Biology
Fundamentals of Genetics
Gregor Mendel Father of genetics Monk Austrian Gardener Mathematician
Gregor Mendel
Trait A physical or behavioral characteristic Coded for by at least one gene
(discrete piece of DNA on one chromosome)
But Mendel didn’t know what it looked like
Garden Peas (Pisum sativum) and some Traits
Height long or short stem Pod color green or yellow Seed texture smooth or wrinkled Flower color purple or white
All peas had either one trait or the other, never something in between.
Mendel’s cross pollination Take the pollen from the anther of one
plant and transfer it to the stigma of another
This is sexual reproduction of flowers Each parent transfers half of its DNA
to the offspring
Dominant Traits If an organism has both genes for a
trait (Such as the gene for yellow peas and the gene for green peas), the one that is expressed or shown is the dominant gene (in this case, the pea was always yellow, never green).
Alleles Alleles are different forms of the same
gene. For example there is one gene for pea
color, but there is a yellow pea allele and a green pea allele
A Human Example There are two basic colors of eyes in
human beings: – Dark (brown, black and hazel) – Light (blue, green and grey)
Dark eyes are dominant over light eyes
Eye Color Continued
So, if a woman received the brown eye gene from her father and the blue eye gene from her mother, she would show the brown eye trait, because it dominates over the blue gene.
The blue gene is called “Recessive”, it is still in the woman’s DNA, but is not seen or expressed.
A Code for Genes
Mendel developed an easy code for genes. He used a capitol letter for the dominant gene and a lower case letter for the recessive gene.
Thus:
B stands for the dark (Brown) gene and…
b stands for the light (Blue) gene for
human eye color
Genotype and Phenotype Genotype is the actual alleles that an
organism possesses and is shown with the letters for the alleles. For example: BB, Bb and bb
Phenotype is the gene that is expressed, it is the trait that is seen in the organism. Examples are brown eyes and blue eyes.
Homozygous and Heterozygous
Homozygous organisms have both of the same alleles for a trait, either BB or bb
Heterozygous organisms have one of each allele, Bb (bB is the same thing so we list the dominant allele first)
Homozygous Dominant Two brown eye alleles Genotype is BB Phenotype is brown eyes
Homozygous Recessive Two blue eye alleles Genotype is bb Phenotype is blue eyes
Heterozygous One brown and one blue eye allele Genotype is Bb Phenotype is brown eyes (the
dominant phenotype is expressed when both alleles are present)
Mendel’s pea crosses P1 generation
– Purebreds
F1 generation– All dominant hybrids
F2 generation– 3:1 ratio
Predicting results of monohybrid crosses
Probabilities– The chance or likelihood that something
will happen a certain way– Example:
What is the probability that you will have a child with blue eyes?
Punnett squaresParent # 1 genotype
Parent # 2 genotype
Offspring #1 genotype
Offspring #2 genotype
Offspring #3 genotype
Offspring #4 genotype
Probabilities:
Dominant Phenotype: 4/4
Recessive Phenotype: 0/4
Heterozygous Genotype: 4/4
Homozygous Genotypes: 0/4
More Punnett squaresParent # 1 genotype
Parent # 2 genotype
Offspring #1 genotype
Offspring #2 genotype
Offspring #3 genotype
Offspring #4 genotype
Probabilities:
Dominant Phenotype: 3/4
Recessive Phenotype: 1/4
Heterozygous Genotype: 2/4
Homozygous Genotypes: 2/4
Homozygous Dominant: 1/4 Homozygous Recessive: 1/4
Incomplete Dominance Neither allele is expressed...exactly Example: Pink carnations
Codominance Both alleles are expressed (not blended) Example: Roan Horses
– Both parent hair colors are visible
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