Download Genetics - Valhalla High School

GENETICS
Gregor Mendel
• Genetics is the study
of heredity.
• Gregor Mendel
(1860’s) discovered
the fundamental
principles of genetics
by breeding garden
peas.
Mendel crosspollinated pea
plants
• He cut away
the male parts
of one flower,
then dusted it
with pollen
from another
What did Mendel conclude?
• He concluded that factors are passed from
one generation to the next.
eye color locus
b = blue eyes
eye color locus
B = brown eyes
The Principle of
Dominance
The Principle of
Dominance – some
alleles are
dominant and other
are recessive.
Paternal Maternal
• Alleles
1. Alternative forms of genes.
2. Units that determine heritable
traits.
Dominant and Recessive alleles
Dominant alleles – upper-case
a. homozygous dominant
(BB – Brown eyes)
Recessive alleles – lower case
a. homozygous recessive
(bb – blue eyes)
b. Heterozygous (Bb – Brown eyes)
Phenotype vs. Genotype
• Arrangement of genes
• Outward appearance
that produces the
• Physical characteristics
phenotype
• Example:
• Examples:
1. TT, Tt
1.Brown eyes
2.blue 2. tt
eyes
Segregation
Alleles separate during meiosis
Recessive traits show up about 1/4th
of the time.
• Because there is only
a 25% chance that
two recessive alleles
will be paired
together.
Punnett square
• A Punnett square is
used to show the
possible combinations
of gametes.
• Monohybrid Cross
Monohybrid Cross
• Example: Cross between two heterozygotes
brown eyes (Bb)
BB = brown eyes
Bb = brown eyes
bb = blue eyes
B
b
B
Bb x Bb
b
female gametes
for
male
gametes
Monohybrid Cross
B
b
B
BB
Bb
b
Bb
bb
Bb x Bb
1/4 = BB - brown eyed
1/2 = Bb - brown eyed
1/4 = bb - blue eyed
1:2:1 genotype
3:1 phenotype
Dihybrid Cross
Dihybrid Cross
RY
RY
Ry
rY
ry
Ry
rY
ry
Dihybrid Cross
RY
RY RRYY
Ry RRYy
Ry
RRYy
RRyy
rY
RrYY
RrYy
ry
RrYy
Round/Yellow:
9
Round/green:
3
Rryy
wrinkled/Yellow: 3
rY RrYY
RrYy
rrYY
rrYy
wrinkled/green:
ry
Rryy
rrYy
rryy
9:3:3:1 phenotypic ratio
RrYy
1
Dihybrid Cross
• Example: cross between round and yellow
heterozygous pea seeds.
R
r
Y
y
= round
= wrinkled
= yellow
= green
RrYy x RrYy
RY Ry rY ry x RY Ry rY ry
possible gametes produced
Independent Assortment
• Chromosomes separate
independently of each
other
B
This means all
gametes will be
different!
B
F
Bb
B
b
Ff
f
sperm
B
Bb
Bb
Ff
B
Bb
diploid (2n)
f
b
b
Ff
B
b
meiosis I
meiosis II
F
haploid (n)
Independent Assortment
• Genes for different traits can segregate
independently during the formation of gametes
without influencing each other
• Question: How many gametes will be produced
following allele arrangements?
• Remember:
1.
2.
3.
2n (n = # of heterozygotes)
RrYy
AaBbCCDd
MmNnOoPPQQRrssTtQq
for the
Answer:
1. RrYy: 2n = 22 = 4 gametes
RY Ry rY ry
2. AaBbCCDd: 2n = 23 = 8 gametes
ABCD ABCd AbCD AbCd
aBCD aBCd abCD abCD
3. MmNnOoPPQQRrssTtQq: 2n = 26 = 64
gametes
Incomplete
Dominance
• One allele is not
completely dominant
over another
R
R
r
Rr
Rr
produces the
F1 generation
r
Rr
Rr
All Rr = pink
(heterozygous pink)
Codominance
• Both alleles are expressed
• Example: blood
1. type A = IAIA
or IAi
2. type B = IBIB
or IBi
3. type AB = IAIB
4. type O = ii
Black cow + white cow =
spotted cow
Which shows more genetic variation.
(more combos?)
• Male CC with female Cc
C
• Male cc with female Cc
C
c
C
C
c
c
c
Genetic Engineering
Gene
Therapy
Is cloning a
possibility?
• Is it right to use cloning
to create an entirely
new human being?
•
Is it ethical to create
an embryonic copy of
John Doe to supply
cells to keep John
alive?
•
Does a multicellular
ball of tissue -- an
embryo -- have the
same rights and status
as a human being?
How does Mendel’s principles apply to
organisms
• The basic principals
can be applied to
humans as well as any
other living organism.
• Consider this
Punnett square:
• Many traits are controlled by multiple alleles
or multiple genes.
– Multiple alleles (more than 2 choices)
– Polygenic (multiple genes control a single trait)
Multiple alleles:
•
the case where three or more alleles of the
same gene exist. Remember, an organism
will have only two of these alleles (one from
mom and one from dad).
– Examples: Coat color in rabbits, blood type in
humans
Multiple alleles:
Polygenic traits:
•
traits that are determined by alleles from
more than one gene; these traits usually
have a range of phenotypes
– Examples: skin color in humans, height in
humans
Mapping Genes
• It’s easy to imagine that genes on different
chromosomes assort independently, but what about
genes that occur on the same chromosome? Don’t
they always appear together?
• Not always due to crossing over. Genes that occur
together on a chromosome will be separated when
homologous chromosomes exchange genes.
• The frequency of genes occurring together can help
us generate a gene map.
• The more often two genes occur together, the
closer they are to each other on the
chromosome.
– If the genes are never separated by crossing over,
they always occur together. All offspring will look
like one of the parents (in reference to the genes
in question).
– If half of the offspring are parental and half are
recombinations of the parents (in reference to the
genes in question), then they are said to be
independent. This means they are either on
separate chromosomes or they are almost always
separated during meiosis.
– You will learn to calculate distances and create a
map in AP Bio, or in college
Human chromosomes
2. There are two types of chromosomes.
a. Autosomes: Of the 46 chromosomes, 44 of them (22
pairs of chromosomes) are called autosomes (non-sex
chromosomes).
b. Sex chromosomes: The last two chromosomes are called
the sex chromosomes because they determine the sex of
the person. Females have two X chromosomes (XX) and
males have one X and one Y chromosome (XY).
3. Gametes
a. All gametes are haploid. In humans, that means
each egg cell and each sperm cell has 1 copy of
each chromosome for a total of 23
chromosomes.
i.
Egg cells: All human egg cells carry 23 chromosomes,
one of which is a single X chromosome. This is
written as 23, X.
ii. Sperm cells: In males, there are two types of sperm
cells- one carries an X chromosome (23, X) and one
carries a Y chromosome (23, Y).
iii. When a sperm and egg
cell combine, half of the
time the fertilized eggs
(also called zygotes) are
female (46, XX) and half
of the time they are
male (46, XY).
X
Y
sperm
X
X
XX
XX
female
female
XY
XY
male
male
eggs
• Sex Linked traits: traits
that are determined by
alleles that are found
on the X or Y
chromosome.
– The Y chromosome is
shorter and does not
carry all the same
alleles as the X
chromosome.
– Females are XX and males are XY.
– Females can be homozygous or heterozygous for
a trait carried on the X chromosome, but males
(having only one X chromosome) are
hemizygous.
– If they inherit a defective gene from the parent,
then they will exhibit the trait because they
cannot inherit a second gene to mask it.
– Conversely, a healthy male cannot be “hiding” a
bad recessive allele because they only have one
X chromosome.
Example of a sex-linked Punnett square:
– XBXb (heterozygous female with normal vision) crossed to XBY
(hemizygous male with normal vision)
XBY
XB
Y
XB XB
XBY
XB
XB Xb
XB Xb
Xb
XbY
Genetics and the Environment
• Characteristics are determined by both genes
and the environment.
• External: While genes will influence the height
of a plant, the amount of water, sun, and
other climate conditions will also affect the
height.
Genetics and the Environment
• Internal: There are recent findings that
proteins involved with DNA can turn genes on
or off based on environmental factors.
– Certain chemical exposure can turn genes on or
off (make the traits show up or not) for
generations after exposure, but there are no
changes to the DNA (no mutations).
– This new understanding of how genes are
expressed is called epigenetics.