Adapted from: Biology Two Activity Manual pages 100
FAMILY PORTRAITS – WHAT PATTERN IS THIS?
Intended learning outcomes
This activity has been designed to enable you to:
 identify the features that distinguish four basic patterns of inheritance
 analyse and synthesise the data in a pedigree, and decide on a likely mode of
inheritance of the trait under consideration
 Draw a pedigree that show a particular pattern of inheritance of a trait
 Recognize that, in some cases, a given pedigree may not contain sufficient
information to reach a conclusion about the mode of inheritance of the trait
concerned.
Introduction
The pattern of occurrence of an inherited trait in a family across several generations
can be shown as a pedigree. In a pedigree, a set of symbols is used to denote features
of the pedigree (Figure 1)
Figure 1
By analysing a pedigree for an unknown inherited trait, it is possible to predict
the likely mode of inheritance of that trait, based on whether the trait is dominant
or recessive and whether the controlling gene is located on an autosome or the X
chromosome.
In this activity, we will focus on the four common patterns of inheritance,
namely:
• autosomal dominant • X-linked dominant
• autosomal recessive • X-linked recessive.
However, you should note that other patterns of inheritance can also occur.
PART A: Pedigree analysis
Your teacher will identify whether you are to work alone or in groups.
Your task is to analyse the pattern present in a pedigree and to identify the
possible mode(s) of inheritance of a trait. In carrying out this task, you will use
both methods of exclusion as well as of confirmation. You should note that in
some cases a pedigree may not contain the critical information needed to reach a
decision and, in that case, the pedigree is said to be non-informative.
Pedigrees may be analysed in several ways. Here is one approach that can assist
this process.
Is the trait dominant or recessive?
To help decide whether a trait is dominant or recessive, a pedigree can be
examined and the following question asked:
A. Does every affected person have at least one affected parent?
If the answer is `No', it is reasonable to conclude that the trait is not dominant and
so is a recessive trait. If the answer is `Yes', we cannot immediately exclude one
or other possibility, and the information in the pedigree is inconclusive.
1. Look at the two pedigrees in figure 2 and answer question A above.
Figure 2
2. Notice that in the case of the Smith family, each affected person does not
have at least
one affected parent; III-2 is the critical person. It is reasonable to conclude
that this trait
cannot be dominant, but is recessive.
2. Notice that in the case of the Jones family, each affected person has at one
affected
parent. We cannot conclude on the basis of this information whether the
trait is
dominant or recessive - both conclusions are valid, so these data are
inconclusive.
Q1. Knowing that the trait in the Smith family pedigree is recessive, ass ign genotypes,
such as RR or Rr or rr, to the persons in that family, as shown, in table 1.
If more than one genotype is possible, indicate both in the table.
9.6A
Person
I-2
I-3
Possible genotype(s)
rr
II-3
III-1
III-2
Table 1
If we are trying to decide whether a trait is dominant or recessive, and the first
question, however, depends on a particular feature being present in the pedigree,
and this will not always be the case.
B. If the pedigree is inconclusive in terms of question A and the pedigree includes
two affected parents with children of both sexes, are all the children affected?
If the answer is ‘No', we can reasonably conclude that the trait is not recessive,
and so is dominant. If the answer is ‘Yes', we cannot immediately exclude one or
other possibility, and the information in the pedigree is still inconclusive.
4. Look at the two pedigrees in figure 3 and answer question B.
5. Notice that in the case of the Liu family, only some of the children (both
sexes) of these
affected parents show the trait; II-5 and II-6 are the critical persons. Since
these children
are not affected, it is reasonable to conclude that this trait is dominant.
Figure 3
Q2. Knowing that the trait is dominant, assign genotypes to each of the Liu
parents (I-1 and I-2) from the following: BB or Bb or bb.
6. Notice that in the case of the Ting family, all the children of these affected
parents show the trait. We cannot conclude on the basis of this information
alone whether the trait is dominant or recessive - both conclusions are valid
Q3. If the trait were dominant, which of the following genotypes would the
Ting parents be expected to have: DD or Dd or dd?
Q4. If the trait were recessive, which of the following genotypes would the
Ting parents be expected to have: RR or Rr or rr?
In summary, analysis of a pedigree using questions A and B may allow one of
the following conclusions:
• the trait is dominant
• the trait is recessive
• the data in the pedigree are inconclusive.
Is the trait autosomal or X-linked?
Assuming that you have been able to identify the dominance or recessiveness of a
trait, the next issue is to try to decide whether the trait is controlled by a gene that
is located on one of the autosomes or whether the gene concerned is located on the
X chromosome.
C. With large numbers, are there equal numbers of affected males and females?
If the answer is ‘Yes' (there is no bias and both sexes are equally affected), then
we can reasonably conclude that the trait is not X-linked but is controlled by an
allele of a gene on an autosome.
If the answer is ‘No' (a bias is apparent and more of one sex is affected than the
other), then we can reasonably conclude that the trait is X-linked - that is,
controlled by an allele of a gene located on the X chromosome.
7. Look at the Boutros family pedigree in figure 4 which shows the pattern for
a dominant trait. What evidence exists in this pedigree to support the
conclusion that this is a dominant trait?
Figure 4: Boutros family
8. To decide whether this is an autosomal or an X-linked trait, answer question
C for the Boutros pedigree. Notice that in the case of this family, a bias is
apparent. The affected father in generation I has passed the trait on to all his
daughters but to none of his sons in generation II. Since this is a large
family, this pattern is more likely to be a bias than a chance effect. It is
reasonable to conclude that this trait is X-linked, so this pedigree shows an
X-linked dominant pattern.
Q5. Are equal numbers of males and females affected in this large
pedigree? Which sex appears to be more likely to be affected by this trait?
Q6. Assume that the father (I-2) has the genotype XDY. Write genotypes for
all other members of this family in a copy of the table 2
.
Person Possible genotype(s)
I-2
XDY
I-1
II-2
II-3
III-2
Table 2
9. Look at the pedigree for the inheritance of a form of red -green colour
blindness in the Jantzen family (figure 5), which shows the inheritance of a
recessive trait. What evidence exists in this pedigree to support the
conclusion that this is a recessive trait?
Figure 5: Jantzen family
10. Now answer question C for the Jantzen pedigree. Notice that in the case of
this family, a bias is apparent. Notice that the affected father (I-2) has
passed the trait on to three of his grandsons in generation III. This is
unlikely to be a chance effect. There is an excess of affected males
compared with females. It is reasonable to conclude that this trait is X linked and this pedigree shows an X-linked recessive pattern of
transmission.
Q7. Do all the daughters of the male I-2 have to be heterozygous carriers
red-green colour blindness?
11. Look at the pedigree for the inheritance of a recessive trait in th e Evans
family (figure 6). What evidence exists in this pedigree to support the
conclusion that the trait is recessive?
Figure 6: Evans family
12. To decide whether this trait is autosomal or X-linked, answer question C for
the Evans pedigree. Notice that in the case of this family, a bias is apparent.
The affected mother (I-1) has passed the trait on to all of her sons, but to none
of her daughters. Overall, more males are affected than females. This is
unlikely to be a chance effect. It is reasonable to conclude that this trait is X -
linked. This pedigree in fact shows an X-linked recessive pattern of
transmission.
In summary, based on the appearance of certain patterns in a pedigree, a
pedigree may be identified as showing an X-linked inheritance. These patterns are:
• transmission of a dominant trait from an affected male to all of his daughters and
none of his sons
• a recessive trait from an affected male to some of his grandsons
• a recessive trait from an affected female to all of her sons and none of her
daughters.
If these patterns are absent, it is reasonable to conclude that the pattern is
autosomal.
In the case of an autosomal trait, affected parents can pass the trait to children of
either sex and, overall, in a large pedigree, about equal numbers of males and
females will be affected
Q8. What is the minimum number of generations required for a pedigree?
Q9. Extend the Jones family pedigree in figure 2 so that it is no longer
inconclusive, but is an example of a dominant trait.