Transcript Mendelian Genetics

Mendelian Genetics
How Genes Work
Who Are You?
Phenotype
– How you
look;
PHysical
appearance
Genotype
– Your genetic
makeup;
GENEs
Father of Genetics

Modern genetics began with Gregor Mendel’s
quantitative experiments with pea plants
Stamen
Carpel
Figure 9.2A, B
White

Mendel crossed pea
plants that differed
in certain
characteristics and
traced the traits
from generation to
generation
 This illustration
shows his
technique for
cross-fertilization
Figure 9.2C
1
Removed
stamens
from purple
flower
Stamens
Carpel
PARENTS
(P)
2 Transferred
Purple
pollen from
stamens of white
flower to carpel
of purple flower
3 Pollinated carpel
matured into pod
4
OFFSPRING
(F1)
Planted
seeds
from pod
Genetics Basics
 Chromosomes
occur in pairs, one from
MOM, one from DAD (homologues)
 Genes are carried on chromosomes
 Genes code for a trait or characteristic
(I.e. hair color)
 Alternate forms of that trait are called
ALLELES (ie. Blond, brown, redhead,
etc)
Alleles…
 Alleles
can be dominant or
recessive
 Heterozygous vs. homozygous
 Only way to “see” a recessive
trait?
Mendel ‘s
basic laws
Law of Segregation
Law of Independent Assortment

The chromosomal basis of Mendel’s principles
Figure 9.17
Walter Sutton’s Theory of
Chromosomal Inheritance
(Mendel’s proof)
 Gametes
contribute to heredity via
nuclear material (chromosomes)
 Homologues segregate during
meiosis
 Homologues separate
independently of other homologous
pairs
Probability
Mathmatical
model of how
often specific events will
happen
#
of occurances/# of attempts
Punnett Square
 Visual
representation or model of
– what alleles can be present in
gametes
– how those alleles can recombine in
offspring
 Used to determine the probability of
offspring’s genetic makeup
Monohybrid Cross
 One
gene
– 2 alleles considered (one from
mom, one from dad
Dihybrid Cross
2
genes
– 4 alleles considered
How can we determine Homovs. heterozygous individuals?
Test
Cross
– Must use homo recessive to
conduct cross in order to “see”
the questionable allele
What Mendel Didn’t See

Multiple alleles
 Codominance
 Epistasis
 Polygenic traits or Continuous Variation
 Pleiotropy
 Incomplete Dominance or “blended
inheritance”
 Environmental Effects
 Sex linked and sex influenced traits
CoDominance
– Both alleles expressed at the
same time; both dominant

Incomplete dominance: neither allele is fully
dominant (blended inheritance)
Figure 9.12Ax
Epistasis

Sequential action of genes
– Product of one gene influences
another (one gene gives
“permission” for another allele to
work
– Gene action acts as a biochemical
pathway & feedback inhibition

Ex: Indian corn coloration
Pleiotropy
 Where
one allele may have multiple
effects on phenotype
–

Normal and sickle red blood cells
Figure 9.14x1
Individual homozygous
for sickle-cell allele
Sickle-cell (abnormal) hemoglobin
Abnormal hemoglobin crystallizes,
causing red blood cells to become sickle-shaped
Sickle cells
Clumping of cells
and clogging of
small blood vessels
Breakdown of red
blood cells
Physical
weakness
Impaired
mental
function
Anemia
Heart
failure
Pneumonia
and other
infections
Pain and
fever
Paralysis
Brain
damage
Accumulation of
sickled cells in spleen
Spleen
damage
Damage to
other organs
Rheumatism
Kidney
failure
Continuous Variation
or Polygenic Traits
 Multiple
genes acting to influence
a characteristic
– Produces gradual changes, not
distinct “borders”
Ie. Height, weight, nose length,
skin pigment
P GENERATION
aabbcc
(very light)
AABBCC
(very dark)
F1 GENERATION
AaBbCc
Sperm
Fraction of population
AaBbCc
Skin pigmentation
F2 GENERATION
Figure 9.16
Environmental Influence
 Example
animals
= color change of fur in arctic
Same eyes, different lighting
Multiple Alleles
More
than one allele per gene
3
alleles (ABO)
– 4 phenotypes (A, B, AB, O)
 Uses
concept of glycoproteins (sugar
name tags or antigens) to mark cells
 Non-recognition of the correct “name
tag” for blood type can cause
agglutination
Landsteiner Blood Groups
– galactosamine – AA
(homo) or AO (hetero)
 Type B – galactose – BB or BO
 Type AB – galactosamine +
galactose (codominant)
 Type O – no sugar marker - OO
 Type A
Blood Donor Facts
Universal Donor
Universal
Recipient
Rh factor
Rh factor can be + or –
 + is like a “seen” name tag or antigen
 - is “invisible”
 Rh- moms that have Rh+ babies are subject
to spontaneous abortions

– Erythroblastosis fetalis
– Controlled with an injectionof RhoGam
to “hide” babies cells
Sex Linked





“Sex on the X”
Specific
trait/disorder is
found on sex
chromosome,
usually the X
Usually recessive
traits
Seen more often in
males than females
Ex: color blindness,
hemophilia
Barr Body inactivation

In females, both
X chromosomes
are not
metabolically
active
 Random
inactivation of
one X
chromosome
may influence
traits expressed