Download Mendel - the father of modern genetics

Mendel - the father of modern genetics
Born 1822 in Czechoslovakia
Became a monk and resumed interest in plant breeding at his
monastery
Used garden peas as his experimental tool
Read his paper at a meeting in 1865, it was published in 1866, but
was not cited until 1900, 16 years after his death
Why Mendel succeeded when previous plant/animal breeders
failed:
1) He studied one character at a time and each character had simple
visual alternatives.
The 7 characters he described are:
1) pea shape; round versus wrinkled
2) internal color; yellow to orange versus green
3. seed-coat & flower color; both white, versus gray seeds and
purple flowers
4. pod shape; inflated versus constricted
5. pod color; green versus yellow
6. flower position; axial versus terminal
7. Plant height; tall (6-7 ft versus dwarf 9-18 inches
2) He kept pedigrees and made several generations of crosses
3) He kept track of the numbers of each type of progeny from each cross
Mendels experimental procedure using Garden Peas
1) Establish that parents are truebreeding (purebreeding) by self
pollination and verify that all progeny resemble the parent
Pea flowers have both a pistil connected to the ovary (female) and
stamens with anthers where pollen (male) is formed; self pollination
can be guaranteed by "covering" a flower. You can see a picture of
the pea flower at:
http://netspace.students.brown.edu/MendelWeb/MWflower.html
Purebreeding cultivars are often referred to as "lines"
2) Make reciprocal crosses between parents that differ in the same
character (Monohybrid Crosses)
P1: Tall female X dwarf male
or - the reciprocal cross:
P1 Dwarf Female X tall male
In either case all the progeny (F1) were tall !!
3) When the F1s were allowed to self pollinate, Mendel planted the
resulting peas and found that in the F2 generation:
787 gave tall plants : 277 gave dwarf plants, which he summarized as
3 tall : 1 dwarf (3/4 tall : 1/4 dwarf)
This meant the dwarf "factor" as he called it was still present in the F1,
but was masked by the tall character.
He concluded that Tall is dominant to Dwarf, which is recessive, and
that each pea has two copies of the "factors" for each trait (peas are
"diploid" meaning they have two sets of all genes)
Mendel used a single letter to designate each trait; in this case T for
the tall character and t for the dwarf.
** note that Mendel used the same letter (T or t) , and not T vs d for the
different forms of one trait. You must try to do the same or you will
confuse yourself and me too!
We now use the term genes rather than factor, and different forms of a
gene are called alleles; thus T and t are alleles of a plant-height gene
If the F1 had 2 copies then the parents should also have 2 copies
each; Rewriting the crosses gives:
TT (tall) female X tt (dwarf) male ⇒ all Tt (tall)
The appearance is the phenotype, the genetic composition TT, Tt, or tt
is the genotype; TT and tt are "Homozygous" Tt is "heterozygous"
Why does selfing an F1 (Tt) plant result in a 3:1 phenotypic ratio in the
F2 generation?
Both the eggs and the pollen, which are formed by
meosis, only have 1 copy of each gene (gametes are
haploid) Thus 1/2 of the eggs and pollen of a
heterozygous parent will be T and 1/2 will be t; when
combined by chance the F2 phenotypic ratio will be:
3/4 tall : 1/4dwarf, as can be seen from the "Punnett Square"
(shown below)
The diagram also shows why the F2 genotypic ratio will be
1/4 TT : 2/4 Tt :1/4 tt
Eggs
1/2
T
1/2
1/2
T
t
1/4 TT
tall
1/4 Tt
tall
Pollen
1/2
t
1/4 Tt
tall
1/4 tt
dwarf
This type of inheritance is referred to as simple dominance, meaning
the trait is controlled by one gene, and one allele is dominant over the
other
Mendel verified that the F2 population did indeed include 1 in
four plants (TT) that produced only tall progeny when self
fertilized, that 1/2 of them were heterozygous, again
producing 3 Tall: 1 dwarf progeny after selfing, and that the
dwarf tt plants were also true breeding.
A legend for this "simple dominance" type of inheritance is:
T_ Tall
tt dwarf
(the blank means either T or t may be present)