Download Rules to Genetics - Effingham County Schools

Dihybrid Crosses
Incomplete Dominance
Codominance
Multiple Alleles
In a nutshell!
Review: Dominant/Recessive
• One allele is dominant over the other
(capable of masking the recessive allele)
– Example: Purple flowers are dominant over white
PP = purple
pp = white
Pp = purple
Review Problem:
Dominant/Recessive
• In pea plants, purple flowers (P) are dominant
over white flowers (p) show the cross between
two heterozygous plants.
GENOTYPES:
- PP (1); Pp (2); pp (1)
- ratio 1:2:1
PHENOTYPES:
- purple (3); white (1)
- ratio 3:1
P
p
P
PP
Pp
p
Pp
pp
Dihybrid Cross:
a cross that shows the possible offspring
for two traits
Fur Color:
B: Black
b: White
Coat Texture:
R: Rough
r: Smooth
In this example, we will cross a
heterozygous individual with another
heterozygous individual.
Step 1: Determine the parents’ genotypes
Parent 1: BbRr
Parent 2: BbRr
Step 2: You must find ALL possible gametes
that can be made from each parent.
Remember, each gamete must have one B
and one R.
Parent 1: BbRr
Parent 2: BbRr
Parent 1
Possible gametes:
BR
Br
bR
br
F
O
Parent 2
Possible gametes:
BR
Br
bR
br
I
L
Step 3: arrange all possible gametes for one
parent along the top of your Punnett
Square, and all possible gametes for the
other parent down the side of your Punnett
Square…
BbRr x BbRr
BR
Fur Color:
B: Black
b: White
BR
Coat Texture:
R: Rough
r: Smooth
Br
Step 4: Find
the possible
genotypes
of the
offspring
bR
br
Br
bR
br
BR
Br
bR
br
BR
BBRR
BBRr
BbRR
BbRr
Br
BBRr
BBrr
BbRr
Bbrr
bR
BbRR
BbRr
bbRR
bbRr
br
BbRr
Bbrr
bbRr
bbrr
BbRr x BbRr
Fur Color:
B: Black
b: White
Coat Texture:
R: Rough
r: Smooth
How many of the
offspring would have a
black, rough coat?
How many of the
offspring would have a
black, smooth coat?
How many of the
offspring would have a
white, rough coat?
How many of the
offspring would have a
white, smooth coat?
BR
Br
bR
br
BR BBRR
BBRr
BbRR
BbRr
Br
BBRr
BBrr
BbRr
Bbrr
bR
BbRR
BbRr
bbRR
bbRr
br
BbRr
Bbrr
bbRr
bbrr
Fur Color:
Coat Texture:
B: Black
R: Rough
b: White
r: Smooth
How many of the
offspring would have
black, rough coat?
How many of the
offspring would have a
black, smooth coat?
How many of the
offspring would have a
white, rough coat?
How many of the
offspring would have a
white, smooth coat?
Phenotypic Ratio
9:3:3:1
BR
Br
bR
br
BR BBRR
BBRr
BbRR
BbRr
Br
BBRr
BBrr
BbRr
Bbrr
bR
BbRR
BbRr
bbRR
bbRr
br
BbRr
Bbrr
bbRr
bbrr
Fur Color:
Coat Texture:
B: Black
R: Rough
b: White
r: Smooth
• On a blank sheet of paper…
In pea plants, yellow seeds (Y) are dominant over
green seeds (y), and rounded peas (R) are
dominant over wrinkled peas (r).
Cross a plant that is heterozygous for both traits
with a plant that is homozygous recessive for
both traits. Draw a Punnett square to show all
possible offspring, and determine the phenotypic
ratio.
YyRr X yyrr
YR
Yr
yR
yr
yr
YyRr
Yyrr
yyRr
yyrr
yr
YyRr
Yyrr
yyRr
yyrr
yr
YyRr
Yyrr
yyRr
yyrr
yr
YyRr
Yyrr
yyRr
yyrr
Incomplete Dominance
• A third (new) phenotype appears in the
heterozygous condition.
• Flower Color in 4 O’clocks
RR = red
rr = white
Rr = pink
Problem: Incomplete
Dominance
• Show the cross between a pink and a
white flower.
GENOTYPES:
- Rr (2); rr (2); RR (0)
- ratio 2:2:0
R
r
r
Rr
rr
PHENOTYPES:
- pink (2); white (2) Red (0)
r
- ratio 2:2:0
Rr
rr
• Humans have 23 pairs of
chromosomes.
• 22 of the pairs are
called autosomes.
• One pair of
chromosomes is related
to the sex of an
individual , these
chromosomes are called
sex chromosomes
• XX = Female
• XY = Male
• Genes that are located on the X
chromosome are called sex-linked genes.
• Traits determined by sex-linked genes are
called sex-linked traits
– Ex: Color Blindness
(c = colorblind, C = normal)
• Color Blindness is recessive.
• FEMALES: both X chromosomes must
have the gene in order for the trait to
be expressed.
• MALES: only have one X chromosome. If
that X chromosome has the colorblind
allele, then the trait will be expressed.
• Colorblindness affects 1 in 100
females; 1 in 10 males
X-linked Recessive Inheritance
Male
X
Female
Y
One copy of an altered gene on
the X chromosome causes the
disease in a male.
= Hemizygote
X
X
An altered copy on one of the X
chromosome pair causes
carrier status in a female.
= Heterozygote
XCXC – Normal Female
XCXc – Carrier Female
XcXc – Color Blind Female
XCY– Normal Male
XcY– Colorblind Male
1. A colorblind male marries a normal
female. What are the offspring
genotypes and phenotypes?
(C = normal, c = colorblind)
Xc
XC
XC
Y
1. A colorblind male marries a normal
female. What are the offspring
genotypes and phenotypes?
(C = normal, c = colorblind)
Xc
XC
XC
XC Xc
XC Xc
Y
XC Y
XC Y
Genotypic Ratio: __________
Phenotypic Ratio: __________
What percentage of sons with
colorblindness? _______
daughters? ______
Multiple Alleles
• There are more than two alleles for
a trait
• Blood type in humans
• Blood Types?
– Type A, Type B, Type AB, Type O
• Blood Alleles?
– A, B, O (in book – IA, IB, I)
Rules for Blood Type
• A and B are codominant
• A and B are dominant over O
Problem: Multiple Alleles
• Show the cross between a mother who has
type O blood and a father who has type AB
blood.
O
O
GENOTYPES:
-AO (2) BO (2)
-ratio 2:2:0:0:0:0
A
AO
AO
PHENOTYPES:
- type A (2); type B (2)
- ratio 2:2:0:0
B
BO
BO
Problem: Multiple Alleles
• Show the cross between a mother who is
heterozygous for type B blood and a father who is
heterozygous for type A blood.
GENOTYPES:
-AB (1); BO (1);
AO (1); OO (1)
- ratio 1:1:1:1:0:0
PHENOTYPES:
-type AB (1); type B (1)
type A (1); type O (1)
- ratio 1:1:1:1
A
O
B
AB
BO
O
AO
OO