Download Genetics Reference Sheet

Genetics Reference Sheet
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Gene- unit of heredity (a segment of DNA)
Allele- different forms of a gene
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Punnett Square- tool for predicting probability of offspring with different
allele combinations
Dominant trait- An allele that expresses its phenotypic effect even when
heterozygous with a recessive allele (the big guy always wins)
Recessive trait- allele that does not express a characteristic effect when
present with a dominant allele. Expresses only when 2 recessive alleles
present. (little guy is rarer)
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Homologous chromosomes-A pair of
matching chromosomes in an organism, with
one being inherited from each parent.
Non-Homologous Chromosomes
Homologous Chromosomes
In order to be homologous, the
chromosomes must code for the same genes
and in the same spots.
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Karotype- picture of a person's chromosomes. Can be used to determine
genetic disorders or diseases.
75% of the offspring will express
the dominant trait & 25% of the
offspring will express the
recessive trait when there are
two heterozygous parents.
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Phenotype- the set of observable
characteristics of an individual resulting from
the interaction of its genotype with the
environment. (Physical appearance)
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Genotypegenetic makeup of a cell, an
organism, or an individual usually with
reference to a specific characteristic under
consideration. Must include 2 alleles.
Homozygous- a genotype with two alleles that
are the same (TT or tt)
Heterozygous- a genotype with two different
alleles (Tt)
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There are 23 pairs of homologous chromosomes with the 23 pair showing
the sex chromosomes (XX= female; XY=male)
This karotype shows 3 chromosomes at the 21 pair spot (trisomy 21). This
karotype shows the chromosome arrangement of a male with Down
syndrome.
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Incomplete dominance- one allele does not completely dominate another
allele, The heterozygous genotype will result in a “blending” of the two
alleles. (mixing paint)
X-linked traits- gene causing the trait or the disorder is located on the X
chromosome. (ie. Color blindness or hemophilia)
Both males &
females can
express X-linked
traits.
In order for a male
to express the trait,
his only X
chromosome must
have the allele.
The Red and White alleles
show incomplete dominance.
The heterozygous
combination expresses a
“blending” of the two alleles
producing a pink phenotype.
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Co-dominance- A heterozygous combination in which both alleles are fully
expressed in the phenotype. (“Co” = Together)
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The Red and Blue alleles are
co-dominant. The
heterozygous combination
expresses both the red and
blue alleles in the phenotype.
However, it is less
common for
females to be
affected because
both of her X
chromosomes must
have the allele.
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Pedigree- a tool used to show how a characteristic might be inherited from
generation to generation
o Determining between Autosomal or X-linked pedigrees
 Autosomal- male & females are usually affected equally
 X-linked- male are affected more frequently than females
o Determining between Dominant or Recessive pedigrees
 Dominant- trait is observed in most generations; cannot be
inherited from unaffected parents.
 Recessive- trait seems to “skip” generations; can be
inherited from unaffected parents.
A and B are co-dominant. If a
person receives an A allele
and a B allele, their blood
type is type AB.
Autosomal Recessive
Autosomal Dominant
X-Linked Recessive