Transcript RR - Noadswood Science

What does this picture show?
What does this picture show?
Boy or Girl?
X
Y
X
“Allele”
Girl
XX
XY
Boy
Genetic Crosses
L.O: To understand
and be able to
construct genetic
diagrams
Matching pairs – characteristics
Using the genotypes of a parent you
can work out what characteristics
offspring will have. e.g. Eye colour
In eye colour the brown eye allele is dominant, so we call it
B, and the blue eye is recessive, so we call it b:
Bb
BB
Homozygous
brown-eyed
parent
bb
Heterozygous
Blue-eyed parent
brown-eyed
parent
What would the offspring have?
Eye colour – one way to work it out!
Example 1: A homozygous
brown-eyed parent and a
blue-eyed parent:
X
BB
Parents:
Gametes:
Example 2: 2 heterozygous
brown-eyed parents
bb
Bb
X
Bb
B
B
b
b
B
b
B
b
Bb
Bb
Bb
Bb
BB
Bb
bB
bb
(FOIL)
Offspring:
All offspring have brown eyes
25% chance of blue eyes
Eye colour
Example 3: A heterozygous brown-eyed
father and a blue-eyed mother:
Bb
bb
B
b
b
b
Bb
Bb
bb
bb
Equal (50%)
chance of
being either
brown eyed or
blue eyed.
Another Method – Punnet Squares
RR x rr – crossing homozygous parents
parental genotype:
gametes:
F1 offspring
genotype:
RR
R
x
R
rr
r
r
r
r
R
Rr
Rr
R
Rr
Rr
What are the phenotypes of the F1 offspring?
?
Rr x Rr – crossing heterozygous parents
F1 generation
phenotype:
X
genotype:
gametes:
F2 offspring
genotype:
Rr
Rr
R
r
R
r
R
r
R
RR
Rr
r
Rr
rr
What are the phenotypes of the F2 offspring?
?
Rr x Rr – F2 phenotypes
parental genotype:
F2 genotypes:
Rr
RR
x
Rr
Rr
Rr
rr
F2 phenotypes:
(Click twice on the buds
to reveal the flowers;
click again to close them.)
In the F2 generation, 3 of the 4 possible offspring are red.
Only one offspring shows the recessive phenotype.
When two heterozygous parents are crossed, the possible
offspring will always show a 3:1 ratio in favour of the
dominant phenotype.
What is a test cross?
A test cross allows you to find out if an organism showing
a dominant characteristic is homozygous or heterozygous
for the dominant allele.
For example the genotype of a red flower could be:
RR
or
Rr
Carrying out a test cross
A test cross is carried out between the flower of unknown
genotype and another flower whose genotype is known.
For example, a yellow flower can only have the genotype rr
because it’s recessive.
So the test cross is:
x
?
(RR or Rr)
rr
Try to work out the phenotypes of the offspring
in both possibilities (RR or Rr crossed with rr)
r
r
R
Rr
Rr
R
Rr
Rr
So if the offspring yield
these results what is
the unknown genotype
of the parent?
r
r
R
Rr
Rr
r
rr
rr
Homozygous cross activity
© Boardworks Ltd 2004
Heterozygous cross activity
© Boardworks Ltd 2004
Test cross activity
© Boardworks Ltd 2004