1. GENETICS

2. GENETICS
Every trait (or characteristic) in your body comes from
instructions from your mother and father
Father
Mother

3. GENETICS
The instructions are coded in the DNA as genes. Genes
are located in chromosomes.
Genes
Segments of DNA that
code for specific traits
For example…
Gene for height
Gene for eye-color
This is not an accurate example. It’s just used to illustrate a point.

4. GENETICS
A closer look…
Half of the offspring’s chromosomes are from mommy, and
half are from daddy.
Father
Mother

5. GENETICS
A closer look…
Humans have 23 different chromosomes. We get 1 of
each from our parents, for a total of 46 in somatic cells.

6. GENETICS
A closer look…
Homologues
Homologues
Not
homologues
A pair of the same types of
chromosomes are called
homologous chromosomes,
or just homologues.
This picture has 22 pairs of
homologous chromosomes.

7. GENETICS
Chromosomes and their genes are passed to the offspring
(children) through sperm and egg cells (gametes)
Egg cells
23 chromosomes
Sperm cells
Father
Mother
23 chromosomes

8. GENETICS
Chromosomes and their genes are passed to the offspring
(children) through sperm and egg cells (gametes)
Father
Mother

9. GENETICS
9 months later…
Father
Mother
The offspring is born

10. GENETICS
Each gene has alternate forms, called alleles. For
instance, the gene for eye colour may have 2 alleles:
brown or blue
Brown eyes
Blue eyes
Father passed-on:
Brown eyes
Mother passed-on:
Blue eyes

11. GENETICS
Some alleles can “mask” the effects of the other allele.
Although the
mother has blue
eyes, the child has
brown eyes.
Father
Brown eyes
Mother
In this case, brown eyes are “dominant”
As a result, blue eyes are “recessive”
Blue eyes

12. GENETICS
Some alleles can “mask” the effects of the other allele.
Dominant – traits that are
expressed more often.
Alleles that are dominant are usually
represented by a capitalized letter
symbolizing that allele (i.e. B)
Recessive – traits that are
expressed less frequently.
Brown eyes
Blue eyes
Alleles that are recessive are usually
represented by a lower-case letter
symbolizing that allele (i.e. b)

13. GENETICS
Dominant and Recessive Alleles:
Daughter’s genetic makeup:
Brown eyes
Blue eyes
How can the daughter’s two alleles (genotype) be written?
Let the allele for brown eyes be B, and the allele for blue eyes be b
Brown eyes
Bb
Blue eyes

14. GENETICS
Dominant and Recessive Alleles:
Brown eyes
Bb
Blue eyes
Notice how the daughter carries the allele for blue eyes, but she does
not have blue eyes.
Thus her phenotype (observable trait) is brown eyes.

15. GENETICS
Homozygous vs. heterozygous
Brown eyes
Bb
Blue eyes
Since she carries two different alleles for eye colour, we
can say that she is heterozygous for eye color.
Heterozygous – describes the genotype of an organism that
contains two different alleles (ex. Bb)

16. GENETICS
Homozygous vs. heterozygous
blue eyes
bb
Blue eyes
If she had blue eyes, we can say that she is
homozygous for eye color.
Homozygous – describes the genotype of an organism that
contains two alleles that are the same (ex. BB)

17. GENETICS
But wait…
Father
Is this possible?!
Mother

18. GENETICS
Yes this is possible
The father could have carried the recessive allele for blue
eyes as well…
Father
Bb
bb
Mother
bb
…although you can’t tell because he has the dominant
brown eye allele (which “masks” blue eyes)

19. GENETICS
The father’s parents could have passed the blue eye allele to
him
Grandpa
Grandma
bb
BB
Father
Bb

20. GENETICS
All too complicated?
Let’s take a look at how it all started…
Gregor Mendel (1822-1884)
- Known as the father of genetics
- Worked with pea plants

21. GENETICS
Mendel’s pea plants
He observed 2 traits for each part of the plant

22. GENETICS
Mendel’s pea plants
Mendel came up with the concept of alleles.
He noticed that alleles are hereditary, and that you can predict
the probability of the offspring having certain alleles.

23. GENETICS
Mendel’s pea plants
He also noticed that some traits dominated over others
For instance, if you “crossed” a yellow-pea plant with a
green-pea plant, you generally get a yellow-pea plant
Mendel Video

24. GENETICS
Mendel’s pea plants
What does “crossing” the pea plants mean?
It means to mate a plant with another plant by pollination.
Garden peas are both self-fertilizing and cross-fertilizing.
Self-fertilizing – a plant’s pollen grains fertilize it’s own egg cells in the
ovary
Cross-fertilizing – a plant’s pollen grains fertilize another plant’s egg cells
in the ovary

25. GENETICS
Mendel’s pea plants
This allowed Mendel to mate pea plants with each other as
well as with itself.
For example, you can mate a
purple flower pea plant with itself.
This is called a
Punnett Square
MATE!

26. GENETICS
Punnett Square
This means that mating a pea
plant that is heterozygous for
flower colour (Bb) with itself will
produce…
F1 GENOTYPE:
25% BB
50% Bb
25% bb
A 1:2:1 ratio
F1 PHENOTYPE:
75% purple flowers
25% white flowers
F1 stands for filius and filia, which in Latin means
“son” or “daughter”

27. GENETICS Square
Constructing a Simple Punnett
Step 1: Draw a square with a 2 by 2 grid

28. GENETICS Square
Constructing a Simple Punnett
Step 2: Choose a letter for your allele and
record this choice
Let the allele for purple flower be represented by the letter B

29. GENETICS Square
Constructing a Simple Punnett
Step 3: Consider all possible gametes produced by the
first parent. Write the alleles for these gametes across
the top of the square
Let the allele for
purple flower be
represented by the
letter B
Bb
B
b

30. GENETICS Square
Constructing a Simple Punnett
Step 4: Consider all possible gametes produced by the
second parent. Write the alleles for these gametes down
the side of the square
Let the allele for
purple flower be
represented by the
letter B
B
bb
b
b
b

31. GENETICS Square
Constructing a Simple Punnett
Step 5: Complete the square by writing all possible allele
combinations from the cross
B
b
b
b
Let the allele for
purple flower be
represented by the
letter B

32. GENETICS Square
Constructing a Simple Punnett
Step 6: Determine the genotypic and phenotypic
proportions of the offspring
Let the allele for
purple flower be
represented by the
letter B
B
b
Bb
b
bb
F1 Genotypes:
50% Bb
50% bb
F1 Phenotypes:
b
Bb
bb
50% of the plants
have purple flowers
50% of the plants
have white flowers

33. GENETICS
A plant that is homozygous for purple flowers is crossed with a plant
that has white flowers. If the purple condition is dominant over the
white condition, what are the genotypes and phenotypes of the F1
generation?
GIVEN:
Let the allele for flower color be presented by the letter P
Parent genotypes:
Parent gametes:
PP
P or P
X
X
pp
p or p
Parent # 1 gametes
Results:
P
Parent # 2
gametes
p
Pp
Pp
25%
p
P
25%
Pp
Pp
25%
25%
Therefore the results of
the PP x pp cross
indicate that:
F1 genotypes: 100% are
Pp (or 4 out of 4 are Pp)
F1 phenotypes: all plants
have purple flowers

34. GENETICS
Sheep ranchers prefer white sheep over black sheep, because black
fur is hard to die and is brittle. The allele for black fur is
recessive. As a result, if a sheep rancher wishes to purchase a
white fur sheep for breeding, how does she/he know if it will make
black fur babies (in other words, how does the rancher know if
her/his sheep is homozygous or heterozygous?)?
A test cross can be performed to determine the genotype of a
dominant phenotype, which involves breeding the unknown
genotype with a homozygous recessive genotype. In this case the
white sheep with an unknown genotype will be bred with a
homozygous recessive black fur sheep.

35. WHY WERE MENDEL’S FINDINGS
IMPORTANT?
Once we find traits that we like in an organism (for
example, a dog), we can maintain these traits by
mating closely related individuals for the purpose of
maintaining or perpetuating these characteristics (this
is called “inbreeding”)

36. WHY WERE MENDEL’S FINDINGS
IMPORTANT?
-We can also mix traits that we like together from
different species (in plants)
-This process is called “hybridization”

37. WHY WERE MENDEL’S FINDINGS
IMPORTANT?
Genetic Screening:
-we can tell if an individual
carries an allele (or two
alleles) for genetic disorders
-Amniocentesis and Chorionic
Villus Sampling (CVS)

38. WHY WERE MENDEL’S FINDINGS
IMPORTANT?
AMNIOCENTESIS:
Looking at fetal
cells from the
amniotic fluid

39. WHY WERE MENDEL’S FINDINGS
IMPORTANT?
CHORIONIC VILLUS
SAMPLING (CVS):
Sampling tiny
fingerlike projections
on the placenta
Can be performed
earlier (10th to 12th
week of pregnancy)
than amniocentesis

40. THE STORY ISN’T AS SIMPLE…
There are often
more than 2
alleles per
gene…
…but each
organism can
ONLY have two
different alleles
for a trait at any
one time
This is called
having multiple
alleles for one
gene
We usually
express these
alleles like this:
E1, E2, E3, E4

41. THE STORY ISN’T AS SIMPLE…
Codominance:
Both alleles are
expressed at the
same time

42. THE STORY ISN’T AS SIMPLE…
Incomplete
dominance: two
alleles are equally
dominant

43. THE STORY ISN’T AS SIMPLE…

44. SEX-LINKED TRAITS
- Traits that are
controlled by genes
located on the sex
chromosomes
(usually the X
chromosome)
Ex: Duchenne
muscular
dystrophy,
hemophilia,
Charcot-MarieTooth disease and
color blindness
-Usually
represented like
this:
XR Xr

45. SEX-LINKED TRAITS
Females get 2 X chromosomes:
Protected by
other X
chromosome
Males get ONE X chromosome:
Disease!!!

46. SEX-LINKED TRAITS
Females get 2 X chromosomes:
-1 gets turned off (called a
Barr Body)
-some cells have one X
chromosome inactive, while
other cells have the other
inactive

47. GENETICS
Dihybrid cross
So far what we have done
is a monohybrid cross,
which only involves one
trait. What if you wanted
to see how two different
traits are passed on to the
next generation?
Male RrYy x Female RrYy

48. GENETICS
Dihybrid cross
So far what we have done
is a monohybrid cross,
which only involves one
trait. What if you wanted
to see how two different
traits are passed on to the
next generation?
Male RrYy x Female RrYy