Intro to Genetics
Genetics is the branch of biology that deals with patterns
of inheritance.
Heredity is the biological process by which parents pass
on genetic information to their offspring through their
gametes.
Gregor Mendel, an Austrian monk,
started the science of genetics by
experimenting with pea plants
between 1856 and 1868.
Mendel came up with the Principles of Dominance,
Segregation, and Independent Assortment.
Mrs. Degl
Based on his findings, Mendel proposed that certain
traits were inherited as a result of the transmission of
hereditary factors.
Mendel’s hereditary factors, called genes , are located
on the chromosomes.
Each gene has a definite position on a chromosome, called a
locus.
Mrs. Degl
A gene is a hereditary unit consisting of a
sequence of DNA that occupies a specific
location on a chromosome and determines a
particular characteristic in an organism.
In the early 1900’s T.H. Morgan carried out
breeding experiments (crosses) with the fruit fly
(Drosophila), that supported Mendel’s findings. He
received the Nobel Prize for Physiology or
Medicine in 1933.
Mrs. Degl
Two alternate genes that control each trait are called
alleles, and they are located on the same positions on
homologous chromosomes. (Example: red or brown
hair)
Homologous chromosomes are a pair of matching
chromosomes in an organism, with one being inherited
from each parent.
Mrs. Degl
GENOTYPE = the genes present in the DNA of
an organism.
Pairs of letters are used to represent genotypes for
one particular trait.
There are always two letters in the genotype
because (as a result of sexual reproduction) one
code for the trait comes from the mother organism &
the other comes from the father organism, so every
offspring gets two codes (two letters).
PHENOTYPE = how the trait physically showsup in the organism.
Mrs. Degl
When we have two capital or two lowercase letters in the
GENOTYPE (ex: TT (tall) or tt (short)) it's called
HOMOZYGOUS
("homo" means "the same"). Sometimes the term "PURE" is
used instead of homozygous.
When the GENOTYPE is made up of one capital letter & one
lowercase letter (ex: Tt) it's called HETEROZYGOUS
("hetero" means "other"). Just to confuse you, a heterozygous
genotype can also be referred to as HYBRID.
Genotype = genes present in an organism
(usually abbreviated as two letters)
TT =
Tt =
tt =
homozygous heterozygous homozygous
= pure
= hybrid
= pure
Mrs. Degl
Test Cross
In genetics, a test cross was first introduced
by Mendel in order to determine if an individual
exhibiting a dominant trait is a homozygous
dominant or a heterozygous dominant. To do
this, he crossed a homozygous recessive
individual. If all offspring display the dominant
phenotype, the individual in question is
homozygous dominant; if the offspring are split
equally between the dominant and recessive
phenotype, the individual is heterozygous.
Mrs. Degl
The Principle of Dominance
In a cross of parents that are pure for contrasting traits,
only one form of the trait will appear in the next
generation. Offspring that are hybrid for a trait will
have only the dominant trait in the phenotype.
Genotype
Symbol
Genotype Vocab
Phenotype
TT
homozygous DOMINANT
or
pure tall
tall
Tt
heterozygous
or
hybrid
tall
tt
homozygous
RECESSIVE
or
pure short
short
Mrs. Degl
Punnett Squares help us
illustrate the crosses that
Mendel did.
T
T
t
T = the dominant allele for tall
stems
t
t = recessive allele for short
stems
A summary of this cross would be:
Offspring
(F1 Generation) = First
Generation
Parent Pea Plants
(P Generation) = Parent
Genotypes:
TT x tt
Phenotypes:
tall x short
(Let’s fill it in)
Genotypes:
100% Tt
Mrs. Degl
Phenotypes:
100% tall
The Principle of Segregation
During the formation of gametes (eggs or sperm), the
two alleles responsible for a trait separate from each
other. Alleles for a trait are then "recombined" at
fertilization, producing the genotype for the traits of the
offspring.
T
t
Mendel now takes two of
the "F1" generation
(which are hybrid tall: Tt)
& crosses them. He
thought he was going to
get all tall again (since tall
is dominant). But no!
T
t
(Let’s fill it in)
Mrs. Degl
Mendel’s F1 cross summary:
Parent Pea Plants
(Two Members of F1
Generation)
Genotypes:
Tt x Tt
Offspring
(F2 Generation)
Phenotypes:
tall x tall
Genotypes:
25% TT
50% Tt
25% tt
Phenotypes:
75% tall
25% short
A helpful thing to recognize:
Any time two parents have the same phenotype for a trait
but some of their offspring look different with respect to that
trait, the parents must be hybrid for that trait.
Mrs. Degl
The Principle of Independent Assortment
Alleles for different traits are distributed to sex cells (&
offspring) independently of one another.
So far we've been dealing with one trait at a time. For
example, height (tall or short), seed shape (round or
wrinkled), pod color (green or yellow).
Mendel noticed during all his work that the height of the
plant and the shape of the seeds and the color of the pods
had no impact on one another. In other words, being tall
didn't automatically mean the plants had to have green
pods, nor did green pods have to be filled only with wrinkled
seeds, the different traits seem to be inherited
INDEPENDENTLY. This is called Independent
Assortment.
Mrs. Degl
Four o'clock plants have a
gene for color and a gene
for height with the
following phenotypes:
Dihybrid Cross
RR: red flower
TT: tall plant
Rr: pink flower
Tt: medium height plant
rr: white flower
tt: dwarf plant
If a dihybrid plant is selffertilized, Let’s see
proportions of genotypes
and phenotypes produced
on the next page.
Mrs. Degl
Number
Genotype
Phenotype
1
RRTT
red-flower, tall plant
2
RrTT
pink flower, tall plant
1
rrTT
white flower, tall plant
2
RRTt
red flower, medium height plant
4
RrTt
pink flower, medium height plant
2
rrTt
white flower, medium height plant
1
RRtt
red flower, dwarf plant
2
Rrtt
pink flower, dwarf plant
1
rrtt
white flower. dwarf plant
Mrs. Degl
With incomplete dominance, a cross between organisms
with two different phenotypes produces offspring with a
third phenotype that is a blending of the parental traits.
R = allele for red flowers
W = allele for white flowers
red x white ---> pink
RR x WW ---> 100% RW
It's like mixing paints, red + white will make pink. Red doesn't totally
block (dominate) the white, instead there is incomplete dominance, and
we end up with something in-between.
We can still use the Punnett Square to solve problems involving
incomplete dominance. The only difference is that instead of using a
capital letter for the dominant trait & a lowercase letter for the recessive
trait, the letters we use are both going to be capital (because neither
trait dominates the other).
Mrs. Degl
With codominance, a cross between organisms with two
different phenotypes produces offspring with a third
phenotype in which both of the parental traits appear
together.
R = allele for red flowers
W = allele for white flowers
red x white ---> red & white spotted
RR x WW ---> 100% RW
The genetic gist to codominance is pretty much the same as incomplete
dominance. A hybrid organism shows a third phenotype --- not the usual
"dominant" one & not the "recessive" one ... but a third, different
phenotype. With incomplete dominance we get a blending of the
dominant & recessive traits so that the third phenotype is something in
the middle (red x white = pink).
In COdominance, the "recessive" & "dominant" traits appear together in
the phenotype of hybrid organisms.
Mrs. Degl