1. Genetics

2. The Science of Genetics
Do you know anyone who has red hair? How about brown, black, or blonde hair? What other colors can you think of? There are literally hundreds of different shades of hair color that a person can have. Color is not the only characteristic of hair that helps determine how someone looks. How thick is your hair? Is it curly or straight?

3. The Science of Genetics
With all the possible variations, it would be very difficult to find two people with the exact same hair color, thickness, and so on. What about your eyes. What color are they? How big are they? Are they set deep in your head, or do they bulge outward?

4. The Science of Genetics
Yet despite the fact that your friends and family all look very different from one another, it is still quite easy for us to tell them apart from dogs, cats, and apricot trees. What determines how someone will look? What insures that all humans look like humans, yet also still look very different from one another? This force is called genetics.

5. History of Genetics
Throughout most of human history, the reason why an organism looks like its parents, while looking very different then a hound dog (unless its parents were hound dogs), was not known.

6. History of Genetics
This changed in the 1860s, when an Austrian monk by the name of Gregor Mendel began experimenting with peas. Mr. Mendel wanted to find out how life forms pass physical characteristics also known as traits, from one generation to the next.

7. History of Genetics
The traits that Gregor Mendel focused his study on were the height of a pea plant, the color of pea seeds, and the shape of pea seeds.
By cross pollinating the pea plants, he carefully controlled which plants reproduced, and tracked how each of these traits was passed on from generation to generation.
Cross pollination means that Gregor Mendel took pollination from a pea plant which he selected, and put it on another pea plant he selected.

8. Genetics of a Pea
To better understand what Mendel learned, Lets explore height, which is one of the pea traits that He experimented with.

9. Punnett Square
One of the easiest ways to calculate the mathematical probability of inheriting a specific trait was invented by an early 20th century English geneticist named Reginald Punnett .
His technique employs what we now call a Punnett square.
This is a simple graphical way of discovering all of the potential combinations of genotypes that can occur in children, given the genotypes of their parents.
It also shows us the odds of each of the offspring genotypes occurring.

10. Punnett Square
Next, you put the genotype of one parent across the top and that of the other parent down the left side.  For example, if parent pea plant genotypes were YY and GG respectively, the setup would be:
    

11. Note that only one letter goes in each box for the parents
Note that only one letter goes in each box for the parents.   It does not matter which parent is on the side or the top of the Punnett square.  
Next, all you have to do is fill in the boxes by copying the row and column-head letters across or down into the empty squares.  This gives us the predicted frequency of all of the potential genotypes among the offspring each time reproduction occurs.

12. In this example, 100% of the offspring will likely be heterozygous (YG).
Since the Y (yellow) allele is dominant over the G (green) allele for pea plants, 100% of the YG offspring will have a yellow phenotype, as Mendel observed in his breeding experiments.

13. Hetero and Homozygous genes
Heterozygous refers to having two different alleles for a single trait.
The gene for seed shape in pea plants exists in two forms, one form or allele for round seed shape (R) and the other for wrinkled seed shape (r). A heterozygous plant would contain the following alleles for seed shape: (Rr).
Homozygous refers to having identical alleles for a single trait.
The gene for seed shape in pea plants exists in two forms, one form or allele for round seed shape (R) and the other for wrinkled seed shape (r). A homozygous plant would contain the following alleles for seed shape: (RR) or (rr).

14. Mendel’s Pea Experiment
Mendel picked common garden pea plants for the focus of his research because they can be grown easily in large numbers and their reproduction can be manipulated. 
Pea plants have both male and female reproductive organs.  As a result, they can either self-pollinate themselves or cross-pollinate with another plant. 
In his experiments, Mendel was able to selectively cross-pollinate purebred plants with particular traits and observe the outcome over many generations.  This was the basis for his conclusions about the nature of genetic inheritance.

16. Pea Flowers
In cross-pollinating plants that either produce purple or white flowers exclusively, Mendel found that the first offspring generation (f1) always has purple flowers.   However, the following generation (f2) consistently has a 3:1 ratio of purple to white flowers

18. Genes
Mendel discovered that some genes are dominate, or stronger then other genes.
When there are two different kinds of genes (a purple and a white flower) the dominate gene will determine how the plant will grow.
In pea plants, the purple gene is dominate, while the white gene is recessive. Recessive is a big word that means “not dominate”.

19. Mendel’s Hypothesis
He came to three important conclusions from these experimental results:
1. That the inheritance of each trait is determined by "units" or "factors" that are passed on to descendents unchanged  (these units are now called genes )
2. that an individual inherits one such unit from each parent for each trait
3. that a trait may not show up in an individual but can still be passed on to the next generation.

20. Mendel’s Hypothesis
When reproducing, each parent can pass only one gene to their offspring, from each gene pair. This means that the offspring will inherit one gene from each parent, making a new gene pair.

21. This means that if your father has two genes for brown hair, and your mother has two genes for blonde hair, what will you inherit?
You will receive one brown hair gene from your father, and one blonde hair gene from your mother. Your gene pair for hair color will be brown – blonde

22. Genes
Scientists write gene pairs by using capital and lowercase letters. (TT, Tt, tt) The capital letter represents a dominate gene, while the lower case letter indicates a recessive gene.

23. Mendel’s Experiments
Now, lets return to our discussion about pea plants. In Mendel's experiments, he put the pollen from true breeding tall pea plants (TT) on true breeding short (tt) pea plants. Remember, each parent can only pass one gene to the offspring.
What will the gene pair look like for the offspring? They will all be (Tt), having one tall gene, and one short gene.

25. So why is it important to know genetics?
The answer is that they can be used as predictive tools when considering having children.
Let us assume, for instance, that both you and your mate are carriers for a particularly unpleasant genetically inherited disease such as cystic fibrosis .
Of course, you are worried about whether your children will be healthy and normal.
For this example, let us define "A" as being the dominant normal allele and "a" as the recessive abnormal one that is responsible for cystic fibrosis.

26. As carriers, you and your mate are both heterozygous (Aa). 
This disease only afflicts those who are homozygous recessive (aa). 
The Punnett square below makes it clear that at each birth, there will be a 25% chance of you having a normal homozygous (AA) child, a 50% chance of a healthy heterozygous (Aa) carrier child like you and your mate, and a 25% chance of a homozygous recessive (aa) child who probably will eventually die from this condition.

27.                                     
         
If both parents are carriers of the recessive allele for a disorder, all of their children will face the following odds of inheriting it:
25% chance of having the recessive disorder
50% chance of being a healthy carrier
25% chance of being healthy and not have the recessive allele at all

28. It is likely that every one of us is a carrier for a large number of recessive alleles.  
Some of these alleles can cause life-threatening defects if they are inherited from both parents. 
In addition to cystic fibrosis, albinism, and beta- thalassemia are recessive disorders.

29. Some disorders are caused by dominant alleles for genes
Some disorders are caused by dominant alleles for genes.  Inheriting just one copy of such a dominant allele will cause the disorder. 
This is the case with Huntington disease, achondroplastic dwarfism, and polydactyly. 
People who are heterozygous (Aa) are not healthy carriers.  They have the disorder just like homozygous dominant (AA) individuals.
                                    
         
If only one parent has a single copy of a dominant allele for a dominant disorder, their children will have a 50% chance of inheriting the disorder and 50% chance of being entirely normal

30. Genetics Activity