Re: What is purebred and how it deals with genetics for eyes..more info below

Diane,

Let's start with the simplest question...what does homozygous mean?

Humans are diploid; that is, they have two sets or 23 pairs of homologous 
chromosomes (46 total chromosomes) since each parent donates one of the 
two chromosomes in a pair to their offspring. At any site along a given 
chromosome, this offspring can be homozygous (have the same allele or 
version of a gene) at that site or can be heterozygous (have different 
alleles) at that site because one chromosome is donated by each parent.

Let's take your eye color example (and simplify it)...We'll assume that 
there is just one locus (or site) on the chromosome that controls brown vs. 
blue eye color. At this locus, you can have one of the two possible 
alleles or version of the eye color gene, brown (B) or blue (b), and we'll 
also assume that brown eye color results from either one or two copies of 
the brown allele. In other words, the allele or gene that produces brown 
eye color is dominant to the blue eye color allele. Any individual that has 
brown eyes is either BB (homozygous) or Bb (heterozygous). Individuals with 
blue eyes must have two copies of the b allele (bb).

Homozygous individuals are "pure breeders"; they can only donate one type 
of allele to their offspring and if each parent is homozyous, only 
homozygous offspring can result. In your example, the grandparents are 
homozygous for brown eyes, that is, they are both BB only. Since each can 
only donate a B allele to their children, all offspring from this union 
(i.e., the parent of the newly-weds) would also be homozygous or pure-
breeding, for brown eyes, BB. If both newly-weds had parents 
such as these, they would both also be homozygous BB and could only produce 
brown eyed children.

 However, if we only know that the grandparents HAD brown eye color, each 
grandparent could have been either BB or Bb genotypically. If one was BB 
and the other Bb, then all offspring would have brown eyes, but 1/2 of the 
offspring would be genotypically Bb.Since 50% of these grandparents' 
offspring are Bb, these individuals are "carriers" of blue eye color and 
can produce blue-eyed offspring if the mate with another "carrier" (Bb). If 
both grandparents were "carriers" or were Bb, then 3/4 would have 
brown eyes and 1/4 would have blue eyes, but of 2 of the 3 brown eyed 
individuals would have the potential to pass their blue eye color allele on 
to their offspring.

Thus, if all four of the original grandparents on both sides of the newly-
wed's families were known to be homozygous for brown eye color (BB); there 
would be no chance of the newly-weds having a blue-eyed child (bb). But, if 
all we know is the original grandparents had brwon eyes, then there is a 
possibility of blue-eyed offspring from the newly-wed couple.