Download Genetics the study of heredity

Genetics
the study of heredity
Gregor Mendel
“Father of Genetics”
• Heredity -the transfer of characteristics
from parents to offspring through their
genes
• Gregor Mendel -used garden peas to
study heredity
Mendel’s Experiments
• cross fertilization: Mendel
crossed two parent plants with
opposite traits (purple x white).
This was the Parental generation
(P). The First generation (F1)
were identical (purple).
• self fertilization: Mendel allowed
the purple flowers from the First
generation (F1) to self-pollinate.
Self pollination produced the
Second generation (F2).
Muskopf, Shannan
Mendel’s Conclusions
• The F1 generation all showed the purple trait (called the
dominant trait)
• In the F2 generation the (white) trait reappears in ¼ of the
flowers (called the recessive trait)
• Each flower has two alleles that determine the appearance
• The alleles are represented by letters (uppercase letter
represents the dominant allele; lowercase letter represents the
recessive allele)
• P is dominant and represents purple
• p is recessive and represents white
• The Dominant Is Expressed No Matter What
• Need 2 Copies Of The Recessive Allele In Order To Be
Expressed
• PP = purple flower Pp = purple flower
pp = white flower
Alleles
– homozygous: organisms that have 2 identical
alleles for a trait (could be two capital or two
lowercase letters)
• PP
• pp
– heterozygous: organisms that have 2 different
alleles for a trait
• ex: Pp (the dominant allele P is expressed so
this flower would be purple)
• Genotype: letters used for the alleles
– ex: PP, Pp, pp
• Phenotype: what an organisms looks like
– ex: purple, white
Punnett square
• A Punnett square is used to show the possible
allele combinations in the offspring of 2 parents.
The four boxes
represent the
four possible
offspring
• Monohybrid cross = cross involving only 1 trait
Example of a Monohybrid Cross
A plant heterozygous with green peas (Gg) is crossed
with a plant that has yellow peas (g).
• Step 1: Choose a letter for the alleles (green is dominant;
yellow is recessive)
• G : green pea
g: yellow pea
• Step 2: Write the genotypes of the parents
• heterozygous plant with green peas : Gg
• plant with yellow peas: gg
• parents: Gg x gg
Step 3: Set up the punnett square with one parent on
each side
G
g
g
g
Step 4: Fill out the punnett square middle
G
g
G
g
g
gg
g
G
g
Gg
gg
g
Gg
g
G
g
gg
g
Gg
gg
gg
g
Gg
gg
• Step 5: Look at the four boxes from Step 5 and determine the
genotypes of the four offspring
– Genotypic ratio: 2 Gg: 2 gg
• Step 6: Look at the genotypes in Step 6 and determine the
phenotypes;
– Green (G) is dominant over yellow (g), plants that have G in
their offspring have green peas
– Phenotypic ratio: 2 green: 2 yellow
Practice-Monohybrid Crosses
Cross an individual with blue eyes with an individual with
homozygous brown eyes. Brown eyes (B) is dominant to blue
eyes (b).
B
B
b
b
Phenotypes:
Genotypes:
Cross an individual with blue eyes with an individual with
homozygous brown eyes. Brown eyes (B) is dominant to blue
eyes (b).
B
B
b
Bb
Bb
b
Bb
Bb
Phenotypes: All Brown Eyes
Genotypes: All Bb
Practice-Monohybrid Crosses
A child is diagnosed with a recessive genetic disease. Neither
parent has the disease. What are the genotypes of the parents?
N
?
N
?
Phenotypes:
Genotypes:
nn
A child is diagnosed with a recessive genetic disease. Neither
parent has the disease. What are the genotypes of the parents?
N
n
N
NN
nn
n
Nn
nn
Genotypes of the parents are Nn
Incomplete Dominance = Blending
In snapdragons, there is not a dominant allele. The
flower color can be red, pink, or white. A heterozygous
flower (Rr) will a blending of red and white (pink).
•Muskopf, Shannan. Online Images. The Biology Corner. 20 April 2007.
http://www.biologycorner.com/bio1/celldivision-chromosomes.html
Codominance: the recessive & dominant traits appear
together (both are dominant, no recessive allele)
– Ex: Cross a red cow with a white cow. What will the
offspring be?
R
W
W
R
R
R
W
RW
RW
W
RW
RW
Muskopf, Shannan. Online Images. The Biology Corner. 20
April 2007. http://www.biologycorner.com/bio1/celldivisionchromosomes.html
– phenotype: all red and white speckled
– genotype: all RW
Polygenic Traits: “many genes” act together
resulting in a range of phenotypes
– Ex: skin, hair, eye color
Skin color is a
polygenic trait because
it shows a range of
colors. There is not a
dominant and recessive
color.
Farabee, M.J. “Skin Pigmentation.” 2001. Online Image. Online Biology Book. 5 May 2007.
http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookgeninteract.html
Multiple Alleles: genes that have more than two alleles
– There are four blood types (phenotypes): A, B, AB, and O
– Blood type is controlled by three alleles: A, B, and O
– Each individual only inherits two alleles (one from each
parent).
– A and B are codominant
– O is recessive, two O alleles result in type O Blood
Dominant
Dominant
Codominant
Recessive
Blood Types
A
B
AB
O
Possible Genotypes
AA or AO
BB or BO
AB
OO
Practice: Cross a Type AB with a Type O.
AB x OO
1. Set up punnett
square with one
parent on each side
O
O
A
B
AO
BO
AO
2. Fill out the punnett
square middle
BO
What are the possible blood types of the four offspring?
Genotype
Blood Type
2 AO
2 Type A
2 BO
2 Type B
Practice
A woman heterozygous for Type A blood marries and a man
with Type AB blood. Show the cross and the possible offspring.
1. Write the genotypes of the parents:
• woman heterozygous for Type A: AO
• man with Type AB: AB
2. Set up punnett square with one parent on each side and fill
in the middle.
A
O
Blood types of
possible offspring:
A
AA
AO
AA: Type A blood
AO: Type A blood
B
AB
BO
AB: Type AB blood
BO: Type B blood
Practice
If a Type O individual marries a Type B individual can they have
offspring with Type O blood? What type of blood can the
offspring have?
B
O
?
Blood types of
possible offspring:
AA: Type A blood
AO: Type A blood
O
AB: Type AB blood
BO: Type B blood
If a Type O individual marries a Type B individual can they have
offspring with Type O blood? What type of blood can the
offspring have?
B
O
O
BO
OO
O
BO
OO
Yes, they can have a
child with type O or
type B blood.
Practice
If 2 individuals with Type AB blood marry, what percentage of
their offspring will have Type AB blood?
A
A
B
B
If 2 individuals with Type AB blood marry, what percentage of
their offspring will have Type AB blood?
50% of their offspring could be Type AB
A
B
A
AA
AB
B
AB
BB
Review of Terms
•
•
•
•
•
•
•
•
Allele
Homozygous
Heterozygous
Homozygous Dominant
Homozygous Recessive
Heterozygous
Genotypic Ratio
Phenotypic Ratio
A form of a gene
Both Alleles are the Same
Alleles are Different
AA
aa
Aa
2 PP : 2 pp
2 Purple : 2 White