This topic came up on a thread in back yard chickens lately and this is a post that I wrote. We have a lot of new members coming in and I thought this would be helpful to someone, so I am re-posting.
Ok, crash course in (very) basic genetics, without getting into meiosis, mitosis, halitosis..or any of those other -osis terms that muddle things up.
There are basically two types of genes - dominant and recessive. Each chicken, for that matter each of us, has two copies of every genetic trait. One side comes from the male parent and one comes from the female parent. A dominant trait is a bossy trait - it's going to show its stuff, even if only one side is the dominant one. A recessive trait is the wallflower - it has to be coaxed out in public, and then only if it is holding hands with its twin.
Without getting into incomplete dominance, crossing over, epistatic, codominance, or any of those other complicated processes by which new, and truly unique individuals are created, lets say that we have a trait called "lavender". Lavender is a true recessive, so it will only show its face in public if it is paired with another just like it. If the dominant "lavender" gene shows up, it keeps the weaker, recessive side in a corner.
When referring to various genetic traits, it is easier to express them as letters, rather than spelling them out completely. And, typically, the recessive is expressed as a small letter, or "l", and the dominant, bully, side is expressed as a capital, "L". So, if both parents can only contribute a dominant "L", the baby will have "LL", or be "homozygous dominant" and will not express, or show, any lavender coloration. Are you with me so far?
Now, let's say that one parent is this homozygous dominant, but the other parent is carrying one dominant "L" and one recessive "l". That recessive "l" is not going to be expressed in that parent, because the bully "L" is keeping it in the corner, but it can share that "l" with its children. Since the other parent can only contribute the dominant "L", those children that receive the recessive "l" from this parent will all be "splits" - or heterozygous with the "Ll". Of course, those children, just liked its mixed parent, will not show lavender coloration.
But, if you take that mixed "Ll" and breed it to another "split" "Ll", then there is a possibility that each parent will contribute the recessive "l" to one, or more, of its children. Those children will be "ll", or homozygous recessive. And guess what color they will be? Lavender. Since there is no bully dominant trait to keep it in a corner, those shy little twins will march right out in public and say, "here we are!".
One of the most useful tools in contemplating genetics is the Punnett Square. It was developed a very long time ago, by an early geneticist named, of course, Reginald C Punnett, who was also the Professor of Genetics at the prestigious Cambridge University. It is a very simple little tool for calculating the odds of, either getting what you want from a breeding or anticipating what you don't want. Here is how it works.
I've put the letters representing each parent's genetic indicators for lavender - the male across the top (in blue) and the female down the side (in red). Remember our male is homozygous dominant, "LL" and our female is heterozygous, or "Ll" - also known as split for lavender.
In each square I copy the corresponding letter from each parent, so in the first "child" row and in the first "child" column, I copy the blue "L" from above and the red "L" from the left. And I continue that process until all the "child" squares are full.
Two of the four child squares are "LL", or homyzygous dominant, just like their daddy. Two of the child squares are "Ll", just like their mom. None of these children will be lavender, but two of them, or 50%, could pass on the lavender trait.
You can play with this Punnett Square to see what other possible combinations can be achieved. Each child square represents, statistically, 25% of the children from that breeding.
Ok, I'm moving away from the podium for a while.
Hope this helps.
