Part 2: Genetics and molecular
biology
Chapter 8: Inheritance
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-1
Inheritance of a single gene
•
•
•
•
•
Blending inheritance was the popular theory in the
late 1800s
Nothing was known of the molecular nature of
genes
Mendel studied pure-breeding lines of pea plants,
in which all progeny are the same as the parent
plants
Traits could be studied one at a time
The question was, if the traits of the two parents
differ, what do the offspring look like?
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-2
Monohybrid cross
•
•
•
•
Mendel studied seven traits of pea plants, each of
which had two alternative forms (see Fig. 8.2)
When pure-breeding lines with each trait were
crossed only one form was present in the offspring
The offspring are called the F1 (first filial)
generation
The form was always the same, regardless of the
strain source of pollen or egg
(cont.)
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-3
Fig. 8.2: Seven traits of garden peas
studied by Mendel (top)
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-4
Fig. 8.2: Seven traits of garden peas
studied by Mendel (bottom)
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-5
Monohybrid cross (cont.)
•
•
•
Plants with yellow seeds crossed with greenseeded plants always had progeny producing
yellow seeds
To determine the fate of the green trait, the yellow
F1 plants were crossed together to produce an F2
generation
In this generation the green trait reappeared in a
proportion of the plants, having been masked in
the F1
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-6
Fig. 8.3: The results of Mendel’s first type
of experiment
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-7
Mendel’s conclusions
•
•
•
•
Each genetic trait must be determined by two
factors—these factors are now known as genes
The two copies of each gene may differ from one
another—copies are known as alleles
Where alleles are the same, the organism is
homozygous for that gene
Where alleles are different, the organism is
heterozygous for that gene
(cont.)
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-8
Mendel’s conclusions (cont.)
•
•
•
•
Alleles do not blend, but remain as discrete units
of inheritance
Where alleles for a single gene are different, only
one is expressed in the phenotype
This allele is said to be dominant over the nonexpressed recessive allele
Because the alleles do not blend, the recessive
allele becomes visible in the F2 generation
(cont.)
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-9
Mendel’s conclusions (cont.)
•
When a trait is produced by a single gene having
two alleles, and one allele is dominant
– the ratio between the dominant and recessive
phenotypes will be 3:1 in the F2 generation
– the ratio of genotypes in the F2 generation is 1:2:1 for the
homozygous dominant, heterozygote and homozygous
recessive respectively
– this ratio was consistent for all the pairs of traits Mendel
studied
(cont.)
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-10
Mendel’s conclusions (cont.)
Principle of Segregation
•
Individuals carry pairs of genes, termed alleles,
that influence particular inherited traits. The alleles
segregate during gamete formation such that any
individual gamete contains only one of each pair of
alleles
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-11
Fig. 8.4: Mendel’s breeding program following the
inheritance of seed colour in peas over two generations
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-12
Dihybrid cross
•
•
•
Mendel also crossed together pure-breeding
strains differing in two unrelated traits e.g. seed
colour and shape
In each case the F1 generation showed the
dominant phenotype of each allele pair: yellow and
round
In the F2 generation the following occurred
– new combinations of traits not present in the parents
– the ratios of different phenotypes were specific and
consistent
(cont.)
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-13
Fig. 8.6: Mendel’s breeding program following the
inheritance of both seed colour and seed shape in peas
simultaneously
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-14
Dihybrid cross (cont.)
•
•
•
Independent assortment is shown in the F2
generation by the presence of every combination
of alleles in equal numbers
There are only four different phenotypes possible
The ratio between double dominant homozygote:
heterozygote (gene 1): heterozygote (gene 2):
double recessive homozygote is 9:3:3:1
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-15
Independent assortment
•
Principle of Independent Assortment
• Alleles of a gene controlling one trait assort into
gametes independently of alleles of another gene
controlling a different trait
• Independent assortment of genes is possible when
the two genes considered are located on different
chromosomes
• The F2 generation phenotype ratio of 9:3:3:1
requires independent assortment
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-16
Multiple effects of single genes
•
Often a single gene affects more than one trait
• The gene allele producing purple pigment in
flowers also produces colour in other parts of the
plant, such as stems
• A coat-colour allele in mammals causes not only
yellow fur but abnormal cartilage development
• This phenomenon is called pleiotropy, where more
than one trait is influenced by a single gene
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-17
Codominance and blood groups
•
•
•
•
Mendel’s analysis required two alleles for each
gene and one to be dominant in the phenotype
Many genes have more than two alleles in a
population
Some alleles are coexpressed in the phenotype
rather than being dominant or recessive
A system which illustrates these points is the ABO
blood group
(cont.)
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-18
Codominance and blood groups
(cont.)
•
•
•
The ABO proteins are antigens on the surface of
red blood cells
A single gene has three alleles, IA, IB and i, of
which each individual has only two
Allele IA produces antigen A, IB produces antigen B
and i has no product—called group O when
homozygous
(cont.)
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-19
Codominance and blood groups
(cont.)
•
•
A and B are separate molecules; when both are
present the blood group is AB since they are
codominant
Either A or B, when present with allele i, will
determine the blood group so each is dominant
over O
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-20
Backcrosses and testcrosses
•
•
•
•
A backcross is a cross between the heterozygous
F1 progeny and either homozygous parent
A cross with the homozygous recessive organism
is called a testcross
Since only the dominant alleles are visible in the
heterozygote, the genotype cannot be
distinguished from homozygous dominant for
those alleles
A testcross reveals the presence of recessive
alleles in the heterozygote
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-21
Mendelian inheritance in humans
•
•
•
Many human traits are inherited by Mendelian
principles
Of particular interest in human genetics are
disease-causing alleles
The inheritance of traits in families is followed
using pedigrees, where people are assigned
symbols depending on their genotype and
phenotype for a particular trait
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-22
Fig. 8.7a: Pattern of inheritance of a
genetic disease: cystic fibrosis
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-23
Fig. 8.7b: Pattern of inheritance of a
genetic disease: Huntington disease
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-24
Patterns of disease inheritance
•
Defined by the pattern of expression of the
disease-causing allele of the gene relative to the
normal one
• Based on the expression of the disease gene in
the phenotype
• Also determined by the location of the disease
gene on autosome or sex chromosome
(predominantly X)
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-25
Sex determination and linkage
•
•
•
Many species, from insects such as Drosophila
melanogaster through to humans determine sex by
chromosomes
These are called sex chromosomes
In each case one sex will have two sex
chromosomes of the same type (homogametic)
and the other will have two different sex
chromosomes (heterogametic)
(cont.)
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-26
Sex determination and linkage
(cont.)
•
•
•
In humans and Drosophila melanogaster , females
have two X chromosomes but males only have
one X and a Y
Males cannot be homozygous or heterozygous for
the alleles on the X—rather they are said be
hemizygous
For sex-linked inheritance the sex of the offspring
matters
– males inherit their X chromosome only from their mother
– females inherit X chromosomes from both parents
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-27
Fig. 8.9: Sex linkage and chromosome inheritance
in Drosophila melanogaster
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-28
X-linkage in humans
Fig. 8.10a: A pedigree showing inheritance of
colour blindness
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-29
X-Linkage in Humans
Fig. 8.10b: A test plate used for detecting colour
blindness
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-30
Linkage on autosomes
•
When genes are located on the same
chromosome they are obliged to travel together
during meiosis—this is called linkage
• During prophase 1 of meiosis, chromatids of
homologous chromosomes exchange information
• These crossing over events are called chiasmata
• Since the homologous chromosomes will be
heterozygous for some genes, alleles will be
recombined
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-31
Recombination
•
•
•
To test for independent assortment a testcross is
done between a double heterozygote and the
double recessive homozygote
If the genes are assorting independently the four
possible phenotypes should be present in the ratio
1:1:1:1
Any deviation from that ratio in the progeny
indicates that the genes are not assorting
independently and may be linked
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-32
Fig. 8.11a: The wild-type Australian sheep
blowfly, Lucilia cuprina
Copyright © Alyscha Hill
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-33
Fig. 8.11b: A mutant white (w) fly
Copyright © Alyscha Hill
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-34
Fig. 8.11c: Bristles on a mutant crooked
bristles fly
Copyright © Alyscha Hill
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-35
Fig. 8.11d: Genotypes
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-36
Recombination
•
•
•
The allele combination present on the original
chromosomes is called the ‘parental’ genotype
New combinations generated by chiasmata are
called ‘recombinant’ genotypes
The presence in the progeny of recombinant allele
combinations indicates that genes concerned are
linked (i.e. on the same chromosome)
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-37
Recombination and linkage
•
The number of the progeny that have recombinant
genotypes is proportional to the distance between
the genes
• Analysis of allele recombination is the basis for
genetic mapping
• Genes are ‘located’ relative to one another by a
series of crosses and measurement of
recombination frequencies between the loci
(cont.)
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-38
Recombination and linkage
(cont.)
•
•
•
The distances are nominal, rather than actual
physical units of distance
The unit is the centimorgan (cM): the number of
recombinant progeny/total progeny x 100
Relative positions of genes have been extensively
mapped by this process
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-39
Fig. 8.12a: Chromosome 1 of Drosophila
melanogaster
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-40
Fig. 8.12b: Human chromosome 1
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-41
More variations
•
Incomplete dominance
– where expression of both alleles leads to an intermediate
phenotype, such as in snapdragon flower colour
•
Gene interactions
– recombined alleles of different genes may interact to
produce new phenotypes (see Fig. 8.14)
•
Gene expression may be conditional, requiring
certain environmental conditions to become visible
– an example is the c coat colour allele in Siamese cats
where the allele is only active at low temperatures (see
Fig. 8.15)
(cont.)
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-42
Fig. 8.14: Eye colour phenotypes of (a) wild-type, and two
mutants (b) brown and (c) scarlet of Drosophila
melanogaster. (d) A different eye colour phenotype, white.
(a)
(c)
(b)
(d)
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-43
Fig. 8.15: A Siamese cat homozygous for
the c pigment allele
Copyright © Jean-Paul Ferrero/AUSCAPE
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-44
More variations (cont.)
•
•
Not all genes are fully expressed in an individual
(expressivity) or in a population (penetrance)
Polygenic traits—influenced by the combined
expression of a number of genes e.g. height and
skin colour in humans
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-45
Epigenetic regulation
•
X-inactivation
– in eutherian mammal females one X chromosome is
inactivated randomly in each cell to equalise the
expression of genes in both sexes
•
Imprinting
– the parental origin of some chromosomes determines the
expression pattern of the genes
– in marsupials the paternal X chromosome is always
inactivated
– an allele on human chromosome 15 can cause different
diseases depending on the parental origin
Copyright  2005 McGraw-Hill Australia Pty Ltd
PPTs t/a Biology: An Australian Focus 3e by Knox, Ladiges, Evans and Saint
8-46