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Contents
Section
Page
Part One
Getting Started – Vocabulary and Definitions
First Steps – Symbols, Phenotypes and Genotypes
The Punnett Grid
The Basic Monohybrid Cross with a Homozygous Genotype
The Basic Monohybrid Cross with a Heterozygous Genotype
Genetic Ratios
The Test Cross
More than one generation
The 3 : 1 Ratio
Ratios and Probability – Predicting Offspring
Problems with only partial information
Codominance
Multiple Alleles and Codominance
The Sex Chromosomes and Sex Linkage
Predicting Probability in Sex Linkage Problems
Multiple Choice questions
Answers to the Multiple Choice questions
Pedigree Charts
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28
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34
35
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Part Two
Getting Started on HL– Definitions
The Dihybrid Cross
Using Genotype Numbers
The 4 x 4 Grid
Autosomal Linkage
Autosomal Linkage and Crossing Over
Recombinants
The Optional Section
Genetics Made Easy HL 2016 © Ashby Merson-Davies
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1
Introduction
Many students find genetics hard going but in fact it is actually easier than you realised once you have
grasped the rules and patterns. Genetics problems usually either start by giving you the parents and asking
you to determine the offspring, or vice versa. All the information you need is actually in the question.
This guide takes you through from the very basics assuming you have done no genetics at all, to some quite
challenging problems. After each new topic there are several problems for you to try and just after them are
the answers.
Genetics problems in exams are usually about fruit flies, mice, peas, humans, or other familiar organisms.
Some of these appear in this guide, but to relieve the repetition most of the organisms have come from my
imagination. This does not matter – the genetics is still the same.
By working your way steadily through the guide, writing your answers in the question boxes and then
checking these against mine, you should end up by wondering what the difficulty was. Good luck!
I would greatly value any feedback on this guide. Please feel free to email me at Oxford Study Courses –
[email protected]
Part One
Getting Started
The first thing you must do is learn the vocabulary and definitions.
Chromosome ……..… Single long molecule of DNA, associated with protein, that carries the genes in linear
order.
Gamete ……………… General name for the haploid sex cells, eggs, sperm cells and pollen grains.
Homologous …….….. Chromosomes in a diploid cell which contain the same sequence of genes but are
derived from different parents.
Gene ……………..….. The basic unit of inheritance by which hereditary characteristics are transmitted from
parent to offspring.
Allele ……………….… One of a number of alternative forms of a gene that can occupy a given gene locus on
a chromosome.
Locus …………….…… The specific position on a homologous chromosome of a gene.
Phenotype ………...…. The characteristics of an organism.
Genotype ……..……… The alleles possessed by an organism.
Dominant ……………... An allele that has the same effect on the phenotype when in either the homozygous
or heterozygous state.
Recessive ………..…… An allele that only has an effect on the phenotype when it is in the homozygous state.
Codominant ………….. Pairs of alleles that both affect the phenotype when present in the heterozygous state.
Homozygous…………. Having two identical alleles at a gene locus.
Heterozygous ……...... Having two different alleles at a gene locus.
Pure breeding ………… Having a homozygous genotype. Usually used to describe a dominant phenotype.
Sex chromosomes …. The pair of chromosomes that control the sex or gender of the individual. In mammals
and some other organisms, eg the fruit fly, these are the X and Y chromosomes.
Homogametic ……….. The gender that has an identical pair of sex chromosomes, XX. This is the female.
Heterogametic ….…... The gender that has two different sex chromosomes in the pair, XY. This is the male.
Sex linkage ……….…. The pattern of inheritance characteristic of genes located on the sex (X)
chromosomes.
Carrier …………..…… A female that has a dominant allele on one X chromosome and one recessive allele
on the other.
Test cross …………… Using a homozygous recessive to test a phenotypically dominant phenotype to
determine if it is heterozygous or homozygous.
Monohybrid cross …… A genetic cross involving only one characteristic,
F1 and F2 ………..…… The first and second generations. The F2 is obtained by crossing F1 x F1.
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Genetics Made Easy HL 2016 © Ashby Merson-Davies
The Basic Monohybrid Cross with a Homozygous Genotype
In a homozygous genetics problem each of the parents produces one type of gamete and these fuse
together to produce the offspring. Let’s look at this problem.
In humans let green eye colour be dominant over blue eye colour. If Mum and Dad both have
blue eyes what colour eyes will their children have?
This allows us to choose
Read the question very carefully and pick out any facts you are given.
letters for the two alleles
In humans let green eye colour be dominant over blue eye colour.
– let G = green because
If Mum and Dad both have blue eyes
it is the dominant allele,
what colour eyes will their children have?
and g = blue because
that is the recessive and
Since blue is recessive this
you use the lower case
tells us that both Mum and
of the same letter.
Dad have to be
homozygous recessive gg.
Now set out your genetics diagram. You should always set this out the same way each time. The reason for
this is that it reduces the possibility of mistakes.
Key: G = green; g = blue.
Mum
Parent phenotypes
blue eyes
Parent genotypes
gg
Gametes
Dad
blue eyes
gg
g
Since both parents only
produce one type of
gamete a Punnett grid for
the F1 is not needed.
g
F1 genotype
F1 phenotype
gg
blue eyes
Note that I have only put one gamete from each parent and not two. This makes it simpler and does not
affect the outcome, as shown below.
Mum
blue eyes
gg
Parent phenotypes
Parent genotypes
Gametes
F1 genotypes and
phenotypes
g
g
Dad
blue eyes
gg
g
g
male gametes
g
g
gg
gg
blue
eyes
blue
eyes
gg
gg
blue
eyes
blue
eyes
female gametes
g
g
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Genetics Made Easy HL 2016 © Ashby Merson-Davies
12 Assume green eyes in humans is dominant to blue eyes. Clive has blue eyes but his parents
Jonathan and Francine both have green eyes. Clive marries Sally, who has green eyes, and
their first baby, Cynthia, has blue eyes. Produce a diagram to show the genotypes of this
family. If Clive and Sally have another baby what is the probability that it will have green eyes.
Jonathan and Francine both have green eyes and therefore must
each have a G allele, but since their son Clive has blue eyes and
therefore the genotype gg, they must also each have a g allele.
Key: G = green, g = blue
First generation
Parent phenotypes
Parent genotypes
Jonathan
green
Gg
Gametes G1
G
Francine
green
Gg
g
g
Second generation
Parent phenotypes
Parent genotypes
Clive
blue
gg
Gametes G1
g
Since Sally has green eyes
she must have a G allele,
but she must also have a g
allele so that Cynthia can
have blue eyes with the
genotype gg.
Sally
green
Gg
g
Third generation
F1 genotypes and
phenotypes
g
G
G
g
G
gg
Gg
blue
Cynthia
green
The Punnett grid shows that there is a 50% chance that the second child of Clive and Sally will have
green eyes.
Codominance
So far we have looked at the situation where one allele is dominant over the other allele. Codominant alleles
both affect the phenotype when heterozygous. Codominance therefore produces a different phenotype for
each of the three genotypes, i.e. the heterozygous genotype has a phenotype that is a mixture of the two
alleles.
There is though another term, incomplete dominance, and there is a slight difference between the two.
Codominance is when both alleles are expressed in the phenotype.
Incomplete dominance is when one allele is only partially expressed over the other allele.
The ABO blood grouping, sickle cell, and the Camelia
Lady Vansittart show codominance.
The Snapdragon (Antirrhinum majus) and the Four
O’Clock flower (Mirabilis jalapa) show incomplete
dominance.
White x red Æ pink.
The Camelia
Lady Vansittart.
X
Æ
The IB group both types of expression as codominance.
Genetics Made Easy HL 2016 © Ashby Merson-Davies
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