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Genetics and Evolution
3.1 Genes
3.2 Chromosomes
3.3 & 10.1 Meiosis
3.4 & 10.2 Inheritance
10.3 Gene pools and speciation
3.5 Genetic modification and biotechnology
Genes
Essential idea: Every living organism inherits
a blueprint for life from its parents.
Genes
• A gene is a heritable factor that consists of a
length of DNA and influences a specific
characteristic.
• A gene occupies a specific position on a
chromosome.
Genes
• The various specific forms of a gene are alleles.
• Alleles differ from each other by one or only a
few bases.
• New alleles are formed by mutation.
Genes
• Application: The causes of sickle cell anemia,
including a base substitution mutation, a change
to the base sequence of mRNA transcribed from
it and a change to the sequence of a
polypeptide in hemoglobin.
Genes
• The genome is the whole of the genetic
information of an organism.
• The entire base sequence of human genes was
sequenced in the Human Genome Project.
Genes
• Application: Comparison of the number of genes
in humans with other species.
Genes
• Skill: Use of a database to determine differences
in the base sequence of a gene in two species
(Genbank database, cytochrome C sequence).
Chromosomes
Essential idea: Chromosomes carry genes in a
linear sequence that is shared by members
of a species.
Chromosomes
• Prokaryotes have one chromosome consisting
of a circular DNA molecule.
• Some prokaryotes also have plasmids but
eukaryotes do not.
• Eukaryote chromosomes are linear DNA
molecules associated with histone proteins.
• In a eukaryote species there are different
chromosomes that carry different genes.
Chromosomes
• Homologous chromosomes carry the same
sequence of genes but not necessarily the same
alleles of those genes.
• Diploid nuclei have pairs of homologous
chromosomes.
• Haploid nuclei have one chromosome of each
pair.
Chromosomes
• The number of chromosomes is a characteristic
feature of members of a species.
• Application: Cairns’ technique for measuring the
length of DNA molecules by autoradiography.
Chromosomes
• Application: Comparison of genome size in T2
phage, Escherichia coli, Drosophila
melanogaster, Homo sapiens and Paris
japonica.
Comparison of genome sizes
Organism
Genome size
(base pairs)
Common name
T2 phage
180,000
Virus that attacks E. coli
E. Coli
5,000,000
Gut bacterium
D. melanogaster
140,000,000
Fruit fly
H. Sapiens
3,000,000,000
Human
P. japonica
150,000,000,000
Canopy plant ( a woodland
plant)
Chromosomes
• Application: Comparison of diploid chromosome
numbers of Homo sapiens, Pan troglodytes,
Canis familiaris, Oryza sativa, Parascaris
equorum.
Comparison of 2n chromosome No.
Organism
2n number
Common name
P. equorum
4
Horse threadworm
O. sativa
24
Rice
H. Sapiens
46
Human
P. troglodytes
48
Chimpanzee
C. familiaris
78
Dog
Chromosomes
• A karyogram shows the chromosomes of an
organism in homologous pairs of decreasing
length.
• Sex is determined by sex chromosomes and
autosomes are chromosomes that do not
determine sex.
Chromosomes
• Application: Use of karyograms to deduce sex
and diagnose Down syndrome in humans.
Chromosomes
• Skill: Use of databases to identify the locus of a
human gene and its polypeptide product.
Meiosis
Essential idea: Alleles segregate during
meiosis allowing new combinations to be
formed by the fusion of gametes.
Essential idea: Meiosis leads to independent
assortment of chromosomes and unique
composition of alleles in daughter cells.
Meiosis
• One diploid nucleus divides by meiosis to
produce four haploid nuclei.
• The halving of the chromosome number allows
a sexual life cycle with fusion of gametes.
• DNA is replicated before meiosis so that all
chromosomes consist of two sister chromatids.
• Chromosomes replicate in interphase before
meiosis. (HL)
Meiosis
• The early stages of meiosis involve pairing of
homologous chromosomes and crossing over
followed by condensation.
Meiosis (HL)
• Crossing over is the exchange of DNA material
between non-sister homologous chromatids.
• Crossing over produces new combinations of
alleles on the chromosomes of the haploid cells.
• Chiasmata formation between non-sister
chromatids can result in an exchange of alleles.
• Skill: Drawing diagrams to show chiasmata
formed by crossing over.
Meiosis
• Orientation of pairs of homologous
chromosomes prior to separation is random.
• Separation of pairs of homologous
chromosomes in the first division of meiosis
halves the chromosome number.
• Homologous chromosomes separate in meiosis
I. (HL)
• Sister chromatids separate in meiosis II. (HL)
Meiosis
• Skill: Drawing diagrams to show the stages of
meiosis resulting in the formation of four haploid
cells.
Meiosis
• Crossing over and random orientation promotes
genetic variation.
• Independent assortment of genes is due to the
random orientation of pairs of homologous
chromosomes in meiosis I. (HL)
• Fusion of gametes from different parents
promotes genetic variation.
Meiosis
• Application: Non-disjunction can cause Down
syndrome and other chromosome abnormalities.
• Application: Studies showing age of parents
influences chances of non-disjunction.
Inheritance
Essential idea: The inheritance of genes
follows patterns.
Essential idea: Genes may be linked or
unlinked and are inherited accordingly.
Inheritance
• Mendel discovered the principles of inheritance
with experiments in which large numbers of pea
plants were crossed.
Inheritance
• Gametes are haploid so contain only one allele
of each gene.
• The two alleles of each gene separate into
different haploid daughter nuclei during meiosis.
Inheritance
• Fusion of gametes results in diploid zygotes with
two alleles of each gene that may be the same
allele or different alleles.
Inheritance
• Dominant alleles mask the effects of recessive
alleles but co-dominant alleles have joint effects.
• Many genetic diseases in humans are due to
recessive alleles of autosomal genes, although
some genetic diseases are due to dominant or
co-dominant alleles.
• Application: Inheritance of cystic fibrosis and
Huntington’s disease.
• Application: Inheritance of ABO blood groups.
CF
Huntingdon's
disease
ABO blood groups
Inheritance
• Some genetic diseases are sex-linked. The
pattern of inheritance is different with sex-linked
genes due to their location on sex
chromosomes..
• Application: Red-green colour blindness and
haemophilia as examples of sex-linked
inheritance.
• Skill: Construction of Punnett grids for predicting
the outcomes of monohybrid genetic crosses.
Inheritance
• Skill: Comparison of predicted and actual
outcomes of genetic crosses using real data.
• Skill: Analysis of pedigree charts to deduce the
pattern of inheritance of genetic diseases.
Inheritance
• Many genetic diseases have been identified in
humans but most are very rare.
• Radiation and mutagenic chemicals increase
the mutation rate and can cause genetic
diseases and cancer.
• Application: Consequences of radiation after
nuclear bombing of Hiroshima and accident at
Chernobyl.
Inheritance (HL)
• Gene loci are said to be linked if on the same
chromosome.
• Unlinked genes segregate independently as a
result of meiosis.
Inheritance (HL)
• Application: Completion and analysis of Punnett
squares for dihybrid traits.
• Skill: Calculation of the predicted genotypic and
phenotypic ratio of offspring of dihybrid crosses
involving unlinked autosomal genes.
• Skill: Identification of recombinants in crosses
involving two linked genes.
• Application: Morgan’s discovery of nonMendelian ratios in Drosophila.
Inheritance (HL)
• Variation can be discrete or continuous.
• The phenotypes of polygenic characteristics
tend to show continuous variation.
• Application: Polygenic traits such as human
height may also be influenced by environmental
factors.
Inheritance (HL)
• Chi-squared tests are used to determine
whether the difference between an observed
and expected frequency distribution is
statistically significant.
• Skill: Use of a chi-squared test on data from
dihybrid crosses.
Critical values table for Chi-squared
Gene pools and speciation (HL)
Essential idea: Gene pools change over time.
Gene pools and speciation (HL)
• A gene pool consists of all the genes and their
different alleles, present in an interbreeding
population.
• Evolution requires that allele frequencies
change with time in populations.
Gene pools and speciation (HL)
• Reproductive isolation of populations can be
temporal, behavioural or geographic.
• Skill: Comparison of allele frequencies of
geographically isolated populations.
Temporal...
Behavioural...
Geographic...
Gene pools and speciation (HL)
• Speciation due to divergence of isolated
populations can be gradual.
• Application: Identifying examples of directional,
stabilizing and disruptive selection.
Gene pools and speciation (HL)
• Speciation can occur abruptly.
• Application: Speciation in the genus Allium
(onion) by polyploidy.
Genetic modification and
biotechnology
Essential idea: Biologists have developed
techniques for artificial manipulation of DNA,
cells and organisms.
Genetic modification and
biotechnology
• PCR can be used to amplify small amounts of
DNA.
• Gel electrophoresis is used to separate proteins
or fragments of DNA according to size.
• DNA profiling involves comparison of DNA.
Genetic modification and
biotechnology
• Application: Use of DNA profiling in paternity
and forensic investigations.
• Skill: Analysis of examples of DNA profiles.
Genetic modification and
biotechnology
• Genetic modification is carried out by gene
transfer between species.
• Application: Gene transfer to bacteria using
plasmids makes use of restriction
endonucleases and DNA ligase.
Genetic modification and
biotechnology
• Application: Assessment of the potential risks
and benefits associated with genetic
modification of crops.
• Skill: Analysis of data on risks to monarch
butterflies of Bt crops.
Genetic modification and
biotechnology
• Clones are groups of genetically identical
organisms, derived from a single original parent
cell.
• Many plant species and some animal species
have natural methods of cloning.
Genetic modification and
biotechnology
• Skill: Design of an experiment to assess one
factor affecting the rooting of stem-cuttings.
Genetic modification and
biotechnology
• Animals can be cloned at the embryo stage by
breaking up the embryo into more than one
group of cells.
Genetic modification and
biotechnology
• Methods have been developed for cloning adult
animals using differentiated cells.
• Application: Production of cloned embryos
produced by somatic-cell nuclear transfer
(Dolly...).