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Ch 9
Patterns in Inheritance
Gregor Mendel
•1860’s
•Crossed pea
plants
•Character vs trait
Genes
• Mendel hypothesized that
there are alternative forms of
genes the units that
determine heritable traits
• Self-fertilization
• Cross fertilization
Flower color
Purple
White
Axial
Terminal
Seed color
Yellow
Green
Seed shape
Round
Wrinkled
Flower position
Pod shape
Inflated
Pod color
Green
Stem length
Tall
Constricted
Yellow
Dwarf
Terms
• True-breeding
• Hybrid
– Mono-hybrid
– Dihybrid
•
•
•
•
•
•
Cross
P generation
F1 Generation
F2 Generation
Phenotype
Genotype
Mendel
Mendel’s Observations of Inheritance Patterns
– 2 alleles inherited
– Mendel’s law of segregation
– If the 2 alleles of an inherited pair differ
• Heterozygous
• Dominant allele
• Recessive allele
– If the 2 alleles are the same, no conflict
• Homozygous
Homologous chromosomes
• Two alleles for each characteristic.
• Alternative forms of a gene reside at the same
locus
Dominance
• Dominant
– Gene that determines phenotype
• Recessive
– Gene that has no noticeable effect
Disorders
• Recessive
– Carriers
• Dominant
– Some cases, homozygous dominant is lethal
Table 9.9
Fetal Testing
– Amniocentesis and chorionic villus sampling (CVS)
Allow doctors to remove fetal cells that can be tested for genetic
abnormalities
Amniocentesis
Chorionic villus sampling (CVS)
Needle inserted
through abdomen to
extract amniotic fluid
Ultrasound
monitor
Ultrasound
monitor
Fetus
Suction tube inserted
through cervix to extract
tissue from chorionic villi
Fetus
Placenta
Placenta
Uterus
Chorionic
villi
Cervix
Cervix
Uterus
Amniotic
fluid
Centrifugation
Fetal
cells
Fetal
cells
Several
weeks
Figure 9.10 A
Biochemical
tests
Several
hours
Karyotyping
Fetal Imaging
– Ultrasound imaging
Uses sound waves to produce a picture of the fetus
Figure 9.10 B
Incomplete Dominance
Intermediate phenotypes
Hypercholestorolemia
Figure 9.12 B
Multiple Alleles/Codominance
• ABO blood type in humans
• Alleles for A and B blood types are codominant
Pleiotropy
• A single gene
which affects
many phenotypic
characteristics
Polygenic
A single characteristic that
is influenced by many
genes
Sex Determination
(male)
(female)
Parents’
diploid
cells
44
+
XY
22
+
X
22
+
Y
44
+
XX
22
+
X
Egg
Sperm
44
+
XX
Offspring
(diploid)
44
+
XY
Other systems of sex determination exist in other
animals and plants
Figure 9.22 B
22
+
XX
22
+
X
Figure 9.22 C
76
+
ZW
76
+
ZZ
32
16
Figure 9.22 D
Independent Assortment
• Mendels Law of Independent Assortment
– Each pair of alleles segregates independently of
other pairs during gamete formation
****Not entirely true
Punnett Squares
• A diagram that is used to predict an outcome
of a particular cross or breeding experiment
• Determine the probability of an offspring
having a particular genotype
Fig. 9-3b
Genetic makeup (alleles)
pp
PP
P plants
Gametes
All p
All P
F1 plants
(hybrids)
All Pp
Gametes
1
–
2
1
–
2
P
Sperm
P
F2 plants
Phenotypic ratio
3 purple : 1 white
p
P
PP
Pp
p
Pp
pp
Eggs
Genotypic ratio
1 PP : 2 Pp : 1 pp
p
Practice problems
• Monohybrid cross
– Blue eyes b, Brown eyes B
– B is dominant
– Both parents heterozygous brown eyed (Bb)
• Homozygous for trait P
– Mom is PP, dad is pp
Experiment
Purple flower
Linked Genes
Phenotypes
• Linked genes are
close to each
other on the same
chromosome and
tend to be
inherited together
PpLI
 PpLI
Observed
offspring
Long pollen
Prediction
(9:3:3:1)
Purple long
Purple round
Red long
Red round
215
71
71
24
284
21
21
55
Explanation: linked genes
PL
Parental
diploid cell
PpLI
PI
Meiosis
Most
gametes
PL
PI
Fertilization
Sperm
PL
Most
offspring
PI
PL
PL
PL
PI
PI
PI
PL
PI
PL
Eggs
PI
3 purple long : 1 red round
Not accounted for: purple round and red long
Sex Linked Genes
• Genes carried on sex chromosomes
• XrX, XrY
• Fruit Fly eye color- carried on X
– R=dominant red eye (R)
– R=recessive white eye (r)
What proportion of offspring will have white
eyes is mom is heterozygous red eyed and dad
is homozygous red eyed?
Fig. 9-21c
Female
Male
XR Xr
XR Y
Sperm
XR
Y
XR
XR XR
XR Y
Xr
Xr XR
Xr Y
Eggs
Sex Linked
• Affect mostly males