Download Mendelian Genetics

Chapter 11: sec 11-1, 11-2, & 11-3
Genetics
Genetics
(sec 11-1 & 11-2)
• Genetics
– The study of heredity
• Genes
– The chemical factors
of inheritance located
on chromosomes, made
of DNA
Mendelian Genetics
How Are Traits Inherited?
Gregor Mendel
• Raised on farm and understood the value of
plant breeding.
• At 21, entered priesthood and studied plant
breeding in a monastery in the Czech Republic.
• Loved to read especially about natural sciences
and was aware of Darwin’s findings.
• Studied the inheritance of traits in pea plants.
Gregor Mendel
• Studied traits that occur
in distinct forms.
• Developed true-breeding
varieties
– When bred amongst
themselves these plants
produced offspring identical
to the parent for that trait.
• Used mathematical
analysis in his studies.
Definitions
• Gametes: reproductive
cells produced by
sexually reproducing
organisms.
– Two types:
• male gametes = sperm
– In plants: contained in
pollen
• Female gametes = eggs
– In plants: contained in
ovules
– Ovules contained in
carpels
Fertilization
• Fertilization: fusion of
egg and sperm
– Self-fertilized: fusion of
sperm and egg from same
plant
– Cross fertilized: fusion of
egg and sperm from two
different plants
• Produced hybrids
• F1: first generation
• F2: second generation
True-breeding:
-Plants allowed to
self pollinate
-Produced offspring
identical to parents
Mendel’s Interpretations
• Modified to incorporate today’s vocabulary
– Genes: the hereditary information that determines a
single trait
– Alleles: alternate forms of a gene
• Homozygous/purebred - When an organism
inherits two of the same alleles for a trait.
• Heterozygous/hybrid - When an organism
inherits two different alleles for one trait.
Homozygous Alleles
Heterozygous Alleles
Why were all the seeds in the F1
generation round?
• Dominant vs. Recessive Allele
– Dominant: an allele that is expressed
whenever it is present
• Symbolized by a upper case letter (R)
– Recessive: an allele that is masked
whenever the dominant allele is present.
• Symbolized by a lower case letter (r)
• Dominant and recessive alleles influence
an organism’s phenotype
Genotype and Phenotype
• Genotype: Genetic makeup of an individual. It
is determined by the alleles present for each
trait. The type of genes an individual has.
• Phenotype: Physical appearance of a trait. It is
the expression of the genotype.
Monohybrid Cross
(One Factor Cross)
Mother contributes:
True Breeding
A
or
A
A
AA
AA
A
AA
AA
Mother contributes:
Cross Breeding
a
or
a
A
Aa
Aa
A
Aa
Aa
Can you look at someone and
determine their genotype?
• Testcross is used to determine the
genotype of an individual
– Testcross: crossing an organism with
unknown genotype with one that is
homozygous recessive for the trait.
– Example:
• Plant that has round (R) seeds crossed with one
that is homozygous recessive (rr) for wrinkled
seeds
R?
x
rr
Test Cross
Genetics
(sec 11-3)
• Mendel performed experiments designed
to follow 2 different genes in pea plants
– Called a two factor cross (Dihybrid Cross)
Mendel’s Laws
• Based on Mendel’s data, he formulated
two laws:
– Law of Segregation: A parent contributes
only one of its alleles for a trait to each
offspring.
– If parent is heterozygous for a trait, the
particular allele donated to the offspring is
random.
• Law of
Independent
Assortment: Alleles
of one gene are
passed to offspring
independently of the
alleles of other genes.
– Applies to the
inheritance of two or
more genes
simultaneously.
Law of Independent Assortment
• Allows for new gene combinations or
genetic recombination
Why Aren’t Members of the Same
Species Identical?
• Sources of Genetic Variation
– Law of Independent Assortment
– Mutation
– Crossing over
Mutations
• Definition = change in DNA sequence
resulting the appearance of a new allele
• Can occur at any time, but not always
beneficial
• Some can be harmful and result in death
• Some can result in the organism having
traits that make them better suited to the
environment
Do Mendel’s Laws Always Apply?
Incomplete dominance
– Some alleles (traits) are neither
dominant or recessive
– Controlled by more than one allele
• The heterozygous phenotype is between the
homozygous dominant or recessive. Both
alleles are expressed.
Example of Incomplete Dominance
Another example
• Shorthorn Cattle: Red male mated to a
White female = Roan calf
• RR X rr = Rr
If a red bull (RR) is mated to a roan (Rr)
cow, what color will the calves be?
Shorthorn: Red X Roan
R
R
R
RR
RR
r
Rr
Rr
What if Both Parents are Roan?
R
r
R
RR
Rr
r
Rr
rr
I.E. Incomplete dominance
– Roan
Codominance
• Both alleles share
dominance and are
expressed
• I.E. speckled
chickens
– Black feathers
dominant
– White feathers
dominant
Codominance
• Two alleles are expressed (multiple alleles) in
heterozygous individuals.
• Example: human blood type
1.
2.
3.
4.
type A
type B
type AB
type O
=
=
=
=
IAIA or IAi
IBIB or IBi
I A IB
ii
Polygenic Traits
• Interaction of several genes
– Human skin color
• More than four genes control this trait
– Human eye color
– Human hair color
Recombination and crossing-over
• Recombination- offspring which have
traits in a different combination from
parents.
• Two kinds of recombination
– Independent assortment of genes on
different chromosomes
– Crossing-over of genes on same
chromosome
Crossing-Over
(sec 11-4)
• Meiosis I
– Starts with tetrads
• 4 chromatids (paired homologous chromosomes)
– At this point crossing over can occur
– Gives us even more variation in living organisms
Determination of
Sex
• Drosophilia - fruit
fly
–
–
–
–
XX - female
XY - male
XXY - female
XO - male
• Humans
–
–
–
–
XX - female
XY - male
XXY - male
XO - female
• Non-X or Y
chromosomes are
known as
Autosomes
diploid
germ cells
in female
diploid
germ cells
in male
eggs
Meiosis, gamete
formation in both
female and male:
sperm
X
x
Y
X
x
X
Fertilization:
XY
X
X
X
XX
XX
Y
XY
XY
sex chromosome
combinations
possible in new
individual
XX
Fig. 12.4, p. 196
Slide 4
Sex linkage
We have 2 sex chromosomes choices:
– X and Y
• Females have XX chromosomes, males XY
• Sex-linked genes located on X, not Y
• To show recessive trait, female needs 2
copies, male only needs 1.
Sex Linked Traits
• Color blindness (recessive)
– Cannot distinguish between certain colors
• Most common form red-green colorblindness
• Hemophilia (recessive)
– A protein needed for blood clotting is missing
• Duchenne Muscular Dystrophy (recessive)
– Progressive weakening and loss of skeletal
muscle