Transcript PPT Chapter 09 - McGraw Hill Higher Education

Part 2: Genetics and molecular
biology
Chapter 9: Inheritance
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9-1
Inheritance of a single gene
• Blending inheritance was the popular theory in the
late 1800s as nothing was known of the molecular
nature of genes
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9-2
Gregor Mendel
• Mendel studied pure-breeding lines of pea plants,
in which all progeny are the same as the parent
plants
• His question was: ‘If the traits of the two parents
differ, what do the offspring look like?’
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Monohybrid cross
• Mendel studied seven traits of pea plants, each of
which had two alternative forms (see Fig. 9.2)
• Traits could be studied one at a time
• 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 F1 form was always the same, regardless of
the strain source of pollen or egg
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9-4
Fig. 9.2 (top): Seven traits of garden peas studied
by Mendel
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9-5
Fig. 9.2 (bottom): Seven traits of garden peas
studied by Mendel
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9-6
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
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Fig. 9.3: The results of Mendel’s first type of
experiment
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9-8
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
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9-9
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
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9-10
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
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9-11
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
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9-12
Fig. 9.4: Mendel’s breeding program following the
inheritance of seed colour in peas over two generations
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9-13
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
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Fig. 9.6: Mendel’s breeding program following the
inheritance of both seed colour and seed shape in peas
simultaneously
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9-15
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
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9-16
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
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9-17
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
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9-18
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
• The ABO blood group system illustrates these
points
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Table 9.2: Characteristics of the human ABO
blood group system
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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 (or no antigens)—called
group O when homozygous
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9-21
Codominance and blood groups
(cont.)
• A and B antigens 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
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9-22
Question 1:
Jill and Tom are concerned because they have
blood types A and B respectively, but their new
daughter, Amanda, has blood type O. Does this
mean that Jill or Tom might not be Amanda’s
parents?
a) Only Jill can be Amanda’s Mother
b) Only Tom can be Amanda’s Father
c) Neither Jill nor Tom can be Amanda’s parents
d) Both Tom and Jill can be Amanda’s parents
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9-23
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
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9-24
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
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Fig. 9.7a: Pattern of inheritance of a genetic
disease: cystic fibrosis
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Fig. 9.7b: Pattern of inheritance of a genetic
disease: Huntington disease
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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 an autosome or sex chromosome
(predominantly X)
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Sex determination and linkage
• In many species, from insects such as Drosophila
melanogaster through to humans, sex is
determined 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)
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9-29
Fig. 9.8: Pattern of inheritance of sex
chromosomes in humans
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9-30
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
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9-31
Fig. 9.9: Sex linkage and chromosome inheritance
in Drosophila melanogaster
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X-linked traits in humans
Fig. 9.10a: A pedigree showing inheritance of
colour-blindness
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X-linked traits in humans (cont.)
Fig. 9.10b: A test plate used for detecting colourblindness
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9-34
Question 2:
Red–green colour-blindness is an X-linked
recessive disorder. What is the probability that a
female child who has a colour-blind father, and a
normal sighted mother (whose father was colourblind), would also be colour-blind?
a) ¼
b) ½
c) ¾
d) 0
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9-35
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
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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
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Fig. 9.11a: The wild-type Australian sheep blowfly,
Lucilia cuprina
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Fig. 9.11b: A mutant white (w) fly
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Fig. 9.11c: Bristles on a mutant crooked
bristles fly
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Fig. 9.11d: Genotypes
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Recombination (cont.)
• 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)
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Linkage and recombination
• 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
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Linkage and recombination (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
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Fig. 9.12a: Chromosome 1 of Drosophila
melanogaster
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Fig. 9.12b: Human chromosome 1
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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
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Fig. 9.13: 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)
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More variations (cont.)
• 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
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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
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Epigenetic regulation
• X chromosome inactivation
– in eutherian mammal females, one X chromosome is
inactivated randomly in each cell to equalise the
expression of genes in both sexes
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Epigenetic regulation (cont.)
• 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 (see Box 9.2 in
the textbook)
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Summary
• Genotype is the genetic constitution of an
organism
• Phenotype is an organism’s observable traits,
which depend on both genotype and environment
• Generally, individuals carry two alleles for each
gene, which separate (segregate) into gametes
• Independent assortment: the segregation of alleles
of one gene into gametes has no influence on the
segregation of the alleles of another gene
• Phenotypes may be dominant, codominant or
recessive
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Summary (cont.)
• Absence of independent assortment indicates
gene linkage through location on same
chromosome
• Linked genes can be separated if crossing over
occurs, resulting in recombination. The frequency
of recombination is related to the distance between
the two loci
• Polygenetic traits: many genes, one trait
• Epigenetic regulation: activity of some genes can
be modified
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