Primer
of Mendelian Genetics
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
analysis typically involves making genetic crosses
between organisms that differ for some trait.
The basic principles were first described by Gregor Mendel in
1867, rediscovered in 1900, and developed over the next 50+
years without knowledge of their molecular basis. Genetics is a science 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." Molecular biology deals with
the inheritance of DNA genotypes: Genetics deals with the
inheritance of phenotypic traits.
1. Alternative forms of genes are called alleles; every individual possesses
two alleles at each locus.
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 "mask" the phenotypic expression of
other alleles. The former are
called dominant and the
latter recessive.
Dominance is
determined by comparison of the heterozygote phenotype
with that of the two homozygotes
Dominant
alleles are often symbolized with a capital letter (A);
recessive
alleles with a lower-case letter (a).
For
example, some people can taste the chemical phenylthiocarbimide (PTC) ("tasters"),
and some cannot ("non-tasters").
The trait "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 also produces characteristic
ratios, which are the product of
the ratios at the separate loci
For example, in
a dihybrid
cross between two "double
heterozygotes" ( AaBb
x AaBb )
The genotypic ratios are (1 AA :
2 Aa : 1
aa) x (1 BB : 2 Bb : 1 bb)
= 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 (3 "A":1 "a")
x (3 "B":1 "b")
= 9
"AB" : 3 "Ab" : 3 "aB" : 1 "ab"
*
Mendel
was unaware that genes (which he called "unit
factors") 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
influenced 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.