More Genetics Stuff

Dominant/recessive, homozygous/heterozygous, you didn't think it was going to be that simple did you? As it turns out, the 25,000 genes that make up the human body are somewhat complicated in the way they code for proteins/traits and the way they interact with each other.

Sometimes there is no dominant or recessive allele; the heterozygous genotype causes a different trait than either of the homozygous genotypes.
Some (most) traits are a product of many genes working together.
Some traits are passed on differently depending on the sex of the parent that has it.
Some traits are expressed differently in males, females because of hormones
Some genes may be turned on or off because of one's life experiences.

Not-So-Simple Dominance

In many cases, a dominant trait codes for a protein, and the recessive gene codes for a non-functioning version of the protein. People with the dominant type A blood make an antigen (ID marker) on the outside of their cells; people with recessive type O blood, do not make any kind of antigen on their blood cells. Heterozygous individuals have type A blood. Sometimes though, neither of the two traits shown by homozygous individuals are what is shown by a heterozygous individual.

Incomplete Dominance - blending the two extremes

HH - curly hair
Hh - wavy hair
hh - straight hair
"Wavy" is a blend of the other two traits.

Codominance - both of the extremes show up in heterozygous individuals

BB - chicken with black feathers
Bb - chicken with black and white feathers
bb - chicken with white feathers
The heterozygous chicken does NOT have gray feathers, they are black in some places and white in others

Multiple Alleles - Some times there are more than just two different forms of a trait (alleles). There are 3 different alleles for blood type - A, B, and O. (A and B are codominant, O is recessive)

Type A blood = AA or AO
Type B blood = BB or BO
Type AB blood = AB
Type O blood = OO

Sex Linked Traits

Genes are sections of a chromosome that code for traits, but males and females have slightly different chromosomes. The 23rd pair of chromosomes are the Sex Chromosomes - X and Y. Different chromosomes have different genes on them.

The "default" for humans is to be female.

XX - female
XY - male
XO (individuals with just one X chromosome and no 2nd sex chromosome) - female (Turners Syndrome)
YO (just a Y and no X) - no zygote can survive without an X chromosome
Basically, you can survive without a Y chromosome, but everyone needs an X - it has genes on it that are necessary for survival and have nothing to do with being male/female.

Females have two copies of the X chromosome.

They can be homozygous or heterozygous for any trait that is on the X chromosome.
Most female cells actually use one X chromosome and deactivate the 2nd into what is called a Barr Body.

Males only have one copy of an X chromosome.

They will express the only copy of the gene that they have.
They cannot be heterozygous
The Y chromosome has completely different genes
Males are more likely to show recessive traits because they cannot hide them with a dominant allele.

Colorblindness is an X linked recessive trait.

For females:
- BB = normal vision
- Bb = normal vision
- bb = colorblind
- females have to get a recessive allele from BOTH parents to be colorblind
For males:
- B = normal vision
- b = colorblind
- males have to get the one recessive allele from their mother to be colorblind (a father gives his son a Y, not an X)

Yet more genetic stuff

Most traits are polygenic; they are controlled by more than one gene.

There are hundreds of genes involved in skin color, eye color, height and all of the other traits that seem like they could be simple.

Some traits are influenced by hormones or other outside signals

Genes for traits considered male or female are present in both sexes, but activated by testosterone or estrogen or other hormones.

Epigenetics - some traits are not determined solely by DNA

Some proteins normally turn genes on or off
- This is why cells in your skin can make sweat and cells in your stomach can make acid
Some proteins turn genes on or off, but are only made as a result of certain life-experiences like stress or diet
- This is why identical twins are never really identical even though they have the same DNA