Download Mendelian Genetics

GENETICS
Beyond Mendel
Gregor Mendel
(1822-1884)
• Worked with pea plants in monastery garden
– This was lucky, because peas have pretty simple genetics, as do
many genes in most organisms.
– His observations of dominant & recessive patterns of heredity
allowed him to develop 2 important “laws”:
• LAW OF SEGREGATION
– 2 alleles for a trait separate (segregate) when gametes
are formed [It’s random: Zz may pass on either Z or z]
• LAW OF INDEPENDENT ASSORTMENT
– Alleles of different genes separate independently of
one another during gamete formation
• [“alleles for gene Z are not linked to alleles for gene W”]
Basic Punnett Squares
• Single trait cross (only 1 letter of alphabet) = 2x2
• In basic Mendelian genetics, heterozygous shows
only the dominant trait.
• Can predict probability of offspring genotypes
R
R
r
r
R
r
RR
RR
Rr
Rr
R
r
Rr
Rr
rr
rr
Dihybrid Cross Punnett Square
• Looks at 2 traits at once = 4X4
(16 boxes)
• Step 1: find parents’ genotypes (example: XXgg, XYGg)
• Step 2: find the 4 different possible gamete combinations for each parent
(You will randomly get 2, FOIL looks at all possibilities)
Dihybrid Cross Punnett Square
• Looks at 2 traits at once = 4X4
(16 boxes)
• Step 1: find parents’ genotypes (example: XXgg, XYGg)
• Step 2: find the 4 different possible gamete combinations for each parent
(You will randomly get 2, FOIL looks at all possibilities)
for ex: (mom: Xg Xg Xg Xg) (dad: XG Xg YG, Yg)
Mom
»
Xg
XG
Xg
Dad
YG
Yg
Xg
Xg
Xg
Dihybrid Cross Punnett Square
• Looks at 2 traits at once = 4X4
(16 boxes)
• Step 1: find parents genotypes (example: XXgg, XYGg)
• Step 2: find the 4 different possible gamete combinations for each parent
(You will randomly get 2, FOIL looks at all possibilities)
for ex: (mom: Xg Xg Xg Xg) (dad: XG Xg YG, Yg)
• Step 3: Fill in squares (should have 4 letters in each box)
»
Xg
XG
How many
heterozygous
girls?
Xg
YG
Yg
Xg
Xg
Xg
XXGg
XXgg
XXGg
XXgg
XXGg
XXgg
XXGg
XXgg
XYGg
XYGg
XYGg
XYGg
XYgg
XYgg
XYgg
XYgg
Life is messy.
There are exceptions to most rules.
Mendel’s genetics only work
for simple traits.
Exceptions...
Some traits are controlled by more than 1 gene.
=Polygenic Traits [poly = many]
(Ex: Height, pigmentation [eye color, hair color, skin color]…)
Some traits are neither dominant, nor recessive.
=Co-dominance or incomplete dominance
Some traits are sex-linked. (on the X chromosome)
Some traits have more than 2 possible choices
(= multiple alleles)
Some traits are NOT actually independent.
Some traits change with the environment!
Sex Linked Traits
• Occur on the X chromosome (pair #23)
• Usually show up in males more than
females, because males only have 1 X.
– Females can be carriers, Males cannot.
– EXAMPLES: Albinism (sometimes),
Hemophilia, Colorblindness,
Duchenne Muscular Dystrophy,
Menkes…
Pedigrees
• A pedigree is a family history, a genetic family tree
• Can show an inheritance pattern
– Very helpful for showing genetic disorders and sex-linked traits
– Lines show mating and offspring
• Horizontal (--) lines show mates, vertical ( l ) lines show offspring
– Each row is a generation (Roman numerals may # generations)
Usually means female
Usually means male
or
Usually indicates an affected individual
May have different color for carriers (heterozygous)
Sex-linked Punnett Square
• The trait is written as a superscript on the X because it is
attached to the X chromosome
Example:
– If N=normal pigment/ A= albino (lack of pigment)
– Mom= carrier (XNXA)
XN
XA
– Dad= normal (XNY)
XN
– How many albino kids?
Y
XNXN XNXA
N
A
X Y X Y
Colorblindness Pedigree
I.
II.
QuickTime™ and a
decompressor
are needed to see this picture.
III.
= female
Colorblindness is sex-linked.
Is it possible for a female to be
colorblind?
= male
Shaded= affected
Incomplete Dominance
• Heterozygous Phenotype is a blend/in-between,
sort of a compromise, with no true dom. or rec.
• Example: pink snapdragons are heterozygous,
and can produce red, pink, or white flowers
when two pink flowers are crossed!
Note: 1 letter used:
• Another example:
Curly hair + straight hair
= wavy hair!
Polygenic Traits
Most traits are controlled by more than 1 gene,
so you get a spectrum of possible
appearances between the parents’ traits
.
Examples: Height, Weight, Skin & Hair color Also: Diabetes,
schizophrenia, and many other conditions
Multiple Alleles
• Some traits are controlled by
3 or more alleles.
• Example: Human Blood Type
– There are 3 possible alleles: A, B, or O
Co-Dominance
• Neither allele is dominant nor recessive
• Both traits are shown at the same time
• Example: Human Blood Type
– There are 3 possible alleles: A, B, or O, but there
are 4 possible bloodtypes: A, B, AB, or O
– A & B are codominant, o is the only recessive
Genotype:
AA
Ao
BB
Bo
AB
oo
Phenotype:
Type A blood
Type A blood
Type B blood
Type B blood
Type AB blood
Type O blood
Blood Type Punnett Square
I= Dominant bloodtype allele (A or B)
written IA or IB to show difference
i= recessive bloodtype allele (o)
What is your blood type?
Ask your parents if they know their
blood types, and maybe you can
see how you got yours.
Bloody Facts:
• Blood Banks always need Type O because
it is a “universal donor”.
– People with type A, B, AB, or O can be given type O
blood without the body rejecting it.
– People with blood type O can ONLY accept type O,
and will violently reject any other blood type.
– Giving a Type A person a Type B blood transfusion,
or vice versa, will probably kill them as it will be
rejected and attacked by their immune system
• Type AB is a “universal recipient”
– Their body will accept any blood type, since their
immune systems are used to both A and B forms,
and the recessive O is non-threatening.
Environmental Influence
• Some phenotypes depend on the environment.
• Example: Hydrangea flower color
– Blue flowers appear on plants in acidic soil
– Pink flowers appear on plants in basic soil
– If you replant a
pink plant in acid soil,
the flowers will be
blue next year!
– Other example:
Artic fox coat is reddish brown in summer, but turns white
in winter as the pigment gene is temporarily “turned off”
to camouflage itself as the landscape changes
Genetic Disorders
• There are 4 ways to get a genetic disorder:
– Autosomal Dominant (from a dominant gene
on a chromosome in pairs 1-22, not on X [#23])
Achondroplasia, Marfan Syndrome, Huntington’s ….
– Autosomal Recessive (not on X chromosome)
Sickle Cell Anemia, Tay-Sachs, Cystic Fibrosis …
– Sex-linked (on X chromosome) Recessive
Hemophilia, Colorblindness, Albinism…
– Mutation: An error in copying DNA
Can usually be seen in a karyotype; Down’s Syndrome, Cri du chat…
Remember: Autosomes= any chromosome other than X or Y
Autosomal disorders occur equally among males and females.
Interesting Fact:
In the U.S., many cases of Huntington’s
disease can be traced back to two
brothers who left England in the 1600’s
because they were accused of witchcraft.
The charges were based on strange
behavior including constant dance-like
movements, which are a symptom of
Huntington’s disease.
You’ll learn about this and many other
conditions in your upcoming
GENETIC DISORDER PROJECT!
Genetic Disorder Project:
Your Mission:
Work with your partner, focus on assignment.
(up to 2 people can sign up for each topic)
Research your assigned disorder
(use at least 3 good online sources)
Create & deliver a PowerPoint presentation.
Write your own Reflective Report
This will be at least 10% of your 3rd Qtr grade!
Do it well!