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Genetics
The study of inherited traits
Jennifer and Homer Simpson
If the two of them were to get married, would
their children look like this?
Definitions
1. Heredity – the passing of traits from parents to
2.
3.
4.
5.
offspring
Gregor Mendel – the father of genetics
Self-pollination – all traits are received from the
one parent
Cross-pollination – seeds are produced as
offspring from two different plants
Purebred – offspring are bred for specific traits
by mating identical parents
6. Traits – an organism’s characteristics
passed on by the biological parents
7. Hybrids – offspring created by crossing
parents with different characteristics
8. Genes – segments of DNA that code for
traits
9. Alleles – letters that are used to express the
frequency of traits
10.Dominance – individual traits that are more
likely to be expressed in a population,
capital letters
11.Recessive – traits less likely to be visible in
a population , lower case letter.
12.Segregation – separation of alleles during
meiosis
13.Punnett Square – a probability chart used
to determine possible outcomes for the
inheritance of traits
14.Phenotype – a description of an organism’s
physical appearance, what it looks like.
15.Genotype – a two letter code using alleles
that expresses the dominance of an
individual, what genes/letter are there.
16.Probability – the % chance of a specific
trait or set of traits being inherited
17.Homozygous – the two alleles that
represent a trait are exactly the same, ex.
HH, hh
18.Heterozygous – the two alleles that
represent a trait are different, ex. Hh
19.One-Factor Cross – only one allele is used
in a Punnett Square, ex. Hair color
20.Two-Factor Cross – two sets of alleles are
used to determine probability of having
two linked traits, ex. Hair and eye color
What is Genetics?
 Genetics is the study of traits and
how they are chemically passed from
one generation to their offspring by
meiosis and fertilization
Where did Genetics get its Start?
 Gregor Mendel (a Biology Teacher and a Catholic Priest) made some
observations while working in the monastery gardens
 He hypothesized that each organism had its own unique set
of traits that could be transferred from one organism to the
next.
 He used pea plants to test and support his hypothesis.
First: peas exhibit a number of contrasting traits that
Mendel could easily study
Second: pea plants can easily be cross pollinated
Third: a relatively large number of seeds are produced
The Start of Genetics cont.
 Mendel's first observations showed that peas self
pollinate.
 Each pea plant had its own egg and sperm.
 He exchanged male and female pollen from two different
plants to force cross-pollination. (hybrid)
 The plants that weren’t switched were purebreds.
Gregor Mendel
1822 - 1884
Genes and Dominance
Every organism has
certain characteristics
called traits.
Each trait is expressed by
a two letter code (alleles)
Some alleles are stronger
than others (dominant)
and others are weaker
(recessive)
Number your paper from 1 to 3
Do Not skip a space!
1. Each trait is expressed by a _________?
2. What are stronger traits called?
3. What are weaker traits called?
1. Two letter code (alleles)
2. Dominant
3. Recessive
Alleles
 Each trait is controlled by two genes (1 from each biological parent)
 Genes can be represented by letters
 An uppercase letter denotes the wild type (usual, dominant)
form of the gene
 A lowercase letter denotes the mutant (unusual, recessive)
form of the gene
 There are three distinct combinations; e.g. TT,Tt, and tt
 If a dominant gene is paired with a recessive gene, the
dominant's trait will usually out power the recessive, but not
always
Questions 4, 5, & 6
4. How many genes control one trait?
5. What are genes represented by?
6. If a dominant and recessive gene are paired together,
which trait is seen?
4. Two
5. Letters
6. Dominant
Punnett Squares
• To set up a Punnett square, draw a large square,
and then divide it into 4 equal sections. It should
look something like this:
Punnett Squares cont.
Now you need two parents to mate with a known genotype.
For example, a red flower (genotype Rr) and a white
flower (genotype rr). Place one of the parents on top, and
one on the left. You should get a something similar to this:
Punnett Squares cont…
• Take each letter in each column and combine it with
each letter from each row in the corresponding
square. You should now have a picture close to this:
Punnett Squares cont….
 Each punnett square is equal to 100% of the possible
genotypes.
 Therefore each box in our punnett square is equal to 25%
probability
• We can then look at the results of our Punnett Square and
determine the probability of traits.
• 50% of the squares are Rr; 50% are rr
• We can assume there is a 50% chance the flowers will be
red or 50% chance of them being white.
Questions 7 & 8
7. What is each punnett square worth in percentage?
8. If there are 10 boxes in a Punnett Square, what would
each individual box represent in percentage?
7. 100%
8. 10%
Try your own Punnett Square
• Find out the probability of having a tall or short
plant using these two genotypes.
• TT and tt (T = Tall t = short)
Genotype and Phenotype
Genotype is the genetic code (alleles) for an organism.
Phenotype is the actual physical appearance.
Blonde Hair = BB
Green Hair = bb
Brown eyes = AA
Yellow eyes = aa
Figure out probability of this problem.
Write down each of their traits for hair and eye color
The Information
The Grinch
Phenotype: Green Hair
Yellow Eyes
Genotype: bb (green hair)
aa (yellow eyes)
Britney Spears
Phenotype: Blonde Hair
Brown Eyes
Genotype: BB
(blonde hair)
AA
(brown eyes)
Try again!
This time, come up with your own set of alleles for
hair color and skin color.
Is this what their kids would look like?