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GENETICS
Beyond Mendel
Basic Punnett Squares Single trait cross (only 1 letter of alphabet) In basic Mendelian genetics, things are simple;
if it’s not dominant, it is recessive (only 2 phenotypes)
Heterozygous shows only the dominant trait, completely.
R R r r
R R
r r
Rr Rr
rr rr
RR RR
Rr Rr
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 sex-linked. (on the X chromosome)
Some traits are neither dominant, nor recessive.=Co-dominance or incomplete dominance
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, Hemophilia, Colorblindness, Duchenne Muscular Dystrophy, Menkes…
Sex-linked Punnett Square The trait is written as a superscript on the X because
the gene is attached to the X chromosome
Example: If N=normal pigment/ n= albino (lack of pigment)
Mom= carrier (XNXn) XN Xn
Dad= normal (XNY) XN
How many albino kids?
Y
XNXN XNXn
XNY XnY
Sex-Linked Pedigree If a trait is SEX LINKED, it will usually be seen in
more males in a family, because males cannot hide these (XY).
Incomplete Dominance
Heterozygous Phenotype is a blend/in-between, sort of a compromise, with no true dominant or recessive.
Example: pink petunias are heterozygous, and can produce red, pink, or white flowers when two pink flowers are crossed!
Note: 1 letter used: one (usually the darker color) is given a capital even though it isn’t truly dominant.
Incomplete Dominance Pedigree
A pedigree for incomplete dominance would include 3 different shades; perhaps a black, a white, and an even inbetween grey.
(like , , and )
(or , , and )
Co-Dominance (Co = together)
Neither allele is recessive. Both traits are shown at the same time in
heterozygous. Both alleles get a capital letters.
Example: “Roan” hair in some cows.
RR= red hairsWW= white hairsRW = red + white (“roan”)
Co-dominance:Red + White = Red &
White spotted
+
=
CoDominance Pedigree A pedigree for codominance would
include 3 different colorings, too; perhaps a black, a white, and a half black/half white shape for heterozygous.
(like , , and )
(or , , and )
Incomplete Dominance (ID) or Co-Dominance
(CD)?
RedRed + WhiteWhite = PinkPink
INCOMPLETE DOMINANCE
(an even blend of the parents)
Incomplete Dominance (ID) or Co-Dominance
(CD)?
RedRed + WhiteWhite =
RReedd + WWhhiittee
CO-DOMINANCECO-DOMINANCE
(BOTH colors are seen)(BOTH colors are seen)
Incomplete Dominance (ID) or Co-Dominance
(CD)?
=+
Incomplete Dominance (ID) or Co-Dominance
(CD)?
Co-Dominance! Both colors are shown full strength.
=+
Incomplete Dominance (ID) or CoDominance
(CD)?
+ =
Incomplete Dominance (ID) or CoDominance
(CD)?
CoDominance! Both colors are shown full strength.
+ =
Incomplete Dominance (ID) or CoDominance
(CD)?
+ =
Incomplete Dominance (ID) or CoDominance
(CD)?
+ =
Incomplete Dominance! An even BLEND between both is seen.
Incomplete Dominance (ID) or CoDominance
(CD)?
+ =
(straight hair + curly hair = wavy hair)
Incomplete Dominance (ID) or CoDominance
(CD)?
Incomplete Dominance! An even BLEND between both is seen.
+ =
Incomplete Dominance (ID) or CoDominance
(CD)?
+ =
Incomplete Dominance (ID) or CoDominance
(CD)?
CoDominance! BOTH brown & green are seen in this hazel.
+ =
Incomplete Dominance (ID) or CoDominance
(CD)?
+ =
Incomplete Dominance (ID) or CoDominance
(CD)?
CoDominance!
Both traits are shown full strength.
+ =
So far…. Sometimes things have just dominant or recessive
traits (Mendellian).(Unless I give you information otherwise, assume it is a
simple Mendellian trait with just 2 phenotypes). Sometimes there is no clear dominant: If so, I will always let you know by just telling you it is
incomp. dom or co-dom. or by telling you that there are more than two phenotypes the heterozygous being a blend or a combination of the other options.
And don’t forget about sex-linked traits on #23!
Multiple Alleles Some traits are controlled by 3 or more alleles. Examples: Human eye color (brown, blue, & green) Human Blood Type:
There are 3 possible alleles: A, B, or O
Polygenic Traits (poly = many)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
But wait, there’s more!
Blood Types: Multiple Alleles AND codominance!
There are 3 possible alleles: A, B, or o, but there are 4 possible bloodtypes:
Type A, Type B, Type AB, or Type O
A & B are codominant, O is the only recessiveGenotype: Phenotype: Produces:AA Type A blood enzyme AAo Type A blood enzyme ABB Type B blood enzyme BBo Type B blood enzyme BAB Type AB blood BOTH A & Boo Type O blood Neither
enzyme
(Rh factor [positive/negative] is controlled by another gene)
Blood Type Punnett Square
You may have 3 different letters in this kind!
A and B are written uppercase (both dominant)
o is always written lower case (= recessive).
o o
o AB x Ao Bo x Ao
Ao Ao
AA AB AB
Bo
Bo
oo
Bloody Facts: Blood Banks always need Type O because
it is rare and 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 change with 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) RecessiveHemophilia, Colorblindness, Albinism…
Mutation: An error in copying DNACan 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 4th Qtr grade!Do it well!
Genetics Unit Projects: C level: All must do a genetic disorder
presentation & reflective paper. Due before spring break We’ll have computers Friday, Mon, Tues.
B level: Do C + 3 online activities A level: Do C + 3B + one of these:
Family pedigree Genetic Disorder Research Paper Genetic Disorder Poster
