Biology Reference
In-Depth Information
have another form. Different forms of the same gene, as mentioned
above, are called alleles. For the present discussion, assume each gene
has only two alleles. This is a reasonable assumption for many genes and
is the basis of classical Mendelian genetics. An easy way to think about
this case is to imagine one allele as representing a functional gene (the
so-called normal gene) and the other allele as representing a
nonfunctional mutant of the gene. As is shown in Figure 3-3, both
chromosomes may have the same allele for the eye color gene (A and A),
different alleles for the size gene (B and b), and the same allele for the
coat color gene (c and c).
Some genes have a dominant allele and a recessive allele. A dominant
allele is expressed if it is present in both copies or in one copy with
a recessive allele on the opposite chromosome. A recessive allele is only
expressed if it is present in both copies. Suppose in an imaginary
creature, amber eyes are dominant to green eyes. The creature will be
amber-eyed if it has either one or two dominant alleles (i.e., AA or Aa)
and will only be green-eyed if it has two recessive alleles (aa). We
distinguish between the genotype of the creature (AA, Aa, or aa) and its
phenotype or appearance (amber-eyed or green-eyed). We call the
genotypes homozygous if they have two of the same allele (AA or aa) and
heterozygous if they have one of each (Aa). In Figure 3-4, we illustrate
Mendel's result with the seed shape gene, found on the Pisum
chromosome number 7. We use A to denote the dominant smooth-seed
allele and a to denote the recessive wrinkled-seed allele.
The parents are both true-breeding and therefore homozygous. They can
only produce a single type of gamete, as shown in Figure 3-4. The F
1
generation, resulting from the combination of egg and sperm in
fertilization, is heterozygous (Aa) and dominant (smooth). The F
1
individuals can produce two different kinds of gametes, A and a,in
approximately equal numbers (see Figure 3-5).
Keeping track of the gametes and ensuring that each one is paired
appropriately can be done with a Punnett square (see Figure 3-6). In a
A (amber eyes)
A (amber eyes)
B (big)
b (small)
c (white coat)
c (white coat)
FIGURE 3-3.
Hypothetical allele composition for a pair of chromosomes of an imaginary organism.
