Transcript 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???
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)
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
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)
11.3 Beyond Dominant and Recessive Alleles
• Genes can act in various ways
1. Dominant vs. Recessive – one allele
completely masks another allele.
2. Incomplete Dominance
1. Definition : one allele is not completely
dominant over another
Ex: flowers – white x red flowers = pink
flowers
11.3 Beyond Dominant and Recessive
• Codominance
– Definition: both alleles contribute to the
phenotype of the organism
Ex: chicken feather – black and white alleles =
black and white feathers
Colors don’t blend like incomplete dominance
11.3 Multiple Alleles
• Definition: more than two alleles
• (more than 2 alleles exist in a population not
an individual) Blood Types
Ex: rabbit’s coat color
Ex: human’s blood type
11.3 Polygenic Traits
• Definition: traits that are controlled by 2 or
more genes
Ex: fruit fly red eyes - 3 genes involved in making
pigment
– Diff. combo of genes produce different eye colors
Ex: Human skin color – more than 4 different genes
Human height – more than 50 genes
11.3 Genetics and the Environment
• Genes provide a plan for development, but
how the plan unfolds also depends on the
environment:
• Ex. Butterflies have different wing colors
depending on when they hatch
• Hydrangea flowers are different colors
depending on soil pH.
Mendel Questions
Using tall and short pea plants and the letters
T = dominant and t = recessive describe
Mendel’s F1 generation and F2 generation.
Provide the genotype and phenotype for each
of four offspring from each generation.
You do not need to use a punnett square.
Meiosis
Ttrr x ttRr
ttRr x TtRr
Questions
1. Genetics is the study of ________.
2. Alternate characteristics of an organism
such as height, hair color, eye color, etc.
are called _______.
3. Why did pea plants make such a good
subject for Mendel to study?
4. What does “true breeding” mean?
Considering the allele related to
short and tall pea plants.
1. Plants of the P generation (tall) will
produce only_____________ if not
crossed with plants having other alleles.
2. Describe the phenotype (what we can see)
of Mendel’s F1 Generation.
3. What was the genotype (use letters “T”
and “t”) for Mendel’s F1 generation?
4. How did the F2 generation suggest that
genes were simply segregated, not lost?