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Biology
Gregor Mendel & Genetics
Who is Gregor Mendel?
 An Austrian monk
who loved to garden
 Through study and
breeding of pea
plants he unlocked
mysteries of heredity
 What is heredity?
http://content.answers.com/main/content/img/webpics/gregor_mendel.jpg
Pea Plants
Have 7 different traits
Traits are found on genes—genes are
separated into alleles
Example:
Height gene has 2 alleles: short & tall
Seed color gene has 2 alleles: yellow & green
Rules of Heredity
The Rule of Dominance: one allele is
dominant to the other recessive allele
Dominant trait will not allow recessive trait to be
displayed
Example: height—tall (T) is dominant to short
(t)
Dominant alleles capitalized
Recessive alleles lower case
Rules of Heredity
The Law of Segregation: the alleles for
the same gene are separate and are
inherited independantly of each other
Example:
T
T
Gene for height with
2 alleles
t
t
During meiosis, 1
allele can go
With one gamete,
the other goes to a
second gamete
How did he figure all this out?
 True-breeding and cross-breeding
 Phenotype: the trait that is displayed—what
you see
 Genotype: the gene combination that displays
the phenotype
 Homozygous: 2 alleles for same gene are
identical
Example: TT or tt
 Heterozygous: 2 alleles for same gene are
different
Example: Tt
Examples
TT is a homozygous genotype for height
Phenotype: tall
Tt is the heterozygous genotype for height
Phenotype: tall
tt is a homozygous gentoype for height
Phenotype: short
Breeding Definitions
P1: parent generation
F1: 1st generation son or daughter
F2: 2nd generation son or daughter
Monohybrid Cross: breeding 1 trait only
Dihybrid Cross: breeding 2 traits at
same time
Monohybrid Cross
Mendel cross pollinated 2 plants:
(P1) 1 short X 1 tall = (F1) all tall
Then crossed all tall F1 generation
(F1) 1 tall X 1 tall = (F2) 3 tall and 1 short
A phenotypic ratio of 3:1
This led to his Rule of Dominance
Dihybrid Cross
2 traits: shape of seeds, color of seeds
(P1) 1 round/yellow seed X 1 wrinkled/green =
(F1) all round/yellow
Then crossed all round/yellow F1
generation
(F1) 1 round/yellow X 1 round/yellow =
(F2) 9 round/yellow, 3 round/green, 3
wrinkled/yellow, 1 wrinkled/green
A 9:3:3:1 phenotypic ratio
This led to the Law of Segregation
LET’S PRACTICE!
PUNNETT SQUARES
Genetics Outside The Box
 Not all traits follow a simple dominance vs.
recessive rule
 For Example:
1. Codominance

2.
3.
4.
5.
Blood types
Incomplete Dominance
Multiple Alleles
Polygenic Traits
Sex-linked Genes
Codominance
Both alleles contribute to the phenotype
No blending of phenotypic traits—both
traits are displayed
For Example:
White Chicken (WW) X Black
Chicken (BB)
Results in a Black AND
White chicken (WB)
http://static.rcgroups.com/forums/attachments/3/3/0/3/6/951792.thumb?qTu6oJVgL7ucL7gyoy3lYzcjMmf0AGR8BGV
Codominance: blood types
There are 4 human blood groups: A, B,
AB, O
The alleles that code for these types are:
IA, IB, i
Rh Factor is a single gene with 2 alleles
(+) is dominant, (-) is recessive
Codominance: blood types
Phenotype
Genotype
Antigen
on RBC
Transfuse
to
Transfuse
from
A
IA IA or IA i
A
A, AB
A, O
B
IB IB or IB i
B
B, AB
B, O
AB
IA IB
A&B
AB
O
ii
NONE
A, B, AB,
O
A, B, AB,
O
O
Codominance: blood types
AB = Universal Recipient
Can receive from everyone
O = Universal Donor
Can donate to everyone
Rh factor example:
AB (+) means you are positive for the Rh allele,
either homozygous or heterozygous
AB(–) means you are negative for the allele,
homozygous recessive
Incomplete Dominance
No allele is dominant over another
Heterozygous phenotype is a blending of
the 2 homozygous phenotypes
For Example:
Four O’Clock Flowers
Red (RR) X White (WW)
Results in pink flowers (RW)
http://thumbs.ebaystatic.com/pict/7725948498_2.jpg
Multiple Alleles
Genes that have more
than 2 alleles
Does not occur in one person, but
throughout a population
For Example:
Rabbits hair color
Full, chinchilla, himalayan or albino
http://rabbit-world.main.jp/rabbitshurui-shop/himalayan_1.jpg
Polygenic Traits
Traits controlled by two or more genes
Genes may be on the same or different
chromosomes
Has a wide range of phenotypes
For Example: human skin color
Sex-Linked Genes
 Sex always determined by Dad
To be a girl you must have XX
chromosomes
To be a boy, you must have XY
chromosomes
Mom will always give an X, the second
chromosome is determined by Dad
Karyotype: picture of grouped chromosomes
Sex-Linked Genes
Genes located on an X or Y chromosome
only
The X chromosome is bigger, therefore it
holds more genes
Because males only have 1 X
chromosome, even recessive disorders
are expressed in men
Examples: color-blindness, hemophilia,
Duchenne Muscular Dystrophy
Common Genetic Disorders
 Chromosomal Disorders
Down Syndrome
 Codominant Disorders
Sickle cell anemia
 Dominant Disorders
Huntingtons’s Disease
 Recessive Disorders:
Albinism
Cystic fibrosis
PKU
Tay-sachs disease
Trisomy 21: Down Syndrome