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Contents
Section
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
Genetics Made Easy SL 2016 © Ashby Merson-Davies
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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]
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, e.g. 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 SL 2016 © Ashby Merson-Davies
Now you try this one.
In mice straight ears is
dominant to folded ears. A
mouse homozygous for straight
ears is mated to a mouse with
folded ears. All the offspring
have straight ears. Explain this
with the help of a genetics
diagram.
Key:
Parent phenotypes
Parent genotypes
Gametes
F1 genotype
F1 phenotype
Here is how you could have tackled this problem.
This allows us to choose the letters.
(Don’t use S). Straight = T; folded = t.
In mice straight ears is dominant to folded ears.
A mouse homozygous for straight ears
is mated to a mouse with folded ears.
All the offspring have straight ears.
Explain this with the help of a genetics diagram.
My answer would therefore be –
This mouse therefore has the genotype
TT because it is homozygous.
This mouse has to have the
genotype tt because folded is
recessive.
Key: T = straight, t = folded.
The mouse with straight ears has the genotype TT because we are told it is homozygous. The mouse with
folded ears has to have the genotype tt because the folded allele is recessive.
Parent phenotypes
Parent genotypes
Gametes
straight ears
TT
folded ears
tt
T
F1 genotype
F1 phenotype
t
Tt
straight ears
All the F1 are heterozygous Tt and therefore show the dominant characteristic, straight ears, which
matches the result given in the problem.
Now try these. Remember always to write down your reasoning – it helps to create good logical patterns in
your mind. I have highlighted the key facts for you to focus on.
Purple eyed fruit flies were
mated to homozygous red eyed
flies. If red is dominant deduce
what the genotype and
phenotype of the offspring
were.
1
Answer on page 9.
A mouse with grey fur was
mated with a mouse with white
fur. Both mice were
homozygous. All the offspring
had grey fur. Explain this.
2
Answer on page 9.
Genetics Made Easy SL 2016 © Ashby Merson-Davies
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Problems with only partial information
Sometimes in longer problems involving two or three generations you are only given bits of information. The
simple way here is to draw out the genetic diagram as normal and fill in all the information you are given.
Look at this problem.
Assume that brown eyes in Dingbats is dominant to purple eyes. A Dingbat with brown eyes
mated with one with purple eyes. Two of their babies had purple eyes and one had brown eyes.
The brown eyed baby Dingbat eventually mated with another with brown eyes and their first
baby had purple eyes. Determine the genotypes of all these Dingbats.
The first step is to do the skeleton diagram and put on what we know.
This has to have one B in order to
have brown eyes but for the moment
we leave out the second allele.
Key: B = brown, b = purple.
First generation
Parent phenotypes
Parent genotypes
brown
B?
Gametes G1
B
purple
bb
Bb
brown
Second generation
Parent phenotypes
Parent genotypes
brown
Bb
Gametes G1
Third generation
F1 genotypes and
phenotypes.
B
bb
purple
brown
B?
b
B
b
Summary
First generation
Brown-eyed parent Bb.
Purple-eyed parent bb.
Purple-eyed babies bb.
Brown-eyed baby Bb.
Second generation
Brown-eyed parents both Bb
Third generation
Purple-eyed baby bb.
Since this has purple eyes it has to
be bb and therefore the second b
has to come from the brown-eyed
parent. Therefore this has to be
heterozygous Bb.
b
F1 genotype
F1 phenotype
B
BB
brown
Bb
brown
B
b
Bb
brown
bb
purple
We know this has to be
homozygous recessive.
?
This has to have one B in order
to have brown eyes but for the
moment we again leave out the
second allele.
This is their first baby
and since it has purple
eyes it must be bb,
and so we can say the
unknown brown-eyed
genotype must be Bb.
Try this one.
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?
Answer on page 19.
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Genetics Made Easy SL 2016 © Ashby Merson-Davies