Transcript Mendelian Genetics Mono and Dihybrid Crosses, Sex

Ch 14 Gregor Mendel and Inheritance Study
Began research late 1850’s
St. Thomas (Augustinian) monastery, Brno, Yugoslavia
• What genetic principles account for the
passing of traits from parents to offspring?
1. “blending” hypothesis
–genetic material from the two parents blends together
(like blue and yellow paint blend to make green)
2. “particulate” hypothesis
–parents pass on discrete heritable units (genes)
•Mendel’s pea breeding data
• supported the particulate mechanism
Scientific approach
Asked question: how are traits inherited?
Used convenient model system: garden peas
-controlled breeding
-a variety of either/or visible traits
-quantitative data
Derived conclusions based on the data
For example
To examine inheritance of flower color
Cross-fertilized (hybridized):
true breeding purple x true breeding white
Measurable character: phenotype
LE 14-2
Removed stamens
from purple flower
Materials & Methods
Transferred spermbearing pollen from
stamens of white
flower to eggbearing carpel of
purple flower
Parental
generation
(P)
Carpel
Stamens
Pollinated carpel
matured into pod
Planted seeds
from pod
First
generation
offspring
(F1)
Examined
offspring:
all purple
flowers
100% purple
phenotype
“True-breeding”
plants that produce offspring of the same
variety when they self-pollinate
Nomenclature
• True-breeding parents
– P (generation)
• Hybrid offspring (cross between 2 different true-breeding
parents)
– F1 (generation)
• Progeny of F1 self-pollination
– F2 (generation)
LE 14-3
Results from
two crosses
P1 x P2
F1 x F1
F2
P Generation
(true-breeding
parents)
Purple
flowers
White
flowers
F1 Generation
(hybrids)
F2 Generation
All plants had
purple flowers
Conclusions
• Only purple flowers in the F1 hybrids
– Purple: dominant trait
– White: recessive trait
• Same pattern of dominant inheritance
- six other pea plant phenotypes, each represented by two
traits
Mendel’s “heritable factors” (= genes)
(though he did not know the existence of DNA)
Mendel’s Model
•
Inheritance patterns obeyed rules of
probability
1. Assumes each trait has two variants
Current knowledge: phenotype is controlled by
genes (genotype)
paternal and maternal alleles (genes)=two variants
positioned at chromosomal locus
LE 14-4
Allele for purple flowers
Locus for flower-color gene
Allele for white flowers
Homologous
pair of
chromosomes
2. Offspring inherit two alleles, one from each parent
3. One of the alleles determines phenotype
P= purple (dominant allele)
p= white (recessive allele)
Homozygous if PP or pp (noun; homozygote)
PP=_____________________________
pp=___________________________
Heterozygous if Pp (noun: heterozygote)
4. Law of segregation
two alleles for any given phenotype separate during
gamete formation.
Thus, offspring inherit one allele from each parent.
When do alleles first separate from each other?
During anaphase I
Convenient quantitative way to predict outcome of crosses
Punnett Square
1. Name alleles with single letter
2. Write down genotype of each parent
3. Fill in boxes with all possible combinations of alleles
according to parental genotypes
On Board
What is the frequency of genotypes of the offspring of the parental cross?
100% heterozygous
What is the frequency of phenotypes of the offspring of a parental cross?
100% purple
What is the frequency of genotypes of the F1 cross progeny?
50% heterozygous; 25% dominant and recessive homozygous
What is the frequency of phenotypes of the F1 cross progeny?
75% purple, 25% white (3:1)
LE 14-5_2
P Generation
Appearance:
Genetic makeup:
Purple
flowers
PP
White
flowers
pp
P
p
Gametes
F1 Generation
Appearance:
Genetic makeup:
Purple flowers
Pp
1
Gametes:
2
1
P
p
2
F1 sperm
P
p
PP
Pp
Pp
pp
F2 Generation
P
F1 eggs
p
3
:1
LE 14-6
3
Phenotype
Genotype
Purple
PP
(homozygous
Purple
Pp
(heterozygous
1
2
1
Purple
Pp
(heterozygous
White
pp
(homozygous
Ratio 3:1
Ratio 1:2:1
1
• How can we determine the genotype of an
individual with a dominant phenotype?
Genotype possibilities (purple flower color)?
PP or Pp
Test cross
Hybridize to individual with homozygous recessive individual (pp)
Do Punnett for each. Outcome?
LE 14-7
Dominant phenotype,
unknown genotype:
PP or Pp?
Recessive phenotype,
known genotype:
pp
If Pp,
then 2 offspring purple
and 1 2 offspring white:
If PP,
then all offspring
purple:
p
1
p
P
p
p
Pp
Pp
pp
pp
P
Pp
Pp
P
P
Pp
Pp
Monohybrid vs dihybrid
Heterozygous for 1 character
Heterozygous for 2 characters
Set up a cross between
heterozygous purple flowered
pea plants
Pp x Pp
Do a Punnett Square: determine genotypic and phenotypic ratios.
What are patterns of inheritance 2 characters?
Given: true breeding P
for pea color & shape
Dominant characters
Yellow
Round
Recessive characters
green
wrinkled
Set up Parental Cross
Set up F1 cross
What is phenotypic frequency of the offspring?
LE 14-8
P Generation
YYRR
yyrr
Gametes YR
yr
YyRr
F1 Generation
Hypothesis of
dependent
assortment
Hypothesis of
independent
assortment
Sperm
1
Sperm
1
2
YR
1
2
yr
1
1
2
2
1
4
Yr
1
4
yR
1
4
yr
YR
4
YYRR
YYRr
YyRR
YyRr
YYRr
YYrr
YyRr
Yyrr
YyRR
YyRr
yyRR
yyRr
YyRr
Yyrr
yyRr
yyrr
YR
YYRR
1
YR
Eggs
Eggs
F2 Generation
(predicted
offspring)
4
YyRr
1
Yr
4
yr
YyRr
3
4
yyrr
1
1
yR
4
4
1
Phenotypic ratio 3:1
yr
4
9
16
3
16
3
16
3
16
Phenotypic ratio 9:3:3:1
• Observation of a dihybrid cross
- Each pair of alleles segregates independently of
other pairs of alleles during gamete formation
- Mendel proposed the law of independent assortment
(Fast forward)
Established at metaphase I & II
•Note:
Applies only to genes on different, nonhomologous
chromosomes
Genes located near each other on same chromosome tend
to be inherited together: linkage
Autosomes (22 pr)
Sex chromosomes
Inheritance of Sex-Linked Genes
Sex chromosomes
Carry genes that determine sexual phenotype & other traits
Sex-linked genes:
Any gene on sex chromosome (usually refers to X-chromosome
because of bigger size, more genes)
LE 15-10
How to show inheritance of sex-linked traits:
follow X and Y chromosomes in a cross
Sperm
Ova
Sperm
Ova
Sperm
Ova
• Sex-linked recessive disorders
–Color blindness
–Duchenne muscular dystrophy
–Hemophilia
Present hemophilia problem
Mother carrier/ father normal
We’re celebripeas!
Questions?