Heredity
the transmission of traits from one
generation to the next
Genetics
the scientific study of heredity
Particulate hypothesis
first proposed by Hippocrates, was an early explanation for inheritance.
suggested that particles called
pangenes came from all parts of
the organism to be incorporated
into eggs or sperm. characteristics acquired during the parents’ lifetime could be transferred to the offspring
The “blending”
hypothesis
the idea that genetic material
from the two parents blends together
(like red and white paint blend to make pink). Does not explain how traits that disappear in one generation can reappear in later ones
Gregor Mendel 1
In 1866, Mendel
– deduced the principles
of genetics by breeding
garden peas, and
– relied upon a
background of
mathematics, physics,
and chemistry.
correctly argued that
parents pass on to
their offspring discrete
“heritable factors” and
– stressed that the
heritable factors
(today called genes)
retain their
individuality
generation after
generation.
Mendel’s Experimental, Quantitative Approach
Advantages of pea plants as a model system for genetic study. There are many varieties with distinct heritable
characters (such as flower colour)
▪ Character variants (such as purple or white flowers) are called traits
Self-fertilization is
common in peas
(and most plants)
* But mating of plants
can be controlled
* Each pea plant has
sperm (pollen)-
producing organs
(stamens) and egg-
producing organs
(carpels)
* Cross-pollination
(fertilization between
different plants) can
be achieved by
dusting one plant
with pollen from
another
* Peas produce lots of
offspring quickly
Mendel produced
varieties that were
true-breeding
plants that
produce offspring
of the same
variety when they
self-pollinate
hybrids
The offspring of two
different varieties
genetic cross
The cross-fertilization is
a hybridization
P generation
True-breeding parental
plants
F1 generation (first
filial) and F2
generation (second filial)
Hybrid offspring and A cross of F1 plants produces F2
monohybrid cross
A cross between two
individuals differing in a
single character. Crossed a plant with purple
flowers and a plant with
white flowers.
* The F1 generation
produced all plants
with purple flowers.
* A cross of F1 plants
with each other
produced an F2
generation with ¾
purple and ¼ white
flowers.
The all-purple F1
generation did not
produce light purple
flowers, thus
rejecting the
blending hypothesis
Mendel reasoned that
only the purple flower
factor was affecting
flower color in the F1
hybrids
Mendel called the
purple flower color a
dominant trait and
the white flower color
a recessive trait
Mendel’s proposition
- There are alternative versions of character factors (aka
alleles) that account for variations in inherited characters - For each character, an organism inherits two
alleles, one from each parent.
- The alleles can be the same (homozygous) or different
(heterozygous). - If the two alleles of an inherited pair differ, then one
determines the organism’s appearance
(phenotype) and is called the dominant allele.
The other has no noticeable effect on the
organism’s appearance and is called the recessive
locus (plural, loci)
The specific location of a gene along a chromosome. For a pair of homologous chromosomes, alleles of a gene
reside at the same locus.
* Homozygous individuals have the same allele on both homologues.
* Heterozygous individuals have a different allele on each homologue.
Mendel’s Law of segregation
The two alleles for a heritable character separate
(segregate) during gamete formation and end up in
different gametes
* Thus, an egg or a sperm gets only one of the two
alleles that are present in the organism
* This segregation of alleles corresponds to the
distribution of homologous chromosomes to different
gametes in meiosis
dihybrids
Crossing two true-breeding parents differing in two
characters produces dihybrids in the F1 generation,
heterozygous for both characters
Mendel’s Law of independent assortment
the inheritance of one character has no effect on the
inheritance of another
* each pair of alleles segregates independently from other (ie. a
dihybrid cross is the equivalent to two monohybrid crosses)
Mendel’s law of independent
assortment is true for the
characters he studied because
the genes responsible for the
characters are located on
separate chromosomes
The domesticated dog: a modern genetic model
Gene for dog size
found in 2007
A single single-nucleotide
polymorphism (SNP) in
Insulin growth factor 1 is
common to all small breeds
and nearly absent from giant
breeds
Dogs display a wide
array of phenotypes
ranging from
appearance,
behavior, and
disease
susceptibility
Dogs can be
grouped into
lineages based on
shared genetic
variations
Epistasis
expression of a gene at one locus
alters the phenotypic expression of a gene at a
second locusBlack fur (B) is
dominant to brown
(b)
* But another allele
E determines
whether pigment
is deposited
* Golden labs are
ee recessive
complete dominance
Mendel’s pea crosses always
looked like one of the two
parental varieties
incomplete dominance
For some characters, the
appearance of F1 hybrids
falls between the
phenotypes of the two
parental varieties.
Codominance
each allele is expressed in
separate distinguishable ways
ABO blood groups: Three alleles IA, IB, IO blood group is universal donor
AB blood group can receive transfusion of any blood group
Donor
Recipient
Pleiotropy
occurs when a single gene affects
many phenotypic characters
▪ Sickle-cell disease is a human
example of pleiotropy.
– The sickle-cell allele affects the
type of hemoglobin produced, and
in turn the shape of red blood
cells, which can lead to anemia
and organ damage.
– Sickle-cell and non-sickle alleles
are codominant.
Why does sickle cell alleles persist in populations?
One theory is that carrying one sickle cell allele may be
beneficial under some situations. Malaria triggers sickling in
people that are heterozygous for sickle disease (HbAS)Parasite infection increases oxygen consumption in
heterozygous HbAS blood cells
RBCs become deoxygenated and form sickles and are
cleared from bloodstream
Clearance of circulating parasitized blood cells protect HbAS
persons from severe infestation
Plasmodium
(malaria parasite)
