X and Y chromosomes and genetic screening (CCEA)

In humans, sex is determined by the sex chromosones.

• Females have two X chromosomes (XX).

• Males have one X and one Y chromosome (XY).

Meiosis forms haploid gametes (sperm and egg cells).

All eggs contain one X chromosome.

Half of sperm contain one X chromosome and half contain one Y chromosome.

A mother always passes on an X chromosome, while a father can pass either an X or a Y.

During fertilisation, there is a 50% chance that a sperm with an X chromosome will fuse with the egg, resulting in a girl (XX), and a 50% chance that a sperm with a Y chromosome will fuse, resulting in a boy (XY).

Some genetic conditions are inherited, meaning they are passed from parent to child.

They are caused by mutations, which are random changes in the DNA or the number of chromosones.

Currently, there are no cures for genetics conditions.

• Haemophilia is a blood disorder where people cannot clot their blood, leading to excessive bleeding.

• It is caused by a recessive allele on the X chromosome, so it is sex-linked.

• Sufferers are almost exclusively males because they only have one X chromosome and need just one recessive allele to show the condition.

• Only in very rare cases a female will have two recessive alleles and be a sufferer.

• Females with one normal and one recessive allele are carriers. They do not have Haemophilia symptoms but can pass the recessive allele to half of their children and pass the condition to their sons.

Alleles:

• H = normal
• h = haemophilia

• Mainly affects the lungs and digestive system, which become clogged with mucus leading to frequent infections.

• It is caused by a recessive allele, which means only homozygous recessive individuals (ff) will be affected.

Alleles:

• F = normal
• f = cystic fibrosis

• Affects nerve cells in the brain, leading to brain damage.

• It is caused by a dominant allele.

• It usually becomes apparent after 40 years old by which time the person may have passed the allele onto their children

Alleles:

• h = normal
• H = Huntington’s

• Individuals with Down's syndrome have reduced muscle tone and learning difficulties.

• It is caused by an extra chromosome at chromosome 21.

• This is not inherited and happens by chance during the formation of egg cells, giving them an extra chromosome (24 total). When fertilised by a sperm cell with 23 chromosomes, this results in a total of 47 chromosomes instead of 46.

Genetic screening involves testing people or groups of people for the presence of a particular allele or other genetic abnormality.

Pregnant mothers are offered a blood test at 10–14 weeks to assess the risk of having a child with Down's syndrome.

High-risk cases may then be offered amniocentesis, which is more accurate but carries a 1% risk of miscarriage, unlike the safer blood test.

Amniocentesis procedure:

• A needle is inserted into the amniotic fluid that surrounds the foetus and fluid is withdrawn.

• Chromosomes in foetal cells from the fluid are examined for genetic abnormalities such as Down's syndrome and cystic fibrosis.

Mothers at greater risk include:

• Those with a family history of genetic conditions.

• Those who previously carried a foetus with a genetic abnormality.

• Older mothers.

• Those with possible problems identified in earlier tests, such as blood tests.

Parents can be faced with difficult decisions if genetic screening shows their foetus has a genetic condition.

Ethical issues are questions about right, wrong and fairness, especially when decisions affect others' rights, privacy, or well-being.

Who decides who will be tested?

• People should have the choice to be tested

• Decisions for unborn babies are made by parents, is this right?

What are the benefits and risks of amniocentesis and blood tests?

• Amniocentesis: more accurate but involves a small risk of miscarriage.

• Blood Tests: safer but less accurate, often needing further testing.

What is the dilemma for carriers of genetic conditions?

• Parents face emotional challenges and difficult decisions if genetic screening shows their foetus has a genetic condition.

• It will allow them to access support.

• Parents may be offered an abortion:

• prevents suffering, poor quality of life, and ease the burden on parents and siblings.

• the unborn child has a right to life and cannot voice their choice

• Abortion is against some religious teachings

Should genetic information be available to wider society?

• Pros: It could advance medical research.

• Cons: Insurers or employers might misuse it, causing discrimination.

• Solution: Strong laws are needed to protect privacy.