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Mendel & Heredity
Chapter 8
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Heterozygous
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Heterozygous – individuals with
different alleles for a particular gene
or trait
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Represented by using one upper case letter
and that letter’s lowercase. (Aa or Tt)
Commonly referred to as a carrier
Example heterozygous tall = Tt
Only the dominant allele is expressed
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HOMOZYGOUS
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Homozygous – individuals with the
same alleles for a particular gene or
trait
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Represent by using two identical lower
case letters (aa, AA,)
Individual possesses two recessive alleles
for the same trait
An organism that has two identical alleles
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Phenotype – physical appearance of a
trait (brown eyes , red hair etc)
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External appearance of an
organism as determine by what
alleles are present
Genotype – refers to the set of alleles
inherited. Gene representation (Aa or
AA or aa)
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Dominant allele – an allele that can
mask the effect of the recessive allele
(represented by a Capital letter- (A))
Recessive allele – the allele that is
hidden and will only be expressed when
homozygous. (represent by a lower
case letter - (a))
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Individuals do not have to exhibit a
trait in order for it to appear in their
offspring.
If both parents are heterozygous for the
trait (Aa * Aa) there will be a 25%
chance of an offspring showing the
trait.
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Punnett square – device used to
show the probability that event could
occur.
Represents the genotype of an offspring
Probability – the likelihood that a
certain event will occur.
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Codominance – two dominant alleles are
expressed at the same time. Ex. Roan color in
cattle (both red and white hairs are present)
Heredity – passing of traits from parents to
offspring
Mutation – a change in a gene due to
damage or being copied incorrectly
Mutation – a change in an organism’s DNA
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Sex-linked
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Sex-linked-A trait that is determined by a
gene that is found on the X chromosome.
Sex-linked = If a characteristic is sex-linked,
it occurs most commonly in males
Since the allele for colorblindness is located
on the X chromosome. Colorblindness is sexlinked
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Self-pollination – a reproductive
process in which fertilization occurs
within a single plant
Cross-pollination – a reproductive
process in which fertilization occurs
between two plants
Pollination – the transference of
pollen
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Monohybrid cross – a cross that involves
one pair of contrasting traits. Ex. Hair color
Dihybrid cross – a cross that involves two
pairs of contrasting traits. Ex. Hair color &
eye color
In Mendel’s experiments, a trait that
disappeared in the F1 generation but
reappeared in the F2 generation was always a
recessive trait
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Alleles – different forms of a particular
gene (represented by alphabetical
letters – A, a)
Probability – the likelihood that a
specific event will occur
Pedigree – Identifying patterens of
inheritance within a family over several
generation
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Multiple Alleles
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Multiple alleles – A trait controlled by three
or more alleles. Ex. Blood type
Incomplete dominance – When a
heterozygous individaul has a phenotype that
is intermediate (blending) between the
phenotypes of its two homozygous parents.
Ex. Red flower crossed with white flower and
all the offspring are pink (intermediate)
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Hemophilia – A genetic disorder resulting in
defective blood clotting
Fragile blood cells with an irregular shape
that may block vessels is a symptom of a
genetic disease known as sickle cell
anemia
Gene technology – current technology
being used today to possibly cure some
genetic disorders.
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Punnett Squares
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Be able to complete and interpet a
punnett square.
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Ie. Determine the offspring’s genotype(s);
phenotype(s); genotypic and phenotypic
ratios.
(work sheets – punnett squares)
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