Mendel's first law

You won't get too far in your fly experiment if you don't know about Mendel's laws. These laws describe the fundamental principles of inheritance. Amazingly, Mendel figured this all out by studying pea plants (over 10,000 for 10 years) way before DNA and Genes were understood. Mendel first reported these laws in 1865 but they faded into obscurity until they were rediscovered in 1900.

A couple of definitions:

GENOTYPE = What genes a cell contains

PHENOTYPE = The characteristics/traits of an organism 

Anyway, that is enough preamble --> to the laws....

Mendel's First Law: The Law of Segregation

 Every individual has a pair of genes (alleles) for any particular trait. 

Each parent passes on only one allele to each of their gametes - by meiosis. 

Gametes randomly come together, 

hence offspring will have again two alleles for a particular trait, 

one from their mother and one from their father.

The dominant allele will determine the phenotype/trait.

Struggling with this? Lets break it down bit by bit.

Every individual has a pair of genes (alleles) for any particular trait. 

In diploid organisms, there are two copies of every chromosome. One copy from the mother and one from the father. This means that there will be two copies of every gene - alleles. Genes decide what an organism will be like, and sometimes only one gene/ pair of alleles will decide something very easy to observe - for example, eye colour in Drosophila.

Each parent passes on only one allele to each of their gametes. 

Remember meiosis? As a result of meiosis you end up with gametes that have only one copy of each chromosome from a pair - these cells are called haploid since they have half of the chromosomes that were in the parent cells. So, the result is that each gamete has only one allele for each gene since there are no longer pairs of chromosomes. Check out the diagram below if you need a little meiosis reminder ...

image taken from: http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa_pre_2011/celldivision/celldivision4.shtml

Gametes randomly come together, 

hence offspring will have again two alleles for a particular trait, 

one from their mother and one from their father.

So, gametes (which are haploid i.e. contain only one copy of every gene) pair up at random (i.e. a sperm with an egg), and once they have paired they will create cells that contain two copies of every chromosome again (i.e. diploid cells)- one copy of every chromosome is from the mother's gamete, and the other copy of every chromosome is from the father's gamete. This means that there will again be two copies/alleles of every gene.

The dominant allele will determine the phenotype/trait.

One of the copies of a gene will be dominant over the other copy. This means that only one of them will decide what the trait/phenotype will be - i.e. hair colour.

Hmmm, it would be really nice to be able to visualize that.....how about some Punnett Squares. These are a really nice visual way to represent Mendelian inheritance. They are used to lay out all of the possible combinations of the alleles of a particular gene from the mother and father so that you can figure out the probability of an offspring having a particular genotype and therefore phenotype....

Take a look at this example:

image taken from: http://course1.winona.edu/sberg/241f02/Lec-note/Mendel.htm

Let me explain this Punnett Square:

• The alleles for the colour gene that decides whether the pea pod is green or white are given the letter G or g. Big G is the dominant allele, little g is the recessive allele. 
• Outside of the square, G and g represent the two possible gametes there might be - so in this case both of the plants being crossed have a G and a g copy of the colour gene (they are heterozygous for the colour gene), hence their gametes contain one or the other of these alleles. 
• The Punnett Square shows all the different combinations you might get of these alleles if these two pea plants were crossed. You might get two big G's coming together - making a homozygote GG. Or a big G might end up with a little g - making a heterozygote Gg. Or, two little g's might pair up, giving a homozygote gg. 
• Because G is dominant over g, when you have a genotype resulting from this cross that is GG or Gg, the pea pod will be green, and only when you have a genotype that is gg will you get a white pod. 
• The Punnett Square allows you to easily see what the chances are from the cross that you might get a green or white pea pod- 3 out of the 4 squares have a genotype that will give a green phenotype....... 
• ......so, if you set up loads of these crosses, you would expect 75% of your plants to have green pods - assuming pea pod colour is inherited according to Mendel's first law that is!

Well, that is a lot for one post, Mendel's second law will be coming next....