Day 29 Sex Linked Genes and Incomplete Dominance

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.

Class Practice:

Sample Boy vs Girl Punnett Squares

Lecture:

-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

Student Activity:

Students compare the color graphs to experience what it would be like to be color blind (Some students might be color blind)

Student Practice:

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

Lecture:

Although 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

-When we were working with dominant and recessive traits we used the letter of the dominant trait. In incomplete dominant you can use two different letters since neither trait is dominant (Although sometimes you will still see only one letter used)

Human hair texture is another example of incomplete dominance

-Genotypes

-ss = Straight

-cc = Curly

-sc = wavy

Student Practice:

We will select students from the class and by the look of their hair see what type of hair their children will have

Lecture:

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