Common terms used in genetics.
One of the founding fathers of the USA, Benjamin Franklin,
profoundly said "by failing to prepare, you are preparing to fail" and
whilst I am pretty sure he did not say this with biology in mind, we are going
to hold him to his word and prepare ourselves by making sure we know what
these keywords all mean. Gametes are reproductive cells, or sex cells, so
female gametes are the ovum, or egg cells, and the male gametes are the sperm cells.
Chromosomes: DNA is tightly coiled around proteins and it's packaged into structures called
chromosomes. They are found in the nucleus of cells. A gene is a unit of
hereditary material. Each chromosome contains thousands of genes made up of
DNA. The gene codes for a particular characteristic of the organism, such as
eye colour or blood group.
An allele is an alternative name for a gene, representing the different versions
of that gene. So whilst the chromosome from your mother and father both have
genes for hair colour, body height and lipoma formation, they may have different
alleles. So your mum may have a red hair allele,
whereas your dad has a brown hair allele. The phenotype is the outward appearance,
or characteristics, of the organism depending upon genes. So the phenotype
for the red hair gene is actually having red hair. The genotype is the set of
genes in our DNA which are responsible for the phenotype.
Homozygous and heterozygous: alternative forms of the given gene are called alleles. When an individual has
two of the same allele, so both coding for red hair, they are homozygous. Both
genes are identical.
Whereas if one allele codes for red hair and the other for brown hair, they
are heterozygous. Two different forms of the same gene are present.
When two different alleles are present, so are heterozygous, one allele will be dominant. This allele will
influence the phenotype. Brown hair is dominant over red hair, so the offspring
is more likely to have brown hair.
So in this example the red hair allele is recessive. This means the gene is not
expressed in the phenotype. Recessive genes are only expressed when the other gene
is also the recessive form. So it needs to be homozygous: both carrying the red hair allele.
Finally, what do haploid and diploid mean? Haploid cells
have one set of chromosomes and is labeled as an 'n'. Diploid cells have
two sets of chromosomes, '2n'. Most cells are diploid, so in humans we have
23 pairs of chromosomes: one from our mother and one from our father.
All of our cells are diploid, other than the gametes: the egg and sperm. Which are haploid.
When the sperm fertilizes the egg, the chromosomes pair up and
become diploid. So they have to start at haploid so that there is room to pair.
Hopefully you now know what these words all mean. Gamete, chromosome, gene, allele, phenotype, genotype, homozygous, heterozygous, dominant, recessive, haploid, diploid.
It is worth taking the time to really know and understand them before you start tricky
topics like mitosis. I promise you it will make everything much easier.
Benjamin Franklin doesn't lie!