Mendelian GeneticsGregor Mendel•Monk•Lived in modern-day Czech Republic•“father of modern genetics”•Wrote a famous paper called Experiments on plant hybridization•In mid-1800s, he studied heredity in pea plants
7 traits of pea plants studied
Round Wrinkled
Yellow Green
Seed Shape
Seed Color
Flower Color
Pod Shape
Pod Color
Purple White
Inflated Constricted
Green Yellow
Flower Position
Stem Height
Axial Terminal
Tall Short
R r
Which color is dominant?
• A purple and white plant are crossbred
• The resulting peas are planted
• All four F1 generation plants are purple
• Which allele do you think is the dominant allele? Remember: each parent donates 1 allele to each offspring
P1
F1
What if he had crossed Pp x pp?
Genotype: Phenotype:
Genotype:
Phenotype:
Genotype:
Phenotype:
Genotype:
Phenotype:
Purple Purple
white White
p p
p
P
Pp Pp
pp pp
This F1 Generation ≠ 4 purple flowered plants, therefore the P1 generation must have been PP x pp.
Now, cross two from the F1 generation.
Genotype: Phenotype:
Genotype:
Phenotype:
Genotype:
Phenotype:
Genotype:
Phenotype:
Purple Purple
purple White
P p
p
P
PP Pp
Pp pp
The white flowers show back up—the gene wasn’t lost. It was just hidden!!!
•If the phenotype is a recessive trait, the organism must be ___ for that trait.
•homozygous recessive (aa)
•If the phenotype is a dominant trait, the organism could be ____ or ___ for that trait.
•Homozygous dominant (AA) or heterozygous (Aa)
aaAA or Aa
Gene: flower position
Possible Allele Combinations:
Axial position of flower Terminal position of flower
Think through it…If the offspring have a
phenotypic ratio of
4 axial: 0 terminal
then the genotype of the dominant parent must be
AA
If the genotype of the dominant parent is Aa, then the phenotypic ratio of the offspring is
2 axial: 2 terminal
_a _a
_a _a
a a
A
A or a
Independent Assortment Animation
Law of Independent Assortment—alleles for different traits are distributed to eggs and sperm independently !!!
Some alleles are located on the sex chromosomes instead of the autosomes (chromosome pairs 1-22 for humans). These traits are called sex-linked traits.
Fruit fly eye color is determined by an allele on the X chromosome. Fruit fly eyes may be red(R) or white(r).
XR = red eyes Xr = white eyes
XRXR or XRXr
Red eyed female
XrXr
White eyed female
XRYRed eyed male
XrYWhite eyed male
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