Mendelian Inheritance - 9
- Gregor Mendel
- credited with first discovering the mode of genetic inheritance
- Replaced the idea of
"___________ inheritance” with the idea of "_____________
inheritance”
- Mendel’s theories from the
1850s have been proven through our modern understanding of genes,
chromosomes, and DNA
Mendel’s
experiments
- Chose seven characters common in the garden pea
- For each character he developed
true-breeding lines for two
alternate traits
- He crossed plants which were
true-breeding for alternate traits of a character
- He then followed these plants
through two generations = monohybrid cross
Monohybrid Cross
- Mendel performed several
monohybrid crosses
- In each case he came up the the same general results, for example:
- Tall
plants X Short plants (= ___________)
- All
hybrid offspring were tall (= ___________)
- F1
individuals were crossed to generate the next generation (= ___________)
- 3/4 of
the F2 generation were tall, 1/4 were short
- Monohybrid
crosses result in a 3:1 ratio in the F2 generation
Definitions
- ____________ = alternative
forms of a gene.
- __________ _________ = an allele that can mask the
expression of other alleles.
- __________ _________ = an allele that has no effect
in the presence of a dominant allele.
- ____________ = an individual
possessing two copies of the same allele
- ____________ = an individual
possessing two different alleles for a specific gene
- ____________ = the physical
make-up of an organism
- ____________ = the genetic
make-up of an organism
Mendel’s Law of
Segregation
- Different alleles lead to
different traits.
- Diploid organisms possess two
copies of each gene - or two alleles.
- The ____________
of an individual determines the _____________.
- In a heterozygous individual,
the two different alleles segregate in equal proportions into the gametes.
- The probability of passing on
either allele to a single offspring is 50%.
- Mendel termed this mode of
inheritance “Particulate Inheritance” as
each parent contributed a particle to the offspring.
Punnett Square - used to determine outcome of a cross
Testcross
- Used to determine genotype of
an individual expressing a dominant phenotype
- Cross the individual in
question with a homozygous recessive individual
- Results will determine if the
test individual is homozygous dominant or heterozygous
- All
dominant phenotype = _____________
- 1:1 dominant:recessive = _____________
Dihybrid cross
- Used to follow two different
characters at one time
- Same setup as before - begin
with true-breeding lines
- homozygous
for each trait
- Resulted in Mendel’s “Law of
Independent Assortment”
- Each character in a dihybrid cross is assorted independently of the other
character
- The resulting 9:3:3:1 ratio in
the F2 generation is the result of independent 3:1 ratios for
each character
Extensions
of Mendelian Inheritance
Incomplete dominance
- Heterozygote has an
intermediate phenotype
Codominance
- Two or more alleles
are equally expressed when present in a heterozygous individual
Pleiotropy
- When a single
allele affects multiple traits
Epistasis
- When an allele of
one gene affects the expression of alleles at a different gene
Polygenic Inheritance
- Traits that are
determined by a two or more genes working in an additive fashion
Environmental effects
- Biotic and abiotic factors affect a genes expression
Penetrance
- Expression
of a trait that is affected by internal environments
Terminology: blending inheritance,
particulate inheritance, monohybrid cross, dihybrid
cross, character, trait, true-breeding trait, P generation, F1
generation, F2 generation, genotype, phenotype, allele, gene,
dominant allele, recessive allele, homozygous, heterozygous, independent
assortment, independent segregation, incomplete dominance, codominance,
pleiotropy, epistasis,
polygenic inheritance, penetrance