Download Mendel and the Gene Idea

Mendel and the Gene Idea
Chapter 14
p. 251-260
Model of Inheritance



Inherited traits are
passed along in a
“particulate” model
Specifically, genes act
as units of heredity,
each coding for a
specific trait
1st discovered by
Gregor Mendel

“Father of Genetics;”
Austrian monk
Mendel’s Approach – why peas??

1) Peas possess distinct
characteristics which
could easily be studied



Characteristic: feature
that varies amongst
individuals (i.e. flower
color)
Trait: form of charac.
(i.e. purple or white)
2) Mendel could control
pea reproduction


Self-pollination: stamen
provides pollen (sperm)
for carpel (ova) of same
plant
Cross-pollination:
pollen from 1 plant is
brushed onto another
Mendel’s Approach (con’t)

3) Mendel chose “true-breeding”
plants



Self-pollination gives rise to offspring
w/same distinct traits
Hybridization: cross-breeding of 2
diff. true-breed plants
4) Alleles segregate during meiosis


White flower trait must be “hidden” in
F1 gen.
Thus, alleles (2 forms of a gene) are
inherited separately
Law of Segregation

1) Alleles: 2
forms of a gene



i.e. purple or white
Account for
different traits
2) 2 alleles are
inherited


1 from each parent
May code for diff.
traits
Law of Segregation (con’t)

3) Alleles may be either:




Dominant: always expressed (“P”)
Recessive: only expressed if dominant
not present (“p”)
i.e. purple trait “hides” white
4) Alleles separate during meiosis


Each sperm/ovum only gets 1
“Law of Segregation”
Punnett Square

If parent plants are
true-breeding, can
predict probability of
offspring traits (F2
gen.)



Dominant trait = “PP”
Recessive trait = “pp”
Since alleles segregate,
have 25% chance of
inheriting recessive trait
Genetics Vocab

Homozygous: pair of
same alleles


Heterozygous: 2
different alleles


i.e. “Pp”
Genotype: genetic
make-up


“true-breeding;” i.e.
“PP” or “pp”
Either homozy. or
heterozy.
Phenotype:
expressed trait

i.e. purple or white
Testcross

If genotype of P
gen is unknown,
can be determined
using testcross:



Cross white “pp”
w/ unknown purple
If any F1 are white,
know purple is “Pp”
If NO white
produced, purple is
probably “PP”
Law of Independent Assortment

Alleles are packaged into gametes
independently of one another

Monohybrid: crossing of 1 genetic trait


Dihybrid: crossing of 2 traits



i.e. yellow pea color (Y) x green pea color (y)
i.e. yellow x green pea color AND smooth (R)
x wrinkled (r) shape
By performing dihybrids, Mendel
determined traits are inherited
independently
“Law of Independent Assortment”
Probability

Scale ranges from 0-1


i.e.:1/2, 3/8, 4/5…
Independent
Events: outcome of 1
event has no impact
on outcome of
another


i.e.: consecutive coin
flips
Allele segregation
into gametes in an
independent event
Rule of Multiplication

Used to determine probability of
simultaneously occurring
independent events





Calculate probability of each event
alone, then multiply together
i.e.: if parent has YyRr, what is prob. of
YR offspring?
1) Yy: prob Y = ½
2) Rr: prob. R = ½
½ x ½ = ¼ YR
Rule of Addition

Used to calculate probability of an
event occurring in different ways





Calculate prob. of each event and add
i.e.: Offspring is heterozygous for a
trait – how many ways can this
happen?
1) R (mom) r (dad) = Rr = 1/4
2) r (mom) R (dad) = Rr = 1/4
1/4 + 1/4 = 2/4 = 1/2
Patterns of Inheritance
Chapter 14
p. 260-266
Incomplete Dominance

Heterozygous
genotypes produce
phenotypes inbetween dom & rec.


1 allele is not
completely dominant
over another
i.e.: red (dom) +
white (rec) → pink
Codominance

Heterozygous
genotypes produce
BOTH phenotypes
equally


Both alleles affect
phenotype
i.e.: M, N, & MN
blood groups
Dominant/Recessive Relationship

1) Is a spectrum:

Complete ← Incomplete → Codominance

Why such a range of phenotypes

I.E.: Tay-Sachs Disease
Lack of enzyme causes inability to
metabolize lipid in brain
 Recessive disorder (rr)
 Heterozygotes do not have disorder, but
only produce ½ amt enzyme

Dominant/Recessive Relationship

2) “Dominance” ≠ domination


Just different forms of a gene
i.e.: pea shape
Smooth: produces enzyme, pea swells
 Wrinkled: no enzyme, pea shrivels


3) “Dominant” ≠ occurs more often

i.e.: Polydactyly (extra digits)
Condition is a dominant trait
 Only occurs 1/400 births

Multiple Alleles

Most genes have >2 alleles


i.e.: ABO Blood Groups
3 alleles, named for presence of
specific carbohydrates
IA: substance A
 IB: substance B
 i: no substance


If foreign blood introduced, antibodies
produced, blood will clump
ABO Blood Groups
Pleiotrophy

Most genes affect
many phenotypes

i.e.: allele for SickleCell Disease causes
many symptoms
Epistasis

When one gene affects
the expression of
another gene on same
chromosome


i.e.: mouse fur color
Black is dom. over
brown BUT if epistatic
gene is rec. homo., NO
color will show (albino)
Polygenic Inheritance

Quantitative
Characters:
characteristic w/ full
spectrum of traits


i.e.: height, skin color
May be caused by
Polygenic Inheritance
 More than 1 gene
affects phenotype
 Environmental factors
also have effect
Environmental Impact on Phenotype


Phenotypes are also
affected by nutrition,
weather, etc.
Norm of Reaction:
range of phenotypes
caused by environ.
influences

Characteristics affected
by both genotype &
environment are called
multifactorial
Pedigree Analysis

Pedigree Chart: “family tree” of
genetic traits





Used to study transmission of traits
through generations
□ = male
● = has trait
O = female
° = does not
l = parent-offspring
have trait
- = marriage
Pedigree Charts
Genetic Disorders
Chapter 14
p. 266-270
Recessively Inherited Disorders




Genes code for production of certain
proteins
Recessive alleles code for no protein or
faulty protein
May result in mild → severe disorders
Heterozygotes “OK” because dominant
allele produces enough protein


Called “Carriers”
If 2 heterozy. reproduce, have ¼ chance of offspring
w/ disorder
Types of Recessive Disorders

1) Cystic Fibrosis



Defective/absent
chloride channels
Causes mucus build up
in pancreas, lungs,
digestive tract, etc. →
death
2) Tay-Sachs


Dysfunctional enzyme
causes failure to break
down gangliosides
(lipids)
Infants suffer seizures, ↓
motor & mental functioning
→ early death
Types of Recessive Disorders

3) Sickle-Cell Disease



Single amino acid
substitution
Hemoglobin can’t bind O2
properly; leads to
multitude of other
symptoms
Sickle-Cell Trait:
heterozygotes


May show some
symptoms (incomplete
Dominance)
Both normal &
abnormal Hb made
Dominantly Inherited Disorders

Often due to mutations;
much more rare


i.e.: Acondroplasia:
dwarfism
Often kills offspring; alleles
will never be passed




May persist if symptoms do
not show until later in life
i.e. Huntington’s
Disease: deterioration of
nervous system
Symptoms begin 35-45
years old
Now possible to detect
early; no cure
Multifactorial Disorders

Have genetic AND environmental
causes


i.e.: heart disease, diabetes, cancer,
etc.
Many times are result of polygenic
effects
Genetic Testing and Counseling

Can use Mendelian concepts to
predict probability of offspring
having disorder


Each offspring is an independent
event
Many ethical/social issue involved
Carrier Recognition

Some methods can detect if parents
are carriers BEFORE having
offspring


i.e.: Tay-Sachs, Sickle-Cell, Cystic
Fibrosis
Information can be potentially
abused

i.e.: Health insurance discrimination
Fetal Testing

1) Amniocentesis: remove amniotic fluid
to detect certain chemicals or create
karyotype


2) CVS: “chorionic villi sampling”;
removes tissue from placenta for
karyotyping


14th-16th week
8th week
3) Ultrasound: provides image of baby

Anytime; better in 3rd trimester
Newborn Screening

Many disorders are
tested for
immediately upon
birth

i.e.: PKU:
Phenylketonuria
 Child can not process
phenylaline
 May lead to mental
retardation
 If detected, can be
controlled trough diet