Mendelian
genetics is also referred to as classical genetics. Gregor Johann Mendel
(1822-1884; check this link out: Mendel
Web) was an Austrian monk/scientist. He was a teacher and aspired to
become a certified teacher, but failed. A contemporary of Charles Darwin.
Mendel was relatively unknown and unrecognized for his work until early
in this century.
- Mendel's studies on garden peas
Mendel didn't start his breeding experiments with
peas, but with animals. The abbot of the monastery preferred he not use
animals, so Mendel went with peas. (This illustrates the interesting yet
erroneous idea that plants are not sexual.) It was a fortuitous switch!
- Characteristics studied by Mendel
Mendel studied 7 characteristics: seed form, color,
flower position, color, etc.. (I don't suspect you'll need to know them
all.) It ended up that all 7 characteristics demonstrated the principles
he was to point out.
- Why garden peas?
Availability
- True breeding varieties
Mendel could obtain the varieties he would need to
test the interbreeding of plants with different characteristics.
- Self-fertile flower
Mendel could control fertilization because the sperm
from one flower could fuse with the egg from the same flower and produce
offspring. Mendel could control the crosses by removing the male part of
the flower, then transporting male gametes from another flower to the emasculated
flower.
- Short generation time
It didn't take as long as it might have to get results.
(Crops were produced rather quickly.)
- Difficulties overcome by Mendel
- No concept of DNA or chromosomes
Mendel had no scientific idea to build upon.
- No concept of meiosis
Same as above regarding the formation of sex cells.
- Some wise decisions made by Mendel
- Select well-defined traits
The traits he selected were easy to quantify.
- Extensive groundwork completed
prior to the establishment of final experimental design
It must have taken extensive preparation to be able
to come up with his ground-breaking work.
- Extensive replication crosses
Mendel counted a lot of characteristics from a lot
of pea plants.
- Understanding phenotype and genotype
Phenotype is what's expressed; what you see: blonde
hair, brown eyes. Genotype is the complete genetic makeup: it includes
both what is expressed AND what is not expressed (not manifest). Example:
Dr. St. Clair and his wife are both blonde (phenotype) but they have a
couple redheaded kids. This is because they both had readhead DNA in their
genotype that was passed down, becoming evident in the kids' phenotype.
- Results of Mendel's work
- The principle of unit characters
(elementum or genes)
What do we call the heritable unit of genetic information
passed from parents to offspring? genes. Mendel came up with the concept
and referred to such units as 'elementum.' (Be good we'll see you on Wednesday.)
- The phenomenon of dominant and
recessive alleles
Dominant alleles take precedence over recessive alleles
when it comes to gene expression. The dominant allele (usually demonstrated
with a capital letter) masks the effect of the recessive allele (lower
case letter).
- Example: Cross a homozygous recessive
dwarf (tt) and homozygous dominant tall (TT) results in all tall offspring
(Tt)
The dominant allele (T) was expressed in the phenotype
of all offspring, while the recessive allele, though part of the genotype,
was not expressed in the phenotype. Could a recessive allele (t) be expressed
in the phenotype of the offspring from two dominant-expressing parents?
What would have to be their genotype? (both heterozygous).
- The principle of segregation: paired
alleles separate from one another during the formation of sex cells (meiosis)
 13.1-3
Genetic Crosses
(During anaphase I of meiosis,) paired alleles (example: TT) on homologouse
chromosomes separate: the T's separate: each T allele goes to a different
gamete (meiosis). So if you had a heterozygous parent (Tt), half of their
gametes would have T and the other half t.
- tt x TT
All offspring: Tt (heterozygous)
- F1 generation: Tt x Tt
3 possible gentypes: (TT, Tt, tt). 2 possible phenotypes:
tall or dwarf.
- Result (F2): Tall (TT, Tt) and
dwarf (tt) individuals
Keep in mind that Mendel knew nothing of meiosis:
so how did he arrive at the principle of segregation? When he looked at
the consistent 3:1 phenotypic ratio in the F1 generation, he reasoned that
the only way it was possible would be if the 'elementum' were organized
and passed down in this way, with paired alleles separating.
- The principle of independent assortment:
paired alleles on DIFFERENT homologous chromosomes separate independently.
Demonstrated in a dihybrid cross (two traits).
- ttrr x TTRR (homozygous parents)
= (all) TtRr
Cross homozygous tall, red-flowered plants with homozygous
dwarf, white-flowered plants: you get, according the law of segregation,
all heterozygous offspring: all tall, red-flowered.
- F1: TtRr x TtRr (heterozygous parents)
=
According to the law of segregation you'll get a
'T' or a 't', an 'R' or an 'r'. The law of INDEPENDENT ASSORTMENT states
that either of the T (T, t) alleles (from one homologous chromosome) will
end up with either of the R (R,r) alleles (from a different homologous
chromosome). Whether you get a 't' or a 'T' has nothing to do with whether
you'll get an 'r' or an 'R': they sort independently. Mendel was able to
reach this conclusion from the following results: (the phenotypic ratio
9:3:3:1) Note the unique combinations of alleles: the possibilities of
how they could be assorted.
- Phenotypic ratio of offspring:
(TTRR, TtRr) = 9
(TTrr, Ttrr) = 3
(ttRR, ttRr) = 3
(ttrr) = 1
-
- The principles of segregation and
independent assortment in light of meiosis
With the luxury of an understanding of meiosis, which
Mendel did not have, we see HOW these principles work within cells: can
you determine at what phase(s) in meiosis these principles come in to effect?
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