Punnet Squares, Linked
Genes and Pedigrees
Predicting offspring genotype
• If we know the genotype of the parents,
we can predict the genotype of the
offspring, using test crosses (punnett
squares)
• Alternatively, parental genotype may be
able to be determined if the offspring
genotype is known
Example
Tracey
John
Phenotypes:
Phenotypes:
Blood group B
Blood group B
Normal skin pigment
Normal skin pigment
Genotypes:
Genotypes:
IBi
IBi
?
?
Samson
Phenotypes:
Blood group O
Albino
Genotypes:
?
Working out the possibilities
• Step 1: work out the possible genes within
the sperm and egg cells
• Step 2: perform monohybrid cross
PUNNETT
SQUARE:
BLOOD TYPES
B
I
i
B
I
i
What is the probability of heterozygotes
producing different phenotypes?
PUNNETT
SQUARE:
HETEROZYGOTES
A
a
Genotype ratio:
Phenotype ratio:
A
a
Complete the following:
• Punnett Squares made easy worksheet
What if we want to work with
combinations?
• We want to find out all the combinations of
skin pigment and blood types that Tracey
and John’s children could have.
• Step 1: work out all the genotype
possibilities within the egg and sperm cells
• Step 2: perform dihybrid test cross
Dihybrid cross
IBA
IBa
iA
ia
IBA
IBIBAA
IBIBAa
IBiAA
IBiAa
IBa
IBIBAa
IBIBaa
IBiAa
IBiaa
iA
IBiAA
IBiAa
iiAA
iiAa
ia
IBiAa
IBiaa
iiAa
iiaa
Genotype ratio:
Phenotype ratio:
Linked genes
• Genes are said to be “linked” when their
loci are found on the same chromosome.
• It means that those alleles are usually
inherited together… but not always!!
The offspring of this couple gets one of
each parental chromosome. The genes
are intact, so the alleles that are inherited
are of the “parental type”
http://www.schenectady.k12.ny.us/putman/biology/data/images/inherita
nce/5vtxrn.gif
Recombination
• Remember we said that in meiosis,
genetic material can move from one
chromosome to another?
• This is called recombination. It results in
offspring having chromosomes that are not
identical to parental chromosomes.
Recombination
You can see that the probability of genes that are close together (a and b)
being separated during crossing over is less than that of genes that are further
apart (a and c)
http://www.sayhelloandsmile.com/parta/paper1knowledge/2_diseasecausationdiagnostic/2d_genetics/2d3_e.jpg
Recombination
• In the example below, the mother is
heterozygous at both loci, while the father
is homozygous at both.
Parental types:
G
G
G
g
a
a
a
A
Ga, gA (the codes found on the parental
chromosomes)
If crossing over occurs in maternal meiosis,
there is no change, as the alleles are the same
on each. HOWEVER, if there is crossing over in
the paternal chromosomes, and the G/g alleles
swap places, there will be RECOMBINANT
offspring with chromosomes with the following
allelic combinations:
ga or GA
Detecting linkage
• If you know the number of recombinant
offspring (those without the parental type
chromosomes), you can work out how far
apart the genes are from each other.
• This is the equation to use:
100 x number of recombinant offspring
total number of offspring
Detecting linkage example
Paternal genotype
Maternal
genotype
Parental type
Ga
gA
Ga
Recombinant
GA
ga
GaGa GagA GaGA gaGa
44%
44%
6%
6%
So, using our formula before (although, we have percentages, it’s much
easier!):
Distance between loci = 100x number of recombinant (6+6=12) = 12 map units
Total number of offspring (44+44+6=100)
Detecting linkage problem
Parent 1
A
Parent 2
A
A
Parent 1
(all parental)
a
Parent 2
Q
q
Q
q
Parental
Recombinant
Number of parental type genotypes:_______
Number of parental type offspring: 212
Number of recombinant genotypes: _________
Number of recombinant offspring: 156
Distance between loci:
AQ
Aq
AQ
AAQQ
AAQq
aq
AaQq
Aaqq
Aq
AAQq
AAqq
aQ
AaQQ
AaQq
Pedigrees
• Family tree style diagrams that show the
presence/absence of particular disorders
within a family.
• Has a distinct set of symbols
Pedigree Key
Carrier (heterozygote, in
recessive disorders)
Autosomal dominant pattern
Characteristics of pattern:
-Both males and female affected
-All affected individuals MUST have
an affected parent
-Once the trait disappears from a
branch of the pedigree, it does not
reappear
-In a large sample, approx. the same
number of each sex will be affected
Autosome: any chromosome that is not a
sex chromosome
Autosomal recessive pattern
Characteristics of pattern:
-Both males and females can
be affected equally
-Two unaffected parents can
have an affected child (why…
and what symbol could be
useful here?)
-All children of two people
with the disorder MUST also
be affected
-The trait may disappear from
a branch of the pedigree, but
reappear later on
http://www.migeneticsconnection.org/genomics/family%20history/AutosomalR
ecessiveOnly1.gif
X linked dominant pattern
Characteristics of pattern:
-Male with trait passes it on to all
of his daughters, and none of his
sons
-Female with trait may pass it to
both daughters and sons
-Every affected person has at
least one parent with the trait
-If the trait disappears from a
branch, it does not reappear
-More affected females than
males
http://www.biolab.co.in/uploads/biolab/5dafedf3f5770fc5aa9654ea05c5c0a3.jpg
X linked recessive pattern
Characteristics of pattern:
-All sons of an affected female
are affected
-All daughters of a father with the
trait are carriers
-None of the sons of an affected
father and unaffected mother will
show the trait UNLESS their
mother is a carrier
-All children of two affected
individuals will also be affected
-More males than females show
the trait
http://journal.hsmc.org/ijnidd/uploadedImages/figure4.GIF
In groups:
• You will be given one of the pedigree
patterns.
– Provide explanations for EACH of the
characteristics of the pattern
– Find out about at least two disorders with this
pattern of inheritance