- Gene: A section of DNA that codes for a specific protein.
- Allele: Each gene comes in different types called alleles. For example, a gene for eye colour may have a gene for blue type allele and a brown type allele.
- Dominant allele: Version of a gene that will always have an effect if it is present.
- Recessive allele: Version of a gene that will only have an effect if the dominant version of the gene is not present (if both are present)
- Homozygous: both alleles for a characteristic are the same the organism is homozygous for that characteristic.
- Heterozygous: if both alleles for a characteristic are different then the organism is heterozygous for that characteristic.
- Phenotype: The physical characteristics that a certain set of alleles cause.
- Genotype: The alleles for a certain characteristic that are found in an organism.
Components of DNA 🧬
- DNA is found in the nucleus of a cell and is in the formation of long strands. Each strand forms a structure called a chromosome.
- A nucleus contains different chromosomes; there are usually two copies of each chromosome.
- In I/GCSE Biology, chromosomes are divided up into genes and each chromosome contains a large amount of them. Each gene has a job.
- For example, many genes control variations in our characteristics. Variation caused by genes is called inherited variation because we inherit genes from our parents.
- In I/GCSE Biology, different forms of the same gene are called alleles. Each allele may contain slightly different instructions to create variations.
- As there are two copies of every chromosome in a somatic cell nucleus there are two copies of every gene. Each copy of a gene could be a different allele.
- Different organisms contain different numbers of chromosomes. Humans have 23 pairs of such, which contain about 25000 different genes in total. There are many alleles for each gene so it is easy to understand variation between humans.
Punnett squares and family pedigrees 👇👇👇
In I/GCSE Biology, this is a punnett square which is used to work out the possible genotypes. It can help work out probabilities of certain genotypes too. For example, we can see here there is a 50% chance of a homozygous recessive and 50% chance of a heterozygous dominant pair of alleles. The ratio would be 1:1.
Family pedigree charts are useful for working out the probability of someone inheriting a genetic disorder. This is called pedigree analysis in I/GCSE Biology. Symbols are used to represent different disorders; for example, the coloured in circle could mean a female with cystic fibrosis and a white one mean a female without it.
Specific genetic disorders 🤒🤕
- Genetic disorders are diseases which are caused by faulty alleles. The allele for sickle cell anaemia is recessive so two copies are needed to suffer from the illness. Sickle Cell Anaemia is bad as the red blood cells do not function properly and they stick together and block up blood vessels. Sometimes, this can be fatal.
- Sickle shaped blood cells also cannot carry oxygen very well as they have a smaller surface area so less haemoglobin.
- Cystic fibrosis is caused by a recessive allele. A person who has this illness will have their lungs clogged up with thick mucus making breathing difficult and leading to infections.
- Mucus can also block the tubes carrying enzymes to the small intestine and this can cause weight loss as food is not digested properly.
That's the end of the topic!
Drafted by Joey (Biology)