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Genetics
Genetics is…
the scientific study of heredity (traits that are
passed from parent to offspring).
The Work of Gregor Mendel
Mendel’s Peas
Genes
• Mendel determined that…
– Traits are inherited as a result of factors passed from
parents to offspring
– We now know that these “factors” are genes
What is a Gene?
With your table group discuss what you
think a gene is. Write or draw your agreed
description on your whiteboard.
What is a
Gene
Genes (contd.)
• Allele – the different or alternate form of a gene.
– For example: Flower color in pea plants has 2 alleles:
purple and white
Different forms of a gene are written
as capitol and lower case letters.
Dominant alleles will be expressed if they are
present.
Violet (purple) flower color is dominant over white
color. The allele for violet is written as P
Recessive alleles are written with
lower case letters. The allele for
white is written as p
Genotype vs. Phenotype
• Phenotype is the physical characteristic of a
trait of an organism.
– Example: Tall or short
• Genotype is the genetic makeup of a trait.
– Example : Tt, TT, or tt.
Homozygous vs. Heterozygous
• Homozygous – two identical alleles for a
particular trait.
– Example : TT or tt
• Heterozygous – two different alleles for the same
trait.
– Example Tt
Why do you have two copies of every gene?
You have two of each chromosome, one from
each parent. Each chromosome has hundreds
to thousands of genes.
Punnett Squares
A Punnett square is a tool to show how the alleles
can combine in order to predict the possible
genotypes and phenotypes of offspring.
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 Square Practice
Draw a punnett square on your paper or
whiteboard.
W
w
WWW Ww
w Ww ww
Usually write the
capital letter first
Lets say:
W- dominant white
w- recessive violet
Complete a cross with two parents that 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)
Work this problem on your
whiteboard or paper.
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.
What are the
percentage of
phenotypes?
t
t
T
Tt
Tt
t
tt
tt
50% tall
50% short
Punnett Square Independent
Practice.
• Use the Zork Genetics trait chart to work out
the following crosses:
1. Cross two heterozygous one leg Zorks.
What are the percentages of each genotype?
What are the percentages of each phenotype?
2. Cross one homozygous green lips with a
heterozygous purple lips Zork. Give the
genotypic and phenotypic percentages
Now do it backwards
3. If you have the following #s of offspring
determine the possible genotypes of the parents:
a. 48 Zork babies with one eye and 12 Zork babies
with three eyes
b. 60 Zork babies with green hair and none with
yellow hair.
c. 54 Zork babies with thick eyebrows and 51 Zork
babies with thin eyebrows
Support your answers with punnett squares
Now predict
4. Two Zorks had 78 offspring. 15 had no wings
and 63 had wings. What are the chances
that their next offspring will not have wings?
Support your answer with punnett squares
Create and show the genotypes for
your Zork
• Your Zork must have 10 traits with at least 3
homozyogous dominant, 3 heterozygous and 3
homozygous recessive traits.
• Make a list of the genotypes and phenotypes
• Draw and color your Zork.