What is a sex cell?  What is a non-sex cell?  What are Chromosomes and what role do they have?  Non-sex cells contain two (2) pair of chromosomes, one set of chromosomes from each parent.  The chromosomes contain a copy of each gene from each parent.  Sex cells are the sperm or egg.  The egg always provides an X chromosome.  The sperm can provide either an X or Y chromosome.  Gender is determined by whether the sperm provides either an X or Y chromosomes.

CHROMOSOMES

          Most humans have forty-six (46) chromosomes.  There are twenty-two pairs, one set from each parent.  Paired chromosomes are autosomes.  Then there are either two XX chromosomes for a female or XY chromosomes for a male.  Some humans have XXY or even XXXY and are either male or female…what?   (See SRY gene below)  Females can have only an X.  The Y chromosome has less genes than the X chromosomes (smaller in illustration)

          An estimated 26,000 genes are contained in each cell and allotted across 22 pairs of chromosomes (figure 1).  Each parent provides a set of genes that have the same specific location and order on each chromosome.  Mutations of these genes during embryogenesis contribute to the characteristics of the newborn.  Chromosomes vary in the number of genes they contain based on function.  Some chromosomes contained millions of genes (multiples of many genes). 

Chromosome Characteristics

Chromosomes provide a backbone to which genes attached.  This backbone is sugar-phosphate substance called Deoxyribonucleic Acid (DNA).  This was discovered in the first half of the twentieth century.  Identifying genes on chromosomes is an ongoing activity in genetic research today.  A more difficult ongoing research activity is identifying the functions associated with each chromosome.  The role of chromosomes is an ongoing research activity.                  

Embryogenesis

          So, how do we get XX or XY?  The embryo development starts without gender recognition and proceeds on a default mode of X X.  During embryo development, if the sperm provided a Y-chromosome, a gene called sex determining region Y (SRY) signals for a gonad hormone to create testosterone thus creating a male.  If the sperm provides an X-chromosome, the default mode continues and signals for the gonad hormone to proceed to create estrogen thus creating a female.

          Abnormalities can occur in cell division during embryogenesis that results in either X, XXY or XXXY.  Thus, if the sperm provided both X and Y, one would be recessive (not dominate-inactive).  If the Y-chromosome were inactive, it would be unable to generate the SRY gene on the Y-chromosomes, thereby leaving a female with an inactive Y-chromosome…likewise for a second X in males.  Could such abnormalities be an influence of gender preference later in life?

 Alzheimer Risk

          Familial AD refers to inheritance of risk gene and/or alleles.  Early Onset of Alzheimer Disease (EOAD) (about age 35) has two genes identified as risk factors.  These genes are Presenilin 1 that is located on Chromosome 14 and Presenilin 2 located on Chromosome 1.  Late Onset Alzheimer’s Disease (LOAD) (symptoms begin showing age 65 and over).  LOAD has identified a mutation in the Amyloid Precursor protein gene (APP) as a risk factor, along with the Apolipoprotein E gene (ApoE) as a risk factor due to a dominate E4 allele that is located on Chromosome 19.  An allele is one of alternative forms of the same gene from either a parent or a mutation.

          ApoE gene allele E4 is a high risk for LOAD.  ApoE gene has an allele option of E2, E3, or E4. If the father has E2/E3 and the mother has E3/E4, then the embryo possibilities are E2/E3, E2/E4, E3/E3, or E3/E4. Therefore, a 50 percent chance of E4 is possible. If the embryo receives E2-E4, then it becomes a question of which allele is dominant? If E2 is dominant over E4, AD may not occur even with the E4 risk allele.  However, if the E4 is dominate, then risks of AD increase.  For the embryo with E2/E4 where the E4 is not dominant, that E4 allele still could pass to the next generation, where it could become dominant.

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