Review
What is a gamete? Why do we need to
know the gamete composition?
1. Aa
21=2
2. AaBB
21=2
3. AaBbCc 23=8
4. AaBbCC
22=4
We need to know the gamete composition so that we can insert
them into the boxes in punnet square
e.g # AaBbCc x AaBbCC
ABC
ABC
AbC
aBC
abC
ABc
AbC
Abc
aBC
aBc
abC
abc
Testcross
• What is a testcross?
• Why do we do testcross?
- homozygous recessive alleles
Sex linked
•
•
•
•
•
2 ways to write it, depending on instruction
Eg, assume that PTC is a sex-linked gene.
Ability to taste is dominant XP+/ XP
Inability to taste is recessive XP/ Xp
So,
XP+XP+,
XP+X ,
XP+Y are tasters and
non-tasters will be XPXP,
XPY
• Always bear in mind that if they talk about
disease, normal means a NONCARRIER XP+XP+, XP+Y
Try this
• The gene for eye in flies (Drosophila
melanogaster) are sex linked. The gene for red
eye is dominant over yellow eye. A red eyed
male is mated with yellow eyed female.
Diagram the cross is the male is heterozygous.
Try this
• The gene for eye in flies (Drosophila
melanogaster) are sex linked. While gene gene
for wings are autosomal. The gene for red eye
is dominant over yellow eye, gene for vestigial
wings is dominant over long wing. A red eyed
male fly with vestigial wings is mated with
yellow eyed female fly with long wings.
Diagram the cross is the male is heterozygous.
Map distance and chi-square
How do we determine the ratio
Green pod, starchy 568
Red pod, wrinkle
525
Green pod wrinkle 154
Red pod, starchy
163
1400
• Recombinant ??
• Ratio ??
• Which one is which ??
Trihybrid cross- determining map
distance
Parent flies were mated
+
• Black body, purple eyes, vestigial wings b
b
• Gray body, red eyes, long wings
Producing
• Black body, purple eyes, vestigial wings
• Gray body, red eyes, long wings
• Black body, purple eyes, long wings
• Black body, red eyes, long wings
• Black body, red eyes, vestigial wings
• Gray body, purple eyes, vestigial wings
• Gray body, red eyes, vestigial wings
• Gray body, purple eyes, long wings
pr+
pr
411
412
60
28
1
30
61
2
vg+
vg
Calculate the distance between genes
• We know that map distance is
Recombinant
Total
X 100
So, we need to calculate the map distance of
• Between body and eye color
• Between body and wings
• Between eye and wing
• Between body and eye color
• Between body and wing
• Between eye and wing
• But.. The actual distance calculated was 18.4
mu, therefore, this is said to experience rare
double crossovers.
• Therefore
Single and Double crossovers