Inheritance
First we must understand some basics.
A gene is a small section of DNA that carries the instructions to make a protein. Our body is made up mostly of proteins. Our genes determine the protein and the protein determines the characteristic. For example, one protein results in green eyes. A subtle change to it and it gives brown eyes. The gene says to make a colour pigment, but it is not always the same. So the gene can have slightly different forms - we call these alleles.
We each have 46 chromosomes - thats 23 pairs. What does this mean?
A pair of chromosome are the same - but different! They are each the same length and carry codes for the same genes, eye colour, hair colour, tall, short etc. However, one chromosome may say blue eyes and the other Brown.
Only one can show itself so they are either brown or blue. The one that is most over powering is called the dominant gene. The opposite is recessive. The genes are represented with letters. The letter is usually taken from the dominant gene - lets say B for Brown. All other colours take the same letter but lower case, so b - mean not Brown.
We have 2 of each gene and therefore 2 letters to represent the types we have. A person with brown eyes may have 2 alleles, both saying Brown - BB. They may have one for brown eyes and one for blue - Bb. However, because brown is dominant their eyes will be brown. A person with blue eyes must have the combination bb - so neither allele is instructing them to have brown eyes. Where the person has two alleles the same we say they are homozygous (homo = the same), where they are different they are heterozygous.
We call the physical characteristic that you see the Phenotype.
We call the combination of alleles (genes) that give the characteristic, the genotype (hetero = different).
A gene is a small section of DNA that carries the instructions to make a protein. Our body is made up mostly of proteins. Our genes determine the protein and the protein determines the characteristic. For example, one protein results in green eyes. A subtle change to it and it gives brown eyes. The gene says to make a colour pigment, but it is not always the same. So the gene can have slightly different forms - we call these alleles.
We each have 46 chromosomes - thats 23 pairs. What does this mean?
A pair of chromosome are the same - but different! They are each the same length and carry codes for the same genes, eye colour, hair colour, tall, short etc. However, one chromosome may say blue eyes and the other Brown.
Only one can show itself so they are either brown or blue. The one that is most over powering is called the dominant gene. The opposite is recessive. The genes are represented with letters. The letter is usually taken from the dominant gene - lets say B for Brown. All other colours take the same letter but lower case, so b - mean not Brown.
We have 2 of each gene and therefore 2 letters to represent the types we have. A person with brown eyes may have 2 alleles, both saying Brown - BB. They may have one for brown eyes and one for blue - Bb. However, because brown is dominant their eyes will be brown. A person with blue eyes must have the combination bb - so neither allele is instructing them to have brown eyes. Where the person has two alleles the same we say they are homozygous (homo = the same), where they are different they are heterozygous.
We call the physical characteristic that you see the Phenotype.
We call the combination of alleles (genes) that give the characteristic, the genotype (hetero = different).
Genotype
BB Bb bb |
Phenotype
Brown eyes Brown eyes blue eyes |
Inheritance of eye colour
When two people have a child, it is possible that their children have characteristics that they don't have. For example two brown eyed parents may have a blue eyed child. This can happen when both parents have a dominant allele for brown eyes - and therefore have brown eyes, but also carry a recessive allele for blue eyes. When they have a child together there is a 1 in 4 (1:3, 1/4 or 25%) chance that the child will have blue eyes.
This is all summed up quite nicely in this amusing little video
If you get all this you actually know more than Gregor Mendel did!!!
Heterochromia
This will not be in your exam, but, someone always asks "why have some people got one blue eye and one brown?" - heres the answer