Transcript Ch 11 Notes - Intro to Genetics

Why is Genetics interesting?
Dominant
BB
Recessive
bb
Recessive Epistasis
ee (B or b)
Introduction to Genetics
- Chapter 11
What is Inheritance?
• Every living thing has a set
of characteristics inherited
from its parent or parents.
• Genetics is the scientific
study of heredity.
11-1 The Work of Gregor Mendel
• He was an Austrian monk
• He worked with different true-breeding
(pure bred) pea plants
• Pea plants were a good choice because:
• They were self-pollinating
• Seed in one season
• Many different true-breeding types
• Mendel worked with 7 different traits:
•
•
•
•
•
Seed Shape & Color
Seed Coat Color
Pod Shape & Color
Flower Position
Plant Height
Genes and Dominance
• What is a trait?
• A characteristic
like eye color
• Example:
• Tall vs.. Short –
Height
• Round vs.
Wrinkled – Pea
Shape
• Mendel called the original plants the
P1 (parent) generation
• Offspring = F1 generation
• If you cross two parents with
different traits, the offspring are
called hybrids.
Mendel’s F1 Cross
This is what Mendel saw.
The offspring had the characteristics of only one of its parents
•
Mendel concluded:
1. Inheritance is determined by factors that are
passed from one generation to the next
2. Chemical factors that determine traits are called
genes
3. Different forms of the same gene are called
alleles
Example: Gene for height
Alleles: tall vs. short
4. The Principle of Dominance : Some alleles
are dominant and some alleles are recessive.
• Dominant Traits are always expressed
• Recessive Traits are can only be
expressed when the dominant allele is
not present
• Mendel wondered if the recessive alleles
had dissapeared or were they still present
in the F1 plants
• He decided to allow all seven kinds of F1
hybrids to produce F2 offspring.
Segregation
• Mendel looked at the results of his
F1 and F2 crosses:
P1
F1
tall plants x short plants
tall plants
( F1 become the parents for the next generation )
P2
tall plants x tall plants
F2
tall plants , short plants
This is What Mendel Saw
• Because the trait, short, reappeared,
Mendel reasoned that the alleles for
tallness and shortness had separated
from each other when gametes
(sex cells) form.
T = Tall
Tt
t = short
T
t
F1 : the F1 plant
produces
2 kinds of
sex cells
11-2 Probability and Punnett Squares
• The chances that a particular event will happen is
called probability.
• The principles of probability can be used to
predict the outcomes of genetic crosses.
Punnett Squares
• Punnett Squares can be used to determine
the genetic combinations that might result
from a genetic cross.
• The letters in a Punnett Square represent
alleles:
• Capital Letters = Dominant Alleles (G)
• Lowercase Letters = Recessive Alleles (g)
Punnett Square Terms
• Homozygous = True Breeding (Pure)
• Heterozygous = Hybrid (Mixed)
• Phenotype = Physical Characteristics
(Tall or short)
• Genotype = Genetic Makeup (T or t)
Predicting Averages
• Probabilities can predict the average
outcome of genetic crosses.
• The larger the number of offspring
resulting from a cross, the closer the
results will be to the expected values.
Ratios:
P - 75% Tall, 25% short
G – 1:2:1
Offspring
11-3 Independent Assortment
• What happens if there is more than
one gene?
• Does inheriting a certain gene for
seed color affect the inheritance of
another trait like plant height?
2 Factor Crosses
• Mendel performed experiment to
follow two different genes as they
passed from one generation to the
next.
• These experiments are known as
two factor (Dihybrid) crosses.
• Mendel crossed plants that were
true-breeding for two different traits.
• Round, yellow peas
• Genotype RRYY
• Wrinkled green peas
• Genotype rryy
Which traits are dominant and
which traits are recessive?
RRYY x rryy
ry
RY
RY
RY
RY
ry
ry
ry
Why are there so many boxes?
•Each parent can
produce 4 different
kinds of sex cells
(gametes)
•Each gamete has an
equal chance of
combining with each
of the other parents
4 types of gametes.
F2: Dihybrid Cross
• Each of the offspring in the example are
hybrids for BOTH traits - Dihybrids
• Mendel crossed these offspring to
produce another generation of plants (F2)
• If the genotype of each parent is RrYy,
What kinds of gametes will each parent
produce?
Gametes
• Parent 1: RrYy
• _____ _____ _____ _____
• Parent 2: RrYy
• ______ ______ _____ _____
RrYy x RrYy
____
____
____
____
____
____
____
____
Results
• The results of the F2 cross showed
that the alleles for the two different
traits segregated independently into
the gametes.
• The offspring from this cross
showed a 9:3:3:1 ratio of the different
phenotypes.
Summary of Mendel’s Principles
• Inherited traits are determined by genes.
Genes are passed from parents to
offspring
• Some forms of the gene may be dominant
and others may be recessive
• The genes segregate during meiosis so
only one copy of a gene goes into the
gamete
• Alleles for different genes usually
segregate independently of one another.
Exceptions to Mendel’s Principles
• Some alleles are neither dominant
nor recessive, and many traits are
controlled by multiple alleles or
multiple genes.
Incomplete Dominance.
• One allele is not completely
dominant over another.
• The heterozygous phenotype is
somewhere in between the two
homozygous phenotypes.
____
•R = red flowers
•r = white flowers
•Rr = pink flowers.
____
____
____
Codominance
• Both alleles contribute to the
phenotype of the organism
• Example: Horses, allele for red hair is
codominant with allele for white hair.
Animals with both alleles have both
red and white hairs. The color is
called roan.
Blue Roan
Red Roan
Multiple Alleles
• A particular trait has more than just two
alleles.
• You only inherit two of those alleles at a
time.
• Examples: coat color in rabbits, hair color
in humans, and human blood types.
Hair Color
Human Blood Types
Polygenic Traits
• Traits produced by the interaction of
many genes.
• Examples: human skin color, height,
cystic fibrosis
Applying Mendel’s Principles
• Albinism in humans is
caused by a recessive
trait.
• If two people with
normal skin color
have a child with
albinism, what are the
odds that a second
child will also have
albinism?
A = normal, a = albino
•Chances for a
normal child?
•___________
•Changes for an
albino child?
•___________
____
____
____
____
11-4 Meiosis
Introduction
• Multicellular organisms use mitosis to
replace cells that are lost due to injury or
damage or to grow.
• These cells (somatic cells) are identical to
the parent cells because all of the DNA is
first copied and then two copies of the
DNA separate when the daughter cells
form.
• The daughter cells are identical to the
parent cells
Meiosis is Different
• Multicellular organisms reproduce
sexually.
• In order to keep the number of
chromosomes the same from generation
to generation, the sex cells have to reduce
the number of chromosomes to one half of
the number that you find in a somatic cell
(body cell)
• Meiosis is the process that reduces the
number of chromosomes to 1/2.
Meiosis I
• The chromosomes have replicated
during S phase of the cell cycle.
• During Prophase I, the chromosomes
become visible and the
chromosomes pair off--that is
chromosomes that carry the same
information called homologs, and
form structures called tetrads.
• Something important happens during
this process--the homologous
chromosomes can twist around each
other and some times they break off.
When they re-attach, they may attach
to the other chromosome.
• This event is called crossing over
and is an important process in
genetics.
After prophase I, the tetrads of
chromosomes line up at the equator
of the cell for metaphase I
In anaphase I, the pairs of
chromosomes separate from each
other. Each daughter cell receives
one copy of each type of
chromosome.
Metaphase I
Anaphase I
• In telophase I, two daughter cells
form. Each of these cells have 1/2 of
the number of chromosomes that we
started with.
Meiosis II
• In this second cell division, the two
daughter cells from meiosis I
undergo one more cell division
• The steps in this cells division are
very much the same as mitosis
except the DNA is NOT copied again
• Prophase II: chromosomes become visible
• Metaphase II: chromosomes line up at
the equator
Anaphase II: centromeres split, and the
copies of the DNA are pulled apart
• Telophase II: 4 daughter cells form.
Products of Meiosis
• 4 daughter cells
• Cells are not genetically identical to the
parent cell
• Crossing over may have shuffled the
genes from the maternal and paternal
chromosomes.
11-5 Linkage and Gene Maps
?