Download Biology I Chapter 9

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 squares
Parent # 1
genotype
Probabilities:
Offspring #1
genotype
Parent # 2
genotype
Offspring #3
genotype
Offspring #2
genotype
Offspring #4
genotype
Dominant
Phenotype: 4/4
Recessive
Phenotype: 0/4
Heterozygous
Genotype: 4/4
Homozygous
Genotypes: 0/4
More Punnett squares
Parent # 1
genotype
Probabilities:
Offspring #1
genotype
Parent # 2
genotype
Offspring #3
genotype
Offspring #2
genotype
Offspring #4
genotype
Homozygous Dominant: 1/4
Dominant
Phenotype: 3/4
Recessive
Phenotype: 1/4
Heterozygous
Genotype: 2/4
Homozygous
Genotypes: 2/4
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