -SH group).
3. Hydrophobic and hydrophilic interactions result in non-polar hydrophobic R groups arranged on the inside of the protein, excluding water from the center, and polar hydrophilic R groups on the outside.
4. Hydrogen bonds are also apparent.

Question 31

2.9 iii) Which proteins have a quaternary structure?

Answer 31

Only proteins with more than one polypeptide chain have a quaternary structure; for proteins made from a single polypeptide chain, the tertiary structure forms their final 3D shape.

Question 32

2.9 iii) What determines the properties of a protein?

Answer 32

The amino acid sequence in a protein’s primary structure determines what bonds will form and how the protein will fold up into its 3D shape. The 3D structure of a protein determines its properties (which relates to its function in the body).

Question 33

2.9 iv) How does the structure of a globular protein relate to its function?

Answer 33

In globular proteins, the polypeptide chains fold up into a compact, spherical shape:

1. The hydrophilic parts of chains are on the outside and the hydrophobic parts of chains face inwards - this makes the protein soluble, so its easily transported in fluids (for metabolic processes).
2. The specific 3D shape of a globular protein is critical to the function of that protein: for example, enzymes are globular proteins, and the shape of their active site is determined by their tertiary structure.

Question 34

2.9 iv) How does the structure of a fibrous protein relate to its function?

Answer 34

Fibrous proteins do not fold up into compact shapes, but instead remain as long chains:

1. Several polypeptide chains can be cross-linked, allowing for high tensile strength.
2. The proteins are insoluble in water, allowing them to be good structural molecules.

Question 35

2.9 iv) How does the structure of haemoglobin relate to its function?

Answer 35

The globular protein haemoglobin acts as an oxygen transport molecule. It is made up of four polypeptide chains, each associated with an iron-containing haem group (making it a conjugated protein) that binds to oxygen. The specific 3D shape is critical to its role in binding to other substances, and its solubility make it easily transported in blood.

Question 36

2.9 iv) How does the structure of collagen relate to its function?

Answer 36

Collagen is an insoluble fibrous protein, with a triple helix structure: three long proteins wrap around one another, held together by hydrogen bonds. Each stand is cross-linked to another, allowing it to have high tensile strength - this is essential to its role as a structural molecule, providing strength (and a degree of elasticity) to the skin, bones and connective tissue.

Question 37

2.10 i) Why is the 3D shape of an enzyme so important? What happens when the shape changes?

Answer 37

Enzymes usually catalyse only one reaction: only a specific complementary substrate can fit into their active site. Each different enzyme has a different 3D structure, determined by the bonding in its tertiary structure (and therefore the arrangement of amino acids in its primary structure) which results in a different active site. The bonds in a tertiary structure can be altered by pH, temperature, or a mutated gene (which changes the arrangement of amino acids in a primary structure). This results in a different shaped active site, which prevents further enzyme-substrate complexes.

Question 38

2.10 i) Explain the lock-and-key theory.

Answer 38

Substrate molecules have a complementary shape that allows them to fit into active sites and form bonds with amino acids. This results in an enzyme-substrate complex. Once the reaction has taken place, the products are released, leaving the enzyme unchanged. Each enzyme will only catalyze one specific reaction because only one shape of substrate will fit into the specific shape of the active site.

Question 39

2.10 i) Explain the induced fit theory.

Answer 39

It has been found that the active site is often flexible. When the substrate enters the active site, the enzyme molecule changes shape slightly, fitting more closely around the substrate. Only a specifically shaped substrate will induce the correct change in shape of an enzyme’s active site, enabling the substrate to react.

Question 40

2.11 ii) What is the function of enzymes?

Answer 40

Enzymes act as biological catalysts, lowering the activation energy (the minimum energy required to start the reaction and break bonds) and therefore speed up the rate of reaction. When a substrate fits into the enzymes’s active site it forms an enzyme-substrate complex - it’s this that lowers the activation energy by:

1. allowing the oppositely charged groups on the active site and substrate to interact, distorting the substrate and aiding bond breakage or formation
2. or perhaps by bringing two substrates closer together, repulsion between the molecules is reduced
3. or some acidic side chains on the amino acids in the active site can create a favorable condition for the reaction.

Question 41

2.10 iii) What are the two types of cellular enzymes?

Answer 41

Enzymes can be intracellular (catalysing reactions inside the cell) or extracellular (produced and secreted by cells to catalyse reactions outside the cell).

Question 42

2.11 i) What is the process of DNA replication?

Answer 42

DNA replicates itself before cell division so that each new cell has the full amount of DNA. The enzyme DNA helicase causes the double helix to unwind by breaking the hydrogen bonds between bases on the two DNA strands. Each original single stand acts as a template for a new strand. Complementary base pairing means that free-floating DNA nucleotides are attracted to their complementary exposed bases on each original template strand. The enzyme DNA polymerase joins the adjacent nucleotides with phosphodiester bonds in condensation reactions, and hydrogen bonds form between the bases in the original template strand and the new complementary strand. This method is semi-conservative replication, because each new DNA molecule contains one strand from the original DNA molecule and one new strand. This ensures genetic continuity between generations of cells.

Question 43

2.11 ii) How did Meselson and Stahl set up an experiment proving semi-conservative replication and disproving conservative replication?

Answer 43

Meselson and Stahl grew bacteria in a broth containing heavy nitrogen (15Ni); as the bacteria reproduced, they took up nitrogen to make nucleotides for new DNA. When the new DNA contained only heavy nitrogen, the bacteria were then grown in a broth containing light nitrogen (14Ni) only. After one round of DNA replication, they extracted and centrifuged the DNA. The test tube showed a single band of medium density DNA - this dismissed the conservative replication theory, where one DNA molecule would contain two original strands (of heavy nitrogen only, producing a band at the bottom) and the other DNA molecule would contain two new strands (of light nitrogen only, producing a band at the top). It proved semi-conservative replication, where both DNA molecule contains an original (heavy) strand and a new (light) strand, producing would produce a single band of medium-density DNA.

Question 44

2.11 ii) How did Meselson and Stahl set up an experiment proving semi-conservative replication and disproving fragmentary replication?

Answer 44

Meselson and Stahl’s experiment disproved the conservative theory, yet both the semi-conservative theory and the fragmentary theory (where both molecules of DNA contain strands with a mixture of (heavy and light) nucleotides) would produce a single band of medium-density DNA. Meselson and Stahl therefore allowed the bacteria to replicate a second time. The fragmentary theory would have produced one medium (or 3/4 length) band. However, the semi-conservative theory was right: one molecule, with an original (heavy) strand and a new (light) strand, gave a middle band and another molecule, with an original (light) strand and new (light) strand, gave a high band.

Question 45

2.12 i) How can errors in DNA replication give rise to mutations?

Answer 45

A change in the base sequence of DNA could be a result of the following errors in DNA replication:
1. Substitution: one base is substituted for another.
2. Inversion: a sequence of bases is reversed.
3. Insertion: an extra base is added.
4. Deletion: one base is deleted.
5. Duplication: one or more bases are repeated.
The order of DNA bases in a gene determines the order of amino acids in a particular protein. If a mutation occurs in a gene, the primary structure of a protein could be altered, changing the final 3D structure and the properties of a protein, preventing it from function correctly. If a mutation occurs in a gene, it could produce a genetic disorder.

Question 46

2.12 ii) How does cystic fibrosis result from a number of possible gene mutations?

Answer 46

Cystic fibrosis is caused by a mutation in the gene that codes for the CFTR protein (Cystic Fibrosis Transmembrane Conductance Regulator) - there are hundreds of different mutations that could give rise to cystic fibrosis, However, it is caused by a recessive allele, and for a person to exhibit the cystic fibrosis phenotype, they must inherit a homozygous recessive genotype (ff). Those with a heterozygous genotype (Ff) are carriers.

Question 47

2.13 i) Define gene and allele.

Answer 47

Gene: a sequence of bases on a DNA molecule that codes for a protein, which results in a characteristic.
Allele: a different version of a gene (most organisms inherit two versions of a gene - each one on a chromosome, inheriting two homologous chromosomes - one from each parent). Different alleles have slightly different base sequences, which code for different version of the same characteristic.

Question 48

2.13 i) Define genotype and phenotype.

Answer 48

Genotype: the combination of alleles a person has e.g. AA, Aa, or aa.
Phenotype: the characteristic displayed by a person, as a result of their genotype.

Question 49

2.13 i) Define recessive and dominant.

Answer 49

Recessive: a type of allele (represented by a lower case letter), whose characteristic only appears in the phenotype when two versions are present.
Dominant: a type of allele (represented by an upper case letter), whose characteristic appears in the phenotype when both one version (masking the recessive allele) and two versions are present.

Question 50

2.13 i) Define incomplete dominance.

Answer 50

Incomplete dominance: when the trait from a dominant allele isn’t completely shown over the trait produced by the recessive allele, so both alleles influence the phenotype.

Question 51

2.13 i) Define homozygote and heterozygote,

Answer 51

Homozygote: when there are two identical versions of an allele for a certain characteristic, e.g. AA or aa.
Heterozygote: when there two different version of an allele for a certain characteristic e.g. Aa.

Question 52

2.13 ii) What is monohybrid inheritance?

Answer 52

Monohybrid inheritance is the inheritance of a single characteristic, as determined by one gene (or one pair of alleles at a single locus).

Question 53

2.14 How does the expression of a gene mutation result in cystic fibrosis?

Answer 53

The CFTR channel protein transports chloride ions into the mucus (out of cells) when it is viscous and lacking water. In a person with cystic fibrosis, the CFTR protein is non-functional: chloride ions (Cl-) won’t be secreted into the mucus, (resulting in no electrical gradient for sodium ions (Na+) to diffuse down). In people with CF, the water would move into the mucus by osmosis (down a water concentration gradient), but without the elevated salt concentration, water does not move into the mucus. The mucus therefore remains viscous.

Question 54

2.14 How does the expression of a gene mutation in people with cystic fibrosis impair the functioning of gaseous exchange?

Answer 54

Mucus prevents lung infection in people by trapping microorganisms, and being removed by the movement of cilia. However, in people with CF, the mucus is too thick and sticky to be removed by cilia. It therefore builds up and blocks the airways, reducing the surface area for gas exchange, and making it less efficient - this causes breathing difficulties. People with CF are also more prone to lung infections, as the mucus containing the microorganisms can’t be removed.

Question 55

2.14 How does the expression of a gene mutation in people with cystic fibrosis impair the functioning of the digestive system?

Answer 55

The abnormally think mucus produced by people with CF blocks the pancreatic duct, preventing digestive enzymes (produced by the pancreas) from reaching the small intestines. The lower concentration of enzymes within the small intestines means that food is not fully digested, and fewer nutrients are absorbed.
If damage occurs to the cells within the pancreas that produce the hormone insulin, which is involved in the control of blood sugar levels, a form of diabetes can occur.

Question 56

2.14 How does the expression of a gene mutation in people with cystic fibrosis impair the functioning of the reproductive system?

Answer 56

Females with CF have a reduced chance of becoming pregnant: thickened mucus in the cervix reduces the mobility of the sperm, inhibiting it from reaching the egg.
Males with CF commonly lack the vas deferens (sperm duct) on both sides, so the sperm cannot leave the testes. When the vas deferens is present, it can become blocked by a thick, sticky mucus layer, resulting in less sperm leaving the testes.

Question 57

2.15 i) What is genetic screening?

Answer 57

Genetic screening involves analysing DNA to see if it contains alleles for genetic disorders.

Question 58

2.15 i) How and why is genetic screening used in the identification of carriers?

Answer 58

Carrier testing is offered to individuals with a family history of genetic disorders. It shows whether people without a disorder carry an allele that can cause a disorder ( this is carried out by extracting some cells and analysing the DNA).
Couples can be tested before having a child to determine the chance of any future children having the disorder. This allows people to make informed decisions (regarding future children, or prenatal testing, for example).

Question 59

2.15 i) How and why is PGD used? What are the advantages and disadvantages?

Answer 59

Pre-implantation genetic diagnosis (PGD) is carried out on embryos produced by in vitro fertilisation (IVF). It involves screening embryos for genetic disorders (by extracting some cells and analysing the DNA), before implanting those without the genetic disorder into the woman. While this reduces the chances of having a baby with a genetic disorder, it raises the potential problem of designer babies, as well as the rick of false results. However, embryos are discarded, which some people consider unethical (preventing human life).

Question 60

2. 15 i) Outline two prenatal tests.

ii) Consider the implications of prenatal genetic screening.

Answer 60

Amniocentesis: a sample of amniotic fluid (which surrounds the fetus) is obtained via the abdomen, using a very fine needle. This fluid contains fetal cells, and the DNA can be analysed.
- this is carried out at 15-20 weeks of pregnancy.
- it has a 1% risk of miscarriage.
Chorionic villus sampling (CVS): a sample of cells is taken from the chorionic villi, and the fetal DNA is analysed. The procedure is done via the abdomen (using a fine needle) or the vagina (using a catheter).
- this is carried out at 11-14 weeks of pregnancy (taking place earlier than amniocentesis, it can be less traumatic both physically and emotionally).
- it has 1-2% risk of miscarriage (higher than amniocentesis).
False results, however, could provide incorrect information, and some people consider it unethical to abort a fetus because it has a genetic disorder.