A comment on last Friday's class: It seems to me that several  issues were raised in our discussion of human sex chromosomes
1. Do genes determine sexuality or sexual preference (as opposed to the appearance of being male or female, and /or fertility).?
2. Is homosexuality due to alterations in neurological functions (Note: homosexuals do not have impaired cognitive abilities)
2. If sexuality is not genetically determined, how is it determined?
3.If the individual has the appearance of one gender but the reproductive organs of another, what then is this person's sex or sexual preference to be?

One of my aims for this class is to give all of us some diverse biological, social and historical perspectives on those questions and we will come back to all of them.  For now, here are web sites about  two researchers who study possible genetic predispositions to sexual and other preferences  Dean Hamer and  Simon LeVay; our scientists of the day.

Please remember to write your position paper on "Is there a biological definition of female?" for Wednesday's class.  This one page paper should focus on how one might define "female-ness".

Today's lecture:

Variation and Natural Selection
Genes on chromosomes determine (in large part) what an organism looks like and some (how much is a topic for discussion) of how it behaves.
The environment may alter an organisms appearance, but those alterations are not (not always. mutations are an exception) hereditary.
The appearance of any one physical characteristic  may be determined by a single gene, or by multiple genes.
We looked at some examples.
Each gene has its partner (or allele) on a look alike chromosome of the other sets.
The two gene partners are often not  identical, but they can be.
Some genes may "dominate" their partner gene, others may be functionally invisible or recessive, still other situations exist in which two or more genes are active on the same trait at the same time.
Some traits are coded for by so many genes that the set any one offspring might get is very very hard to predict, in the, for example care of  human height or skin color.
Mutations are the result of enviromental modifications of DNA.
Mutations are heritable, but only if the reproductive cells carry them.
All this genetic variation leads to physical (phenotypic) variation, which is heritable.
Some variants can survive and reproduce in a given environment others don't do as well, thus the environment "selects" those that can reproduce,  natural selection.
Only those that survive pass their genes on to their offspring.
Changes in the heritable capacity (gene pool) of species over time, is evolution.
Evolution leads to adaptation. (see figure 13.4C).
New species arise only when populations are isolated from one another, and thus can't interbreed.
Biologists tend to think about the advantages and disadvantages to survival and reproduction of an organism's genetic variation and of the species' adaptations, this includes behavioral adaptations, to its environment.
The exsistence of genetically predetermined female and male individuals may not be adaptive for every species.
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