Day 32 Punnett Squares

Dihybrid Cross Quiz

In case you did not do it, watch these videos

Intro to Genetics

http://www.khanacademy.org/science/biology/heredity-and-genetics/v/introduction-to-heredity

Punnett Squares

http://www.khanacademy.org/science/biology/heredity-and-genetics/v/punnett-square-fun

If the heterozygous offspring (Pp) reproduce, their offspring can be any of the four possible combinations

PP, Pp, pP, pp

We can figure our the possible allele combinations of offspring by using a Punnett Square

Mendel then started testing other traits in the pea plants

Lets do some practice Punnett Squares using these traits!

What if we want to do a Punnett Square for more than one trait?

-We can do what is called a dihybrid cross. A cross between two traits

Lets practice!

Lets take a look at crossing over and genetic recombination

How is sex in mammals determined?

-As we know males have the XY chromosomes and females have the XX chromosomes

-We have been told that there is a 50/50 chance of having a boy or a girl but lets do a punnett square for it an see why there is a 50/50 chance.

-Because we are diploid we have two copies of every allele. If one of them were broken hopefully the other one takes over.

-With males they only have one copy of every allele on the X chromosome and they only have one copy of every allele on the Y chromosome

-Because of this there are genetic disorders that are common in men but not in women

The Y chromosome is very small and does not contain many genes. For the most part it only has the genes that give males their characteristics

-If a gene is located on one of the sex chromosomes it is considered a Sex-Linked Gene

-A common Sex-Linked gene is the one for color blindness

Now lets do a Punnett Square for color blindness and see why it is more common in males

All though Mendel paved the way for genetics not all traits follow Mendelian genetics.

-Mendelian genetics only follows the dominant/recessive rules

There are other types of alleles that are not dominant or recessive

-Incomplete dominance

-Incomplete dominance is when both alleles blend together in the phenotype

-For example the flower color of snap dragons is an incomplete dominant trait

-If a white snap dragon reproduces with a red snap dragon you can get pink flowers

-Genotypes

-rr = Red

-ww = White

-rw = Pink

-Human hair texture is another example of incomplete dominance

-Genotypes

-ss = Straight

-cc = Curly

-sc = wavy

-Another human trait that has incomplete dominant alleles is skin color

-Except skin color is a little more complicated because there are 3 different genes for human skin color

-Since we are diploid that means we have a blend of 6 alleles to determine our skin color