Three Point Linkage Problems
1. Purebred male white mice with short tails and big ears are crossed to purebred black mice
with long tails and small ears. All of the progeny are black, with short tails, and big ears.
These progeny are crossed with white mice with long tails and small ears. The results of
this second cross are given below.
Fur color
White
White
White
White
Black
Black
Black
Black
Tail length
Short
Short
Long
Long
Short
Short
Long
Long
Ear size
Big
Small
Big
Small
Big
Small
Big
Small
Number of Progeny
666
14
6
314
314
6
14
666
a. Define the inheritance pattern of each trait
Fur color: Dominant = ___Black_______
Recessive = __white______
- Represented by the letter __B___
Tail Length: Dominant = __Short________
Recessive = long__________
- Represented by the letter ___S_______
Ear size: Dominant = __big Ears______ Recessive = __small ears____
- Represented by the letter ____E______
Using the letters you just assigned:
What are the parental chromosomes?
bSE and Bse
Which chromosomes represent the double crossovers?
bsE and BSe
Which gene is in the center of the other two?
S
Draw a map of your chromosome below. Then use the data to calculate the distances between
each gene. SHOW YOUR WORK.
(314+314+6+6)/2000 = .32 = 32% recomb = 32m.u. between B and S
(14 +14 +6+6) /2000 = .02 = 2% recomb. = 2 m.u. between S and E
2. In garden peas, the following cross was made between purebred plants.
tall, red flower, round seed
X short, white flower, wrinkled seed
The progeny were all medium height, pink flowered, wrinkled seed. These progeny peas were
crossed to short, white flowered, round seed peas. The results of this cross were:
Height
Color
Short
White
Short
Pink
Medium
White
Medium
Pink
Medium
White
Medium
Pink
Short
White
Short
Pink
Define the inheritance pattern of each trait
Seed
Wrinkled
Wrinkled
Round
Round
Wrinkled
Wrinkled
Round
Round
Number of Progeny
340
340
340
340
160
160
160
160
b. Height: Incomplete dominance = ________________
i. Represented by the letter __H = tall, Hh= med, h = short__
c. Color: Incomplete dominance = ________________
ii. Represented by the letter __R = red, Rr = pink, r = white ________
d. Seed size: Dominant = ___wrinkled___
Recessive = round______
i. Represented by the letter ___W_______
What type of cross was done to get the F2 generation? Test cross
What are the expected ratios of phenotypes? 1:1: 1: 1: 1: 1: 1
Are these genes linked? Explain. If so, map out your genes and calculate the distance
between them. Hint. Try looking at pairs of trait as dihybrids
Parents:
HRw and hrW
Height/color RrHh x rrhh;
Pink Med
Pink short
White med
White short
Expect out of 2000
500
500
500
500
These two loci are independently assorting and NOT linked.
Observed out of 2000
500
500
500
500
Color/Seed
RrWw x rrww
Expect out of 2000
500
500
500
500
Pink wrink
Pink round
White wrink
White round
Observed out of 2000
500
500
500
500
These two loci are independently assorting and NOT linked.
Height/Seed
HhWw x hhww
Med wrink
Med round
short wrink
short round
Expect out of 2000
500
500
500
500
Observed out of 2000
320
680
680
320
These two loci are NOT independently assorting and therefore ARE linked.
Parents were either Hw or hW
So all HW (Med wrink) or hw (short round) are recombinants. The distance between
them would be the percentages
320 + 320 / 2000 = .32 = 32% recombination = 32 m.u. between the two
3. In warthogs, the following autosomal map has been determined.
Tusk
5 m.u.
hair______10 m.u.
Face
The trait variations for the genes are as followed:
Tusk: points up or out
Hair: long or short
Face: ugly or uglier
A purebred up tusked, long haired, ugly boar is crossed with a purebred out tusked, short
haired, uglier sow. The piglets have upturned tusks, medium hair, are uglier. What are the rules
of expression (inheritance patterns) for these genes? (which is dominant, which is recessive)
Up tusks (T) is completely dominant over out tusks (t)
Hair (H) is Incompletely dominant
Uglier Face (F) is completely dominant over ugly face (f)
The piglets grow up, and someone is foolish enough to let them breed. 80 offspring are
produced. What gametes are made and in what frequency? (in other words, work backwards.
You are filling numbers into the data chart that we usually give you!)
Remember distances between loci represent recombination events!!! So…. 5% of offspring
represent crossovers between Tusk and Hair and 10% demonstrate cross overs between Hair and
Face. Another way to put it is that there is a probability of .05 that crossing over will happen
between T and H and a probability of .95 that it will not.
Parent chromosomes:
THf
and
thF
F1 chromosomes:
THf (crossing over did not happen between either T-H or H-F) Probability = (.95)(.90)(.5)
-
The last .5 is because THf represents ½ of the gametes that can result
from no crossing over (thF being the other ½)
thF probability = (.95)(.90)(.5)
tHf (crossing over between T and H, but not H and F) (.05)(.90)(.5)
ThF (crossing over between T and H, but not H and F) (.05)(.90)(.5)
THF (crossing over between H and F, but not T and H) (.95)(.10)(.5)
thf (crossing over between H and F, but not T and H) (.95)(.10)(.5)
Thf (crossing over between BOTH H and F and T and H) (.05)(.10)(.5)
tHF (crossing over between BOTH H and F and T and H) (.05)(.10)(.5)
4. In the flowering plants called Four O'clocks, there is a single gene controlling flower color,
another controlling leaf color, and a third controlling height. The following cross is done.
Purebred White flowers, green leaves, short plants rr ll tt
X
Purebred Red flowers, purple leaves, tall plants RR LL TT
F1
Pink flowers, purple leaves, tall plants RrLlTt
The F1 plants were crossed to short plants with red flowers, green leaves (RR ll tt). The progeny
of this cross are:
Flower
Leaves
Height
36% Red
purple
tall
36% Pink
green
short
rlt
9% Red
green
9% pink
purple
short
rLt
4% Red
green
short
Rlt
4% pink
purple
tall
rLT
short
RLt
tall.
rlT
1% red
1% pink
purple
green
tall
RLT
RlT
A. Define the rules of expression for these alleles of these genes. Make sure you define your
symbols clearly.
B.
Define the linkage relationships for these genes.
- 9% have one type of chromosome made by single crossing over between R and T and 9% have
the other. That’s 18% so far. PLUS we need to remember that the double cross overs are ALSO
cross overs between R and T so we need to add these in. Total = 20% show crossing over
between R and T.
R____ 20 m.u. __________ T____ 10m.u.______ L