Genetics
the study of hereditary variation
© 2007 Paul Billiet ODWS
Phenotype
Organisms have characteristic
appearances
 These appearances may vary from one
individual to another
 The characteristics shown by an
organism is called its phenotype
 (From the Greek phainein = to show and
typos = type)

© 2007 Paul Billiet ODWS
Variations
Variations between organisms may be:
 interspecific – variations between
different species (e.g. tigers have stripes
and leopards have spots)
 intraspecific – variations within a species
(e.g. blood type or height in humans)
It is intraspecific variations that concern us
here.
© 2007 Paul Billiet ODWS
Nature or nurture?


Variations may be influenced by what is inherited
from the parents (the genotype)
Variations may be influenced by the environment
the organism encounters as it grows and develops.
Environment
(nurture)
Genotype
(nature)
Phenotype
© 2007 Paul Billiet ODWS
Studying heredity

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
The Neolithic
revolution
Breeding from the
animals or plants
Gregor Mendel
http://history.nih.gov/exhibits/nirenberg/popup_htm/01_mendel.htm
© 2007 Paul Billiet ODWS
Pea plants for genetics


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Peas have many
recognisable characteristics
(e.g. seed shape)
They are easy to cultivate
Their life cycle is reasonably
short so results can be
obtained quickly
Peas produce a large
number of offspring (seeds),
which makes results easier
to verify
© 2007 Paul Billiet ODWS
http://www.ppdl.purdue.edu/PPDL/images/pisum-sativum.jpg
Pea plants for genetics

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Peas have
hermaphroditic flowers
Self fertilisation is
possible.
The male parts can be
pulled out to
emasculate the
flowers, preventing
self fertilisation
© 2007 Paul Billiet ODWS
jeantosti.com/fleurs4/pois.htm
Mendel’s breeding
experiments
Taking one character only as an example, seed
colour
Female sex cells
Male sex cells in
Parents (P)
from a yellowpollen from a
seeded plant
green-seeded
plant
Cross fertilised (crossed)
First
generation (F1)
© 2007 Paul Billiet ODWS
All seeds produced turned
out yellow
The reciprocal cross
Mendel tried the cross the other way
round
 Green seed female plant x yellow seed
male plant
 The same results were obtained

© 2007 Paul Billiet ODWS
Dominant and recessive
traits
The green seed coloured trait had
disappeared but it reappeared in later
generations as though it were hidden
 Traits that disappear and reappear (e.g.
green seed colour in peas) are called
recessive
 Those that hide them are called dominant
traits (e.g. yellow seed colour in peas)

© 2007 Paul Billiet ODWS
Selfing
Mendel produced a second generation of
plants using the first generation.
 He brushed the male pollen grains onto
the female parts of the same flower
 This is called self pollination and it leads
to self fertilisation or selfing

© 2007 Paul Billiet ODWS
Selfing
First generation (F1)
Yellow seed
producing plants
Selfed
Second generation (F2)
Yellow
seeds
6022
About 75% show the
dominant trait
© 2007 Paul Billiet ODWS
Green
seeds
2001
About 25% show the
recessive trait
Selfing
Second generation (F2)
Yellow
seeds
6022
Green
seeds
2001
Selfed
Selfed
Third generation (F3)
33% produce
yellow seeds only
Pure breeding
© 2007 Paul Billiet ODWS
66% produce a
mixture of yellow
& green seeds
All green
seeds
Pure breeding
Pure breeding
Those plants that only produce one type are
called pure breeding (or true breeding)
© 2007 Paul Billiet ODWS
The particulate theory of
inheritance: Genes
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Mendel concluded from this and other similar
experiments, that characters are controlled by factors
(later called genes)
These genes like separate particles, passed on from
generation to generation
They are not changed or diluted to give intermediates
The gene is the unit of hereditary information
© 2007 Paul Billiet ODWS
Genes and variation
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A character is controlled by a gene that may come in
different types called allelomorphs (meaning “other
forms”) or alleles
These different alleles produce the different traits in a
character
In the above example
 Pea seed colour is controlled by the seed colour
gene
 There are two alleles of this gene (dialleleic), the
yellow allele and the green allele.
 The yellow allele is dominant and the green allele is
recessive.
About 30% of human genes are thought to be diallelelic
© 2007 Paul Billiet ODWS
Symbols for genes

Dominant alleles are given CAPITAL CASE
LETTERS

Recessive alleles are given small case letters

Use letters that look different when written as
small case and capital case (e.g. avoid C, O, P, S,
U)
© 2007 Paul Billiet ODWS
Monohybrid inheritance
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The behaviour of the alleles controlling different
traits revealed patterns in the way they are
inherited
These patters always seemed to be the same so
they are considered as a scientific law
If we only consider the inheritance of one
character (e.g. seed colour) and ignore all the
others (such as flower colour, seed shape etc)
this is called monohybrid inheritance
© 2007 Paul Billiet ODWS
The Law of Segregation

Organisms seem to possess two genes
for each character (they are diploid). This
pair of genes segregate (separate) when
gametes are made (they are haploid)

Pairs of genes are reformed when the
gametes fuse and they recombine in
definite proportions (e.g. 75% to 25%)
© 2007 Paul Billiet ODWS
Mendel and meiosis
Mendel worked this out in 1866
 Though he no doubt understood
fertilisation, meiosis was not observed
until 30 years later

© 2007 Paul Billiet ODWS
Meiosis & Mendel
Meiosis 1: Anaphase 1
 Maternal and paternal
chromosomes
segregate (pulled
separate on the
spindle)
 They move to opposite
poles
© 2007 Paul Billiet ODWS
The Law of Segregation revisited
Organisms seem to possess two genes for
each character (they are diploid). This pair
of genes segregate (separate) when
gametes are made (they are haploid)
= meiosis
 Pairs of genes are reformed when the
gametes fuse and they recombine in
definite proportions (e.g. 75% to 25%)
= fertilisation

© 2007 Paul Billiet ODWS
Combinations of genes
The combination of alleles in an individual
is called the genotype
 If the two alleles are the same it is
homozygous
 If the two alleles are different it is
heterozygous

© 2007 Paul Billiet ODWS
Genotypes



Phenotypes
YY
Homozygous
Yellow
Yy
Heterozygous
Yellow
yy
Homozygous
Green
Pure breeding
Pure breeding
In human genetics heterozygotes who have a
dominant and a recessive allele are called carriers
They are carrying a recessive allele without
expressing it
Many genetic diseases are caused by recessive
alleles
© 2007 Paul Billiet ODWS
Genetic diagrams
P
Phenotypes
Yellow seed
Genotypes
YY
Gametes
F1
Y
X
yy
Y
y
Phenotypes
Yy
Genotypes
Yellow
Proportions
100%
© 2007 Paul Billiet ODWS
Green seed
y
Genetic diagrams
F1
Phenotypes
Yy
Genotypes
Yellow
Proportions
100%
Gametes
Y
y
Where there are several
possible gametes a Punnett
square should be used
F2
Selfed
Genotypes:
Y
y
Y
YY
Yy
y
Yy
yy
Phenotypes:
Yellow
Green
Proportions:
75%
25%
© 2007 Paul Billiet ODWS