Gregor Mendel conducted experiments with pea plants in the 1850s and 1860s to study inheritance of traits. He observed that traits were inherited in predictable patterns, with dominant traits masking recessive traits in the offspring of crosses. His work established the fundamental laws of genetics, including segregation of alleles, independent assortment of traits, and dominance. Mendel's work laid the foundation for the field of molecular genetics and our modern understanding of genes and inheritance.

1. GENETICS
CHAPTERS 9 & 12

2. I. The History of Genetics
Genetics = field of biology devoted to
understanding how traits are passed from
parent to offspring

3. Heredity
= the transmission of characteristics from
parent to offspring

4. Mendel’s Experiments
Gregor Mendel – the father of genetics

5. Mendel’s Experiments
Continued…
Mendel’s Experiments (1856 – 1863)
 Observed characteristics of pea plants
(7) over a long period of time (28,000
plants over 8 years!)
 Initial
observation = when he planted
purple pea plants some white pea plants
grew

8. Mendel’s Methods:
1. grew pure purple and pure white plants for
several generations (P1 generation = parent
generation)

9. Mendel’s Methods continued…
2. cut off anthers from purple plants (male
part)

10. Mendel’s Methods continued…
3. cross-pollinated purple and white pure strains
4. F1 generation grew
(first filial generation) and
he counted the number of
purple and white plants
5. all F1 plants
were purple

11. Mendel’s Methods continued…
6. allowed self-pollination of F1
generation, producing the F2
generation
75% were purple
7.
25% were white

12. Mendel’s Conclusions:
1. something within the pea plants
controlled the characteristics he observed =
“factors”
2. each trait was inherited by a separate
“factor”, since he observed 2 forms of each
trait he hypothesized each characteristic
came in pairs

13.  Mendel’s traits were carried by “factors”
in each plant
 Each trait had 2 possible “factors”
(pairs)

14. Mendel’s Conclusions
continued…
3. since 1 trait completely disappeared in
the F1 generation, but appeared in a 3:1
ratio in the F2 generation, he hypothesized
that one of these “factors” was dominant
and one was recessive = Rule of
Dominance

17. Laws that Evolved from Mendel’s
Work
The Law of Segregation = The two alleles
(“factors”) for each trait must separate
during gamete formation (meiosis). There
will only be one allele for a trait in each

18. Laws the Evolved from Mendel
The Law of Independent Assortment =
the alleles for each trait must behave
independently of alleles for other traits
during gamete formation. We have no way
of determining which allele will go into which
gamete, it is random!

20. Mendel’s Work led to…
The Scientific Field of Molecular Genetics
= study of structure and function of
chromosomes and genes
**Mendel’s “factors” = genes

21. MOLECULAR GENETICS
 Mendel did not know the structure of DNA or
that there were genes during his research
 Gene = segment of DNA on a CH that controls
a trait
 Chromosomes (CH’s) occur in pairs = there are
2 genes responsible for each trait
 1 from mom
 1 from dad

22. Molecular Genetics continued…
 Allele = each different form of a gene,
during meiosis each gamete gets one
form of this gene
Rr
R = dominant allele
r = recessive allele

23. Example of Alleles

24. How to follow allele inheritance
= Genetic Crosses

25. Genetic Crosses Objectives
1. Phenotype of the parents & offspring
= how a trait shows up physically in the
organism

26. Genetic Crosses Objectives
2. Genotype
= the alleles (genes) that the organism has
inherited
 homozygous (the same alleles)
 recessive rr
 dominant RR
 heterozygous (different alleles) Rr

27. Genotype

28. Type of Crosses and Inheritance
1. Monohybrid cross
= following inheritance of 1 trait from generation
to generation

29. Types of Crosses continued…
2. Dihybrid
= following inheritance of 2 traits from generation
to generation

30. Crosses
3. complete dominance
= dominant phenotype always masks the
recessive phenotype when the dominant
allele is present

31. Crosses
4. incomplete dominance
= both phenotypes mix together in a
heterozygous individual

32. Crosses
5. codominance
= both phenotypes show up equally in a
heterozygous individual B = Black cat
W = White cat

33. In order to set up a cross, you use
1. Monohybrid Cross Punnett Square
1 trait!
2. Dihybrid Cross Punnett Square
2 traits!

34. What if you don’t know the
genotype of an organism…only the
phenotype
You Perform A Test Cross
= used if the individual of interest shows the
dominant phenotype (physical
characteristic) but you don’t know if it is
heterozygous or homozygous dominant
genotypically (what alleles the individual
actually inherited)

35. A TEST CROSS
= cross that unknown individual with a
homozygous recessive and look at their
offspring

36. Test Cross Results
Look at offpring of test
individual and the
homozygous recessive
parent
All dominant offspring = test
individual is homozygous
dominant (PP)
Any recessive offspring at all
= test individual is
heterozygous (Pp)

37. Another Test Cross Example

38. Probability
Probability
= likelihood that a specific event will occur,
expressed as a decimal, percentage or
fraction
# times an event is expected to happen
# of opportunities for an event to happen

39. Chapter 12 – Human
Inheritance
Types of Genetic Traits/Disorders & how they
are inherited:
1. Single-allele traits
= controlled by 1 allele of a gene (200)
i.e. Huntington’s Disease (HD), Cystic fibrosis,
sickle cell anemia

41. Human Inheritance Types…
2. Multiple-allele traits
= controlled by 3+ alleles of the same gene that
code for a single trait i.e blood types

42. AA = buds SS = spikes AS = spikes & buds NN=
none
AN = buds SN = spikes

43. Universal
Acceptor
Universal
Donor

44. Human Inheritance Types…
3. Polygenic Traits
= a trait controlled by several genes i.e. skin
color, eye color, human height

45. Human Inheritance Types…
5. Sex-Linked/Sex-Influenced traits
= presence of male or female chromosome and
sex hormones
Sex-Linked – the gene for trait is on the X or Y
v.
Sex-Influenced – the gene isn’t on the X or Y, but
the expression (phenotype) is affected by the
hormones made by being XX (female) or XY
(male)

46. Sex-Linked

47. Examples of Sex-Linked Traits
i.e. hemophilia, colorblindness, muscular
dystrophy, baldness

48. Sex-Linkage
= presence of a gene on a sex chromosome (X or
Y)
1) X-linked – genes found on the X chromosome
and inherited when the individual receives an X
during fertilization
 Most common type
 MostX-linked diseases/traits are recessive
(need 2 copies of allele to show disease/trait
physically)

49. X-linked continued…
 Father to son transmission is IMPOSSIBLE!
 Youcan be a “carrier” and not have the
disease
 Morecommon in males! (they only need 1
copy!)
Examples: muscular dystrophy, hemophilia,
color-blindness, fragile X-syndrome, protanopia,
Aicardi syndrome

50. Male Female
The "a" The "a"
recessive recessive
allele will be allele will not
expressed in be expressed
his in her
phenotype phenotype

51. Y-Linked Traits
= genes found on the Y chromosome and
inherited when the individual receives a Y (and
become a male) during fertilization
 Only males can have these diseases/traits
 Very rare
Examples: Klinefelter syndrome (XXY),
Jacobs Syndrome (XYY)

53. What does it mean to “carry” a
trait?
Carrier
= when an organism has 1 copy of an allele that
causes a recessive disorder but does not present
that disorder/trait physically
XdX = carrier, female
XdXd = has disorder, female
XdY = has disorder, male
XY = doesn’t have disorder and is not a
carrier!, male

55. = A PEDIGREE

56. What is a PEDIGREE??
= family record that shows how a trait is inherited
over several generations ~ is actual
inheritance
Shows:
and not “possible” 1. Sex of individuals
2. “marriages”
3. Number of
offspring
4. Type of trait
a. Single-
allele
b. Sex-linked

57. How to Read a Pedigree