Laws of Dominance, Segregation & Independent
Systematic study of the laws of inheritance requires analysis of crosses
These were first undertaken by Gregor
Mendel in the 1860's [IG1 ResBrief 15.1, pp.
Mendel worked with seven
phenotypic characters in Garden Peas(Pisum sativum) [HOMEWORK]
Mendel arranged controlled crosses, analyzed the results
correctly inferred the rules governing
Rediscovery of Mendel's Laws in 1900 signalled
the start of modern genetics
Phenotype (outward appearance)
is influenced by genotype
(hereditary makeup) or, individual characters are influenced by
or, individual genes are expressed in such a way that they
BTW: Genes are made of DNA
located in chromosomes,
each at a particular physical location (a locus: plural, loci)
Genes are typically expressed as proteins Alternative
Genes exist in numerous allelic variants
Any one (diploid)
individual possesses two alleles
for each gene.
with two identical alleles
is a homozygote
and is described as homozygous;
with two dissimilar alleles
and is described as heterozygous.
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 (called dominant) mask the phenotypic expression of other alleles
[Mendel's Law of Dominance] (IG1
Research Briefing 15.1, pp. 292-293)
That is, the phenotype of the
resembles that of the dominant
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 are described by
the letter of the expressed allele: "A" or "a"
Ex.: the "taster"
allele (T) is dominant to the "non-taster"
allele (t) [HOMEWORK]:
Homozygous TT or
heterozygous Tt individuals show the "T"
phenotype ("taster"): only the homozygous tt individual shows the "t"
phenotype ("non-taster") Ex.:
allele (Y) masks the "green"
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-
& wrinkled peas are ggww]
Do not confuse inheritance
of a genotype and expression of a phenotype
Mendel showed experimentally: Alleles separate (segregate)
during the formation of gametes
(eggs & sperm)
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 )
an expected genotypic ratio
of 1 : 2
: 1 among AA, Aa, & aa
genotypes. The genotypic ratios produce
characteristic phenotypic ratios,
the dominance relationship
of the alleles involved. Ex.: if A is
dominant to a, the cross between heterozygotes produces
expected phenotypic ratio
of 3 : 1
and "a" phenotypes.
at separate loci are
inherited independently[Mendel's Law of
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) ]