Transmission (Classical, Mendelian)
Genetics Ch 11
• Gregor Mendel
– Experiments in Plant Hybridization, 1865
• Controlled experiments, mathematical
analysis
Pisum sativum, the garden pea
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What makes this a good model organism?
easy
100s offspring per cross
short generation time
can self fertilize or cross
– Paint pollen (sperm) from one plant onto the
female parts of another (emasculated plant)
Mendel’s conclusions
1. Genes physical units
– 2 alleles/gene
– 1 allele inherited from each parent
Genes and alleles of Pisum sativum
Gene
• Pea color
• Flower color
• Pod shape
• Pea surface
• Stem height
Alleles
?
white, purple
constricted, inflated
?
tall, dwarf
2. Principle of Dominance
- The dominant allele is expressed in the phenotype
- Recessive is not
Gene for flower color
P allele = purple
p allele = white
GENOTYPES
Homozygous dominant =
Heterozygous =
Homozygous recessive =
PHENOTYPE
3.
Random segregation of alleles into
gametes
– gamete receives 1allele/gene
– random segregation of alleles 50/50
P generation
PP
pp
What is the phenotype of all offspring in
F1 generation?
Note that the P generation is true breeding
P
p
Genotype
Phenotype
p
P
How did Mendel do it?
Monohybrid cross
YY yy
Which allele is dominant?
Genotype of the f1 generation?
Cross 2 f1 plants (or let one self-fertilize)
What is the ratio of phenotypes?
Results of Mendel’s monohybrid
crosses
Parental Strains
Tall X dwarf
Round seeds X wrinkled
Yellow seeds X green
Violet flowers X white
Inflated pods X constricted
Green pods X yellow
Axial flowers X terminal
gene = ?
alleles = ?
F2 progeny
Ratio
787 tall, 277 dwarf
5474 round, 1850 wrinkled
6022 yellow, 2001 green
705 violet, 224 white
882 inflated, 299 constricted
428 green, 152 yellow
651 axial, 207 terminal
Test cross (one gene)
• In cats, white is the dominant fur color.
fur, what are its 2 possible genotypes?
How can a test cross be used to determine
genotype?
\
All possible one gene crosses
Parents
ratio offspring
Autosomal recessive inheritance (bb)
• unaffected parents can
have affected offspring
• May “skip” generation
• Two affected parents
cannot have unaffected
child
• Not sex related
Autosomal recessive traits
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Sickle cell disease
Albinism
Cystic fibrosis
O blood type
Phenylketonuria (Ch.4)
• PKU (1/12,000) Mutation in gene encoding
phenylalanine hydroxylase enzyme needed for
phe metabolism
Chromosome 12
12q24.1
phenylalanine
hydroxylase
block
toxic to brain
tissue
All US babies tested at birth (Guthrie test)
These contain phe
Why are PKU babies normal when born?
Pleiotropy = multiple effects from
one gene
no tyrosine (little melanin)
slow growth
retardation
blue eyes
low adrenaline
Ch 4
1902 Archibald Garrod:
One gene: one enzyme
“Inborn errors of metabolism”
PKU
Albinism
Alkaptonuria
Tyrosinemia
Black urine
arthritis
Fill in genotypes. If II,1 and II, 4 mate, what is the
chance of offspring having PKU?
How do we know
this is autosomal
recessive?
II, 1
X
II, 4
•p(aa AND a girl)?
p(aa)
If III-3 and II-1 mate p (normal child)
Product rule:
p (affected boy)?
All people have harmful recessive alleles, small chance
That 2 people with same rare alleles will mate
Consanguinous marriage?
Bedoin intermarriage
Autosomal dominant disorders
Aa and AA =affected
aa =unaffected
•Tend to show up in every generation
•2 affected parents can have unaffected child
•2 unaffected parents cannot have an affected
child
Dominant pedigree
Achondroplasia -1/20,000 births
• Mutation in FGFR3 gene Chromosome 4
• Affects cartilage growth needed for bone
lengthening
• Affected individuals Aa  why not AA?
• Most cases spontaneous
(associated with increasing paternal age)
what is the genotype of parents in this case?
P(III, 3 and III, 5 have a child of normal
height)
P ( II, 3 and III, 7 have a boy with
achondroplasia)
Dihybrid cross – 2 genes Ch. 11
Mendel’s Law of Independent assortment each allele for a trait is inherited independently
of other alleles
Seeds:
G = yellow allele
g = green allele
W = round allele
w = wrinkled allele
gene?
gene?
Parents =
GGWW X
phenotype?
gametes?
F1 genotype
?
F1 phenotype
?
F1 Gametes?
ggww
Note that each gene gives the 3:1 ratio of a
monohybrid cross
Yellow/green ratio =
Round/ wrinkled =
Forked line method for phenotypes
GgWw X GgWw
Test cross
A pea is round and yellow. What is its genotype?
G-W-
X
ggww
Note the cross of the “unknown” to a homozygous recessive
If all yellow and round:
If all yellow and some wrinkled:
If all round and some green:
If 1:1:1:1:
Probability
Product rulethe probability that two outcomes occur simultaneously is
product of their individual probabilities
assumes independent assortment of genes
GgWw X GgWw
• What is the probability of a yellow AND wrinkled?
p(G-ww)
A female lilac tabby X A male black siamese genotype:
ww bb CC dd Aa X ww Bb CsCs Dd Aa
p( black tabby cat)
p (brown solid color cat)
W = white
B = black
C = solid (not siamese)
D = not dilute (full color)
A = striped (tabby)
Trihybrid cross
AaBbCc X AaBbCc
p(A-B-cc)
AabbCcDD X AaBbCcDd
p(triply recessive)