2. Then we reviewed the questions asked in the first test, handed in at the start of the class.  (While you were only to answer two of the questions asked, you need to understand all of them when preparing for the final exam.) 

3. Then we went on to the biological (genetic) foundations of development.

I.                    Genetics would be simple to understand if  reproduction were simply due to self-replication (cloning) in which  a new organism was simply an identical copy of the original. However, sexual reproduction provides for the immense array of differences between one individual and another. These normal genetic variations ensure that, over time, species can adapt to new conditions, and most genetic variations that are passed from one generation to the next over hundreds of years have some adaptive function. (For instance, sickle cell anemia, a painful and often deadly genetic disease, is maintained in populations who live in tropic zones because having one gene for sickling and one paired normal gene conveys immunity to malaria, which also can be deadly.)

II.                 Mechanisms of genetic transmission:

Genetic information is combined and transmitted through gametes (i.e., ovum and sperm), which contain the ‘blueprint’ for an individual in molecular structures called called chromosomes. (Every cell of the body contains the full complement of that individual’s genetic code, but different genes are activated in the different tissues  made up by the cells.)
Humans have 23 pairs of chromosomes, with one of each pair inherited from the mother and one from the father.

1.      The chromosomes are made of deoxyribonucleic acid (DNA) molecules, which have a double helix (spiral) chemical structure that ‘unzips’ to allow their division and duplication.

2.      The chromosomes are made up of precisely ordered sequences of DNA molecules that code all the information for making the individual human in about 30,000 specific genes. The Human Genome Project is currently mapping for the first time, the location of the specific genes on the various chromosomes.

3.      These precisely ordered sequences of DNA molecules transmit the information for making a unique individual genetic at conception, when the sperm, with one complete set of chromosomes from the father, unites the mother’s ovum, which contains another set, creating a zygote with 46 paired chromosomes.

4.      Each gamete (ovum and sperm) contains a unique subset of the parent's genes, obtained in the process of meiosis, in which not only do the chromosomes divide into single sets, but also parts of the individual chromosomes are rearranged into new combinations, ensuring a vast range of variations.

            The genes paired along the chromosomes may not be identical (homozygous); the various forms are called alleles and when not identical (heterozygous), can have unequal influences over the resulting traits. Sometimes an allele is dominant and controls the resulting trait, while other alleles may be recessive and unexpressed. Other times, alleles are codominant, interacting together to influence the resulting traits.
           Most traits, however, are the result of a number of genes in interaction, not just a single pair. We discussed blue eyes (recessive) and brown eyes (dominant), as well as green and hazel eyes (modifier genes).

 What can go wrong?  Defective genes:  A number of diseases are genetic in origin. Most are recessive and continue to exist not because they are adaptive but because the gene ‘hides’ behind the dominant normal gene. We discussed why a deadly disease due to a dominant  one is rare: the individuals have to live long enough to reproduce in order to pass along the deadly gene. Also, normally healthy genes can be altered (mutated) by mistakes during cell division or by environmental influences (such as Xrays). Such mutations are very rarely beneficial or adaptive; instead they are almost always destructive or deadly. Genetic mutations occur frequently in various cells of mature organisms but do not contribute to affect future generations unless they occur in the reproductive cells. If a disease-causing mutation which occurs in the precursors to sperm or ova is dominant over the normal form of a gene, the organism with the mutation will usually die before passing it on to the next generation, but some deadly genetic diseases do not show up until after sexual maturity has been reached, allowing the gene to be maintained down the generations (Examples: Huntington disease, Fatal Familial Insomnia).  Other dominant genetic 'mistakes' such as Marfan syndrome cause serious defects but may not always be fatal, again, allowing the gene to be passed on to the next generation. These dominant defects, however, are rare.  Recessive genes that cause disease are more common, as they do not affect the individual unless inherited from both the  mother and father. Still, in stable isolated populations where inbreeding is more likely, a fatally defective gene will usually eventually disappear from the gene pool unless it has some beneficial  role (as in sickle cell genes). However, not all genetic mutations are fatal, and even potentially fatal genetically-caused illnesses can be sometimes be modified by environmental factors (example: PKU). 

Sex-linked genetic diseases: Some recessive genetic problems are the result of being located on the sex chromosomes where males have a greater chance of being affected, as they have only one X chromosome  (X-linked) and if they inherit the gene from their mother have no normal dominant gene from the father to override the recessive gene.  A woman may have the disease if she inherits the recessive gene from both mother and father, but is more likely to be a 'carrier' in which the one recessive gene is masked by the dominant normal allele. 

Study the table of genetically-caused diseases on page 60-61.

Chromosomal abnormalities. Chromosomal abnormalities are far more deadly than genetic mutations, as they affect many genes at the same time; and mistakes can easily occur in the stages of meiosis, with parts of chromosomes breaking off or not separating properly, etc. However, we are not aware of these common occurrences as most such cells die before reproduction can take place.  Some defects however are not necessarily fatal and result in the conception and birth of individuals with chromosomal defects such as Down syndrome ( trisomy 21, in which the individual has three copies of the 21st chromosome)  or defects related to abnormalities of the sex chromosomes. Down syndrome is most commonly due to problems occuring during meiosis in older mothers, but sometimes are the result of the father's chromosomal contribution. A woman's ova are present in her ovaries from before her birth, while sperm are being constantly produced by the testes; environmentally-caused mutations are therefore much more commonly associated with maternal age, although aging may have some deleterious effects on male reproduction as well.
Study the table on Page 65 of the common sex chromosome disorders.

  Hmm. Not done yet. I'll finish by Friday PM....