Chapter 15: The Chromosomal Basis of Inheritance
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Genes (Mendel’s “factors”) are located along chromosomes
15.1: Mendelian inheritance has its physical basis in the behavior of
chromosomes
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Chromosomal theory of inheritance – Mendelian genes have specific loci
(positions on chromosomes) on a chromosome, and the chromosome
undergoes segregation and independent assortment
- See figure 15.2
Morgan’s Experimental evidence: Scientific inquiry
- Morgan’s choice of experimental organism
o Fruit fly – drosophila melanogaster
o Prolific breeders – single mating produces hundreds of offspring and
new generations can be bred every two weeks
o Only 4 pairs of chromosomes (3 autosomes, 1 sex)
o Wild type – phenotype for a character most commonly observed in
natural populations (red eyes)
o Mutant phenotypes – alternatives to the wild type (white eyes)
o Invented notation for symbolizing allele s
 w – white eyes
 w+ - red eyes (+ symbolizes the wild type)
- Correlating behavior of a genes alleles with behavior of a chromosome pair
o White eye trait showed up only in males
o Concluded the gene involved in white-eyed mutant was located
exclusively on the X chromosome, with no corresponding allele on the
Y chromosome
o See figure 15.4
15.2: Sex-linked genes exhibit unique patterns of inheritance
The chromosomal basis of sex
- See figure 15.6
- Mammals have a 50% chance of being female, 50% chance of being male
- On the Y chromosome, 78 genes code for about 25 proteins
- Sex-linked gene – gene located on either sex chromosome
- Very few Y-linked genes, very few disorders transferred on Y chromosome
- X chromosome contains ~1100 genes
Inheritance of X-Linked genes
- See figure 15.7
- If an X-linked trait is due to a recessive allele, a female will express the
phenotype only if she is homozygous
- Because males have an X and Y, they have only one locus, therefore the term
hemizygous is used
o Any male who receives the recessive allele from his mother will
express the trait
- More males have X-linked disorders (e.g. colorblindness)
- Duchenne muscular dystrophy, hemophilia are other examples
X inactivation in female mammals
- Most of one of the X chromosomes in each cell in female mammals becomes
inactivated during embryonic development
- Barr body – inactive X in each cell of a female condenses and lies inside the
nuclear envelope
o Genes on the Barr body are not expressed
- See figure 15.8
15.3: Linked genes tend to be inherited together because they are located near
each other on the same chromosome
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Linked genes – genes located near each other on the same chromosome tend
to be inherited together in genetic crosses; genetically linked
How linkage affects inheritance
- See figure 15.9
- Body colour and wing size in drosophila are inherited together because
genes are located near each other on the same chromosome
- Genetic recombination – production of offspring with combinations of traits
that differ from those found in either parent
Genetic recombination and linkage
- Recombination of unlinked genes: Independent assortment of chromosomes
o Parental types – offspring that inherit a phenotype that matches
either of the parental phenotype
o Recombinant types (or recombinants) – offspring that don’t match
either of the parental phenotype
- Recombination of linked genes: Crossing over
o Morgan’s flies – suggestion that genes are on the same chromosome,
since occurrence of parental type had a frequency of higher than 50%
 17% exhibited recombinant phenotypes
o Crossing over – accounts for the recombination of linked genes
o See figure 15.10
New combinations of alleles: Variation for natural selection
- Abundance of genetic variation provides raw materials for natural selection
to work
- If traits are better suited for a given environment, organisms possessing
those genotypes will thrive and leave more offspring
Mapping the distance between genes using recombination data: Scientific inquiry
- Genetic map – ordered list of genetic loci along particular chromosome
- The further apart two genes are, the higher the probability that a crossover
will occur between them and therefore the higher the recombination
frequency
- Linkage map – genetic map based on recombination frequencies
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Map unit – one map unit is equivalent to a 1% recombination frequency
See figure 15.11
Linkage map does portray the order of genes along a chromosome, but does
not portray the precise location of the genes
Cytogenetic maps – locate genes with respect to chromosomal features
See figure 15.12
15.4: Alterations of chromosome number or structure cause some genetic
disorders
Abnormal chromosome number
- Nondisjunction – members of a pair of homologous chromosomes do not
move apart properly during meiosis I or sister chromatids fail to separate
during meiosis II (See figure 15.13)
- Aneuploidy – if an aberrant gamete unites with a normal one at fertilization
- Monosomic – zygote is missing a particular copy of a chromosome
- Trisomic – chromosome is present in triplicate
- Polyploidy – more than two complete chromosome sets in all somatic cells
(3n or 4n cells)
Alterations in chromosome structure
- Deletion – chromosomal fragment is lost
- Duplication – Deleted fragment can attach as an extra segment to a sister
chromatid
- Inversion – chromosomal fragment reattaches to the original chromosome
but in reverse orientation
- Translocation – fragment joins a nonhomologous chromosome
Human disorders due to chromosomal alterations
- Down syndrome
o Trisomy 21
o Correlation between mother’s age and frequency of down syndrome
- Aneuploidy of sex chromosomes
o XXY – Klinefelter syndrome
o XYY – no well-defined syndrome
o XXX – no unusual physical characteristics
o X – Turner syndrome
- Disorders caused by structurally altered chromosomes
o Cri du chat – deletion in chromosome 5
o CML – chromosomal translocation between chromosome 22 and 9
(see figure 15.16)
15.5: Some inheritance patterns are exceptions to standard Mendelian
inheritance
Genomic imprinting
- Genomic imprinting – variation in phenotype depends on whether an allele is
inherited from male or female parent
- Most imprinted genes are on autosomes
- See figure 15.17
- Seems to be a methyl group added to cytosine nucleotides of one allele
Inheritance of organelle genes
- Extranuclear or cytoplasmic genes – genes located outside of nucleus
- Mitochondria and chloroplasts
- Some genes in plastids are responsible for colouration of plant (See figure
15.18)
- Some mitochondrial mutations from mother may contribute to diabetes,
heart disease, and Alzheimers