Transcript Genetics - My Teacher Pages

GENETICS IN A
NUTSHELL
THE BASICS
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Dominant: represented by a large
letter; trait that is expressed
Recessive: represented by a small
letter; trait that is masked unless in
the homozygous form
Genotype: genes in an organism
Phenotype: outer appearance from
the genes
MORE BASICS
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Living things are made of millions of
tiny self-contained components
called cells.
Inside these cells are long and
complex molecules called DNA
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DNA stores information that tells the
cells how to create that living being.
Parts of this information that tell how
to make one small part or
characteristic of the living thing – red
hair, or blue eyes, or a tendency to
be tall – are known as genes.
MORE BASICS
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Every cell in the same living thing
has the same DNA, but only some of
it is used in each cell. For instance,
some genes that tell how to make
parts of the liver are switched off in
the brain. What genes are used can
also change over time.
MORE BASICS
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A living thing has two copies of each
gene, one from its mother, and one
from its father.
There can be multiple types of each
gene, which give different
instructions: one version might cause
a person to have blue eyes, another
might cause them to have brown.
Alleles
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These different versions are known
as alleles of the gene.
Alleles
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Since a living thing has two copies of
each gene, it can have two different
alleles of it at the same time.
Often, one allele will be dominant,
meaning that the living thing looks
and acts as if it had only that one
allele.
Alleles
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The unexpressed allele is called
recessive.
In other cases, you end up with
something in between the two
possibilities. In that case the two
alleles are called co-dominant.
BASICS
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Most of the characteristics that you
can see in a living thing have
multiple genes that influence them.
Many genes have multiple effects on
the body, because their function will
not have the same effect in each
tissue.
MORE BASICS
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The multiple effects of a single gene
is called pleiotropism.
The whole set of genes is called the
genotype.
The total effect of genes on the body
is called the phenotype.
MORE BASICS
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AA=homozygous dominant
Aa=heterozygous
aa=homozygous recessive
AA or Aa=dominant trait
aa=recessive trait
Autosomal Trait
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An autosome is a gene found in the body
cells, i.e. not found on sex chromosomes.
Genetics follow basic heredity rules
regarding dominance and recessiveness.
Combine TT (homozygous tall) and Tt
(heterozygous tall)…What do you get?
Welcome the Punnett Square
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A Punnett Square explains the
possible outcomes for a mating.
T
T
T
TT
TT
t
Tt
Tt
So…
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A cross between a homozygous tall and a
heterozygous tall would yield…
– 50% probability of getting homozygous tall
plants as offspring
– 50% probability of getting heterozygous tall
plants as offspring
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HOW do you get a short plant?
Well…
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Either cross tt x tt or Tt and tt
Try the Punnett Square for each…
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T
t
t
t
Sex-Linked
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Trait on a gene found on X or Y
chromosome
Usually are recessive disordersapparent only in the homozygous
recessive form
Common example: red-green
colorblindness in boys or hemophilia
Sex-Linked Trait Notation
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Notation:
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Male with trait: XhY
Male without trait: XHY
Female with trait: XhXh
Female without trait: XHXH
Carrier female: XHXh
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TRY ONE: Cross a hemophiliac male and a carrier
female.
Incomplete Dominance
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Autosomal traits BUT the heterozygous
phenotype is a combination of the two
homozygous phenotypes
Example: Color in snapdragons
– RR=red
– Rr=pink
– rr=white
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TRY ONE: Cross a pink snapdragon with a red
flower.
Co-dominance
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Autosomal traits BUT the
heterozygous is BOTH homozygous
traits. NO RECESSIVE
Example: Horse Coloring
– RR=Red
– RR’=Red/White (Roan)
– R’R’=White
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TRY ONE: Cross a two roan horses.
Blood Types
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Combination of Co-dominance and
dominant and recessive patterns
– AB blood: IA IB
– A blood: IA IA or IA i
– B blood: IB IB or IB i
– O blood: ii
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TRY ONE: Cross a heterozygous A blood
female with a homozygous B blood male.