1. The document discusses Gregor Mendel's experiments with pea plants which helped establish the fundamental principles of heredity and genetics. 2. Mendel observed that traits passed from parents to offspring in predictable patterns, with some traits being dominant over others. 3. Through his experiments with cross-pollination and tracking traits over generations, Mendel discovered that traits are passed from parents to offspring through discrete units that we now call genes.

1. Inheritance
Chapter 5 Lessons1-2
p.153-170

2. Heredity
• Organisms usually look like their parents.
• Parents pass some of their characteristic,
or inherited traits, to their offspring
• Include: hair/fur color, height, eye color
• The passing of inherited traits from parent
to offspring is know as heredity.

3. Genetics
• Genetics is the study of heredity.
• Gregor Mendel discovered the
rules for inheritance.

5. Mom Dad
Gene for
Eye Color
We have at least two genes for every trait; one
from mom and one from dad.

6. Gregor Mendel The Father of Genetics
• Gregor Mendel was an Austrian monk in
the 1800s.
• He experimented with pea plants
• Helped to understand how traits were
passed from parent to offspring

7. Early Ideas about Heredity
• Blending Theory
– Each parent contributed “factors” that were
blended in the offspring

8. Blending theory is not true for most
traits.

9. Mendel’s Experiment
• Mendel did
controlled breeding
experiments with
pea plants

10. Mendel’s Experiments
• Why pea plants?
– Reproduce quickly
– Easily observable traits (flower color, pea
shape)
– Mendel could control which pairs of plants
reproduced

12. Pollination
• Sperm cells from the pollen fertilize egg
cells in the pistil
• 2 Types

13. Self-pollination vs. Cross-pollination
• Self-pollination
– Pollen from one plant lands on the pistil of a
flower on the same plant
– This is still sexual reproduction

14. Cross-pollination
• Pollen from one plant lands on the pistil of
a flower on a different plant

16. Mendel’s Experiments: Self-
pollination
• Purple flower parent = 100% purple flower
offspring

17. Mendel’s Experiments: Self-
pollination
• White flower parent = 100% white flower
offspring

18. Self-pollination Conclusion
• Mendel called these plants true-breeding
plants
• True-breeding means that when a true-
breeding plant self-pollinates, it always
produces offspring with traits that match
the parent
– White flower parent = 100% white flower
offspring
– Purple flower parent = 100% purple flower
offspring

19. Mendel’s Experiment: Cross-
pollination

20. Conclusion from 1st generation
cross
• Purple flower x white flower = 100%
Purple flowers

21. Conclusion from 1st generation
cross
• Mendel called these plants hybrids, because
they contained both the purple and white genes

22. Conclusion from 1st generation
cross
• Why did only the purple trait show?

23. Conclusion from 1st generation
cross
• Let’s have these hybrid plants self-pollinate

24. Second Generation (Hybrid)
Crosses
• For every three purple flower plant, there
was 1 white flower plant

25. Second Generation (Hybrid)
Crosses
• The trait had disappeared in the 1st
generation, but reappeared in the 2nd

26. Hmmmm…
I wonder why it
skipped a
generation.

27. OMG!
There’s a
3:1 pattern!

28. Mendel’s Conclusions
• Traits are control by two genetic factors
(genes) that are inherited (passed down)
from the parents.
• Not all genetic factors are equal
• Purple flowers appeared even if the plant
had one gene for purple and one for white.

29. Mendel’s Conclusions
• Mendel decided that purple gene blocked
the white gene.
• He called it a dominant trait
• The other gene (white) would be called the
recessive trait, because it was weaker.

30. Genetics Today

31. Genes and alleles
• Today, we know the “genetic
factors” that Mendel
discovered are genes.
• Genes are sections, or part,
of a chromosome that has
genetic information for one
trait.
• One gene can have different
forms (white flowers and
purple flowers)
• Different forms of a gene are
called alleles

33. Phenotype
• Pheno- = “to show”
• Geneticists call how a trait appears,
or is expressed, the trait’s phenotype.
• Example:
–blue eye/ brown eyes
–Purple flowers/ white flowers
–Black hair/ blonde hair

34. Genotype
• Geno = genes
• The two alleles that control the
phenotype of a trait
• Cannot see an organism’s genotype
• Example:
–PP, Pp, pp
–BB, Bb, bb

35. Symbols for Genotypes
• The alleles for a trait are given a letter
symbol based on the dominant form of
the trait.
• Example:
– Purple flowers / White flowers = P, p
– Yellow seeds / Green seeds = Y, y
– Tall stem / Dwarf (short) stem = T, t

36. Symbols for Genotypes
• An uppercase letter represents the
dominant trait
• A lowercase letter represents a
recessive trait
• Examples:
– Purple flower (P); white flowers (p)
– Yellow seeds (Y); green seeds (y)
– Green pods (G); yellow pods (g)

38. Homozygous means the organism
has two alleles that are the same.

39. Heterozygous means that the
organism has two different alleles for
that gene.

40. Predicting Inheritance with
Punnett Squares

41. Punnett squares
• Diagrams that help to predict possible
genotypes and phenotypes of offspring.

42. How to make a punnett square?

43. Using Ratios
• Ratio is 3:1 yellow seeds to green
• Does this mean if you grow 4 offspring 3
will have yellow seeds and 1 will make
green seeds?

44. Using Ratios
• The phenotype of one offspring does not
effect the phenotype of the next.
• For each offspring there would be a 75%
chance of it having the yellow phenotype
and a 25% chance of it having the green
phenotype.

45. Using Ratios
• However, if you had a lot of time, like
Mendel, and you grow 30,000 pea plants,
the ratio would be near 3:1

46. Pedigrees
• Other than a Punnett square, pedigrees
are also used to model inheritance.
• Pedigrees show phenotypes of genetically
related family members

47. Complex Patterns
of Inheritance

48. Types of Dominance
• For Mendel’s pea plants, the presence of
one dominant allele produces a dominant
phenotype
• However, not all allele pairs have a
dominant-recessive interaction

49. Incomplete Dominance
• Traits appear as a combination
• Definition: The offspring’s phenotype is a
combination or mix of the parent’s
phenotype.

50. Codominance
• When both alleles can be observed in a
phenotype

51. Codominance
• When both alleles can be observed in a
phenotype

53. Codominance
• When both alleles can be observed in a
phenotype

55. Multiple Alleles
• Some genes have more than two alleles
• Human blood types have 3 alleles
– IA, IB, i
• This results in 4 blood types
– A, B, AB, or O
• IA and IB are codominant
• i is recessive to IA and IB

58. Polygenic Inheritance
• For Mendel, each trait he studied was
determined by only one gene
• However, a trait can be affected by one
than one gene
• Definition: Multiple genes determine the
phenotype of a trait
• Examples:
– Skin color, height, weight