Download Genetic Crosses - Beaver Local High School

Genetic Crosses
Section 9.2
Genotype
 The genetic makeup of an organism
 Consists of the alleles that the organism
inherits from its parents
 Example: white flowering pea plants
(recessive trait pp)
Phenotype
 The appearance of an organism as a
result of its genotype
 Example: pea plants that are PP or Pp
will have purple flowers
Homozygous
 Both alleles of a pair are alike
 May be homozygous dominant (PP) or
homozygous recessive (pp)
Heterozygous
 The two alleles in the pair are different
 Example: a pea plant that is
heterozygous for flower color
would be Pp (purple flowers)
Probability
 The likelihood that a specific event will
occur
 May be expressed as a decimal, a
percentage, or a fraction
 Probability = number of times an event is
expected to happen_____________
number of opportunities for an event
to happen
Example: yellow pea seeds appeared 6022 times and
green pea seeds appeared 2001 times in the F2
Total (6022 + 2001 = 8023)
What is the probability that the dominant trait will appear
in a similar cross?
P = 6022 = .75
8023
(or 75% or ¾ )
Monohybrid Cross
 A cross between individuals that involves
one pair of contrasting traits
 Example: crossing a pea plant that is
pure for white flowers (pp) with one that
is pure for purple flowers (PP)
Punnett Square
 Diagram used to predict the outcome of
certain crosses
 Predicts the probability of inherited traits
Homozygous X Homozygous
white flower
(pp)
p
p
purple
flower (PP)
P
Pp
Pp
P
Pp
Pp
Homozygous X Heterozygous
black coat
(Bb)
B
b
black coat
(BB)
B
BB
Bb
B
BB
Bb
Heterozygous X Heterozygous
black coat
(Bb)
B
b
black coat
(Bb)
B
BB
Bb
b
Bb
bb
Testcross
 An individual of an unknown genotype is
crossed with a homozygous recessive
individual
 Can be used to determine the genotype
of any individual whose phenotype is
dominant (example: BB or Bb)
Incomplete Dominance
 Occurs when two or more alleles
influence the phenotype
 Example: four o’clock flowers
RR = red
Rr = pink
rr = white
Incomplete Dominance in
Four O’clock Flowers
Pink flower
(Rr)
R
r
Pink flower
(Rr)
R
RR
Rr
r
Rr
rr
Phenotypic ratio: 1 red: 2 pink:1 white
Codominance
 Occurs when both alleles for a gene are
expressed in a heterozygous offspring
 Neither allele is dominant or recessive
 Example: roan horses
(RR`)
R = red coat
R` = white
Dihybrid Cross
 A cross between individuals that involves
two pairs of contrasting traits
 Must consider how the four alleles from
each parent can combine
Homozygous X Homozygous
wrinkled & green
(rryy)
round &
yellow
(RRYY)
ry
ry
ry
ry
RY
RrYy
RrYy
RrYy
RrYy
RY
RrYy
RrYy
RrYy
RrYy
RY
RrYy
RrYy
RrYy
RrYy
Ry
RrYy
RrYy
RrYy
RrYy
Heterozygous X Heterozygous
round & yellow
(RrYy)
round &
yellow
(RrYy)
RY
Ry
rY
ry
RY
RRYY
RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
Four Phenotypes
9/16 round & yellow
(genotypes RRYY, RRYy, RrYY, and RrYy)
3/16 round & green
(genotypes RRyy and Rryy)
3/16 wrinkled & yellow
(genotypes rrYY and rrYy)
1/16 wrinkled & green
(genotype rryy)
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