Primer
of Mendelian Genetics
The outward appearance of an organism (phenotype)
is influenced by its hereditary makeup (genotype).
Many individual characters
(morphological,
behavioral, biochemical, molecular, etc.) of organisms may be
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).
Chromosomes are made up of DNA*.
Genetics is
the
science of analyzing phenotypes
to
infer the nature of their
underlying
genotypes. It 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, molecular biology is sometimes called "reverse genetics."
1. Alternative forms of genes
are
called alleles; every
individual
possesses
two alleles for each gene*.
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).
Dominant
alleles are
symbolized with a capital letter (A)
&
recessive
alleles with a lower-case letter (a).
For
example, some
people can taste the chemical phenylthiocarbimide
(PTC) and some cannot.
The
character
"PTC
sensitivity" is influenced by a gene
with two alleles,
one
for "taster" (T) and one for
"non-taster" (t).
The "taster"
allele (T) masks the expression of the "non-taster"
allele (t):
a
homozygous TT individual
or
a heterozygous Tt individual will show the "T"
phenotype ("taster"):
only
a homozygous tt individual
will
show the 't" phenotype ("non-taster").
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"
*The principles of genetics were first
described by Gregor Mendel
in 1867.
Mendel was unaware of DNA or
that genes reside on chromosomes
Genes that occur on the same chromosome are said to be linked
Gene loci located near
each other on a single chromosome will not assort independently.
The characteristic ratios will be modified,
according to how close they are.
The
modified ratios can be used to create a genetic map
of the chromosome
For example, sex in humans is
determined by genes on sex
chromosomes (X and
Y)
females are XX
have two
alleles (one on each X)
males are XY and
have only one allele on the single X (hemizygous)
Characters on the X
(or Y) chromosomes are sex-linked