Science of HEREDITY
Key Concepts*
Informational Transfer
of the physical characteristics
of cells and organisms from generation to
generation.
Heredity studies how the FORM [morphology]
and APPEARANCE [phenotype]
of an organism as established by its genes & as influenced
by the environment.
Should one look at the genetic ancestry of a
prospective mate ????
Essence of heredity...
of the thousands of
known different species,
each reproduces
itself, faithfully,
yielding
progeny that belong to the SAME
species, i.e., the parent organism hands
down information specifying, in
exacting detail, the characteristics of its
offspring.
The phenomena
of Heredity
is a central definition
of Life...
it is the
ability of cells to faithfully copy its DNA
instructions into identical progeny
passing traits of parents or ancestors to
offspring, while allowing genetic variations.
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parents
a genetic
pedigree
offspring
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Two aspects of heredity: we observe patterns &
analyze mechanisms...
(the essence of the
Scientific method)
Mendelian
Genetics - (observes patterns of
appearance - PHENOPTYPES)
Quantitative analysis
of inheritance of morphological
characters in organisms...
we'll look at experiments which established the
basic principles &
laws
that predict the
pattern of inheritance from generation to
generation
often
done with a genetic pedigree
Molecular Genetics -
(molecular mechanisms of the
appearance - GENOTYPES)
physical properties of molecules (DNA & RNA
& proteins) provide the
molecular mechanisms that affect the observed
patterns of inheritance.
Central Dogma
of Molecular Biology
DNA --> transcription --> RNA -- translation
--> Protein
MENDELIAN GENETICS...
Mendel
Museum - Mendel's Legacy
Gregor
Johann Mendel was an Augustinian monk (later abbot) at
the
the Abbey of
St. Thomas in Brno (today part of
the Czech Republic)
Mendel did his experiments between 1856 & 1863
breeding 29,000 Psium
sativum pea plants...
the flowering*sweet pea can produce progeny via
self
pollination* & cross pollination*
Mendel made experimental crosses between true
breeding parents
for a
single genetic character (morphological trait)
with 2 different phenotypes (appearances)
Mendel's 7 TRUE BREEDING morphological traits in pea plant via
genetic crosses*
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the design
of a typical Mendelian experimental MONOHYBRID CROSS... |
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generation
experimental
genetic cross
Parental
round
seed
x wrinkled
seed [ true
breeding ]
F1
round seed
[
x themself - self-pollinate]
F2
3
round* seed to 1
wrinkled* seed |
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another
example of monohybrid cross using
flower color in peas*
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Mendel described his observations in terms of Particulate Inheritance...
Physical characters are
discernible as "PARTICLES"
or "FACTORS",
which are:
-
discrete entities with own
integrity
(no blending of traits)
-
a trait is preserved through inheritance
(over many generations)
- a
trait occurs within the individual in PAIRS
(now
known as diploid)
-
some particles mask
expression of
others
(round
masks wrinkled in
F1)
Law of
Mendelian Segregation...
his hypothesis:
- when gametes
are formed...
- 2 particles
of inheritance in an individual SEGREGATE from each other
-
each progeny receives 1 particle (in a gamete) from each parent
Current
Interpretation of
Mendel's Particulate Inheritance Hypothesis...
-
Mendel's particles are
GENES... entities definable in molecular
terms [a DNA segment]
- we represent the different forms
of a trait by alphabetic symbols = A & a, B
& b,
etc...
- a trait (as shape) can have
different morphological
forms... each called an ALLELE
allele - a DNA gene
segment which codes for different form of a
similar protein,
that
governs that gene's
phenotype (round, wrinkled, purple, etc...)
alleles
occur on chromosomes at a specific place = a GENE LOCUS*
-
- some alleles mask expression of others
= DOMINANT & RECESIVE allelic traits
geneticists use upper case = dominant
and lower case = recessive trait
recessive -
trait that disappears
in the F1 generation (a is masked by
A)
dominant
- trait that does
not disappear in the
F1 (wild type A is dominant to a)
- each individual possesses [only] 2 alleles for one
specific trait, thus 3 combinations...
RR
- homozygous dominant
(only 1 allele = round)
Rr
-
heterozygous
(dominant allele = round)
rr
- homozygous recessive (both
alleles to be wrinkled)
animation of Mendel's
monohydrid cross with Punnett Squares*
Mendelian Monohybrid Genetic Cross &
Meiosis |
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parental |
RR
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x
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rr
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TRUE BREEDING
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round phenoytpe
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wrinkled
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diploid |
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locus
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gametes (4)
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x |
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haploid |
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MEIOSIS
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F1*
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Rr
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diploid
(R is expressed
r is
not)
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makes 4 gametes |
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but,
show only 2 different |
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F2*
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Punnett Square |
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lists the probabilities of
offspring genotypes
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flower color* |
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Results of a Monohybrid Cross...
ratios*
PHENOTYPE
(APPEARANCE)
3
round to
1 wrinkled
GENOTYPE (ALLELE
MAKEUP)
1
(RR) to
2 (Rr) to 1
(rr)
Mendel's
hypothesis [that the F1 is Aa] is scientifically
testable...
- his premise [a
testable hypothesis]... hypothesized F1 should be a heterozygote [Rr]
(2 particles segregate & each gamete receives 1
particle of each type from each parent)
- a
testable experiment: we can do a Testcross/Backcross*
a cross of
unknown F1 genotype [RR
or Rr]
to
homozygous recessive [rr]...
since 2 recessive alleles are necessary to see
the recessive phenotype
one can easily see
if the recessive is present in the unknown
genotype.
▼
| prediction
1:1 ratio |
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r |
r |
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if F1
heterozygote
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R |
Rr |
Rr |
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r |
rr |
rr |
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| prediction
all the same |
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r |
r |
| if F1 homozygote |
R
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Rr |
Rr |
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either RR or
rr
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R |
Rr |
Rr |
Mendel wrote a 2 part
scientific paper on Experiments on
Plant Hybridizations
He read part 1 to
the Natural History
Society of Brno on Feb., 8, 1865
and part 2 on March 8, 1865.
He published his results in 1866; but they were
largely ignored.
The true
value of Mendel's experiments was his approach to the scientific method...
1. his experimental results were given numerically
2. he used statistical
analyses
3. he used large sample
sizes
4. he followed his results for several
generations
5. he used an inquiry based
approach... & did experimental test crosses...
These are the hallmarks of the scientific method
in genetics and
clearly established that traits are not blended, but separate distinct
characters.
The predominant theory of
inheritance of the 19th century was blended
inheritance, where the inherited characteristics of
progeny is the average of the parents' values of
that characteristic.
Blended Inheritance
vs. Incomplete Dominance -
snapdragons*
can it still be explained by Mendel's Law's?
Yes
- incomplete dominance of traits*
- yes. neither gene is dominant,
i.e., alleles do NOT MASK expression of the other.
Mendel's Law
of Independent Assortment
Different genes sort independently from one
another during progeny reproduction.
a dihybrid
cross demonstrates
principle of Independent
Assortment
dihybrid
crosses involves 2 characters or traits, that
result in...
new combinations of traits, not exhibited by
either parent.
parentals -
progeny look like parents
non-parentals -
progeny don't look like either parent
DOMINANT
RECESSIVE
flower
color
W = red
w = white
seed
color
G = green
g = yellow
P1
WWGG
x wwgg
true breeding [homozygous]
F1
WwGg
c (x WwGg self
cross) what's
the progeny phenotypes
F2
phenotypes
flower color seed color
ratio
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parental |
red |
green |
9 of 16 |
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non-parental |
white |
green |
3 of
16 9:3:3:1 |
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non-parental |
red |
yellow |
3 of 16 |
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parental |
white |
yellow |
1 of 16 |
Each trait alone red (3)
= 12 of 16 white (1)
= 4 of 16
3:1
green (3)
= 12 of 16 yellow (1)
= 4 of 16
Rediscovery of Mendel's
hypotheses:
1900 - Hugo deVries, Carl Correns
& Erich von Teschermak, were
botanists who
studied heredity and all reference Mendel's
earlier works.
1902-1903 Walter
Sutton
& Theodor Boveri
analyzed spematogenesis
and oogenesis,
especially in grasshopper & sea
urchins
and described chromosomes &
observed MEIOSIS and
noted...
- hereditary traits are carried in sperm & egg cells
-
on chromosomes
-
chromosomes obey
Mendel's laws
> chromosomes
occur in pairs, as
do Mendel's "particles"
(homologs)
> chromosomes
segregate at
meiosis, as do the "particles"
via Mendel's Law of
Segregation (homologs/chromatids
separate)
>
chromosomes sort independently of each other
via
Mendel's Law of Independent
Assortment - fig
15.4 (laws)
Sutton and Boveri propose Chromosomal
Theory of inheritance, which states
that
genes
are found at specific locations on
chromosomes and that the behavior of
chromosomes
during meiosis involves
segregation, independent assortment, and
occasionally,
linkage of genes.
Autosomal
Linkage...
Linked Genes
& crossing
over allowed mapping
of gene loci.
-
two traits are carried on a single
chromosome
___R__Y__
-
thus
they do not sort independently; but, how do
we know if they are linked?
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generation |
genotype |
phenotype |
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parental |
RRYY x rryy |
round/yellow x
wrinkled/green |
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F1 |
RYry
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round/yellow |
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TESTCROSS * |
RYry
x
ryry |
round/yellow x
wrinkled/green |
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for
recessives (ry - linked or
not) all
gametes will have same
alleles* |
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if linked
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if
not linked |
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ry |
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F1 |
ry |
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RY |
RYry |
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RY |
RYry (RrYy) |
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ry |
ryry |
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Ry |
Ryry (Rryy) |
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rY |
rYry (rrYy) |
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ry |
ryry
(rryy) |
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1 :
1
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1 : 1 : 1 : 1 |
►
genes occur at a specific locus* &
are linear along chromosomes
which allowed for Genetic
mapping or linkage
maps to indicate
the relative order of genes on a chromosome.
Sturtevant,A.H. J. Exp.Zool.
(1913) 14:43-59.
some Human
Genetic examples
Some
Examples of Human Genetic Traits...
- Widow's
peak* &
free
ear lobes*,
& tongue
roll*
& a cross
with Freckles*,
-
Blood
Groups* -
4 phenotypes O, A, B, and AB
due to RBC
surface carbohydrates
- Sex
linkage - trait carried on
X-chromosome
red-green colorblindness, Duchene muscular
dystrophy, & hemophilia
There are some 7,000 chromosomally
inherited Mendelian diseases
known...
Generally of
two major forms:
- Inborn Errors
of Metabolism - defective
enzymes due to recessive mutations
- Mistakes of
Meiosis -
due to abnormal chromosome structures, such
as...
as numerical (Down's syndrome)
or structural
abnormalities (deleted portions)
Inborn Errors of Metabolism
- mutations
= non-functional enzymes
[genes & diseases]
PHENYLKETONURIA
- [PKU]
pp
1/10,000
births
disfunctional phenylalanine
hydroxylase
on
chromosome
12
PHE
--x-->
tyrosine -->
--> degraded & excreted
toxic accumulation --> blood/brain
barrier ≈
mental retardation
In U.S. a PKU screening test is required all
newborns 24–72 hours after birth.
PHE is essential
(dietary only) aa, so PKU* is treatable by
dietary restriction
TAY SACHS
tt
1/300,000 births
defective (hexosaminidase-A) enzyme -->
high ganglioside fats in nerves -->
fatal
Ashkenazic Jewish ancestry shows an incidence
of 1/ 3,600 births &
1/30 is Tt
Founder Effect - in populations a very small breeding
size recessives can be high
SICKLE CELL
ss pic*
1/6,000 births
defective beta globin polypeptide of
Hb -
DNA
Learning Center video
a single nucleotide change = improper
folding of Hb --> clogs
capillaries
in 1998 - 1st umbilical
cord blood stem cell transplant in children - Keone
Penn update
in 2014 - stem cell bone marrow transplants
reverse Sickle
cell in adults
in 2023 - gene editing drug (Casgevy)
cuts in a particular gene that can reactivate
the production
of fetal hemoglobin, which dilutes the faulty
red blood cells caused by sickle cell disease.
Chromosomal
Abberations -
chromosomal
mistakes of Meiosis & Ploidy...
ANEUPLOIDY is a
chromosomal aberration due to a variation in chromosome #
monosomy
- 1 less disomy = normal (diploid)
trisomy + 1
more than normal
non-disjunction* forms
trisomies where some or all cells have an extra
chromosome
Common trisomies* =
trisomy 21*- Down
Syndrome -
frequency
vs. age*
Other ex:
4 common types of structural
changes - animation*
&
figure
15.14
1) deletions - chromosome
fragment without centromere is lost
2) duplications - a segment
is repeated...
3) inversions - a segment
reverses is linear sequence
4) translocations - both
segments moves from one chromosome to
another.
Eugenics
-
[Gk: eu-true & genos-race,
stock, kin] social improvement of human
genomes.
Eugenics movements aim to improve Humans, often
via selective breeding or
non-breeding.
Historically... proposed
to be done by elimination of "inferior" genes in the
populations...
[ex: forced sterilizations of the
mentally ill - in U.S. in 1950 over 1,526
individuals
were sterilized by legal statutes].
Amniocentesis
- fetal
diagnosis* -
biochemical and cytological analysis of the
amniotic fluid surrounding a fetus for genetic
abnormalities.
preimplantation
tests & Umbilical
Cord Banking
Human Chromosome Map: of human
diseases, disorders, &
syndromes.
back
next lec - Molecular Genetics*
Sample
Genetics Problems
copyright
c2024
-
Charles Mallery,
- Biology 150, Department of Biology, U.
of Miami, Coral Gables, FL
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Ex:
Aneuploidy of SEX chromosomes: (in
> non-disjunction of XX) |
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X X
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0
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+
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XX
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X0  |
X
or Y |
Y0 XXX
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X or
Y |
XXY |
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Turner syndrome
1/5000 births
90% abort -
appears female
often sterile TSS
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dies
triple X
1/1000 births
no visible symptoms
may be sterile
developmental issues |
Kleinfelter
1/2000 births
appears male
sterile
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karyotypes* |
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Sex &
Sports
Hyperandrogenism
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F1 parents of a F1 testcross :
RYry x ryry
linked on same chromosome
not linked on different chromosomes
____R__Y___
____R____ ____Y___
F1
____r__y___
____r____ ____y___
* for the recessive parent: makes no
difference whether genes are linked or not
____r__y___
or
___r___
& ___y___
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F1 gametes
Linked
2 kinds gametes =
2 progeny types
(parentals)
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F1 gametes: not-linked
4 kinds gametes =
4 progeny types -
(parentals & non-parentals_
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recessive
parent
gametes* |
| ____R__Y___ |
___R___
___Y___ |
ry |
| ____R__Y___ |
___R___
___y___ |
ry |
| ____r__y___ |
___r___
___Y___ |
ry |
| ____r__y___ |
___r___
___y___ |
ry |
back
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AB |
Ab |
aB |
ab |
| AB |
AABB |
AABb |
AaBB |
AaBb |
| Ab |
AABb |
AAbb |
AaBb |
Aabb |
| aB |
AaBB |
AaBb |
aaBB |
aaBb |
| ab |
AaBb |
Aabb |
aaBb |
aabb |
9
: 3 : 3 : 1
Easy - How many are Homozygous for one allele
and Heterozygous for
the other allele?
Harder - How many are wrinkled & yellow?
back
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