Mendelian genetics

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Mendelian genetics
Its all about jargon, ratios, and nomenclature
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Mendels peas
fig. 2-3
advantages of peas self pollination and
cross-pollination - chose characters with only
two states, or phenotypes - created pure-bred
lines
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• cross-pollination of parental generation (P) to
  create first filial (F1)
• generation
• - also did reciprocal crosses

figs. 2-4 and 2-5
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cross-fertilized peas to obtain first filial (F1)
generation all offspring the same
self-crossed F1 to obtain second filial (F2)
generation missing type reappeared
all progeny in 31 ratio
deduced presence of dominant and recessive traits
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Mendels postulates 1. there are hereditary
particles that determine traits (genes) 2.
genes are in pairs (alleles) 3. members of a gene
pair segregate equally into the gametes 4. each
gamete carries only one allele 5. gametes combine
at random with respect to allele type
heterozygotes have different alleles homozygotes
have same alleles
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Probability rules Multiplication rule the
probability of two independent outcomes occurring
simultaneously is equal to the product of each of
the two outcomes taken separately. independent
probabilities can be multiplied
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Probability rules Multiplication rule the
probability of two independent outcomes occurring
simultaneously is equal to the product of each of
the two outcomes taken separately. independent
probabilities can be multiplied what is the
probability of a litter of three being all
female?
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Probability rules Multiplication rule the
probability of two independent outcomes
occurring simultaneously is equal to the product
of each of the two outcomes taken
separately. independent probabilities can be
multiplied what is the probability of a litter
of three being all female? probability than any
one offspring will be female ½ probability
that all three will be female ½ x ½ x ½ 1/8
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Punnett square is used to work out progeny of a
cross
Figure 2-6
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phenotypic ratio yellow green Y
y yellow Y/Y Y/y
Y green Y/y y/y y
ratio 3 yellow 1 green
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genotypic ratio yellow green Y
y yellow Y/Y Y/y
Y green Y/y y/y y
phenotypic ratio yellow green Y
y yellow Y/Y Y/y
Y green Y/y y/y y
ratio 1 YY 2 Y/y 1 y/y
ratio 3 yellow 1 green
note Y/y y/Y
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Pause for thought how many generations are
required to determine which trait is dominant?
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What causes dominance? Genes code for
proteins Proteins are either structural or
functional (enzymes) An error in the genetic
code may yield a non-functional enzyme, or no
protein, or a less efficient enzyme
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a bit of notation dominant trait takes capital
letter recessive is lower case A/a letter is
determined by phenotype of the dominant
trait yellow peas are dominant, therefore Y/y
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a bit of notation dominant trait takes capital
letter recessive is lower case A/a letter is
determined by phenotype of the dominant
trait yellow peas are dominant, therefore
Y/y Alternative notation wild type trait is
mutant is - a/a-
or a/a dominant genotype (when we dont
know what the second allele is) can be
abbreviated a/ this includes a/a and a/a-
genotypes
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Probability rules Addition rule the overall
probability of any combination of mutually
exclusive outcomes is equal to the sum of the
probabilities of the outcomes taken
separately. mutually exclusive probabilities
can be added
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Probability rules Addition rule the overall
probability of any combination of mutually
exclusive outcomes is equal to the sum of the
probabilities of the outcomes taken
separately. mutually exclusive probabilities
can be added What is the probability that a
litter of three will have at least one female?
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Probability rules Addition rule the overall
probability of any combination of mutually
exclusive outcomes is equal to the sum of the
probabilities of the outcomes taken
separately. mutually exclusive probabilities
can be added What is the probability that a
litter of three will have at least one female? 0
females ½ x ½ x ½ 1/8 1 female ½ x ½ x
½ 1/8 x 3 3/8 2 females ½ x ½ x ½ 1/8 x
3 3/8 3 females ½ x ½ x ½ 1/8 sum of
probabilities containing a female 7/8
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dihybrid cross (more ratios)
F1 round, yellow X wrinkled, green
R/R, Y/Y X r/r, y/y
round, yellow R/r, Y/y
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dihybrid cross (more ratios)
R/r, Y/y
R/Y R/y r/Y r/y R/Y R/y r/Y r/y

R/r, Y/y
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dihybrid cross (more ratios)
R/r, Y/y
R/Y R/y r/Y r/y R/Y R/y r/Y r/y
R/RY/Y R/RY/y R/rY/Y R/rY/y R/RY/y
R/R,y/y R/rY/y R/ry/y R/rY/Y R/rY/y
r/rY/Y r/rY/y R/r Y/y R/r,y/y
r/rY/y r/ry/y
round, yellow round, green wrinkled,
yellow wrinkled, green
R/r, Y/y
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dihybrid cross (more ratios)
R/r, Y/y
R/Y R/y r/Y r/y R/Y R/y r/Y r/y
R/RY/Y R/RY/y R/rY/Y R/rY/y R/RY/y
R/R,y/y R/rY/y R/ry/y R/rY/Y R/rY/y
r/rY/Y r/rY/y R/r Y/y R/r,y/y
r/rY/y r/ry/y
round, yellow round, green wrinkled,
yellow wrinkled, green
R/r, Y/y
phenotype ratio 9 round, yellow 3 round,
green 3 wrinkled, yellow 1 wrinkled, green
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Fig. 2-10
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Pause to consider all Mendels pea traits
behaved the same way (life is not that
simple..)
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Gene interactions
one gene may affect several phenotypes
(pleiotropy) PKU (phenylketonuria)
single gene for enzyme phenylalanine
hydroxylase) that converts phenylalanine to
tyrosine loss results in mental
retardation, lower pigmentation, etc. several
genes may produce the same phenotype
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Gene interactions
incomplete dominance heterozygote has
intermediate phenotype RR red flower Rr
pink flower rr white flower
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Gene interactions
incomplete dominance heterozygote has
intermediate phenotype (both alleles are
expressed) RR red flower Rr pink
flower rr white flower usually found
when phenotype is a continuous trait
(quantitative) e.g., weight, height, fecundity,
amount of enzyme produced alternative is
discrete traits e.g., round vs. wrinkled,
yellow vs. green peas
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Gene interactions
co-dominance intermediate phenotype is formed
when two dominant alleles are
present in heterozygote - characterized by
having three phenotypes genotype blood
type A/A and A/i A produces A antigen
B/B and B/i B produces B antigen i/i
O produce neither antigen A/B AB produce
both antigens
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Gene interactions
Note that terms are somewhat arbitrary depend on
level of analysis RR red flower Rr
pink flower rr white flower
Incomplete dominance if R allele produces
pigment, r does not Co-dominance if R produces
red pigment, r produces white pigment
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Gene interactions
epistasis two or more genes interact
to form a phenotype genotype
flower color 9 W/-, M/- blue 9 3
W/-, m/m magenta 3 3 w/w, M/-
white 1 w/w, m/m white 4
Fig. 4-13
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Gene interactions
sickle-cell anemia genotype phenotype A/A no
anemia normal blood cells A/S no anemia
sickle only with low O2 S/S severe or fatal
anemia with sickle cells dominantreccessive
for expression of anemia incomplete dominance
for cell shape codominance for production of
hemoglobin
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ratios so far 13 (phenotypic result of a
mono-hybrid cross in dominance recessive
traits) 121 (genotypic result of a
mono-hybrid cross) and phenotypic result for
incomplete dominant traits 9331 (dihybrid
cross in dominance recessive traits) 12 if
lethal allele is present A/A ¼ A/a
2/4 a/a ¼ - but they are all
dead 151 (dihybrid cross with duplicate genes)
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penetrance whether genotype is expressed in
phenotype due to modifiers, epistatic
genes, suppression expressivity degree to
which genotype is expressed in the phenotype
due to other alleles, or environment
Fig. 4-23
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was Mendel honest? all dominant recessive
traits only two alleles at each gene no gene
interactions no sex-linked traits all
independent traits (no linkage pea has 7
chromosomes)
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How do we evaluate all this???? (how close
must the data be to the ratios?) Chi square
?2 (observed expected)2
expected
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Monohybrid cross with incomplete dominance
dev. from exp. (obs-exp)2 phenotype
observed expected (obs-exp)
exp red 19 25 -6
1.44 pink 57 50 7 0.98 white 24
25 -1 0.04 total 100 100 2.46 ?2
degrees of freedom 2 ( N 1)
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