1. Understanding
inher itance

3. What controls traits?
• Mendel concluded that two factors control
each trait.
• He also concluded that they can be
dominant or recessive.
• One factor comes from the egg cell and
one factor comes from the sperm cell.
• What are these factors? How are they
passed from parents to offspring?

4. What controls traits?
chromosomes
• Inside each cell is a nucleus that has
chromosomes. Chromosomes contain
genetic information that controls traits.
• What Mendel called “factors” are parts of
chromosomes. We know them as genes.

5. What controls traits?
chromosomes
• Each cell in an offspring contains
chromosomes from both parents.
• These chromosomes exist in pairs — one
chromosome from each parent.

6. What controls traits?
genes & alleles
• Each chromosome can
have information
about 100’s or 1000’s
of traits.
• A gene is a section
on a chromosome
that has genetic
information for one
trait.

7. What controls traits?
genes & alleles
• An offspring inherits two genes for each
trait, one from each parent.
• The genes can be the same or different,
such as purple or white for pea flower
color.
XX

8. What controls traits?
genes & alleles
• Another example of a trait and its forms
is having dimples or not having dimples.

9. What controls traits?
genes & alleles
• The different forms of a gene are called
alleles.

10. What controls traits?
genot ype & phenot ype
• Geneticists call how a trait appears, or is
expressed, the trait’s phenotype.
• A person’s eye
color is an
example of
phenotype. The
trait of eye
color can be
expressed as
blue, brown,
green, or other
colors.

11. What controls traits?
genot ype & phenot ype
• Mendel concluded that there were two
alleles that controlled the phenotype of
each trait.
• The two alleles that control the
phenotype of a trait are called the trait’s
genotype.
• You cannot see an organism’s genotype.
But you can make guesses about a
genotype based on its phenotype.

12. What controls traits?
genot ype & phenot ype

13. What controls traits?
genot ype & phenot ype
• Scientists use symbols to represent the
alleles in a genotype.
• Uppercase letters represent dominant
alleles and lowercase letters represent
recessive alleles.
• The dominant allele, if present, is written
first.

14. What controls traits?
genot ype & phenot ype

15. What controls traits?
homoz ygous & het er oz ygous
homozygous:
heterozygous:
RR
rr
Rr

16. modeling inheritance
• Plant breeders and animal breeders use
two tools to help them predict how often
traits will appear in offspring.
• These tools, Punnett squares and
pedigrees, can be used to predict and
identify traits among genetically related
individuals.

17. modeling inheritance
Punnet t Square
Pedigree

18. modeling inheritance
Punnet t Squar es
• If the genotypes of the parents are
known, then the different genotypes and
phenotypes of the offspring can be
predicted.
• A Punnett square is a model used to
predict possible genotypes and
phenotypes of offspring.

19. modeling inheritance
Punnet t Squar es

20. modeling inheritance
Pedigr ees
• A pedigree shows phenotypes of
genetically related family members.
• It can also help determine genotypes.

21. modeling inheritance
Pedigr ees
• Let’s look at the phenotypes of the
children.....

22. complex patterns of inheritance
t ypes of dominance:
• Mendel studied traits influenced by only
one gene with two alleles.
• We know now that not all traits are
inherited this way. Some traits have more
complex inheritance patterns.

23. complex patterns of inheritance
t ypes of dominance:
incomplete dominance
• Alleles show incomplete dominance when
the offspring’s phenotype is a blend of the
parents’ phenotypes.
• Here is an example with 4 o’clock flowers.

24. complex patterns of inheritance
t ypes of dominance:
incomplete dominance

25. complex patterns of inheritance
t ypes of dominance:
incomplete dominance

26. complex patterns of inheritance
t ypes of dominance:
codominance
• When both alleles can be observed in a
phenotype, this type of interaction is
called codominance.

27. complex patterns of inheritance
t ypes of dominance:
codominance
• For example, if a
cow inherits the
allele for white
coat color from
one parent and
the allele for red
coat color from
the other parent,
the cow will have
both red and
white hairs.

28. complex patterns of inheritance
mul t iple alleles
• Unlike the genes in Mendel’s pea plants,
some genes have more than two alleles, or
multiple alleles.
• Human ABO blood type is an example of a
trait that is determined by multiple alleles.
• There are three alleles for the ABO blood
type—IA, IB, and i.

29. complex patterns of inheritance
mul t iple alleles
• In the pea plants there were two possible
phenotypes, ie round pea or wrinkled pea.
• The way the alleles combine for blood
types, results in one of four blood types—A,
B, AB, or O.

30. complex patterns of inheritance
mul t iple alleles
• The IA and IB alleles are codominant to
each other, but they both are dominant to
the i allele.
• Even though there are multiple alleles, a
person inherits only two of these alleles,
one from each parent.

31. complex patterns of inheritance
mul t iple alleles

32. complex patterns of inheritance
polygenic inher i tance
• Mendel concluded that only one gene
determined each trait. We now know that
more than one gene can affect a trait.
• Polygenic inheritance occurs when
multiple genes determine the phenotype of
a trait.

33. complex patterns of inheritance
polygenic inher i tance

34. complex patterns of inheritance
polygenic inher i tance
• Because several genes determine a trait,
many alleles affect the phenotype even
though each gene has only two alleles.
• Therefore, polygenic inheritance has many
possible phenotypes.
• Eye color, height, weight and skin color
are all examples of traits that are
determined by polygenic inheritance.

35. complex patterns of inheritance
polygenic inher i tance

36. genes and the environment
• Genes are not the only factors that can
affect phenotypes. An organism’s
environment can also affect its
phenotype.
• For example, the flower color of one type
of hydrangea is determined by the soil in
which the hydrangea plant grows.
• Acidic soil produces blue flowers.
Basic soil produces pink flowers.

37. genes and the environment
• The flower color of one type of hydrangea
is determined by the soil in which the
hydrangea plant grows.
• Acidic soil produces blue flowers.
Basic soil produces pink flowers.

38. genes and the environment
• Himalayan rabbits have
alleles for dark-coloured
fur, which is only
displayed at lower
temperatures.
• The fur in areas further
from the rabbit’s body
heat i.e. nose, ears, feet,
are therefore darkly
coloured.

39. genes and the environment
• For humans, healthful choices can also
affect phenotype.
• Many genes affect a person’s chances of
having heart disease.
• However, what a person eats and the
amount of exercise he or she gets can
influence whether heart disease will
develop.

40. recessive genetic disorders
• Disorders
caused by
recessive genes.
You must have
both recessive
alleles in order
to have the
disorder.

41. recessive genetic disorders

42. sex-linked disorders
• Red-green colour blindness is a recessive
sex-linked disorder.
• It occurs on the X chromosome.
XX XX XX
XY XY
female
not color blind
female
not color blind
male
not color blind
female
color blind
male
color blind

43. sex-linked disorders

44. recessive genetic disorders