Download 11-1 The Work of Mendel

11-1 The Work of Mendel
• What does every living thing inherit from
their parents?
• Genetics – the study of heredity
Look around at your classmates and make a
list of some of the traits that are inherited.
What did Mendel already know…
• Each flower produces pollen (sperm) and
egg cells
• Cross fertilization (sexual) – male and
female cells join
• Self-pollination (asexual) – pollen fertilizes
eggs from same plant
• Mendel’s pea plants were true-breeding
– A tall plant with green seeds would produce a
tall plant with green seeds
Genes, Alleles and Dominance
• Trait – specific characteristic
– Mendel studied 7 different pea plant traits
• What are some examples? Plant Height, Seed
Shape, Pod Color
Genes, Alleles and Dominance
• Genes – the chemical factors that determine traits
(the segment of DNA) ex. pea plant: height
• Alleles – different forms of a gene from each
parent ex. pea plants: tall and short (T or t)
Principle of Dominance
• Principle of dominance: some alleles are
dominant and others are recessive
• Dominant allele – the trait is always shown
– Capital letter (T = tall)
• Recessive allele – the trait that will only
show if there is no dominant allele
– Lower case letter (t = short)
• Heterozygous – organisms that have 2
different alleles for the same trait
– Hybrid for that trait
– Ex: Tt
• Homozygous – organisms that have 2
identical alleles for a particular trait
• True-breeding for a particular trait
– Ex: TT or tt
Genes, Alleles and Dominance
• Phenotype – physical characteristics
– Ex: tall, short, yellow, green
• Genotype – genetic make-up
– Ex: TT, Tt, tt
• Tall plants have the same phenotype (tall), but
not the same genotype (TT or Tt)
• Why are TT and Tt genotypes for tallness, but
tt is not???
Mendel crossed plants to study offspring
• P generation – (parents, originals) – Tall x short
• F1 generation – (offspring of P) tall x tall
• F2 generation – (offspring of F1) 3 tall, 1 short
11-2 Probability and Punnett
Squares
•Mendel realized…the principles of
probability could be used to explain the
results of genetic crosses.
Genetics and Probability
• Probability – the likelihood a particular event
will occur.
– Ex: probability of flipping a coin to heads = ½ or
50%
– Probability of head 3 times in a row = ½ x ½ x ½ =
1/8
– The greater the number or trials, the closer to the
expected ratio
– Past outcomes do not affect future outcomes
• Alleles segregate randomly (like a coin)
Probabilities Predict Averages
• To get an accurate prediction of flipping a
coin – the coin should be flipped many
times and an average taken.
• In genetics …the more offspring you get,
the closer to the predicted ratio.
Principle of Segregation
• Segregation =
separation
• The alleles for tall
vs. short separate
during the
formation of
gametes – sex cells
• Each gamete
carries one allele
for each gene
Principle of Independent Assortment
• Mendel questioned how alleles segregate
• Does it happen independently?
Ex: Does the seed shape gene influence the seed
color gene????
INDEPENDENT ASSORTMENT
• Need to follow 2 diff. alleles from one generation
to the next.
The Two-factor Cross: F1
• 4 alleles (2 genes) at the same time
• Provides the hybrid plants (F2 generation)
• Crossed a homozygous RRYY (round yellow peas)
with a homozygous rryy (wrinkled, green peas)
The two
factor cross:
F2
• F1 – yields all offspring
heterozygous for both
traits
• F2 – heterozygous parents
crossed
• F2 – yields 9:3:3:1
ratio
Mendel discovered the principle of
• Independent assortment
– genes segregate
independently during
the formation of the
gametes
– Helps account for many
genetic variations in
organisms
– Seed shape & color
gene do not influence
each other
Meiosis
Ttrr x ttRr
ttRr x TtRr