Transcript Theoretical Genetics

4.3&10.2 Theoretical Genetics
IB Biology HL I
Mrs. Peters
Spring 2014
Genetic History
 Genetic History:
Gregor Mendel: Austrian
monk who worked with
garden peas in the 1860’s,
developed fundamental
principles of inheritance
 Mendel’s Laws:
 Law of segregation: two
alleles for a specific
character are packaged into
separate gametes
 Law of Independent
Assortment: pairs of alleles
for different characters
segregate independently of
each other in the formation
of gametes (they line up at
the metaphase plate in
metaphase I randomly)

Important Terms
 Genotype: all alleles of




an organism
Phenotype: the
characteristics of an
organism
Allele: one specific form
of a gene
Homozygous: having two
identical alleles of a gene
Heterozygous: having
two different alleles of a
gene
Important Terms
 Dominant Allele: an allele
that has the same effect on
the phenotype whether it is
present in the homozygous
or heterozygous state
 Recessive Allele: an allele
that only has an effect on
the phenotype when
present in the homozygous
state
 Codominant alleles: pairs
of alleles that both affect the
phenotype when present in
a heterozygote (formerly
incomplete dominance)
Practice
 Determine if these are genotypes or
phenotypes:


Red hair
TT
tt
brown eyes
Tt
hitchiking thumb
 Determine if these are homozygous or
heterozygous:

Tt
tt
TT
 Determine if these are dominant or recessive
traits:

Tt
tt
TT
Practice Answers
 Determine if these are genotypes or phenotypes:


Red hair
TT
brown eyes
tt
hitchiking thumb
Tt
 Determine if these are homozygous or heterozygous:

Tt
tt
TT
 Determine if these are dominant or recessive traits:


Tt
tt
TT
Tt will show the dominant trait, but carries the recessive
trait also.
Genetic Crosses
 Monohybrid cross: a cross
involving the inheritance of a
single character (only one
trait)
 Each parent provides two
alleles which are
separated during meiosis
 A cross looks at the
possible combinations of
alleles are for the next
generation
 Punnett square: a
diagramming device used to
predict simple genetic
crosses
Genetic Crosses
Monohybrid Cross
Trait: Seed Shape
Alleles: R: round; r: wrinkled
R
r
R
RR
Rr
R
RR
Rr
Parents: RR x Rr
Offspring??? Let’s find out!
1: draw a chart with 4 sections
2: separate each parent’s alleles
RR= R; R
Rr = R; r
3: place alleles at top and left side of chart
4: Fill in each inner box, with two letters, the
one from the top and the one from the left
5. Determine the possible offspring ratios:
Genotype: 2RR:2Rr
Phenotype: all round
Genetic Crosses
 Dihybrid Crosses: A
cross involving the
inheritance of two
characters (two traits)


This is looking at two
traits that are generally
not located on the
same chromosome.
Looking at the possible
combinations of two
traits or 4 alleles in the
next generation.
Genetic Crosses
Dihybrid Cross
Traits: Seed Shape & Seed Color
Alleles: R: round; r: wrinkled; Y: yellow; y:
green
RY
Parents: RrYy x RrYy
Offspring??? Let’s find out!
RY
1: draw a chart with 4 columns and 4 rows
2: separate each parent’s alleles
RrYy = RY; Ry; rY; ry
RrYy = RY; Ry; rY; ry
3: place alleles at top and left side of chart
4: Fill in each inner box, with two of each
letter, the one from the top and the one
from the left (alpha/cap order)
5. Determine the possible offspring ratios:
Genotype: 1RRYY,
Phenotype:
Ry
Ry
rY
ry
RRYY RRYy RrYY RrYy
RRYy
RRyy
RrYy
Rryy
RrYY
RrYy
rrYY
rrYy
RrYy
Rryy
rrYy
rryy
rY
ry
Practice Time!
Genetic Crosses
 Codominance: (the old
incomplete dominance)
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Two alleles
If homozygous
produce a certain
color, if heterozygous
produce a combined
color
Ex: Alleles for Red
(RR) and White (WW)
flowers, when
heterozygous (RW)
are Pink
Genetic Crosses
 Multiple Alleles: genes
that exist with more
than 2 alleles

Blood Type Alleles:
 A, B, O
iA
i
i
iB
iAi
iBi
iAi
iBi
Genetic Crosses
 Sex Linked Traits: genes
located on X or Y
chromosomes

color blindness and
hemophilia in humans are
sex linked traits
 Seeing color is
dominant and not
seeing certain colors is
recessive
 Hemophilia is a
recessive trait
 Both are carried on the
X chromosome
Xb
Xb
XB
Y
XBXb
XbY
XBXb
XbY
B is not color blind, b is color
blind
Genetic Crosses
 Polygenic Inheritance:
when two or more
genes are part of the
phenotype
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
Skin color and height
in humans
At least three genes
contribute to
determining skin color
Genetic Crosses
 Test Cross: testing a
suspected heterozygote
by crossing it with a
known homozygote
recessive
Important Terms
 Locus: the particular
position on a homologous
chromosome of a gene
 Pedigree: a family tree
with the history of a
particular trait shown
across generations
 Carrier: an individual that
has one copy of a
recessive allele that
causes a genetic disease
in individuals that are
homozygous for this
allele
Pedigrees
 Members without trait
are usually not colored
in
 Members with a trait are
colored in
 Carriers can either be
left without color, have a
lighter color, or have a
split box half colored in
 Always look at all the
information provided
before making
statements about
pedigrees
Pedigree for Hemophilia
Genetic Testing
 Testing for disorders
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Amniocentesis
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A needle is inserted into the
uterus and extracts about
10ml of amniotic fluid, cells
in the fluid are cultured and
analyzed using karyotyping
(takes several weeks to get
results)
Chorionic villus sampling
(CVS)

A narrow tube is inserted
through the cervix of the
uterus and a tiny tissue
sample from the placenta is
suctioned out, karyotyping
can happen immediately
(takes a few hours to get
results, more advantageous
procedure)