Transcript Introduction to Genetics

Introduction to Genetics
Or, the things that made you who you
are!
What is Genetics?
The study of heredity.
Heredity is the passing of traits from parents
to offspring.
How was Genetics “discovered?”
Gregor Mendel
• Austrian Augustinian Monk
• Grew up on a farm
• Studied bee keeping and gardening while in the
monastery
Known as the “Father of Genetics”
• Discovered how traits were inherited
• Used pea plants
Mendel and His Peas
While working with the pea plants in his
garden, he noticed that certain traits would
“disappear” and return from generation to
generation.
Mendel and His Peas (con’t.)
Mendel also noticed
that he could get
certain traits to breed
true
• True-breeding plants
always create plants
that look like
themselves
• Mendel used 6 truebreeding traits
Mendel and His Peas (con’t.)
Mendel liked working
with pea plants
because the plants
could be self-fertilized
(same plant pollinating
itself) or crossfertilized (one plant
pollinating another
plant)
Mendel and His Peas (con’t.)
Mendel would cross-fertilize a true breeding
plant of a certain trait, like height, with
another true breed plant of the opposite trait
• He would breed a tall plant with a short plant
• This created hybrids: offspring of two different
true breeding plants of the same trait
Mendel and His Peas (con’t.)
Mendel then cross the
offspring and noticed
that traits disappeared
• For instance, when he
crossed a true breeding
purple flowered plant
with a true breeding
white flowered plant,
the offspring were all
purple flowered…he
expected some to be
white and he didn’t see
this.
Mendel and His Peas (con’t.)
Mendel performed this
with all six true
breeding traits he
found and always got
the same results, one
of the traits
disappeared
Mendel and His Peas (con’t)
Mendel then decided
to self-fertilize the
offspring (hybrids)
The white colored
flowered reappeared!
This is what
happened!
Mendel and His Peas (con’t.)
Mendel performed this same experiment
with all six true breeding traits and saw
similar results: for example
Mendel and His Laws
From the pea plant experiments, Mendel concluded that
• Rule of Unit Factors: Each organism has two factors that control
each trait
• Rule of Dominance: Recall what happened when a true breeding
purple plant was crossed with a true breeding white plant, and the
white flower color “disappeared” in those offspring. However, the
white reappeared when the offspring self-pollinated. Mendel
called the purple color dominant and the white color recessive
• Law of Segregation: Every organism has two alleles of each gene
and when gametes are produced, each gamete receives only one of
these alleles.
Phenotype vs. Genotype
Genotype is the make up of alleles that
create the trait (gene)
• TT vs. Tt (tall); tt (short)
• PP vs Pp (purple flower); pp (white flower)
• RR vs Rr (round pea); rr (wrinkled pea)
Phenotype is how an organism looks
• Tall or short, purple or white flower, round or
wrinkled pea
Punnett Squares
In 1905, Reginald Punnett, an English
biologist devised a method of finding
possible genotypic outcomes in crosses.
Punnett Squares: Monohybrid Cross
Monohybrid crosses involve only one trait
•
•
•
•
In this case it is height
T: tall (dominant)
t: short (recessive)
Female tt
• Across the top
• Male Tt
• Along the side
Punnett Squares: Monohybrid Crosses
(con’t.)
1)Separate alleles
2) Filling in the top-left
box:
Punnett Squares: Monohybrid Crosses
(con’t.)
3) Filling in the
bottom-left box:
Filling in the topright box:
Punnett Squares: Monohybrid
Crosses (con’t.)
Filling in the bottomright box:
Results:
• Genotype
• Percentage TT
(homozygous dominant)?
• Percentage Tt
(heterozygous)?
• Percentage tt
(homozygous recessive)?
• Phenotype
• Percentage tall?
• Percentage short?
Punnett Squares: Monohybrid
Crosses
Practice Problem A
• In pea plants, purple
flowers (P) are
dominant to white
flowers (p). Cross 2
heterozygous purple
flower plants
Practice Problem B
• Widow’s peak (W) is
dominant to a smooth
hairline (w). Cross a
woman who is
homozygous for
widow’s peak with
her husband, who has
a smooth hairline.
Punnett Squares: Dihybrid Crosses
Dihybrid crosses involve
two traits
RR: homozygous
dominant for round
rr: homozygous recessive
for round (looks wrinkled)
YY: homozygous
dominant for yellow
yy: homozygous recessive
for yellow (looks green)
Cross two heterozygous
RrYy X RrYy
Punnett Squares: Dihybrid Crosses
(con’t.)
Results of Genotype
•
•
•
•
•
•
•
•
•
RRYY:
RRYy:
RRyy:
RrYY:
RrYy:
Rryy:
rrYY
rrYy
rryy:
/16
/16
/16
/16
/16
/16
/16
/16
/16
Results of Phenotype
•
•
•
•
Round/Yellow
/16
Round/Green
/16
Wrinkled/Yellow /16
Wrinkled/Green /16
• Ratio: 9:3:3:1
Punnett Squares: Dihybrid Crosses
(con’t.)
Dihybrid practice problem: In garden
peas, tallness (T) is dominant to shortness
(t) and axillary flowers (A) are dominant to
terminal flowers (a). What are the expected
ratios for the genotypes and phenotypes of
the offspring if a heterozygous tall,
heterozygous axillary plant is crossed with a
heterozygous tall, terminal plant?
Punnett Squares: Dihybrid Crosses
(con’t.)
What Mendel learned from dihybrid crosses
are is that each trait are inherited
independently from each other.
• For example, height is inherited independently
from seed color.
• Law of Independent Assortment
Recap
Who is Gregor Mendel?
Why is Mendel’s work important to know?
Explain what a trait is.
Explain what an allele is.
What are Mendel’s four laws and explain them.
What is a Punnett Square, and how is it used?
What is the difference between monohybrid crosses and
dihybrid crosses?
What did the monohybrid crosses tell Mendel (hint: look
back at the four laws)?
What did the dihybrid crosses tell Mendel?