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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 to grow
hundreds of 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 are physical units
– 2 alleles for each 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
- One allele dominant, the other recessive
- The dominant allele is expressed in the phenotype
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 ONE allele per 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?
The Monohybrid cross
YY yy
Which allele is dominant?
What is the 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
In mice, black fur is dominant over white fur
Cross heterozygous mouse with homozygous
recessive mouse B = black b = white
Test cross (one gene)
• A mouse has black fur, what are its 2
possible genotypes?
Test cross mouse to homozygous recessive
mouse
If black mouse is BB 
If black mouse is Bb 
A mouse was test crossed and 7 offspring
black, 2 white. What is mouse’s
genotype?
All possible one gene crosses
Parents
ratio offspring
Pedigree Analysis Ch 11
Autosomal recessive inheritance (bb)
• unaffected parents can have
affected offspring
• May “skip” a generation
• Two affected parents cannot
have an 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
missing
phenylalanine
hydroxylase
enzyme
If plasma phe level is too high, phe is converted
into a phenylpyruvate toxic to brain tissue
Why are these babies normal when born?
Pleiotropic effects
no tyrosine (little melanin)
slow growth
retardation
blue eyes
low adrenaline
No nutrasweet
low phe diet
($5K/yr)
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 increases the chance
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)
• Mutation in dog FGF4 gene