Download Fundamentals of Genetics Part I

Fundamentals of Genetics Part I
Gregor Mendel
• Tended garden in
monastery in Austria
and saw many plants
grow
• Later entered college
studying math and
science
• Is considered to be the
Father of Heredity:
transmission of
characteristics from
parents to offspring
Mendel’s Peas
• Observed characteristics (a heritable feature) of pea
plants
– Height, flower position, pod appearance, seed texture, seed
color (yellow, green), flower color (purple, white)
– Each characteristic occurred in two contrasting traits: a
genetically determined variant (variety) of a characteristic
• Used his knowledge of statistics to analyze observations of
characteristics
• Collected seeds from pea plants, recording characteristics
of plant from which each seed was collected
• The following year he planted these seeds
– Saw purple flowered plants grew from the seeds he collected
from purple flowered plants
– Also saw some white flowered plants grew from the seeds
from purple flowered plants
Mendel’s Methods
• Able to document traits of each
generation’s parent by controlling
how pea plants were pollinated
– Pollination: occurs when pollen
grains produced in the male
reproductive part (anther) of a
flower are transferred to the female
reproductive part of a flower
(stigma)
• Self-pollination: occurs when pollen is
transferred from the anthers of a
flower to the stigma of either the same
flower or a flower on the same plant
– Pea plants usually do this
• Cross-pollination: involves flowers of
two separate plants
• Self-pollination can be stopped
and cross-pollination performed
(remove anther and manually
transfer pollen to stigma of
another plant)
– By doing this Mendel was able to
protect his flowers from receiving
any other pollen via wind or
insects controlling the experiment
Mendel’s Experiments
• Grew plants “pure”, or true-breeding, for each trait
– Pure: always produce offspring with that trait
– Ex. Pea plants pure for trait of purple flowers self-pollinate to produce offspring with all
purple flowers
• Mendel cross-pollinated pairs of plants that were true-breeding for contrasting
traits of a single characteristic.
– He called the true-breeding parents the P generation.
– Transferred pollen from the anthers of one plant to the stigma of another plant.
• For example, to cross a plant that was true-breeding for the trait of yellow pods with one that was
true-breeding for the trait of green pods:
–
–
1. remove the anthers from the plant that produced green pod
2. Dust the pollen from a yellow-podded plant onto the stigma of a green-podded plant and allowed the seeds to develop.
• When the plants matured, Mendel recorded the number of each type of
offspring produced by each cross.
– He called the offspring of the P generation the first filial generation, or F1 generation.
• He then allowed the flowers from the F1 generation to self-pollinate and
collected the seeds, and grew plants from the seeds.
– Mendel called the plants in this generation the second filial generation, or F2 generation.
Following this process, Mendel performed hundreds of crosses and documented the
results of each by counting and recording the observed traits of every cross.
Mendel’s Experiment in Picture
Mendel’s Results and Conclusions
• In one experiment Mendel crossed a plant pure for
green pods with a plant pure for yellow pods
– F1 generation was all green
• Next, Mendel allowed F1 to self-pollinate and
planted the resulting seeds
– F2 generation plants grew and ¾ were green, ¼ yellow
pods 3:1
• He hypothesized that something within the pea
plants controlled the characteristics he observed
(called these controls, “factors”)
• He reasoned that there must be a pair of factors
controlling each trait (not just one factor)
We Now Know
• Mendel’s “factors” = genes
• What do genes do?!
• There are a pair of factors controlling each trait because peas (like
humans!) are diploid.
• They have TWO of each type of chromosome. Each chromosome carries
one copy of the gene in question. SO: The pea plant has TWO genes for
flower (or pod) color – one on each homologous chromosome. The genes
may be the same, or different. In the example below, they are different.
– We call the alternate versions of the genes alleles.
Dominant and Recessive Trait
• Whenever Mendel crossed plants that were pure for a
characteristic, one of the P traits failed to appear in the
F1 plants but then reappeared in F2 generation.
– The F2 generation always had a 3:1 ratio.
• Concluded that one factor in a pair may prevent the
other from having effect
• Hypothesized that trait appearing in F1 generation was
controlled by dominant factor because it masked
(dominated) the other factor for specific trait.
• Trait not appearing in F1 generation but reappeared in
F2 was controlled by recessive factor
Law of Segregation
• Mendel concluded that paired factors separate
during formation of reproductive cells. What
process forms reproductive cells?!
– Each gamete receives only one factor of each pair
– When two gametes combine to form the offspring
they then have two factors controlling specific trait
• Law of Segregation : a pair of factors is
segregated, or separated, during the formation
of gametes
Law of Independent Assortment
• Mendel crossed plants that differed in two
characteristics (ex. Flower color and seed
color)
– proved that traits produced by dominant factors
do not necessarily appear together
– Factors for different characteristics are not
connected
• Law of Independent Assortment: factors for
different characteristics are distributed to
gametes independently