Reproduction and Genetics (intro)

A gamete is a sex cell (reproductive cell) that joins with another gamete during fertilisation to form a new individual. Each gamete carries half the normal number of chromosomes.

The male gamete is the sperm (spermatozoon); the female gamete is the egg cell (ovum).

A full-term human pregnancy lasts about 40 weeks (around 9 months), counted from the first day of the last menstrual period.

An average menstrual cycle lasts about 28 days, although this varies between individuals (typically 21–35 days).

Benefit: The teenage pregnancy rate fell substantially, from $30$ to $18$ per $1000$, and reported contraceptive (condom) use rose from $45\%$ to $68\%$. This means fewer teenagers are likely to face the health risks and disruption to education associated with an unplanned early pregnancy, and more are protecting themselves during sexual activity.

Drawback: Delivering a comprehensive programme requires significant teacher training, curriculum time, and funding, which must be taken from other parts of the timetable. In addition, reported contraceptive use, while much improved, is still not universal at $68\%$, meaning a substantial number of students may remain at risk of unplanned pregnancy or sexually transmitted infection without further improvement.

Benefit: IVF gives people who cannot conceive naturally the chance to have a genetically related child. The data shows this chance is highest for younger mothers ($32\%$ per cycle under $35$), so many people can realistically expect success, especially if they start treatment while younger.

Drawback: Success rate falls sharply with age, down to just $5\%$ per cycle at $40$+, meaning many older patients need multiple costly cycles, each involving hormone injections and a minor surgical procedure, with no guarantee of success. The emotional and financial cost of repeated unsuccessful cycles can be considerable, and IVF is not affordable or accessible to everyone.

Benefit: NIPT can detect certain chromosomal conditions from a simple maternal blood sample as early as $10$ weeks into pregnancy, without the small risk of miscarriage carried by more invasive tests such as amniocentesis. This gives parents earlier, safer information to prepare for their child's needs or to make informed decisions about their pregnancy and medical care.

Concern: Because the test provides very personal and sensitive genetic information early in a pregnancy, it raises ethical concerns about how the information might be used — including concern that it could increase pressure on parents to end pregnancies for conditions that would still allow the child a full and meaningful life. There are also concerns about the cost of the test and its availability differing between families and countries, which can make this information accessible to some parents but not others.

Variation is the differences that exist between individuals of the same species.

Continuous variation can take any value within a range (e.g. height or body mass). Discontinuous variation falls into distinct, separate categories with no in-between values (e.g. blood group — A, B, AB or O).

A chromosome is a thread-like structure made of tightly coiled DNA, found in the nucleus of a cell, that carries genetic information.

A gene is a short section of DNA (found on a chromosome) that carries the instructions/code for a particular characteristic.

An allele is a different version of the same gene (e.g. the gene for seed colour might have a 'green' allele and a 'yellow' allele).

The genotype is the genetic make-up of an organism (the alleles it carries, e.g. $Rr$). The phenotype is the observable characteristic that results (e.g. round seeds).

A dominant allele is one whose characteristic is expressed in the phenotype whenever it is present, even with only one copy. A recessive allele is only expressed in the phenotype when two copies are present (i.e. no dominant allele is present).

Benefit: Yield increased substantially, from $4.2$ to $7.8$ tonnes/hectare — an increase of about $86\%$. This means significantly more food can be produced from the same area of farmland, which is important for feeding a growing population.

Drawback: The table shows genetic variation in the population has fallen from high to low. Because almost all the wheat now shares very similar genes, the whole crop is much more vulnerable to being wiped out by a single new disease, pest, or change in climate that this narrow gene pool cannot resist — there is little genetic variation left for natural selection to act on to produce resistant survivors.

Benefit: If both partners are tested and found to be carriers of the same recessive condition, they can use this genetic information to make informed decisions before starting a family — for example, seeking genetic counselling, considering prenatal testing, or being prepared to arrange specialist medical care immediately after birth, all of which can improve outcomes for an affected child.

Concern: Genetic test results are highly personal and sensitive information; there are concerns about privacy, such as who else might gain access to the results, and about the emotional impact on a couple of learning they are carriers, which can create anxiety or difficult decisions about starting a family — even though being a healthy carrier of a single recessive allele does not affect the carrier's own health at all.

Benefit: Because the modified maize produces its own natural pest resistance, farmers need to spray far less chemical insecticide on the crop. This reduces costs for farmers, reduces harm to helpful insects such as bees that are not the target pest, and reduces the amount of chemical residue that ends up in the environment and water supply.

Concern: Introducing new genes into crop species raises concerns about long-term ecological effects, such as the modified gene spreading to wild relative plants through cross-pollination, or insect pests eventually evolving resistance to the inserted protein — in the same way that overusing an antibiotic leads to resistant bacteria. There are also concerns among some consumers and countries about the safety and labelling of genetically modified food, which is why regulations around GM crops vary widely around the world.