Download Introduction to Genetics using Punnett Squares

Genetics Using
Punnett Squares
Early Genetics
• The study of genetics
began with observations
made by Gregor Mendel.
• After noticing that the
flowers his pea plants
were either violet or
white, Mendel began to
study the segregation of
heritable traits.
Between 1856 and 1863
he cultivated and tested
at least 28,000 pea
plants.
Remember that Mendel worked almost 150 years ago when nobody
knew about genes or even the structures (chromosomes) that carry
genes.
Mendel’ Pea Plants
Mendel based his laws on his studies of garden
pea plants. Mendel was able to observe
differences in multiple traits over many
generations because pea plants reproduce rapidly,
and have many visible traits such as:
Pod color
Seed Color
Plant Height
Green
Green
Tall
Short
Yellow
Yellow
Seed Shape
Pod Shape
Wrinkled
Smooth
Pinched
Round
Mendel’s Experiments
Mendel noticed that some plants always produced offspring that had a form of a
trait exactly like the parent plant. He called these plants “purebred” plants. For
instance, purebred short plants always produced short offspring and purebred tall
plants always produced tall offspring.
X
Purebred Short Parents
Short Offspring
X
Purebred Tall Parents
Tall Offspring
Mendel’s First Experiment
Mendel crossed purebred plants with opposite forms of a trait. He called these plants
the parental generation , or P generation. For instance, purebred tall plants were
crossed with purebred short plants.
X
Parent Tall
P generation
Parent Short
P generation
Offspring Tall
F1 generation
Mendel observed that all of the offspring grew to be tall plants. None resembled
the short short parent. He called this generation of offspring the first filial , or F1
generation, (The word filial means “son” in Latin.)
Mendel’s Second Experiment
Mendel then crossed two of the offspring tall plants produced from his first
experiment.
Parent Plants
Offspring
X
Tall
F1 generation
3⁄4 Tall & 1⁄4 Short
F2 generation
Mendel called this second generation of plants the second filial, F2, generation.
To his surprise, Mendel observed that this generation had a mix of tall and short
plants. This occurred even though none of the F1 parents were short.
Mendel’s Law of Segregation
Mendel’s first law, the Law of Segregation, has three parts. From his
experiments, Mendel concluded that:
1. Plant traits are handed down through “hereditary factors” in the sperm
and egg.
2. Because offspring obtain hereditary factors from both parents, each plant must
contain two factors for every trait.
3. The factors in a pair segregate (separate) during the formation of sex
cells, and each sperm or egg receives only one member of the pair.
Dominant and Recessive
Genes
Mendel went on to reason that one factor (gene) in a pair may mask, or hide, the
other factor. For instance, in his first experiment, when he crossed a purebred
tall plant with a purebred short plant, all offspring were tall. Although the F1
offspring all had both tall and short factors, they only displayed the tall factor.
He concluded that the tallness factor masked the shortness factor.
Today, scientists refer to the “factors” that control traits as genes. The different
forms of a gene are called alleles.
Alleles that mask or hide other alleles, such as the “tall” allele, are said to be
dominant.
A recessive allele, such as the short allele, is masked, or covered up, whenever
the dominant allele is present.
Homozygous Genes
What Mendel refered to as a “purebred” plant we now know this to mean
that the plant has two identical genes for a particular trait. For instance, a
purebred tall plant has two tall genes and a purebred short plant has two
short genes. The modern scientific term for “purebred” is homozygous.
short-short
short-short
short-short
X
Short Parents
Short Offspring
According to Mendel’s Law of Segregation, each parent donates one height gene to
the offspring. Since each parent had only short genes to donate, all offspring will also
have two short genes (homozygous) and will therefore be short.
Hybrid Alleles
In Mendel’s first experiment, F1 offspring plants received one tall gene and one short
gene from the parent plants. Therefore, all offspring contained both alleles, a short
allele and a tall allele. When both alleles for a trait are present, the plant is said to be
a hybrid for that trait. Today, we call hybrid
tall-tall
alleles heterozygous.
short-tall
short-tall
short-short
X
Parent Short
P generation
Parent Tall
P generation
Offspring Tall
F1 generation
Although the offspring have both a tall and a short allele, only the tall allele is
expressed and is therefore dominant over short.
Dominant Alleles
Mendel observed a variety of dominant alleles in pea plants other than the
tall allele. For instance, hybrid plants for seed color always have yellow
seeds.
Green & Yellow Allele
Yellow Seed
However, a plant that is a hybrid for pod color always displays the green allele.
Green & Yellow Allele
Green Pod
In addition, round seeds are dominant over wrinkled seeds, and smooth pods are
dominant over wrinkled pods.
Law of Independent Assortment
Mendel’s second law, the Law
of Independent Assortment,
states that each pair of genes separate independently
of each other in the production of sex cells. For
instance, consider an example of the following gene pairs:
According to Mendels’ Law of Independent Assortment,
the gene pairs will separate during the formation of egg or
sperm cells. The plant will donate one allele from each
pair. The plant will donate either a yellow or green seed
allele, either a yellow or green pod allele, and a wrinkled
or round seed allele. It will always donate a wrinkled pod
shape. The donation of one allele from each pair is
independent of any other pair. For example, if the plant
donates the yellow seed allele it does not mean that it will
also donate the yellow pod allele.
Lets consider a single gene…
• A gene carries
information that
determines your traits.
Traits are
characteristics you
inherit from your
parents.
• Genes are located in
chromosomes.
• Chromosomes come in
pairs and there are
thousands, of genes in
one chromosome.
Continued…
• In humans, a cell’s nucleus
contains 46 individual
chromosomes or 23 pairs of
chromosomes.
• Half of the chromosomes
come from one parent and
half come from the other
parent.
Here is the detailed
structure of a
chromosome
This is a human
karyotype
representing the 23
pairs of
chromosomes in a
male
Definitions
• Allele- discrete version of the same gene
• Genotype- the genes of an organism for one
specific trait
• Phenotype- the physical appearance of a trait in
an organism
Definitions
• Dominant trait refers to a genetic feature
that “hides” the recessive trait in the
phenotype of an individual.
• The term "recessive” describes a trait that
is covered over (or dominated) by another
form of that trait and seems to disappear.
• Homozygous= two alleles that are the same
for a trait (Pure)
• Heterozygous= two different alleles for a
trait (Hybrid)
Practice
• We use two letters to represent the genotype.
A capital letter represents the dominant form
of a gene (allele) and a lowercase letter is the
abbreviation for the recessive form of the
gene (allele).
• Example below: P=dominant purple and p=
recessive white
The phenotype for this
flower is violet while
its genotype (if
homozygous) is PP.
The phenotype for this
flower is white while
its genotype is pp (to
be white the flower
must have two of the
recessive copies of the
allele).
Punnett Squares

The Punnett square is
the standard way of
working out what the
possible offspring of
two parents will be.
– It is a helpful tool to
show allelic
combinations and
predict offspring ratios.
Before we go further lets review how to set
up a Punnett Square…
We begin by constructing a grid of two
perpendicular lines.
Next, put the genotype of one parent across
the top and the other along the left side.
For this example lets consider a genotype of BB crossed with bb.
B
b
b
B
• Notice only one
letter goes above
each box
• It does not matter
which parent’s
genotype goes on
either side.
Next, fill in the boxes by copying the column
and row head-letters down and across into
the empty spaces.
B
B
b
Bb
Bb
b
Bb
Bb
Punnett Squares
 Now
that we have learned the
basics of genetics lets walk
through some examples using
Punnett Squares.
W
w
WWW Ww
w Ww ww
Usually write the
capital letter first
Lets say:
W- dominant white
w- recessive violet
Parents in this cross are heterozygous (Ww).
Note: Make sure I can tell your capital letters from
lowercase letters.
What percentage of the offspring will have violet
flowers?
ANSWER: 25% (homozygous recessive)
Red hair (R) is dominant over blond hair (r). Make a
cross between a heterozygous red head and a
blond.
R
r
r
r
Rr
rr
Rr
rr
What percentage of the offspring will have red hair? 50%
Let’s try some more…
In pea plants, tall pea plants (T) are dominant
over short pea plants (t). Construct a Punnett
Square for a heterozygous tall pea plant and a
short pea plant.
t
t
T
Tt
Tt
t
tt
tt
What are the
percentage of
phenotypes?
50% tall
50% short
Black eyes (R) is dominant over red eyes (r)
in rats. Make a cross between a homozygous rat
with black eyes and a rat with red eyes.
r
r
R
Rr
Rr
R
Rr
Rr
What is the possibility of
a red eye off springs?
0%

References
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http://www.athro.com/evo/gen/punnett.html
http://www.kidshealth.org/kid/talk/qa/what_is_gene.html
http://brookings.k12.sd.us/biology/ch%2011%20genetics/punnettpr
actice.ppt#1
http://www.usoe.k12.ut.us/CURR/Science/sciber00/7th/genetics/sci
ber/punnett.htm
http://www.biotechnologyonline.gov.au/images/contentpages/karyo
type.jpg