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Basic Concepts in Genetics

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•LCSC06 directed study 2013 2014
Read through the following slides
• You may find that you need more of an
introduction to the topic if this aspect of
bioscience is totally new to you, so I
suggest the relevant pages in Atkinson &
McHanwell or alternatively Tortora et al as
good sources
• Some of the slides have notes attached,
so you might like to print off as ‘notes
pages’ to assist you
•2
•3
•4
Human Karyotype
Most human cells
contain 46 chromosomes:
• 2 sex chromosomes (X,Y):
XY – in males.
XX – in females.
• 22 pairs of chromosomes
named autosomes.
Genotypes
Phenotypes
• In autosomes chromosomes exists in
homologous pairs.
• At each locus (except for sex chromosomes)
there are 2 gene alleles. One allele at an
individual locus on each of the pair of
chromosomes. This constitutes the individual’s
genotype at that locus and for that individual
gene.
• The expression of a genotype is
termed a phenotype. For example, hair
color, weight, or the presence or
absence of a disease.
•6
Haploid and Diploid cells
Remember that body (somatic) cells have
chromosomes arranged in pairs (diploid),
whilst gametes (sperm and ova) have only one
member of each pair (haploid).
Diploid cells will therefore have gene alleles
arranged in pairs, whilst the gametes will
have only a single gene allele.
except in males where the Y chromosome is
shorter than the X
Dominant and recessive alleles
Many genes exist in two allelic forms in which one
allele is dominant to the other which is termed
recessive.
For example in human eye colour – brown eyes (B)
is dominant to blue (b).
Sperm
B
Eggs
b
B BB Bb
b
Bb bb
BB - Brown eyes
Bb - Brown eyes
bb - Blue eyes
In diploid somatic cells the genotype with
respect to an individual gene (e.g. the gene for
eye colour) can be described as homozygous if
both alleles for the gene are the same – so
could be homozygous dominant (BB) brown or
homozygous recessive (bb) blue.
Note that: the convention is that the
dominant allele is denoted with a CAPITAL
letter and the recessive allele a lower case
letter…
If the alleles are different (Bb) the condition is
called heterozygous and in this example the
individual would be brown eyed because brown
(B) is dominant to blue (b).
Try this example:
Two brown eyed parents have three
children – two brown eyed and the other
blue.
Q. What must the genotypes of the
parents be?
•10
NB this gene is on an autosome not the sex
chromosomes.
A ‘Punnett’ diagram is used to work out the
various combinations of gene alleles.
So…..If both parents are homozygous brown
eyed……
Sperm
…the possible
So: all
B B
combinations
children
B BB BB
will be
are:
Eggs
brown
B BB BB
eyed…….
So another possible combination might be:
if one parent is homozygous (BB) and the other
heterozygous (Bb)
Sperm
B
Eggs
b
B BB Bb
B BB Bb
SO: all children
still brown eyed
but 50% chance
homozygous
and 50%
heterozygous
• In the former case , the children who are
heterozygous are carriers but do not
display the phenotype of blue eyes
themselves….remind yourself why this is
the case…..?
•13
The final possibility is if both parents are
heterozygous (Bb)
..this results in the possibility of having both blue
eyed as well as brown eyed children….
Sperm
B
Eggs
b
B BB Bb
b
Bb bb
So : 25% chance
of child being
blue eyed
NB:
Brown / blue eye colour useful as a simple example,
but in fact the genetics of eye colour is actually less
straight forward.
Medical Genetics
•16
Medical Genetics
When studying genetic disorders, 6 general
patterns of inheritance are observed:
•
•
•
•
•
Autosomal recessive
Autosomal dominant
Autosomal C0-dominant
X-linked recessive
X-linked dominant
• Mitochondrial
•17
Medical Genetics
Autosomal dominant
• The disorder appears when only
one of the two genes of a
chromosome pair is defective.
• Affected males and females
appear in each generation of the
pedigree.
• Affected mothers and fathers
transmit the phenotype to both
sons and daughters.
• The risk of a mutation carrier
transferring disease to offspring
amounts to 50%.
• e.g. Huntington’s disease.
Medical Genetics
Autosomal recessive
•
•
•
Recessive inheritance
means both genes in a
pair must be defective
to exhibit the defect.
The disease appears in
male and female
children of unaffected
parents.
e.g. cystic fibrosis
•19
FOXP2 gene
• FOXP2 is a protein that in humans is
encoded by the FOXP2 gene, which is
located on human chromosome 7
• In humans, mutations of FOXP2 cause a
severe speech and language disorder.
• The gene is transmitted in an autosomal
dominant pattern.
• The following article provides some
background to the gene and its expression
in humans.
• Read more about FOXP-2 and link to SLI
in Cummings, L p284
•20
Co-dominant or multiple allele inheritance (ABO blood
groups)
The gene has three allelic forms A and B are co dominant and O is
recessive
Genotype
Phenotype
AA
A
AO
A
BB
B
BO
B
AB
AB
OO
O
Genes present on the sex chromosomes.
• The X chromosome is larger than the Y
chromosome and has a segment for which
there is no equivalent on the Y chromosome.
This segment contains genes where the
alleles are not matched by alleles on the Y
chromosome.
• In males the single allele on the X chromosome
will always show itself in the character
(phenotype) of the individual.
• In females with two X chromosomes a
recessive allele may be masked by a dominant
allele on the other X chromosome.
Keep in mind:
Boys gain their X chromosome from their mother –
never their father. Girls gain one X chromosome
from each parent.
Sperm
X
Eggs
Y
X
XX XY
X
XX XY
When considering sex linked traits we must take account
of the chromosome type (X or Y) as well as the allele
present (or not present) on each.
For example consider a gene on the X chromosome, which
has a ‘Normal’ dominant form (N) and a mutant recessive
form (n).
NB mutated genes are not always recessive or damaging
Sperm
Eggs
XN
Y0
XN
♀NN
♂ N-
Xn
♀Nn
♂ n-
X linked recessive traits
• Many more males than females show the
condition.
• All the daughters of an affected male will
carry the condition and are ‘carriers’.
• None of the sons of an affected male show the
condition or are carriers. Remind yourself why
this must be so….
• An example of an X linked recessive trait is
haemophilia
•28
X-linked dominant trait – e.g. fragile X syndrome
•
Affected males pass the disorder to all daughters but
to none of their sons.
• Affected heterozygous females and unaffected males
pass the condition to half their sons and daughters
Fragile X syndrome, or Martin-Bell syndrome, is a genetic
syndrome which results in a spectrum of characteristic
physical, intellectual, emotional and behavioural
features which range from severe to mild in
manifestation. The syndrome results in a failure to
express a protein which is required for normal neural
development.
Mitochondrial inheritance
e.g. Leber's hereditary optic neuropathy (LHON)
In sexually reproducing organisms, mitochondria are
normally inherited exclusively from the mother. The
mitochondria in mammalian sperm are usually destroyed by
the egg cell after fertilization.
Mitochondrial disorders can appear in every generation of
a family and can affect both males and females, but
fathers do not pass mitochondrial traits to their
children…because…?
Leber’s hereditary optic neuropathy (LHON) or Leber
optic atrophy is a mitochondrially inherited (mother to all
offspring) degeneration of retinal ganglion cells (RGCs) and
their axons that leads to an acute or sub acute loss of
central vision.
•34
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