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MENDELIAN GENETICS
WHAT IS INHERITANCE?
Inheritance is the transfer of genetic material from parent to offspring.
-Each offspring receives ½ of their genetic material/DNA from each parent.
A trait is an inherited distinguishing feature or characteristic…what you see.
-Can you name a common trait of a dog?
WHAT IS HEREDITY?
Heredity is the passing of traits from parent to offspring
Genetics is the study of heredity
HUMAN TRAITS
Little fingers: straight or bent
Ears: free or attached
Tongue: roll or not
HUMAN TRAITS
Hairline: widow peak or not
GENES AND ALLELES
Allele: is the section on the chromosome that represents a trait (these will be written as letters)
Gene: the two alleles work together to express a trait.
allele
gene
allele
GENETICS
Genotype: the code that determines the trait.
Phenotype: the physical characteristic expressed…how it looks.
Dominant: the alleles that are more influential (written in upper case letters).
Recessive: the alleles that are less influential (written in lower case letters).
GENETICS
Homozygous (purebred): Alleles are the same. (AA, aa).
Heterozygous (hybrid): Alleles are different. (Aa)
Genotypical ratio: Prediction of genotype
Phenotypical ratio: Prediction of phenotype
10
MEIOSIS: TWO PART CELL DIVISION
Meiosis I
Meiosis II
Gametes
A HISTORY LESSON
In the 19th century, the leading theory on inheritance was the “blended theory” where the traits from each parent were blended from generation to generation A red rose with a white rose gives you a pink rose
+ =
GREGOR MENDEL THE FATHER OF GENETICS
Central Europe (Czech) scientist turned monk
Conducted research on pea plants
Developed basic principles of heredity for all complex organisms
Traits are passed down from generation to generation; not blended
Work published in 1866; died in 1884; recognized in 1900
MENDEL’S PEA PLANT EXPERIMENTS
13
WHY DID MENDEL USE PEAS PISUM SATIVUM?
Peas can be grown in a small area
Peas produce lots of offspring
Peas will produce pure plants when allowed to self-pollinate over several generations
Peas can be artificially cross-pollinated
14
REPRODUCTION IN FLOWERING PLANTS
15
Pollen carries sperm to the ovary for fertilization
Self-fertilization can occur in the same flower
Cross-fertilization can occur between different flowers
pollen
MENDEL’S EXPERIMENTAL METHODS
Mendel hand-pollinated flowers using a paintbrush then he would snip the stamens to prevent self-pollination
Then he covered each flower with a cloth bag
He traced traits through the several generations
16
THE PEA EXPERIMENTS
Eight traits observed by Mendel and experimented on
21,000 hybridized plants:
THE EIGHT PEA PLANT TRAITS MENDEL TRACED
Seed shape --- Round (R) or Wrinkled (r)
Seed Color ---- Yellow (Y) or Green (y)
Pod Shape --- Smooth (S) or wrinkled (s)
Pod Color --- Green (G) or Yellow (g)
Seed Coat Color ---Gray (G) or White (g)
Flower position---Axial (A) or Terminal (a)
Plant Height --- Tall (T) or Short (t)
Flower color --- Purple (P) or white (p)
18
THE PEA EXPERIMENTS
Let’s take a look at the yellow vs. green
seeds:
3:1 ratio
showed up
in later
generations!
WHAT MENDEL HYPOTHESIZED…..
The inheritance of each trait is determined by “elementen” (or genes) that are passed onto offspring unchanged and not blended
An individual inherits one elementen from each parent for each trait
The alleles of the gene are the different version or forms of the gene and determine the traits that are expressed.
Example: “men have facial hair (gene); some have black hair (allele)”
21
Mendel stated that physical traits are inherited as “factors” or “particles”
Mendel did not know that the “particles” were actually chromosomes & DNA
PARTICULATE INHERITANCE
DOMINANT VS. RECESSIVE
Mendel noticed that one form of the traits dominated over the other A trait may not show up in an individual but can still
be passed on to the next generation
Dominate doesn’t mean “better” but only that it masks the expression of the other trait.
Dominant allele (or gene)-Denoted with capitalized letter ( ex. B)
Recessive allele (or gene)- Denoted with lowercase letter (ex. b)
THINGS MENDEL ALSO CONSIDERED….
The parent plants were homozygous for pea seed color=each had identical alleles for the trait 2 yellows or 2 greens
True or pure breeding
The f1 generation were heterozygous = 2 different alleles from each parent Hybrid
GENOTYPE VS. PHENOTYPE
Genotype is the genetic set for the trait Alleles and denoted with
letters representing the trait (example eye color would be letter “E”
Phenotype is the physical expression of the genotype Trait or characteristic
GENOTYPES
Homozygous genotype - gene combination involving 2 dominant or 2 recessive genes (e.g. PP or pp) also called pure
Heterozygous genotype - gene combination of one dominant & one recessive allele (e.g. Pp); also called hybrid
26
GENOTYPE & PHENOTYPE IN FLOWERS
27
Genotype of alleles: P = purple flower p = white flower
All genes occur in pairs, so 2 alleles affect a characteristic
Possible combinations are:
Genotypes PP Pp pp
Phenotypes Purple Purple white
Parental P1 generation = the parental generation in a breeding experiment.
F1 generation = the first-generation offspring in a breeding experiment. (1st filial generation) From breeding individuals from the P1 generation
F2 generation = the second-generation offspring in a breeding experiment. (2nd filial generation) From breeding individuals from the F1 generation
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Generations
FOLLOWING THE GENERATIONS
30
Cross 2 Pure Plants
TT x tt
Results in all
Hybrids Tt
Cross 2 Hybrids get
3 Tall & 1 Short TT, Tt, tt
31
LAW OF DOMINANCE
• In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation.
• All the offspring will be heterozygous and express only the dominant trait.
• RR x rr yields all Rr (round seeds)
32
LAW OF SEGREGATION
During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other.
Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.
33
LAW OF INDEPENDENT ASSORTMENT
Alleles for different traits are distributed to sex cells or gametes independently of one another.
This law can be illustrated using dihybrid crosses.
MODE OF INHERITANCE
Principles state that inheritance is predictable based on the rules so how can we predict what traits will be passed and/or expressed? Answer: by determining the mode of inheritance
Pedigree
(Phenotype and genotype)
Punnett Square (Genotype)
But this isn’t Geometry!
PEDIGREE
Similar to a family tree but used to trace traits
Can be combined with genotype information
Squares for male
Circles for female
Filled/darkened means “affected” or expressing the trait
Unfilled could mean unaffected or carrier
IMPLICATIONS OF INBREEDING
The Fugate
Family
PUNNETT SQUARE OR GENETIC CROSS
- Calculate probability
of homozygosity or
heterozygosity
-Calculate probability
and frequency of
phenotypic expression
of trait
Example of red and
black coats on
domestic cattle
TYPES OF GENETIC CROSSES
Monohybrid cross - cross involving a single trait e.g. flower color
Dihybrid cross - cross involving two traits e.g. flower color & plant height
38
MONOHYBRID CROSS
Female genotype listed vertically
Male genotype listed horizontally
Problem:
A widow’s peak hairline is
dominant to straight hairline.
Cross a heterozygous widow’s
peak hairline person to a
straight hairline person.
39
DIHYBRID CROSS
Traits: Seed shape & Seed color
Alleles: R (round) r (wrinkled) Y (yellow) y (green)
40
RrYy x RrYy
RY Ry rY ry RY Ry rY ry
All possible gamete combinations
DIHYBRID CROSS
41
RY Ry rY ry
RY
Ry
rY
ry
DIHYBRID CROSS
42
RRYY
RRYy
RrYY
RrYy
RRYy
RRyy
RrYy
Rryy
RrYY
RrYy
rrYY
rrYy
RrYy
Rryy
rrYy
rryy
Round/Yellow: 9
Round/green: 3
wrinkled/Yellow:3
wrinkled/green: 1
9:3:3:1 phenotypic ratio
RY Ry rY ry
RY
Ry
rY
ry
DIHYBRID CROSS
43
Round/Yellow: 9 Round/green: 3 wrinkled/Yellow: 3 wrinkled/green: 1
9:3:3:1
INCOMPLETE DOMINANCE
When they have an appearance somewhat in between the phenotypes of the two parental varieties.
Example: snapdragons (flower)
red (RR) x white (rr)
RR = red flower
rr = white flower
44
R
R
r
r
45
INCOMPLETE DOMINANCE
CODOMINANCE
Two alleles are expressed (multiple alleles) in heterozygous individuals.
Example: blood type
1. type A = IAIA or IAi
2. type B = IBIB or IBi
3. type AB = IAIB
4. type O = ii
46
CODOMINANCE PROBLEM
Example: homozygous male Type B (IBIB)
x heterozygous female Type A (IAi)
47
IAIB IBi
IAIB IBi
1/2 = IAIB
1/2 = IBi
IB
IA i
IB
SEX-LINKED TRAITS
Traits (genes) located on the sex chromosomes
Sex chromosomes are X and Y
XX genotype for females
XY genotype for males
Many sex-linked traits carried on X chromosome
48
SEX-LINKED TRAITS
49
Sex Chromosomes
XX chromosome - female Xy chromosome - male
fruit fly
eye color
Example: Eye color in fruit flies
SEX-LINKED TRAIT PROBLEM
Example: Eye color in fruit flies
(red-eyed male) x (white-eyed female) XRY x XrXr
Remember: the Y chromosome in males does not carry traits
RR = red eyed Rr = red eyed rr = white eyed XY = male XX = female
50
XR
Xr Xr
Y
SEX-LINKED TRAIT SOLUTION:
51
XR Xr
Xr Y
XR Xr
Xr Y
50% red eyed female 50% white eyed male
XR
Xr Xr
Y
HEMOPHILIA
XH XH = female normal
XH Xh = female carrier
Xh Xh = female hemophiliac
XH Y = male normal
Xh Y = male hemophiliac
52
53
SEX-LINKED TRAIT PROBLEM
Example: Eye color in fruit flies
(red-eyed female) x (white-eyed male)
XR XR Xr Y
How many? Red eyed males?
White eyed males?
White eyed females?
Red eyed females?
Xr
XR XR
Y
54
SEX-LINKED TRAIT PROBLEM
Example: Eye color in fruit flies
(red-eyed female heterozygous) x (red-eyed male)
XR Xr XR Y
How many ? White eyed males?
Red eyed males?
White eyed females?
Red eyed females?
XR
XR Xr
Y