Primer
of Mendelian Genetics (1865)
The appearance of an organism (phenotype) is influenced by its
heredity (genotype). Many individual
characters (morphological,
behavioral, biochemical, molecular, etc.) of organisms are
influenced more or less directly by individual hereditary elements
called genes. Genes are
located on chromosomes, each
at a particular physical location called a locus (plural, loci).
Genetics is the science of analyzing phenotypes to
infer the nature of their underlying genotypes. The basic method is the analysis of crosses. The
basic principles were first described by Gregor Mendel in
1865. Genetics operated as
a distinct science from the
rediscovery of Mendel's work in 1900, without knowledge of
the genetic material until 1953. Genetics is distinct from molecular
biology, which analyzes genotypes (in a DNA
molecule) to predict phenotypes (which are often direct or indirect products of
proteins). For this reason, the so-called Central
Dogma of molecular biology (DNA » RNA »
Protein) is sometimes called "reverse genetics."
Terminology here has been updated to reflect 20th & 21st
century sage.
1. Alternative forms of genes are called alleles; every individual possesses
two alleles for each gene.
[Mendel called these units "elementen": the term gene was
not used until 1909]
An
individual with two identical alleles is a homozygote and
is described as homozygous;
an
individual with two dissimilar alleles is a heterozygote and
is
described as heterozygous.
2. Some alleles (called dominant)
mask the phenotypic expression of other alleles (called recessive).
Dominance is determined by comparison of the
heterozygote phenotype with that of the two homozygotes
Dominant
alleles are symbolized with a capital letter (A);
recessive
alleles with a lower-case letter (a).
[Mendel's Law of
Dominance]
For
example, some people can taste the chemical phenylthiocarbimide (PTC) ("tasters"),
and some cannot ("non-tasters").
The character
"PTC sensitivity"
is influenced by a gene with two alleles,
one
associated
with "taster" and one with "non-taster".
The "taster"
allele masks the expression of the "non-taster"
allele in heterozygotes:
Homozygous TT or
heterozygous Tt individuals both
show the "T" phenotype ("taster"):
only
a homozygous tt individual
show
the "t" phenotype
("non-taster").
Because the phenotype of the Tt individual resembles
that of the TT individuals,
the T allele is
described as dominant to the t allele.
3. The two alleles separate (segregate) during the
formation of gametes (eggs
& sperm);
half
of the germs cells carry one allele & half carry the other [Mendel's
Law of Segregation].
4. Random union of gametes produces zygotes that develop into new
individuals.
Zygotic genotypes occur in characteristic ratios, according to the genotypes
of the parents.
For
example, a cross between two heterozygotes (Aa x Aa)
produces
an expected genotypic ratio of 1:2:1
among AA, Aa, & aa genotypes.
5. The genotypic ratios produce
characteristic phenotypic ratios,
according
to the dominance relationships
of the alleles involved.
For
example, if A is dominant to a, the cross between
heterozygotes produces
an
expected phenotypic ratio of 3:1
among "A" and "a" phenotypes.
6. Alleles at separate loci are inherited independently [Mendel's Law of Independent Assortment]
This produces characteristic
genotypic and phenotypic ratios.
For example, in
a dihybrid
cross between two "double
heterozygotes" ( AaBb
x AaBb )
The genotypic ratios are 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1
for the genotypes AABB
AABb AAbb AaBB AaBb Aabb aaBB
aaBb aabb
and the phenotypic ratios are 9 "AB" : 3 "Ab" : 3 "aB" : 1 "ab"