Download IB Genetics Probem Set 3 2016 Genetics Probem Set 3

IB Genetics Problem Set #3 Gene Linkage and Pedigrees
At the end of this problem set make a list of all the genetic conditions (ex. Simple dominance) you have learned about, from
reading, lecture, all three problem sets and the text handout. Try to make an example problem for each.
There are two kinds of gene linkage problems. There are problems where you do crosses with linked
genes and use lines to represent the chromosomes. There are problems where you calculate or use
recombination frequencies. For recombination frequency problems remember you use the “classic
testcross”—heterozygous, heterozygous crossed with homozygous recessive, homozygous recessive.
1. A. Use a punnett square to show the following cross among linked genes. Assume there is no
crossing over.
Cs
cS
Cs
X
cS
B. Circle the recombinant phenotypes (look different than either parent).
C. Now do the same cross, but assume that there is crossing over. This gives you a 4 x 4 punnett
square.
2. If you do the following cross show what combinations the recombinants would have:
TB
tb
X
Tb
tb
3. A wild-type fruit fly (heterozygous for gray body color and normal wings) is mated with a black fly
with vestigial wings. The offspring have the following phenotypic distribution: wild type, 778; blackvestigial, 785; black-normal, 158; gray-vestigial, 162.
A. Just by looking at these numbers take a guess if the genes are linked or not.
B. What is the recombination frequency between these genes for body color and wing size?
4. In another cross, a wild-type fruit fly is mated with a black fruit fly with purple eyes. The offspring
are as follows: wild type, 721; black-purple, 751; gray-purple, 49; black-red, 45. What is the
recombination frequency between these genes for body color and eye color?
5. Two genes are linked and the recombination frequency is 10% based on a total of 1000 plants and
crossing a plant heterozygous for blue petals and round stamens with a plant that has white petals and
oval stamens. The genes are B for blue petals and b for white petals. R is for round stamens and r is
for oval stamens. How many plants of each of the four phenotypes do you expect?
6. A 1:1:1:1 ratio of offspring from a dihybrid testcross indicates that
a. the genes are linked
b. the dominant organisms was homozygous
c. crossing over has occurred
d. the genes are not linked
7. Tomato plants usually have flowers in small groups and produce fruits that are round and smooth.
Two mutations were found:
peach—hair fruits
oblate—flattened fruits
Pure breeding varieties with peach oblate fruits and smooth round fruits were crossed together. The
F1 hybrids, which all had smooth round fruits, were test-crossed with plants that were homozygous for
peach oblate fruits. The numbers in the F2 generation were:
Smooth round 421
Peach oblate 369
smooth oblate 112
peach round 98
A. Identify, with reasons, which tomato plants are recombinants in the F2 generation.
B. Draw a diagram to show how recombinants were produced.
C. Calculate the percentage of recombinants in the cross.
8. If you are in class when you are doing this assignment there is a pedigree on the document camera
from a real IB exam. Do the following with it:
A. Explain, using information from the chart, whether the allele causing the disease is
dominant or recessive.
B. The pattern of inheritance shown in the chart suggests that the condition is sex-linked.
Evaluate the evidence provided by the chart for sex-linkage.
9. For each of pedigree below determine the genotypes of the numbered individuals. Write “AA or
Aa” if it is unknown if the person is homozygous dominant or heterozygous. #1 is an X-linked recessive
disease, #2 is a recessive disease (not sex-linked), and #3 is a sex-linked recessive condition.