LESSON 7 - Punnett Square

Report
Objective
1.Use the punnett square method to determine the possible
genotypes of different monohybrid crosses
2.Determine the phenotypic ratio of a monohybred cross.
3.Explain how the process of meiosis and fertilization account
for the transmission of inherited characteristics.
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The transfer of characteristics (traits) from
one generation to another
Your inherited characteristics (traits) are
determined by the genes located on your
chromosomes
Dominant
Trait
Description
Eye colour
Hair colour
Hairline
Freckles
Earlobe
Fingers
Ear rim
Thumb joint
Folded hands
Tongue rolling
Chin dimple
# of students
with Dominant
# of students
with Recessive
Class
Class
Ratio of
Dominant
to
Recessive
Percentage
with
Recessive
traits
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a short segment of a chromosome coding for
one trait
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chromosomes are found in pairs
in humans, there are 46 chromosomes, made
up of 23 pairs of homologous chromosomes
a pair of chromosomes consists of two
homologous chromosomes which look alike
and carry genes for the same traits
you receive one homologous chromosome of
the pair from each parent, therefore you
receive 2 genes for each trait

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This term is used to describe the physical or
visible appearance of an individual as
determined by the gene combination you
inherit from your parents.
Example: in the case of eye colour, having
brown eyes is the phenotype and is dominant

the gene combination that produces the trait

these genes may be the same or different
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genotypes are represented by upper and/or
lower case letters e.g. Bb
there are three possibilities for genotypes for
a single trait.
e.g. BB or Bb or bb
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alternate forms of a gene
same location on a each chromosome of pair
affects the same trait but differently
alleles are different forms of a gene that carry
different instructions
eg. Brown eyes or blue eyes
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Characteristic that is always expressed its
gene is present
only one dominant allele needs to be present
for the dominant trait to be expressed
an upper case letter designates a dominant
allele
eg. Tall is dominant. It is represented with a
“T”
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Characteristic that is only expressed when
two genes for that trait are present
two recessive alleles need to be present for a
recessive trait to be expressed
if a dominant allele is present it “masks” the
recessive allele
eg: Short is recessive. It is represented with a
 “t”
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“homo” means the “same”
the condition where both alleles for a trait are
the same
there are two conditions for homozygous
genotype:
homozygous recessive genotype “aa”
homozygous dominant genotype “AA”
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“hetero” means “different”
the condition where both alleles for a trait are
different
the genetic information inherited for a trait
from both parents is different
example: heterozygous genotype “Aa”
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Summary (example eye colour)
Genotype
BB
Bb
bb
Condition
Homozygous
dominant
Heterozygous
Homozygous
recessive
Phenotype
Brown eyes
Brown eyes
Blue eyes
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Crossed two purebred parents (One tall -T
and one short -t)
Pure breeding plants always produce identical
offspring.
All offspring were tall!!
Mendel concluded that some traits were
___________ and some traits were ___________.

When individuals with different traits are crossed,
the offspring (F1 generation) will express only the
dominant trait.
Parents: Tall
x
Short
F1 (Offspring): Tall Tall Tall Tall
What must the genotype of the offspring be if the
parents were pure breeding?
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Mendel crossed the F1 generation.
He crosses two hybrid pea plants.
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Hybrids have contrasting traits.
F1:
Tall plants x
Tall plants
F2 (Offspring of F1): Tall Tall
Tall Short
 F2 generation = 3:1 ratio
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What must the genotype of these offspring be if
the parents were hybrids (Tt and Tt)?
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Each F1 parent starts with two hereditary factors
(alleles); one is dominant and one is recessive
Each parent contributes only one factor (allele)
Each offspring inherits one factor (allele) from
each parent
If the dominant factor (allele) is present it will be
expressed.
If the recessive factor (allele) is present it will
only be expressed if only recessive factors are
present.
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Mendel crossed two purebred plants
TT – tall plant
Tt – short plant
Monohybrid cross – only one trait is being
tested
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Punnett Square
Is a grid system resembling a checkerboard,
used in computing possible results of various
genetic combinations
Simply stated, it is a way of representing the
possible combinations of genes when an egg
and sperm unite in fertilization
Characteristic hair texture –
◦ Dominant allele (gene) is curly hair - C.
◦ Recessive allele (gene) is straight hair - c
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Mother
◦ Phenotype – straight hair
◦ Genotype- homozygous recessive - cc
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Father
◦ Phenotype – curly hair
◦ Genotype – homozygous dominant – CC
Egg
c
c
C
Cc
Cc
C
Cc
Cc
Sperm
All of their
children will
have curly
hair. They all
have a
different
genotype from
their parents.
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Bikini Bottom Genetics – Monohybrid Crosses
HOMEWORK: Find out what the following
words mean…
Hybrid
Pure-breeding/Purebred
Why was it important that Mendel used
purebred plants in his experiment?
How does meiosis ensure that you inherit
your mothers and fathers characteristics?
Objectives:
1. Explain what co dominance and incomplete
dominance is. Give an example of each.
2. Use a Punnett square to solve basic
incomplete and co dominance crosses
3. Use a Punnett square to solve basic dihybrid
crosses.
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Incomplete Dominance: When two alleles are
equally dominant, they interact to produce a
new phenotype.
Codominance: When both alleles are
dominant and are expressed at the same
time.
Pg 145 # 1-4
When a red bull is
crossed with a white
cow, the offspring will
be roan.
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When a red flower is crossed
with a red flower,
the resulting offspring
will be pink.
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A type of cross that involves two genes, each
consisting of non identical alleles.
In his second experiment Mendel crossed a pea plant with
round/yellow (RRYY) seeds with a pea plant with
wrinkled/greed seed (rryy).
RY
RY
ry
RrYy
RrYy
ry
RrYy
RrYy
Genotype:
What
genotypes RrYy
resulted?
Ratio
100%
What phenotypesRound/Yellow
Phenotype:
resulted?
Ratio
100%
F1 - Generation
RY
Ry
rY
ry
RY
RRYY
RRYy
RrYY
RrYy
Ry
RRYy
RRyy
RrYy
Rryy
rY
RrYY
RrYy
rrYY
rrYy
ry
RrYy
Rryy
rrYy
rryy
Genotype: RRYY RRYy Rryy RrYY
RrYy
rrYY
rrYy
rryy
Ratio:
1
2
2
2
4
1
2
1
Phenotype:
Round/Yellow
What genotypes
resulted?Round/Green Wrinkled/Yellow Wrinkled/Green
Ratio:
9
3
3
1
What phenotypes resulted?
In the resulting F2 – Generation, the phenotypic ratio will
always be 9:3:3:1
In other words, the likelihood of obtaining each of the
above phenotypes from crossing two hybrid pea plants is
9:3:3:1.
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The inheritance of alleles for one trait
________ (does/does not) affect the
inheritance of alleles for another trait.
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Bikini Bottom
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Questions

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