Gregor Mendel was an Austrian monk who experimented with pea plants, and inadvertantly caused the basis of modern genetics.
He bred a tall pea plant with a dwarf pea plant and noted how the offspring were all tall pea plants. Then, taking two of the tall second generation pea plants, he crossbred them and noted that this time about a quarter of the offspring were actually dwarfs.
This meant that the characteristic that caused a plant to be tall was more effective, or dominant, than that which caused a plant to be a dwarf.
We use this idea today to explain genetics, as now we know that the hereditary units that control our characteristics are genes, made of dominant and recessive alleles.
Phenotypes and Genotypes
Your genotypes are the alleles you have present.
Phenotypes are what characteristic is expressed.
Recessive alleles are normally represented by a lower case letter eg b, while dominant alleles are represented by the capital letter eg B
Genetic diagrams show the possible outcomes of the offsprings genes.
Because sexual reproduction involves fusion of gametes, and the mixture of parents DNA, we cannot tell which alleles of a gene the offspring will inherit. For example if both parents are heterozygous carriers of a disease caused by a recessive allele, there is a chance the child will be a carrier or a sufferer of the disease.
If the child receives 2 copies of the dominant allele, they will be neither a sufferer or a carrier.
If they receieve 1 copy of the recessive allele and 1 of the dominant allele from either parent, they will still be a carrier, but because the dominant gene overrules the recessive, they will not be a sufferer. They could, however, have a child who is a sufferer, if their partner also carries the recessive gene.
If the child receives 1 copy of the recessive allele from each parent, they will be a sufferer. The only way for a recessive allele to be expressed is if both copies are recessive.
In total, 25% chance non carrier, 50% chance carrier, 25% chance sufferer.
Before an embryo develops into a foetus and into a child, it is possible to screen the cells to see whether the child will suffer from a certain disease or not. This is useful for people who know that they are carriers of a disease and do not want to pass this disease on to their offspring.
It often involves creating an embryo using IVF, and deciding after embryos are screened whether to implant into the womb or terminate the pregnancy. This could be said to be less stressful than having to have an abortion as the pregnancy has not technically started.
However some people think it is immoral to pick and choose which babies get to be born. Embryos are a potential human life that is being destroyed. It could promote a prejudiced mindset that disabled people are 'undesirable' because of their handicap.
On the other hand, screening could prevent a child being born with a terrible disease and a poor quality of life. A lot of money is spent on helping people with disorders. Most embryos created during IVF are destroyed anyway, this method ensures that the one picked is healthy and not a waste of money.