Transcript Mendel`s genetics

Mendel’s genetics
Mendel and his
experiments
Gregor Mendel



Austrian monk
Mid 1850’s
experimented with
pea plants
Interested in various
forms found in
plant’s seeds,
flowers, and height
The garden pea
Why did he choose peas?





Easy to grow
They reproduce sexually (male and
female parts on same flower)
Petals tightly enclose reproductive parts
(so pollination can be controlled)
Seed to crop in a short time
Many characteristics that can be studied
(flower color, color of seeds, height, etc.)
Mendel’s experiment – step 1
Mendel carefully
selected a group of
pea plants that came
from a line that
produced only tall
pea plants.
Step 1 continued

Another group of pea
plants was selected
that came from a line
that produced only
short plants.
Step 2



Mendel cross-pollinated the pollen from a tall
pea plant onto the female part of a short plant.
This pollination would later produce pea seeds
that he planted and grew.
The tall plant and the short plant that were
crossed were called the P1 generation.
(P for parents)
Step 3

Mendel took the seeds produced from
the tall plant-short plant cross, planted
them, and grew them into mature pea
plants.
Results of first experiment



The seeds that grew from the tall-short
cross all grew into tall plants
No short plants were produced
These plants were called the F1
generation (F for filial, meaning
“offspring”)
Experiment 2




Mendel allowed the F1 generation plants
to self-pollinate.
He collected the seeds from this cross,
planted them (about 1000 seeds), and
grew them to maturity
These plants were known as the F2
generation
75% turned out tall, 25% turned out short
Ratio



The F2 generation had 3 tall plants for
every 1 short plant
Mathematically, this is known as a “ratio”
We say that the F2 generation had a
ratio of 3 tall to 1 short, written as 3:1
Other pea crosses


Mendel repeated his experiments using
6 other traits (illustrated on p. 262)
This type of cross is known as a
monohybrid cross, because only 1 trait
at a time is being studied
Mendel’s conclusions



Each plant has 2 “factors” that control each of
it’s traits (height, seed color, etc.)
One “factor” comes from one parent, one
“factor” comes from the other
The factor that disappears in a generation is
called recessive, and the factor that shows up
in each generation is called dominant
What Mendel didn’t know




Today we use the word gene instead of
factor
Genes are found on chromosomes
(humans have 46; 23 from each parent)
Each different form of a gene that can be
expressed is called an allele
Each copy of an organism’s alleles
comes from it’s parents
Alleles



We use letters to stand for each allele
In Mendel’s pea plants, we use T
(capital) for tall and t (lower-case) for
short
So Mendel’s P1 generation tall plants
would be TT, his P1 short plants would
be tt, and the hybrid F1 generation would
be Tt.
Mendel’s experiment (using
letters to stand for alleles)

P1
TT x tt
tall x short

F1
all Tt
all tall

F2
¼ TT, ½ Tt, ¼ tt
(3/4 tall, ¼ short)
3 tall:1 short
Other important terms







Heredity – the passing of characteristics from parents to
offspring
Genetics – the branch of biology that studies heredity
Traits – inherited characteristics
Gametes – sex cells
Hybrid – organism with 2 different forms of an allele
Pollination – transfer of male pollen to female part of plant
Fertilization – when male and female gametes unite
Monohybrid crosses


Heterozygous means both alleles are
different
ex: Tt, Bb, Rr
Homozygous means both alleles are
alike
ex: ee, HH, LL, or ff
phenotype



The physical expression of a gene
ex. Green peas, purple flowers, tall
plant, albino skin
Phenotype is always a description
Phenotype is always an adjective
genotype

Genotype is always a combination of
letters to describe an organism
ex: Pp, Rr, NN, mm
Monohybrid crosses

In peas, purple flowers are dominant to
white. Predict the potential offspring of a
cross between 2 heterozygous purpleflowered plants.
Monohybrid cross

In summer squash, white fruit color is
dominant to colored fruit. Predict the
offspring of a cross between a
homozygous white-fruited plant and a
heterozygous squash plant.
Monohybrid cross

In humans, colored skin is dominant to
skin with no pigment. Describe the cross
between 2 heterozygous people.