Blueprint of Life
Topic 8: Solving Genetic Problems
Biology in Focus, HSC Course
Glenda Childrawi, Margaret Robson and Stephanie Hollis
DOT POINT(s)
 solve problems involving monohybrid crosses using
Punnett squares or other appropriate techniques
Introduction
Mendel’s laws can be directly applied to solve genetic problems.
In this lesson we are going to explore this.
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Introduction
The model of Mendelian
inheritance is used to make
predictions of the possible
outcomes when certain
individuals are cross-bred. His
laws provide a very good
problem solving tool to
calculate the probability of a
particular genetic trait (such as
blue eyes or dimples) being
passed on.
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Introduction
This is particularly important because it can be used to predict
the inheritance of genetic defects that arise as a result of a faulty
gene, e.g. albinism, cystic fibrosis, Tay-Sachs disease, night
blindness and short-sightedness. Some genetic defects are lethal
in the homozygous recessive condition and so parents may be
counselled on the probability of their having offspring inheriting
this genotype if the genotypes of both parents are known.
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Punnett Squares
Genetic problems may be solved using Punnett squares or
branching diagrams. Both methods use standard genetic
notation. Letters of the alphabet are used to represent genes:
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Punnett Squares
To represent a genotype that
follows the law of dominance,
only one letter is used to
represent the gene (e.g. the
letter T for height). The
dominant allele is represented
by a capital (upper case) letter
(e.g. T ) and the recessive allele
is represented by the lower
case form of the same letter
(e.g. t).
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Punnett Squares
At the start of a genetic
problem, there should always
be an outline of which letters
will be used to represent the
genotype. For example to
investigate the probability of a
person inheriting curly hair,
one would begin by stating:
Let C = curly hair and c =
straight hair.
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Punnett Squares
The different stages of the cross being investigated are outlined
by clearly labelling the generations:
P = the parent generation
F1 = the first filial generation
F2 = the second filial generation
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Indicate at which stages in the cross segregation occurs, gametes
are formed and fertilisation occurs.
Punnett Squares
Step 1: Designate letters that will represent the genes/traits.
For example:
R = wrinkled seeds, r = round seeds
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Punnett Squares
Step 2: Write down the
genotypes (genes) of each
parent. These may be stated in
the outline of the problem or
you may be expected to
determine the genotypes by
logical deduction from other
information that has been given.
P: Rr × Rr (both heterozygous
or hybrid)
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Punnett Squares
Step 3: List the genes that each parent can contribute to the
gametes.
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Punnett Squares
Step 4: Draw a Punnett square to
show all the possible combinations
of genes when gametes fuse. A
Punnett square consists of nine cells
in a table (three cells in each row
and each column). Write the
possible genes in the gametes of one
parent across the top and the genes
from the gametes of the other
parent along the side of the Punnett
square see (blue letters below).
R
R
r
r
Punnett Squares
Step 5: To show how genes
recombine during fertilisation,
complete each cell of the
Punnett square by combining
the letters above and beside the
appropriate cell. As a general
rule, the capital letter goes first
and a lowercase letter follows
(see red letters on the previous
page).
Punnett Squares
Step 6: Write out the genotypic ratio and phenotypic ratios for
the offspring (see orange text beneath the Punnett square).
Punnett Squares
An alternative to using a Punnett square is to use a branching
diagram. The use of these has been demonstrated in the
explanation of Mendel’s laws. Steps 4 and 5 would be replaced
by drawing branching lines from gametes to offspring.
Genetic Problems
Genetic problems come in
many forms. To solve a
genetics problem, you need to
use the information given—
read it, analyse it, see what
deductions you can make from
it and then perform a cross to
work out the information you
do not know.
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Genetic Problems
It is important to determine
which trait is dominant—this
information may be given to
you in the question, or you may
be expected to work it out by
deductive reasoning. Some
common dominant and
recessive traits are listed in
Table 2.4.
Task
Using the table, identify the dominant and recessive traits
evident in the photographs below.
Activity/Homework
-Students to solve Problems 1-5