Chapter 14
Genetics
Introduction
• Pre 1800s – blending hypothesis
• 1850s – Gregor Mendel
Pisum sativum – what makes pea good genetic
model?
-
• Pea character (gene) =
• Pea trait (allele) =
• self fertilization
Vs. cross fertilization
The monohybrid cross
• Cross true breeding parental plants (P)
• produces F1 generation
-character (gene)
-trait (allele)
- dominant phenotype
- recessive phenotype
F1 plants all purple
• Cross F1 plants
F2 plants = ~250 white ~750 purple
A monohybrid
Is this quantitative or qualitative data?
Mendel’s Conclusions
Dominance
Allele = Alternative version of gene
Dominant allele
P
Recessive allele
Each individual inherits 2 alleles/gene
Genotype
Phenotype?
PP
homozygous dominant
pp homozygous recessive
Pp
heterozygous
p
Dominant allele masks expression of recessive allele
Find the
dominant
allele
Pick symbols
What is the phenotype of plant with genotype Rr?
What is genotype of a green plant?
Genotype?
Segregation
• The 2 alleles segregate in gametes during
meiosis
• Random segregation
Pea chromosome 1
Genotype of plant?
Phenotype of plant?
Human chromosome 11 gene for beta globin
Genotype Hbb/Hbb
• Mendel did not know about:
– DNA/chromosomes/genes
– Crossing over or the cell cycle
– Linked alleles
In mice, black fur is dominant over white fur
Cross heterozygous mouse with homozygous
recessive mouse
Possibilities?
In mice, black fur is dominant over white fur
Cross heterozygous mouse with homozygous recessive mouse
1. Make an allele key
2. Show the cross
3. Write the phenotypes
under the genotypes
4. Determine the parental
gametes (random
segregation)
5. Add to Punnett square
6. Determine offspring
Punnett square
What is the
expected ratio of
phenotypes?
Genotypes?
In a litter of 3,
how many are
expected to be
white?
Which phenotype is the recessive phenotype?
• In cats striping is dominant over non-stripes. Mate
two heterozygous cats.
The Scottish fold is a dominant trait
Genotypes FF
Ff
ff
Phenotypes?
The original Scottish Fold was a
white barn cat named Susie, found
in Scotland
Test cross
Scottish fold cat -> Is she Ff or FF?
If FF, then cross to
ff  kittens?
If Ff, then cross to ff  kittens?
A test cross is always a cross to a homozygous
individual.
Review
• All possible one gene crosses
B = black fur allele
b = white fur allele
• BB X BB  all black
• BB X Bb 
• BB X bb 
• Bb X Bb 
• Bb X bb 
• bb X bb  all white
The Dihybrid cross (2 genes)
Independent Assortment
2 genes
Y = yellow pea allele
y
= green pea allele
Phenotypes of peas
R = round pea allele
r = wrinkled
True-breeding yellow, round X true-breeding green, wrinkled
X
RRYY
rryy
R = round allele Y = yellow
Allele key?
r = wrinkled
y = green
P Genotype?
Gametes? (always 4 per parent in 2 gene cross)
RY and
ry
Parental generation: RRYY
Gametes ?
F1 generation?
Gametes of F1 generation?
F2 generation?
X
rryy
F2 generation
Ratio of genotypes?
Ratio of phenotypes?
Extensions of Mendel
Codominance
Both alleles expressed in phenotype
Ex. Blood type I gene
Alleles IA IB
Phenotype of IA IB
?
multiple alleles
Ex. blood type
Alleles IA IB
IA I A
IAi
IB I B
IBi
i
IA IB
ii
polygenic inheritance
multiple genes affect characteristic
Ex. skin – at least 3 genes
Height
• environmental effects
A particular genotype may have a phenotypic range
• Ex
– Does blood color have a norm?
– Skin color?
?
Recessively inherited disorders
A allele  normal protein
a allele  malfunctioning or no protein
AA
Aa heterozygotes are carriers
aa
disorders range from fetal lethal to mild
Ex. albinism
a allele does not code for functional melanin protein
Chromosome 11
Recessive traits
bb = blond hair
bb = blue eyes
ii = type O blood
Dominantly inherited disorders
A allele  abnormal protein
a allele  normal protein
AA
Aa
aa
Achondroplasia D allele causes dwarfism
Multifactorial disorders
Genetic component + environment (aging, diet,
chemicals, radiation, etc.)