Mendelian Genetics:
Laws of Dominance, Segregation & Independent
Assortment
In Principle:
Systematic study of inheritance requires analysis of crosses
These were first undertaken by Gregor
Mendel in 1860's
Mendel worked with characters in Garden
Peas(Pisum
sativum) [HOMEWORK]
Mendel arranged controlled crosses, analyzed results numerically,
inferred the laws governing
their outcomes
Rediscovery of Mendel's Laws in 1900 signaled start of modern genetics
Genetics was a scientific discipline for 50
years before DNA
Phenotype (external appearance) is influenced
by genotype (hereditary
makeup) or, individual characters are influenced by
particular genes
or, individual genes are expressed in such a way that they
influence characters (traits) IMPORTANT: A gene "for" a
trait is usually an oversimplification BTW: Genes are made of DNA
located in chromosomes,
at a particular physical location (a locus: plural, loci)
Genes are often [but not always] expressed as proteins Molecular phenotype: a gene "for"
an enzyme
Alternative
forms
of
genes are
called alleles;
Most genes exist in multiple allelic variants
Any diploid
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.
Ex.:
some people can taste the chemical phenylthiocarbimide (PTC)
The character"PTC sensitivity"
may be due to a gene with two alleles,
one for "taster"
and one for "non-taster"
Ex.:
Pea seeds have alternative
phenotypes green / yellow, or round / wrinkled Some alleles mask the
phenotypic expression of other alleles
We call the former dominant,
and the latter recessive
[Mendel's Law of Dominance] (IG1
Research Briefing 15.1, pp. 292-293)
That is, the phenotype of the
heterozygote
resembles that of the dominant
homozygote
Dominant alleles are
symbolized with capital letters
(A) Recessive alleles
with lower-case letters (a) Genotypes
are described by giving both alleles: AA or Aa or
aa Phenotypes can be described by
the letter of the expressed allele: "A" or "a"
Ex.: the "taster"
allele (T) is dominant to the "non-taster"
allele (t) :
Individuals that are either homozygous
TT or heterozygous Tt express
the "T" phenotype ("taster"): only the homozygous tt individual express the "t"
phenotype ("non-taster")
Or, TT
homozygotes and Tt heterozygotes show the taster
phenotype, tt homozygotes are non-tasters
Ex.:
the "yellow"
allele (Y) masks the "green"
allele (y)
the "round"
allele (R) masks the "wrinkled"
allele (r) Yy and Rr peas are yellow and round, respectively yy and rr peas are green and wrinkled, respectively
[Alternatively, yellow peas
are GG or Gg, round peas are W-
or Ww
and green
& wrinkled peas are gg ww]
Do not confuse inheritance
of a genotype and expression of a phenotype Dominance is a relationship between alleles,
not between phenotypes Yellow does
not dominate Green
Mendel showed experimentally: Alleles separate (segregate)
during the formation of gametes
(eggs & sperm)
in meiosis
half carry one allele & half carry the other [Mendel's Law of Segregation].
[Mendel did not know
about chromosomes, meiosis / mitosis, or DNA] The random union of gametes produces zygotes that develop into new
individuals. The zygotic genotypes occur in characteristic ratios, according to the parental
genotypes Ex.: a monohybrid cross between two
heterozygotes ( Aa x Aa )
produces
an expected genotypic ratio
of 1 : 2
: 1 among AA, Aa, & aa
genotypes. The genotypic ratios produce
characteristic phenotypic ratios,
according
to
the dominance relationship
of the alleles involved. Ex.: if A is
dominant to a, the cross between heterozygotes produces
an
expected phenotypic ratio
of 3 : 1
between "A"
and "a" phenotypes.
Alleles
at separate loci are
inherited independently[Mendel's Law of
Independent Assortment]
This produces characteristic genotypic and
phenotypic ratios. Ex.: a dihybrid cross between two
"double heterozygotes" ( AaBb x AaBb ) produces genotypic ratios of 1 : 2 : 1 : 2 : 4 : 2 : 1 : 2
: 1 for genotypes AABB
AABb AAbb AaBB AaBb Aabb
aaBB aaBb aabb
and therefore phenotypic ratios
of 9
"AB" : 3 "Ab" : 3 "aB" : 1 "ab"
[ Homework: calculate the
genotypic & phenotypic ratios for a trihybrid cross (AaBbDd x AaBbDd) ]