Download 18.1

Chapter 18
Section 18.1
Gregor Mendel – Pioneer of Genetics
Gregor Mendel
• Gregor Mendel (1822-1884) was an Austrian
monk whose experiments with garden peas laid the
foundation for the science of genetics.
• He tracked and recorded the transmission of seven
visible genetic traits through seven generations of
garden peas.
• The reason that Mendel chose to work with garden
peas was because most characteristics of the garden
pea have only two alternative forms. (Ex. Round or
Wrinkled seeds)
• Mendel used cross-pollination to breed his
garden peas.
• He would remove pollen (sperm cells) from the
male part of one plant and place it on the female
part of a second plant.
• The pollen grains then fertilize the female eggs,
resulting in a new organism called the progeny
or the offspring.
• To refer to his seven generations of peas he used
the following system:
▫
▫
▫
▫
First cross = Parental Generation (P)
Offspring of first cross = First Filial Generation (F1)
Offspring of second cross = Second Filial Generation (F2)
F3, F4, F5, F6, and F7
The 7 Traits
•
•
•
•
•
•
•
Flower colour
Flower position
Seed colour
Seed shape
Pod colour
Pod shape
Stem length
Some Genetics Terms
• Progeny – new individuals that result from
reproduction; offspring.
• Dominant trait – a characteristic that is
expressed when one or both alleles in an
individual are present.
• Recessive trait – a characteristic that is
expressed only when both alleles in an
individual are present.
• Alleles – the alternative forms of genes.
▫ Ex. Purple flowers or White flowers
▫ Each of these is an allele for the flower colour
gene.
• Homozygous – having identical alleles for the
same gene.
▫ Ex. P = purple flowers, p = white flowers.
▫ Homozygous would be PP, or pp.
• Heterozygous – having different alleles for the
same gene.
▫ Heterozygous would be Pp.
• Genotype – the genetic code of an organism.
▫ Ex. PP, pp, Pp
▫ This is the genetic code for flower colour.
• Phenotype – the observable characteristics of
an organism.
▫ The phenotype is the colour itself, purple or white.
Mendel’s Experiments
• Mendel discovered 2 important concepts about
genetics through his experiments:
1.The Principle of Dominance
2.Mendel’s Principle of Segregation
The Principle of Dominance
• When Mendel used pollen from a pea plant with
round seeds to fertilize a pea plant with wrinkled
seeds, he found that all the offspring in the F1
generation had round seeds.
• He tested this many times and always got the
same results.
• This trend suggested to him that the allele for
round seeds, masks the allele for wrinkled seeds.
• He referred to this idea as the principle of
dominance because the round seeds were always
dominant over the wrinkled seeds.
• From then on the round seeds were known as
the dominant trait and the wrinkled seeds
were known as recessive trait.
• The allele for dominant traits is always indicated
by an uppercase letter (R for round seeds).
• The allele for recessive traits is designated by a
lowercase letter (r for wrinkled seeds).
Mendel’s
Traits
Mendel
eventually
discovered the
dominant and
recessive traits
for each of his 7
characteristics.
Mendel’s Principle of Segregation
• Mendel next let the F1 plants self-fertilize to see
what would happen in the F2 generation.
• Mendel was expecting the same result, that 100% of
the plants would have round seeds.
• He was astonished to find that 25% of the F2
generation had wrinkled seeds instead of round.
• He again repeated this several times and found that
the F2 generation always consisted of 75% round
and 25 % wrinkled, a 3:1 ratio.
• To explain these ratios, Mendel reasoned that
each plant must carry two copies (alleles) of
each gene, that can be the same or different.
• An individual with round seeds can have two RR
alleles or one of each because it is dominant, Rr.
• An individual with wrinkled seeds can only have
two rr alleles.
• RR and rr = homozygous
• Rr = heterozygous
• He also reasoned that the alleles must separate
during meiosis, one allele going to one sex cell
and the other allele going to the other sex cell.
• He called this segregation – the separation of
alleles during meiosis.
• Only one of these alleles from a parent can get
passed to one individual offspring.
• The offspring will receive two alleles, one from
‘mom’ and one from ‘dad’.
Segregation of P Generation
Round Seed
Wrinkled Seed
RR
rr
R
Crossing a pure round
seeded plant with a
pure wrinkled seeded
plant will produce a
round seeded plant
100% of the time.
R
r
Cross Pollination Occurs
Rr
Round Seed
r
Only one round
allele is needed
because round is
dominant over
wrinkled.
Segregation of F1 Generation
Round seeds
can appear
when
homozygous
or
heterozygous.
Rr
R
Rr
r
R
RR
Rr
Rr
Round
Round
Round
Wrinkled
seeds only
appear when
homozygous.
r
rr
Wrinkled
18.1 textbook questions
• Pg. 600 #1-5