genetics page 3

Who's the boss?

You may have wondered how come children may have traits (= characteristics) that you don't see in their parents. Some people say that traits can "skip a generation" This is true (but doesn't have to happen - as some people think). The reason why traits can do this is because some alleles will be masked or hidden when a different allele is also present.

Remember that we usually have two alleles for each gene. One from each parent.
Those alleles may be the same, but they can be different.

For example, a person might have two alleles that code for freckles. They will have freckles.
If they have two alleles that code for no freckles they won't have freckles.
What happens if they have an allele for freckles and an allele for no freckles? Well they will have freckles.
The allele for freckles is "stronger" than the other allele.

Geneticists say that the "stronger" allele is dominant and the allele that can be hidden is recessive.
For many genes in all living things there are two or more alleles, some dominant, some recessive and there are some that are in between - but we won't worry about those ones at this level.
Summary - Dominant alleles always show up whhen one of them is present. While recessive alleles will show up only if two of them are present.

Using symbols

It gets boring writing down things like the dominant allele that gives freckles so we use letters to symbolise alleles.
Capital letters are used to show dominant alleles.
Small (lower case) letters show recessive alleles.
We should use the same letter for different alleles of the same gene.

Example: for freckles we could use F = the allele for freckles, and f = the allele for no freckles.
A person with freckles could have alleles FF or Ff.
A person without freckles must have ff.

Genotype and phenotype

I'll bet you have noticed that scientists like making up lots of complicated words for you to learn. The two above are used when talking about what alleles an individual has compared to what they look like or what gene seems to be having an effect.

Genotype means "what alleles are present" for a certain gene.

Most cats have two dominant alleles for being hairy (=furry). Their genotype is HH and their phenotype is hairy.
Some cats have two recessive alleles for this gene. Their genotype is hh and their phenotype is hairless. No kidding! Here are a few of photos of "sphinx" cats: A B C.
What would be the genotype and phenotype of a cat with one dominant and one recessive allele?

Another pair of big words!

If you like learning big words such as genotype and phenotype, you will love the next two big words. They are homozygous and heterozygous.

Homozygous means having two alleles the same.

Homozygous dominant means having two dominant alleles of a gene, e.g. HH

Homozygous recessive means having two recessive alleles of a gene, e.g. hh.

Heterozygous means having one dominant and one recessive allele, e.g. Hh.

If you are saying "Oh my gosh, this is all Greek to me". You are right! The prefixes homo- and hetero- are from the Greek language. You may well have heard of them used with other words where you want to show "same" or "other". E.g. Remember homologous chromosomes? Meaning chromosomes having the same shape. Homogenised milk has been treated to make it the same all the way through to stop its cream separating out. Heterogeneous means varied in content or non-uniform.

The -zygous bit relates to the zygote which is the cell formed when sperm and egg meet. So heterozygous means zygote formed from unlike gametes. Homozygous means zygote formed from similar gametes.

And by the way, if you are clever and know that the scientific name for humans is Homo sapiens, you might also know that this Homo comes from Latin, not Greek and means Man.