Biology 11E
3.A.4 The inheritance pattern of
many traits cannot be explained
by simple Mendelian genetics.
Name: __________________
15.1, 15.2, 15.3, 15.5
• Sex-linked genes reside on sex
chromosomes (X in humans)
• In mammals and flies, the Y
chromosome is very small and
carries few genes
• In mammals and flies, females
are XX and males are XY; as
such, X-linked recessive traits are
always expressed in males
• Some traits are sex limited, and
expression depends on the sex of
the individual, such as milk
production in female mammals
and pattern baldness in males
Chapter 15 – The Chromosomal Basis of Inheritance
How does Mendel highlight the importance and necessity of prediction and hypothesis in
science? Mendel’s “hereditary factors” were merely an abstract concept when he proposed them
in 1860. HE proposed his laws of independent assortment and segregation before it was proved
that heredity had a physical basis in chromosomal behaviour.
What do the yellow dots in fig.15.1 indicate?
They mark the locus of a specific gene on a
homologous pair of human chromosomes.
The location of a particular gene can be
determined by tagging chromosomes with
fluorescent dye that highlights the gene.
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15.1 The Chromosomal Basis of Mendel’s Laws:
*U7. relate chromosome activity to Mendelian segregation and independent assortment
According to the Chromosome Theory of inheritance: Mendelian genes have specific loci along
chromosomes which undergo segregation and independent assortment. Fig 15.2 below shows the
behavior of chromosomes during independent assortment in meiosis and gamete formation.
Figure 15.2
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15.2 Experimentation with wild and white-eyed fruit flies led to the mapping of this gene to
the X-chromosome:
Which famous scientist did the initial genetic experiments with fruit flies in the early 20th
century? _Thomas Hunt Morgan____ Why are fruit flies a good organism to use for genetic
studies? Prolific breeders, a single mating produces hundreds of offspring, a new
generation every 2 weeks, only has 4 pairs of chromosomes easily distinguishable with a
light microscope, (three autosomes and one pair of sex chromosomes)
Wild type Drosophila flies had red eyes. The first mutant that Morgan discovered was a male
with white eyes. He was able to trace the gene for eye colour to a specific chromosome.
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Here is a diagram to illustrate one of Morgan’s first experiments:
Drosophila melanogaster cross between a wild type female and a mutant white eyed male.
Both were homozygous.
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Sex-linked Inheritance 15.2
Genes carried on the X chromosome are inherited differently in males and females.
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The transmission of sex-linked recessive traits follows certain patterns:(15.7)
Albinism
Fig 15.7 Colour blindness
Name 2 other sex linked disorders in humans.
Haemophilia
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Muscular Dystrophy
X-inactivation
Since females inherit 2 X chromosomes do females make more proteins encoded for by X
chromosomes?
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A test cross involves breeding an organism of unknown genotype with a homozygous
recessive individual to determine the unknown genotype. The ratio of the phenotypes in the
offspring reveals the unknown genotype.
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15.3 Gene Linkage p. 293
*U8. calculate the map distance of a particular gene from a chromosome’s centre or between two genes
Linked genes are genes located on the same chromosome that tend to be inherited together. (Do not
confuse with sex linked genes referring to a single gene on a sex chromosome. ) The behavior of linked
genes is different from Mendel’s Law of Independent Assortment.
In the experiment below Morgan investigated gene linkage.
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Using a test cross can determine how closely two genes are linked on the same
chromosome:
Other possible offspring must be a result of crossing over between the two genes and leading to
recombinant types. How can we tell how close the two genes are to each other on the
chromosome? The recombination frequency is related to the distanced between linked genes.
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Note: If genes are not linked then, in a test cross, all possible offspring are possible in equal
proportions:
Crossing Over produces recombinant chromosomes during prophase I
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Frequency of recombination is used to calculate map distance between linked genes
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Summary of how linkage affects inheritance
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Use of recombination frequencies to map genes onto a chromosome
Examine the research Method in Fig. 15.11 Constructing a Linkage Map. A linkage map shows
the relative locations of genes along a chromosome.
Explain the results below
Here is a partial genetic Linkage Map of a Drosophila Chromosome
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15.4 Errors in Chromosomal Inheritance
Write an intro:
1. Meiotic Non-disjunction
http://www.biostudio.com/d_%20Meiotic%20Nondisjunction%20Meiosis%20I.htm
Meiotic non-disjunction leads to abnormal chromosome numbers in the gametes
What is aneuploidy?
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______________________________________________________________________________
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Give some examples of genetic disorders resulting from aneuploidy:
Trisomy 21
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Klinefelter syndrome (XXY)______________________________________________________
XYY males __________________________________________________________________
Trisomy X ____________________________________________________________________
Monosomy X0 (Turner syndrome)__________________________________________________
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2. Polyploidy See Campbell’s Act 15C
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In which type of organisms is polyploidy relatively common? ___________________________
Fig 15.14 A tetraploid mammal
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3. Chromosomal Mutations
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Some genetic disorders caused by chromosomal mutations (chromosomes are structurally
altered):
Cri-du-chat (rey of the cat) - a deletion mutation (chromosome 5)
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Chronic myelogenous leukaemia - a reciprocal translocation involving parts of chromosomes
22 and 9
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Down’s syndrome can be caused by translocation of chromosome 21 onto another chromosome,
or by meiotic non-disjunction resulting in 3 complete chromosome 21s.
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Epigenetics is the study, in the field of genetics, of cellular and physiological phenotypic trait
variations that are caused by external or environmental factors that switch genes on and off and
affect how cells read genes instead of being caused by changes in the DNA sequence.
What is genomic imprinting? ________________________________________________
Genomic imprinting is the epigenetic phenomenon by which certain genes are expressed in a
parent-of-origin-specific manner. If the allele inherited from the father is imprinted, it is thereby
silenced, and only the allele from the mother is expressed. If the allele from the mother is
imprinted, then only the allele from the father is expressed. Forms of genomic imprinting have
been demonstrated in fungi, plants and animals. Genomic imprinting is a fairly rare phenomenon
in mammals; most genes are not imprinted
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