Transcript Patterns of inheritance

Patterns of Inheritance
K.Chamberlain 2008
Mendelian Genetics
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Mendelian genetics - the analysis of the inheritance of a trait this is
controlled by a single gene
Gregor Mendel, an Austrian monk, was the first person to suggest how
individual traits were inherited.
He bred peas and suggested that by looking at just one trait at a time
he could know whether the pea had green pods or yellow pods.
His model allowed the accurate prediction of how offspring of a
particular cross would look.
Mendel’s work was the starting point for how we understand genes
today.
K.Chamberlain 2008
Patterns of inheritance
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Dominant traits and alleles - will appear in the
offspring if one parent contributes it. They are
represented by a capital letter (A, for example)
– Eg, if one parent has brown hair and the other light hair, the
child will get brown hair, as it is the dominant trait
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Recessive traits and alleles - the offspring will only
get the trait if both parents contribute the trait. These
traits can be carried in the persons genes, without
appearing in the person. They are represented by a
lower case letter (a, for example)
– Eg, a dark-haired person may have one gene for dark hair,
which is a dominant trait and one gene for light hair, which is
recessive. It is thus possible for two dark-haired parents to
have a light-haired child, provided each parent contributes a
gene for light hair.
K.Chamberlain 2008
Patterns of inheritance
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Phenotype - is the observable or measurable
characteristics of an individual
Genotype - genetic make-up of the individual
For example, if an individual has brown eyes, they
may have the genotype BB or Bb.
Often it is not possible to tell the genotype of the
individual by looking at the phenotype
An individual with 2 copies of the same allele is called
homozygous.
– Recessive traits are always homozygous (aa)
– Dominant traits can be homozygous (AA)
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An individual with 2 different alleles is called
heterozygous.
– Dominant traits can also be heterozygous (Aa)
K.Chamberlain 2008
Patterns of inheritance
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Complete dominance When the dominant trait
is expressed as the
phenotype
Co-dominance or
incomplete
dominance - when
both traits are
expressed in a
heterozygote. 2 alleles
of a gene are
expressed in the
phenotype
– ABO gene
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Patterns of inheritance
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Sex-linked - the genes are located on one of the sex
chromosomes (X or Y) but not the other. The X
chromosome is longer, it bears a lot of genes not
found on the Y chromosome, thus most sex-linked
genes are X-linked genes. Often inherited in
predictable patterns.
– Haemophilia (X-linked - can be inherited by M or F)
– Kleinfelter’s or XXY Disease (Y-linked - can only be inherited
by M)
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Autosomal - a non-sex chromosome. It is identical in
males and females. Relates to a characteristic that
does not have a predictable inheritance.
– Cystic Fibrosis (recessive)
– Huntington’s Disease (dominant)
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Punnett Squares & Monohybrid Crosses
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Punnett squares are
used to predict the
outcomes of a cross
between 2 individuals
The top row has the
gametes for 1
individual, and the left
column, the gametes for
the other individual
In the body of the table,
the genotype that could
be formed from each
gamete combination is
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written
Pedigrees
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Pedigrees are family
tree diagrams that chart
whether a characteristic
is present.
They can also help
determine if a trait is
dominant or recessive,
sex-linked (either X or
Y) or autosomal
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Pedigrees
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Royal Pedigree
K.Chamberlain 2008