Transcript N.2 Punnet Square Notes
Punnet Square Notes
Phenotype & Genotype
•
Phenotype
– the
physical characteristics
of an organism (example: blue eye color)
Phenotype & Genotype
•
Genotype
– the
alleles possessed
by an organism (example: inheriting two blues alleles on from each parent)
Dominant & Recessive Allele
Dominant Allele
– An
allele that dominants
over a recessive allele. Represented in genetics with a
capital letter
(example the capital letter B could represent the dominant allele for brown eyes).
Dominant & Recessive Allele
Recessive Allele dominate
- An allele that
does NOT
over other traits. Represented in genetic by a
lower case letter
(example the lower case letter b could represent the allele for blue eyes)
Types of Genotypes
•
Homozygous Dominant
: An organism having
two identical dominant alleles
. Represented in genetic by having
two upper case letters
. (example: BB represents a person that has two alleles for brown eyes)
Types of Genotypes
•
Homozygous Recessive
: An organism having
two identical recessive alleles
. Represented in genetic by having
two lower case letters
. (example: bb represents a person that has two alleles for blue eyes)
Types of Genotypes
Heterozygous:
An organism having
two different alleles
for a trait,
one recessive and one dominant
. Represented in genetic by having
one upper case
and
one lower case letters
. (example: Bb represents a person that has a dominant alleles for brown eyes and a recessive allele for blue eyes).
Punnet Square/ Grid
Punnet Square / Grid
is a grid or table that shows all the possible outcomes between a cross between a male in female organisms
Example
Cross a
Heterozygous
pea plant with smooth seeds with a
homozygous recessive
plant with wrinkled seeds.
1.
Select Symbols the alleles: 2.
3.
Write the cross (alleles parents can give): Make a punnet square. – – – What percent are heterozygous?
What percent are smooth?
What percent are wrinkled?
Example
Cross a
Heterozygous
pea plant with smooth seeds with a
homozygous recessive
plant with wrinkled seeds.
1.
Select Symbols the alleles:
S = Smooth; s= wrinkled
2.
Write the cross (alleles parents can give) :
Ss X ss 3.
Write down what each parent can give, also called Gamete or sex cell S; s x s ; s
1.
Make a punnet square. S s S s s S s s s s – – – What percent are heterozygous? 50% What percent are smooth? 50% What percent are wrinkled? 50% s s
P-square Practice Question #1
Let's say that in seals, the gene for the length of the whiskers has two alleles. The dominant allele (W) codes long whiskers & the recessive allele (w) codes for short whiskers. Cross two long-whiskered seals, one that is homozygous dominant and one that is heterozygous?
– The Symbols (letters you will use) ______________________ – The Cross (the parent ’ s genotypes)_________________________ – The Punnett square:
P-square Practice Question #2
In purple people eaters, one-horn is dominant and no horns is recessive. Draw a Punnet Square showing the cross of a purple people eater that is heterozygous for horns with a purple people eater that does not have horns. – – – The Symbols (letters you will use) ______________________ The Cross (the parent ’ s genotypes)_________________________ The Punnett square:
Test Crosses
Problem:
• When an organism has a shows a
dominant phenotype
(example: Brown Eyes) it is not always possible to know if this organism is has a
heterozygote genotype (Bb) genotype (BB)
or
Homozygote dominant
because the phenotype (physical appearance) it the same for both genotypes.
Test Cross
Solution:
• To
determine the genotype
of an organism showing a dominant phenotype a
test cross
is used.
What is a test cross?:
In a test cross an individual that might be heterozygous or homozygous dominant is
crossed with an individual that is homozygous recessive.
How it works?
If you cross a homozygous recessive organism with a homozygous dominant organism
all (100%) the offspring will have the dominant phenotype
.
Example:
Black fur color is dominant to white fur color in sheep. A homozygous dominant black sheep is cross with a homozygous recessive white sheep.
• The Symbols (letters you will use) ______________________ • The Cross (the parent ’ s genotypes)_________________________ • The Punnett square:
If you cross a homozygous recessive organism with a heterozygote
50%
of the offspring will have the
dominant phenotype
and
50%
will have the
recessive phenotype.
• • •
Example:
Black fur color is dominant to white fur color in sheep. A heterozygous black sheep is cross with a homozygous recessive white sheep.
The Symbols (letters you will use) ______________________ The Cross (the parent ’ s genotypes)_________________________ The Punnett square:
Practice 3 problem.
• First try this problem on your own then check your answer with the next slide.
• • P-square practice Question #3 A green-leafed luboplant (
I made this plant up
) is crossed with a luboplant with yellow-striped leaves. The cross produces
185 green-leafed luboplants.
• Summarize the genotypes & phenotypes of the offspring that would be produced by crossing two of the green-leafed luboplants obtained from the initial parent plants.
• A green-leafed luboplant (
I made this plant up
) is crossed with a luboplant with yellow-striped leaves. The cross produces
185 green-leafed luboplants.
Summarize the genotypes & phenotypes of the offspring that would be produced by crossing
two of the green-leafed luboplants
obtained from the
initial parent plants. Note: Green plant is crossed with yellow plant results in all green plants. THUS, green is dominant, and initial green parent ’s genotype homozygous dominant (GG). Initial yellow parent is homozygous recessive (gg), because the only way you can show the recessive phenotype is if you are homozygous recessive. Cross of initial parents: GG x gg ALL offspring from the initial parents are green and heterozygous (Gg).
• • •
This question asks you to cross the offspring from the initial parents. Thus you will cross brother and sister produced from initial parents (Gg x Gg) The Symbols (letters you will use) G= green g = yellow The Cross (the parent ’s) genotype: Gg x Gg The Punnett square:
•
Summary of offspring phenotypes
–
Percentage that will have yellow-striped leaves: 25%
–
Percentage that will have green-leaves: 75%
Problem 4
• First try this problem on your own then check your answer with the next slide Mendel found that crossing
wrinkle-seeded plants
with
pure round seeded plants
produced
only round-seeded plants.
What genotypic & phenotypic ratios can be expected from a cross of a wrinkle-seeded plant & a plant heterozygous for this trait (seed appearance)?
• • • Mendel found that crossing
wrinkle-seeded plants
with
pure round-seeded plants
produced
only round-seeded plants.
What genotypic & phenotypic ratios can be expected from a cross of a wrinkle-seeded plant & a plant heterozygous for this trait (seed appearance)?
NOTE: Crossing a wrinkled seed plant with a round seed plant produced all round seeds. Thus you can assume that round is dominant to wrinkled. Remember the only way the recessive phenotype will show is if the organism is homozygous recessive for that trait.
– – – The Symbols (letters you will use
) R = round; r = wrinkled
The Cross (the parent ’s genotype): Rr x rr The Punnett square: • Summary of offspring – Percentage that will have heterozygous:
50%
– Percentage that will have rounded-seeds:
50%
– Percentage that will have wrinkled-seeds:
50%
Problem 5 & 6
• Do these problems on your own.