Download Chapter 14 Genetics

Chapter 14
Genetics
Introduction
• 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
- gene
-allele
-ratio of phenotypes
- dominant trait
F1 plants all purple
• Cross F1 plants
F2 plants = ~250 white ~750 purple
A monohybrid
Is this quantitative or qualitative data?
Mendel’s Conclusions
1. Dominance
Allele = Alternative version of gene
Dominant allele P
Recessive allele p
Each individual inherits 2 alleles/gene, 1 from each
parent
PP
homozygous dominant
pp homozygous recessive
Pp
heterozygous
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?
2. Random Segregation
• The 2 alleles segregate into gametes during
meiosis
• Random segregation
Pea chromosome 1
Genotype of plant?
Phenotype of plant?
Human chromosome 11 gene for beta globin
Genotype Hbb/Hbb
try it
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
B = black allele
b = white allele
Bb X bb
Black
Gametes
B and b
white
b and b
Punnett square
b
b
B
Bb
Bb
b
bb
bb
What is the
expected ratio of
phenotypes?
Genotypes?
In a litter of 3,
how many are
expected to be
white?
Test cross – mate to homozygous recessive individual
• In cats striping is dominant over non-stripes. Mate a
homozygous striped cat with a non-striped cat. What
% are expected to be striped? S= stripes s = not
SS
X
s
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
We have a Scottish fold cat
Is she Ff or FF?
If FF, then cross to
ff  kittens?
If Ff, then cross to ff  kittens?
Review
• All possible one gene crosses
B = black fur allele
b = white fur allele
• BB X BB  all black
• BB X Bb  all black
• BB X bb  all black
• Bb X Bb 
• Bb X bb 
• bb X bb  all white
The Dihybrid cross (2 genes)
2 genes
Independent Assortment
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
P Genotype?
Gametes? (always 4 per parent when 2 gene cross)
RrYy =Round Yellow X RrYy == Round Yellow
Gametes: RY, Ry, rY, ry
Allele key
R = round allele
r = wrinkled
RY Ry rY ry
Y = yellow
y = green
F1 generation
ry
RY
RrYy
These dihybrid plants are heterozygous
for both traits
• Cross two dihybrid F1 plants
RrYy
X RrYy
Gametes:
Percentage of each gamete type?
F2 generation
Ratio of genotypes?
Ratio of phenotypes?
Extending Mendel;s work
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 IA
IAi
IB IB
IBi
i
IA IB
ii
polygenes
multiple genes affect characteristic
Ex. skin – at least 3 genes
Height
• environment
A genotype may produce a phenotypic range
• Ex
– Blood type has no norm
– Skin color has a norm
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
Dominantly inherited disorders
A allele  abnormal protein
a allele  normal protein
AA
Aa
aa
Achondroplasia D allele causes dwarfism