Transcript 9.1.1 Pedigrees - PrelimBio

Blueprint of Life
Topic 9: Pedigrees
Biology in Focus, HSC Course
Glenda Childrawi, Margaret Robson and Stephanie Hollis
DOT POINT(s)
 perform an investigation to construct pedigrees or
family trees, trace the inheritance of selected
characteristics and discuss their current use
Introduction
In order to study inheritance patterns in humans, it would be
ethically unacceptable to carry out controlled breeding or test
crosses to determine their genotypes. A pedigree chart
(biological family tree) is constructed, based on the phenotypes
present in related individuals, and this is used to trace the
inheritance of any particular characteristic.
magazine.100percentrock.com
Pedigrees
These pedigree charts or
diagrams are drawn up in a
universally accepted scientific
format, using standard symbols.
They show an individual’s
biological relatives and their
partners as a series of circles
and squares, linked by lines. The
occurrence of a particular trait
within the family is represented
by shading.
biobook.nerinxhs.org
Pedigrees
A pedigree can therefore be
defined as a graphical
representation of the
inheritance patterns of a
particular trait in related
individuals over a number of
generations.
scisquadtmms.wikispaces.com
Pedigrees
Pedigree analysis is then done to
determine:
■ how many family members
have the disorder
■ the gender (male or female)
of the affected individuals
■ how individuals are related
ghr.nlm.nih.gov
Pedigrees
This information is used to:
■ determine inheritance
patterns
■ assign genotypes to individuals
where possible
■ make predictions about the
risks of an individual inheriting a
genetic disorder, abnormality or
disease.
www.yorku.ca
Current Use of Pedigrees
Pedigree charts allow easy scientific
analysis of the inheritance of genetic
traits within families and are useful
for studying heredity patterns in
humans and other animals.
 In humans, most pedigrees are
analysed to identify and trace
genetic disorders
 In animals, they are useful for
selecting individuals with desirable
traits for breeding purposes.
www.nidcd.nih.gov
Human Pedigrees
By assigning genotypes to individuals and making predictions from
pedigrees, human pedigrees may be used to:
■ determine if particular family traits are genetically inherited
■ trace the occurrence of a genetic disorder, abnormality or
disease within a family over several generations
■ deduce genotypes: determine the probability that prospective
parents are heterozygous for (and therefore carriers of) a
particular defective allele
■ predict the likelihood of a family member inheriting a trait or
developing a disorder.
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Human Pedigrees
This type of information is used by genetic counsellors to advise
parents on minimising or avoiding the risks of producing a child
with the defect. This pedigree information may also be used by
researchers to develop a program to eliminate the inherited defect
in the population.
northwest-hospitals.kaiserpermanente.org
Human Pedigrees
To study what gene causes a
disorder, researchers use
pedigrees to identify and then
select only those individuals that
are affected or at risk, to refine
the study and to limit gene
analysis to relevant individuals.
geneticcounselingcareer.com
Human Pedigrees
For example pedigrees were used in recent Australian studies of
breast cancer in families and revealed that affected individuals all
have two specific low-risk genes that may significantly increase the
probability of a person getting breast cancer when they occur in
combination within an individual.
www.jewishgenetics.org
Animal Pedigrees
Pedigrees are useful in animal breeding to:
■ select suitable individuals for stud breeding by identifying any
desirable traits
■ predict the distance in relatedness (and therefore genetic
difference) between sires and dams to prevent inbreeding. A
pedigree index is calculated, based on the distance in relatedness
of the parents and this is assigned to the offspring. Offspring that
are distantly related tend to be healthier than inbred organisms.
■ verify the thoroughbred status of animals by breeding societies.
a
Limitations of Pedigrees
Pedigrees do, however, have their limitations. They are useful only
when studying animals that do not produce too many offspring,
e.g. mammals.
www.ml4.org
Limitations of Pedigrees
In humans, the usefulness of pedigrees relies on accurate and
reliable recordkeeping within families (e.g. of deceased relatives).
If a family is small or has too few affected members, conclusions
may be ambiguous.
duckman.pettho.com
Limitations of Pedigrees
Sometimes, the same genetic
defect may be due to changes in
different genes, so unrelated
carriers of a similar defect may
be inaccurately assessed as being
at high risk of having a
‘defective’ child.
learn.genetics.utah.edu
How to Construct a Pedigree
Step 1: Gather phenotypic records of that trait in family members
over several generations. Use symbols to represent the various
family members, showing whether they are male (square) or
female (circle) and whether or not they possess the trait being
studied (shaded = trait being studied is present).
a
How to Construct a Pedigree
Step 2: Assign a number (Roman numeral) to each generation of
the family tree and another number (Arabic numeral) to each
individual in that generation.
a
How to Construct a Pedigree
Step 3: Use linking lines to represent relationships between
people —marriage (horizontal line) and offspring (vertical line).
a
Activity: Constructing and analysing a
family pedigree
Aim: To construct a pedigree or family tree and to analyse the
chart to trace the inheritance of a selected characteristic.
Procedure
Step 1: Select a genetically determined trait or characteristic
that can be studied in your family.You may use one of the
characteristics in the table you received earlier or any other
easily distinguished trait that shows Mendelian inheritance.
Procedure
Step 2: Using members of your family and their knowledge of
other family members, draft a family tree. Ensure that your
family tree consists of at least three generations and no less than
12 individuals.
Procedure
If you do not have access to direct data about your family
members, make deductions about the probable trait for missing
family members, using any information you already have (e.g.
traits of offspring, photos and family stories)—indicate this on
the family tree with an asterisk (*)
Procedure
Step 3: Analyse the data that you have collected for each
individual and determine which allele is dominant. Assign a
suitable letter of the alphabet to represent the dominant and
recessive alleles for this trait.
Procedure
Step 4: Draw a key at the top or side of your pedigree chart to
indicate clearly which form of the trait is dominant and what
letters you have assigned to each allele.
Procedure
Step 5: Interpret the data collected to allow you to assign
genotypes to each individual in the pedigree. It may not be
possible to determine whether an individual is heterozygous or
homozygous for the trait (e.g. an individual whose phenotype
resembles the dominant parent and the evidence from the genotypes of
relatives is inconclusive). If this occurs, indicate both possible
genotypes (e.g. GG or Gg) on the pedigree.
Procedure
Step 6: Calculate the frequency (%) of the trait in your family:
Frequency (%) =
𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑝𝑒𝑜𝑝𝑙𝑒 𝑤𝑖𝑡ℎ 𝑡ℎ𝑒 𝑐ℎ𝑎𝑟𝑎𝑐𝑡𝑒𝑟𝑖𝑠𝑡𝑖𝑐
𝑡𝑜𝑡𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑦𝑜𝑢𝑟 𝑠𝑎𝑚𝑝𝑙𝑒
x 100
Procedure
Step 7: Make predictions about the chance of your future
offspring inheriting the affected trait as described below.
Highlight the symbol that represents yourself in the pedigree.
Predict the chance of your possible offspring inheriting the trait
studied if you were to partner:
(a) a homozygous recessive individual
(b) a heterozygous individual
(c) a homozygous dominant individual.
Show your working for each answer by drawing a genetic cross
(use a Punnett square or a branch diagram).
Activity
-Students to complete pedigree activity