Mendelian Genetics:
Laws of Dominance, Segregation & Independent
Assortment
In Principle:
Systematic study of genetic 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 as to influence characters (traits) IMPORTANT: A gene "for" a
phenotypic trait is almost always 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)
Suppose character"PTC sensitivity" influenced by a gene with
two alleles,
one for "taster"
and one for "non-taster"
Ex.:
Pea seeds have alternative
phenotypes green / yellow, or round / wrinkled Mendel's Law of Dominance
Some alleles *mask" the
phenotypic expression of other alleles in in heterozygous combination
Call the former dominant,
the latter recessive (IG1 Research
Briefing 15.1, pp. 292-293)
That is, heterozygotephenotype is identical
to that of one
of the homozygotes Call allele in
that homozygote "dominant",
call other "recessive"
Dominant alleles
symbolized with capital letters
(A) Recessive alleles
with lower-case letters (a) Genotype
described by giving both alleles: AA or Aa or
aa Phenotype 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 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 WW 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's Law of Segregation
Mendel showed experimentally: Alleles separate (segregate)
during the formation of gametes
(eggs & sperm)
in meiosis half carry
one allele,half carry the other
[Mendel did not know
about chromosomes, meiosis / mitosis, or DNA] Random
union of gametes produces zygotes
that develop into new individuals. Zygotic genotypes occur in characteristic ratios, according to parental
genotypes Ex.: a monohybrid cross between two
heterozygotes ( Aa x Aa )
produces
expected
genotypic ratio of 1 : 2 : 1
among AA, Aa, & aa genotypes. The genotypic ratios produce
characteristic phenotypic ratios,
according to dominance
relationship of alleles involved. Ex.: if A
dominant to a, cross between heterozygotes produces
expected phenotypic ratio
of 3 : 1
between "A"
and "a" phenotypes.
Mendel's Law of Independent
Assortment
Alleles
at separate loci
inherited independently
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: What genotypic & phenotypic ratios result
for a cross AAbb x aaBB ? AABB x aabb ?
Calculate the genotypic & phenotypic ratios
for a trihybrid
cross (AaBbDd
x AaBbDd)