Download Linked genes: sex linkage and pedigrees

Linked genes: sex linkage and pedigrees
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Linked Genes
Gene Mapping
Sex Linked
Inheritance
◦ X linked recessive
◦ X linked dominant
◦ Y linked
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Pedigrees
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Autosomal Recessive
Autosomal Dominant
X linked Recessive
X linked Dominant
Variation
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Linkage and recombination
Some genes do not assort independent of
each other, but rather are inherited together.
We call these genes linked. They are on the
same chromosome and are generally
inherited together.
However, because of crossing over this
linkage is never quite complete.
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Linkage and gene
mapping
It is possible to
measure the relative
position of various
linked genes by
comparing the ratio
of the parental
genotypes to
recombined
genotypes.
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A special type of monohybrid inheritance is
called sex linkage. Genes carried on either of
the chromosomes responsible for sex (X or
Y) are said to be sex-linked. A trait can be
said to b X- linked inheritance involving
genes carried on the X chromosome or Ylinked inheritance involving genes carried on
the Y chromosome.
One common example in humans is the
inheritance of Colour blindness. Colour
blindness is a recessive trait linked to the X
Xb
chromosome and is more common in males
than females. If we cross a Colour blind male
with a heterozygous normal for colour vision
Y
female then we would have the following
Punnett Square and outcomes.
This would result in one normal female and
one colour blind female and one normal male
and one colour blind male.
XB
Xb
XBXb
XbXb
XBY
XbY
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X linked dominant
In the case of X linked dominant traits the
trait is more common in females than X
linked recessive traits.
A father will always pass such a trait to his
daughters.
A heterozygous mother will have a 50%
chance of passing the trait to all her children
regardless of sex.
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Y linked inheritance
The Y chromosome carries a small number of
genes concerned with male fertility and sex
determination.
Traits carried on these chromosomes pass
from father to son through the generations.
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Pedigrees are used
to show the
inheritance of a trait
or disease in a
family and are often
used in combination
with Punnett Squares
to investigate how a
certain trait has
been passed on or
could be passed on.
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Things to remember:
◦ For an individual to
express an autosomal
recessive trait both copies
of the allele must be
present
◦ If both parents affected
then all offspring will also
be affected.
◦ Recessive traits tend to
skip generations with few
individuals affected.
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Things to remember:
◦ For an autosomal
dominant trait to be
expressed only one copy
of the allele is required.
◦ Usually present in each
generation, many affected
individuals.
◦ If the homozygous
dominant condition is
lethal the phenotypic and
genotypic ratios will be
different – good exam
trick!
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Things to remember:
◦ Males only require one
allele to express the
trait, therefore more
males than females will
show the trait.
◦ Females require both
alleles to show the
trait.
◦ Heterozygous females
are described as
carriers of the trait
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Things to remember:
◦ Any individual with the
trait must have a
parent with the trait
◦ Females may be
heterozygous and show
the trait
◦ This condition is much
harder to detect, so
again a good exam
trick question!
¾Variation is the degree of difference that exists between members
of a population. The greater the degree of variation, the more
genetically healthy is a population, ie more able to respond to
change in the environment.
¾If there is only one gene with two alleles for a particular trait, you
either express the trait or you don’t. Example: the allele for tongue
rolling is dominant to the allele for not being able to roll your tongue.
That means within a population there is very little variation, you
either can roll your tongue or you can’t. This is known as discrete
variation.
¾If there is more than one gene for a particular trait and if each
gene has multiple alleles, such as those that control height, or skin
colour, there is much greater variation within the population and it
usually follows a normal distribution pattern. This is known as
continuous variation.
Discrete
Variation
Continuous
Variation
¾Multiple Alleles and Poly Genes – as discussed above the
more alleles present for a particular gene and the more genes
involved in a certain trait, then the greater the source of
variation.
¾Crossing Over in Meiosis – during meiosis it is possible for
chromatids to become entangle and to break and reform
differently causing variation by rearranging the normal groups of
alleles present in the gametes
¾Environment – has an influence over the phenotypic
expression of the genotype, thus increasing variation within a
species, eg. Diet and nutrition affect the size of individuals.
¾Independent Assortment – normal fertilization sees the alleles
being assorted independently – thus sexual reproduction assists
with the increase in variation.
¾Random Fertilisation – plants are more able to hybridise by
mixing gametes between like species and so further variation
occurs.
¾Mutations