BIOLOGY UNIT 4
INHERITANCE SAC
NAME:
Suggested solutions
Be aware that for some questions there would have been more than one acceptable answer.
Q1. Using the data from all the groups determine the phenotypic ratio of orange fish to black fish.
Class data
Oranage
1
:
Black
3.2
Q2. How does the ratio of a particular group’s data compare to that of the class data?
The ratio for each group varied quite a bit. Some were greater than the class data, some below. For
example, Group 4’s ratio was 1 (orange) : 9 (black) whereas group 1’s was 1 (orange) : 1.5 (black).
Q3. Which set of data is likely to be more reliable-group or class? Explain.
The class data is more reliable as it uses a larger sample size. Therefore, chance has less of an
influence on data varying from that which is theoretically expected.
Q4. The fish eggs were obtained from two heterozygous parents that displayed the black phenotype.
Is black body the dominant or recessive characteristic? Explain.
Heterozygous individuals contain one of each allele. The phenotype seen in such an individual will
be the dominant phenotype because it is displayed even in the presence of the allele responsible for
the recessive phenotype.
Q5. Use appropriate notation to assign genotypes to the heterozygous parents that the experimental
eggs were obtained from.
B – black colour
b – orange colour
Q6. Based on your answers to questions 4 and 5, what are all the possible genotypes of the newly
hatched fish?
Possible genotypes are
BB
Bb
bb
Q7. To be able to compare the group results and combine them into class data, the experimental
procedure was kept consistent. List three factors that would have been controlled to allow this to
happen.
Amount of time (5 days) before making observations
Water temperature of the aquariums
Same number of eggs in each group
pH of the water in each aquarium
Q8. In any typical monohybrid cross between two heterozygous individuals that involves an
autosomal dominant-recessive pattern of inheritance, what is the expected ratio of phenotypes in the
offspring?
Expected ratio =
3 (dominant) : 1 (recessive)
BIOLOGY UNIT 4
INHERITANCE SAC
NAME:
Q9. The inheritance of curled wings and ebony body colour is said to be linked. Explain the
meaning of ‘linked’.
Linked means the two genes for the two characteristics are found on the same chromosome and
therefore inherited together.
When pure breed flies that are wild for both traits are crossed with pure breed flies that have black
body and vestigial wings, no F1 offspring have black bodies or vestigial wings.
Q10. List which characteristics are dominant and which are recessive and assign suitable symbols
for each of the alleles responsible.
N – normal colour (wild) – dominant
n – black colour – recessive
R – normal wings (wild) – dominant
r – vestigial wings – recessive
Q11. Imagine these two traits are controlled by genes on separate chromosomes. Two F1 flies are
crossed and they produce 1600 offspring. What phenotypes would you expect in the F2 generation
and in what numbers?
900
normal colour
normal wings
300
300
black colour
normal wings
normal colour
vestigial wings
100
black colour
vestigial wings
A group of students decide to determine whether the two genes are linked or on separate
chromosomes. They cross an F1 fly with a pure breed fly that has black body and vestigial wings.
Q12. What term is given to this procedure?
Test cross
Q13. Do you results from the cross between an F1 fly and a pure breed fly with black body and
vestigial wings support the notion that the two genes are linked? Explain.
Yes the results support linkage between the two genes. In a test cross we would expect each
phenotype to occur in same numbers if they are not linked, ie 1:1:1:1 ratio for all four phenotypes.
This is not the case as two phenotypes have higher numbers than two others. When linked we
expect the offspring to have same numbers of the parent phenotypes and lower numbers o the
“unexpected” phenotypes which is the case here.
Q14. What term is given to the offspring that have phenotypes different to that of the parents used
in the cross?
recombinant offspring
BIOLOGY UNIT 4
INHERITANCE SAC
NAME:
Q15. Imagine if the locus of the black body gene was in between the dumpy wings and curly wings
genes. Predict what sort of results you would now expect from the cross that was carried previously
by completing the table below (use values based on a total of 500 offspring).
Phenotypes of the offspring
No. of
offspring
Wild type for body
and wings
Black body and
vestigial wings
Black body and
wild type wings
Wild type body and
vestigial wings
170
180
70
80
Any numbers will do here, as long as the recombinants are greater than in the previous table and the
first two phenotypes are in similar numbers and higher than the recombinants. All must add to 500
too.
Q16. Explain your table of predicted results and any differences/similarities with the original table.
Seeing as the two genes black and vestigial are now further apart we would expect more
opportunity for crossing over to occur therefore more recombinant gametes being produced as a
result of meiosis. Therefore the recombinant phenotypes from a test cross should be greater than
previously, as a result the numbers of the “expected” phenotypes would have to be lower. The table
is similar to the previous table in that irrespective of how far apart the two genes are on a
chromosome, the two “expected” phenotypes (wild for both and black/vestigial) should occur in
equal numbers and the two recombinant phenotypes should also be approximately equal.
Q17. A genetic counsellor was investigating the pattern of inheritance of a genetic trait in a family.
She drew up the following pedigree, where shaded individuals have the trait.
a) From the pedigree is it possible to determine definitively whether the trait is autosomal dominant
or autosomal recessive Explain using evidence from the pedigree.
No it is not possible to distinguish between dominant and recessive. The trait occurs in each
generation therefore it satisfies dominant inheritance as all affected individuals must have at least
one affected parent. But it can also be recessive. Individual I1 can be heterozygous and therefore
there is 50% chance that children can either have the tait or not.
b) Can you discount the mode of inheritance as being X-linked recessive? Explain using evidence
from the pedigree.
It cannot be X-linked recessive as the II1 is affected therefore all her sons will get the allele for the
recessive trait and show the trait. Only one son has the trait the other isn’t affected.
Q18. The following pedigree shows the inheritance of a particular trait in a family that is controlled
by a gene on chromosome 10. Shaded individuals have the trait.
a) Explain why the inheritance cannot be autosomal recessive.
It cannot be recessive as the parents in generation I which both show the trait would have had to
produce offspring that all display the trait. As parents would only have one type of allele. This is not
the case; therefore it must be an autosomal dominant condition.
BIOLOGY UNIT 4
INHERITANCE SAC
NAME:
b) The type of inheritance shown is autosomal dominant. Explain why individual II-4 does not
show the trait despite having two parents that have the trait.
The parents are both heterozygous, let’s say Mm. It is possible for them to have a child that is mm
and therefore does not show the trait.
c) Using appropriate symbols, determine the genotypes of each of the individuals in the pedigree.
I-1 Mm
I-2 Mm
II-1 M-
II-2 M-
II-3 M-
II-4 mm
III-1 mm
III-2 mm
II-5 mm
Note that it is okay to write MM or Mm as an answer for the ones that have MQ19 Scallops can show three different phenotypes with respect to shell colours: orange, yellow and
black. It is known that the colour is under the control of one gene locus with two alleles found on an
autosome.
a) Define the term phenotype.
Characteristic or trait observed in an individual as a result of the expression of genes interacting
with the environment.
b) Suggest a possible mode of inheritance to explain how three phenotypes are possible when there
are only two alleles for a single gene.
Possible mode of inheritance: co-dominance or incomplete (partial) dominance
c) Based on the results, explain why all three phenotypes were observed when two orange shell
scallops were crossed. Assign suitable alleles and show all relevant working/diagrams.
Orange shelled individuals are heterozygous, containing one of each allele. When mated they will
produce three possible phenotypes as shown in the punnet square in a ratio of 1: 2: 1.
B – black
Y – yellow
Orange individuals – BY
gametes
gametes
B
Y
B
BB
BY
Y
BY
YY
BB = black, BY = orange and YY = yellow in a ratio of 1:2:1 respectively.