Transcript 2 Intro to Mendelian Genetics 2013

Introduction to Mendelian Genetics
Some History
?
• For thousands of years farmers and herders have been
selectively breeding their plants and animals to produce
more useful hybrids
• It was somewhat of a hit or miss process since the
actual mechanisms governing inheritance were unknown
• Knowledge of these genetic mechanisms finally came
as a result of careful laboratory breeding experiments
carried out over the last 150 years
• Started with the studies of an Austrian monk = Gregor
Mendel
Who was Gregor Mendel?
• He is known as the
“FATHER OF GENETICS”
• He discovered how traits were
inherited – the 1st scientist to
obtain successful results from
inheritance studies due to his
methodology
• Lived: 1822 – 1884
• His work was “unimportant”
until early 1900’s
GENETICS – study of heredity
HEREDITY – the passing of traits from parents to
offspring
Mendel’s Home & Garden
• Mendel did his
study on 29,000
pea plants over 14
years
• Pea plants have
many easily
observed traits
(tall/short, purple
flowers/white
flowers)=phenotypes
• Pea plants can be
self-fertilized or
cross-fertilized
(cross pollinated)
Mendel’s Peas
Cross Pollination
(1) The pollen contains the male gamete and can be “picked up” by a
fine brush
(2) The carpel is the female reproductive structure in a flower and
contains the ovary where fertilization of the female gamete occurs
(3) Fertilization occurs after pollen has landed (or placed) upon the
carpel
Genetic Terminology
1.True-Breeding – (PURE BRED) these
organisms ALWAYS create offspring that
look like themselves (same phenotypes or
traits)
2.Hybrids – offspring from different truebreeding organisms
Tall purebred x Short prurebred = Hybrid
Ideas about traits during Mendel’s time
Most scientists thought that traits blended in
offspring to produce “middle ground” offspring
Parent
Offspring
Parent
Mendel’s Observations & Ideas
While observing many generations of pea plants,
Mendel found that sometimes traits seemed to
disappear for a generation and then reappear in later
generations.
He determined:
 Some factor MUST be passed from generation
to generation for this to occur.
(We now know these as GENES)
 Those traits that disappeared were recessive to
other traits that were dominant.
(GENES can be in different forms = ALLELES).
Genes and Alleles
 Genes – factors that determine your traits
 Mendel determined that each trait is
controlled by two factors (alleles)
 One from male, one from female – they
can be different)
Mendel’s Experiments
Pure Bred
Parents = P
Hybrid
Hybrid
First generation
offspring = F1
Observable trait = phenotype
Second generation
offspring = F2
Genetic makeup = genotype
Genotype is represented by capital and lower case letters
Explaining the Cross Results
When a parent makes sperm or eggs, their
genes separate randomly
(PRINCIPLE OF SEGREGATION)
The GAMETES (egg or sperm) contain either
a T allele (tall) or a t allele (short)
Three important conclusions
to Mendel’s research
1. Principle of Dominance and Recessiveness
One allele in a pair may mask the effect of the other
2. Principle of Segregation
The two alleles for a characteristic separate during the
formation of eggs and sperm
3. Principle of Independent Assortment
The alleles for different characteristics are distributed to
reproductive cells independently