Download Fundamentals of Genetics

Fundamentals of
Genetics
Regular Biology
Fundamentals of Genetics
• Genetics
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The field of biology that deals with the
study of heredity.
( the transmission of traits from the
parents to the offspring )
Gregor Mendel
• Gregor Mendel
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The father of modern genetics
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A monk in the 1850’s
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worked with pea plants
Gregor Mendel (1822-1884)
Why Peas?
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Easy to raise
Short life span
He could control reproduction
Large # of offspring
Characteristics easy to identify
His Experiments
• P1 (parental generation)
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Pure tall X Pure short
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 All TALL ( F1 )
• F1 (first filial generation)
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F1 X F1
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 ¾ tall (787 tall), ¼ short (277 short)
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Mendel’s conclusions
• Law of Dominance
One factor (gene/allele) of a pair may mask
the effect of the other, preventing it from having
an effect (being shown)
• Law of Segregation
A pair of factors (genes) are separated during
meiosis so that only one from each pair passed
on to the offspring.
Genetic crosses
• Alleles (genes) are designated by the FIRST
letter of the adjective of the dominant trait.
ex: tall is dominant over short
T = tall
t = short
Phenotype
physical appearance of the trait.
ex: tall, short
…Genetic crosses
• Genotype
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The genes (letters) that an organism has in
it’s cells that determine the trait.
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-2 genes (letters) for each trait
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-1 from Dad
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-1 from Mom
Genotype
• Homozygous (pure)
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when both letters are the same
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ex: TT, tt
• Heterozygous (hybrid)
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when letters are different
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always write the big letter first
• ex: Tt , Xx , Bb
Punnett Square
Shows probability/likelihood of offspring
• Monohybrid cross
( involves 1 trait)
• R = red fur (dominate)
• r = white fur (recessive)
• Rr X Rr
• 1 red pure, 2 red hybrid,
1 white pure
How do you write it ?
• Genotypic ratio
– RR : Rr : rr
-- 1 : 2 : 1
G
– This is in the genes
• Phenotypic ratio
– Dominate/recessive
--3 : 1
P
• This is what we see
Remember……………….
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Dominate = CAPITAL letter (goes first)
Recessive = small letter (comes second)
Homozygous (pure) all dominant or recessive
ex:
RR rr
Heterozygous (hybrid) 1 dominant, 1 recessive
ex:
Rr
Punnett Sq. Mom on side, Dad on top side
Cross (of 2 heterozygous) = Rr x Rr
Cross of homozygous Dominant vs. Resessive
RR x rr
Pedigrees
-Used to show relationships w/in families
-Shaded individuals have a different
phenotype than unshaded
Pedigree
Pedigree
male =
= female
Mating
Steps in solving pedigree problems
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Figure out which trait is dominant/recessive
Fill out all recessive genotypes in pedigree as “nn”
Fill out all dominant genotypes “N__”
(See blackboard)
Varying types of dominance
1. Complete dominance –one allele is completely
dominant over the other
• RR = Red
• Rr = Red
• rr= white
2. Incomplete dominance
• Blending between “dominant” and “recessive” alleles,
resulting in a 3rd possible phenotype which is between the
other 2 phenotypes
• RR = Red
• Rr = pink
• rr = white
*** 3 phenotypes & 3 genotypes…genotypic and phenotypic
ratios are the same (3 “options”)
Ex: 1:2:1 (G & P)
3. Codominance
• Both alleles are expressed in heterozygous
offspring…
– NO Blending…NO Recessive Alleles
• Ex: Horse Coat Color
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R = Red, W = White
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RR = red, WW = White,
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RW = Roan (both red and white hairs present)
http://www.mustangs4us.com/Horse%20Colors/roan.htm
More on Codominance…
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***3 phenotypes and 3 genotypes
What does this mean in regards to the ratios?
Same genotypic and phenotypic ratios
You MUST label these ratios…can’t just write
the #’s, must include letters in order to “label”
• Ex: 1 RR: 2 RW: 1 WW (G and P)
Dihybrid Crosses
• Crosses involving 2 traits in parents & offspring
• Genotypes consist of 4 letters, 2 genes for each trait
• Mendel’s 3rd Law/ Law of Independent Assortment
• The inheritance of genes for one trait (ex: hair color)
is not affected by the inheritance of genes for another
trait (ex: eye color)
• ***only true if genes are on different Chr’s***
Making a dihybrid punnet square
– cross:
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TtRr x ttrr
(1234)
(1234)
– Use the FOIL (1-3, 1-4, 2-3, 2-4)
– 2 letters on sides of P. square, 4
letters inside each box…keep the
same letters together
– See Blackboard for example
Order for Phenotypic Ratio
• Both dom : 1st dom/2nd rec : 1st rec/2nd dom : Both rec
• TTRR
: T__rr
: ttR__
• should add up to 16
• What do you do to get F2 generation?
• Cross two F1 individuals
: ttrr
Sex-Linked Traits
• Traits controlled by genes carried on X Chr
• Males XY, Females XX
• Males have only 1 X sex chr so they have only
one gene for those traits (makes it EASIER for
males to inherit sex-linked recessive traits)
• B/c males = have just 1 of the recessive gene,
the recessive trait will be expressed
• females must have both recessive XX’s (like
what we previously learned)
…Sex linked traits
• Males inherit recessive sex-linked traits from
their mother
• EX: Colorblindness is a recessive sex-linked trait
• A female can be:
• XNXN = homozygous for normal vision
• XNXn = carrier of colorblindness gene (hetero)
• XnXn = colorblindness
…Sex Linked Traits
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A male can be:
XNY = normal vision
XnY = colorblind
***males cannot be carrier of recessive sexlinked traits***
Phenotypic ratio for Sex-linked traits
• Dom Females: Rec. Females: Dom Males: Rec Males
• XNX?
: XnXn
: XNY
: XnY
Blood Type
• Multiple alleles – trait controlled by more than 2
alleles (such as blood type)
• Blood type is controlled by 3 alleles: IA IB i
• I = antigen on surface of blood
– Antigen = Proteins capable of stimulating specific
responses in one’s immune system
– Antibody = produced by the body to fight off foreign
substances
• i = no antigen
…Blood Type
• IA and IB are codominant over i (i = recessive)
• 4 blood types (phenotypes) possible: A, B, AB, O
• (actually 8 possible b/c of A+, A- …due to
another antigen called Rh factor on the surface of
some red blood cells…we will NOT consider this
in our crosses)
• The letters A and B refer to the 2 types of antigens
found on the surface of red blood cells
• AB = both antigens present A = A antigen present
• O = neither antigen present B = B antigen present
6 Genotypes possible for blood type:
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IAI A, IAi = A
IBIB, IBi = B
IAIB = AB
Universal Recipient, can receive from anyone
ii = 0
Universal Donor, can give blood to anyone
Why is knowing about blood type
antigens important?
• Critical for blood transfusions…
• If a person with Type B blood (has B antigens) donates
to Type A blood.
• Type B = donor, Type A = recipient
• The person with Type A will produce antibodies to
fight off the B antigen.
• These antibodies bind to the B antigens and cause that
person’s blood cells to clump together…the recipient
could die.
Blood Types Genotype(s)
(phenotypes)
A
B
AB
O
Antibodies
Anti________
Transfusion
from whom?
Phenotypic Ratios for Blood Type
• A :
B
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AB :
O