Download Patterns of Non Mendelian Inheritance

Review of Patterns of Inheritance
Gregor Mendel’s Principles
• 1. Principle of Dominance
• 2. Law of Segregation – each parent has 2 alleles for a
trait but these are separated (segregated) during the
formation of gametes (meiosis)
• 3. Law of Independent Assortment – gene pairs
separately independently of each other (unless they are
closely linked on the same chromosome)
5/10/2017
Monohybrid (one trait) crosses
Homozygous Dominant x Homozygous Dominant TT x TT
•
T
T
•
T
•
T
• 100% Tall: 0% Short
4:0
• 100% TT: 0% Tt: 0% tt
4:0
5/10/2017
Monohybrid (one trait) crosses
Homozygous Dominant x Homozygous Recessive TT x tt
•
T
T
•
t
•
t
• 100% Tall: 0% Short
4:0
• 0% TT: 100% Tt: 0% tt
0: 4:0
5/10/2017
Monohybrid (one trait) crosses
Homozygous Dominant x Heterozygous TT x Tt
•
T
T
•
T
TT
•
t
Genotypic Ratio
Phenotypic Ratio
5/10/2017
Tt
TT
Tt
50% TT: 50% Tt: 0% tt
100% Tall: 0% Short
4:0
Monohybrid (one trait) crosses
Heterozygous x Heterozygous Tt x Tt
•
T
t
•
T
•
t
TT
Tt
Tt
tt
Genotypic Ratio: 25% TT: 75% Tt: 25% tt
Phenotypic Ratio: 75% Tall: 25% Short
5/10/2017
1:2:1
3:1
Monohybrid (one trait) crosses
Heterozygous x Homozygous recessive Tt x tt
•
T
t
•
t
Tt
•
t
Tt
tt
tt
Genotypic Ratio: 0% TT: 50% Tt: 50% tt
Phenotypic Ratio: 50% Tall: 50% Short
5/10/2017
0:1:1
1:1
Variations to
Mendelian
Genetics
• Not all genetic traits strictly follow the
laws discovered by Gregor Mendel.
• Five types of non-Mendelian inheritance:
– Incomplete dominance
– Codominance
– Multiple alleles
– Polygenic inheritance
– Sex linked traits and/or Sex Influenced
5/10/2017
Variation #1: Incomplete Dominance
• Dominant allele does not completely “mask” the
recessive allele.
• Results in a “BLENDING” of the two forms of the trait.
• This intermediate phenotype is expressed in the
heterozygous condition (Bb)
Example: B = black and W = white forms of hair color trait
BB=
WW=
BW=
5/10/2017
black
white
gray - blended form of the 2 trait forms
Incomplete Dominance
Example 1
• In rabbits black fur is incompletely dominant over white
fur. Cross a black fur male with a white fur female
Black= BB
5/10/2017
White=WW
Incomplete Dominance Example 2
• In carnations, red is incompletely dominant over white.
• Blended inheritance results in pink flowers.
•
•
•
RR
RR
BB
RW
WW
5/10/2017
Variation #2: Codominance
• A condition in which both alleles are expressed
equally because they have equal dominance
• The two alleles don’t blend but are present and can
be seen in full form at the same time in the
offspring
• Ex. Checkered chickens, variegated clover, roan
horses and cows
5/10/2017
Variation #2: Codominance
Example: B = black and W = white
BB= Black
WW= White
BW= Black and White
Both forms of trait are expressed
5/10/2017
Variation #2: Codominance
• In Rhododendrons, pink is codominant with white. This plant
displays codominance. Cross a pink rhododendron P with a
white rhododendron W.
X
5/10/2017
=
Example 3: Codominance
• In cattle brown B is codominant over white W.
5/10/2017
–
BB
–
WW
–
BW
• Cross two roan cattle BW x BW
5/10/2017
Incomplete Dominance vs.
Codominance
• Incomplete dominance – Heterozygote’s traits are a
blend of the two alleles
• Codominance – Both alleles for gene are equally strong
and are both seen
• With incomplete dominance, a cross between
organisms with two different phenotypes
produces offspring with a blending of the
parental traits.
• Ex. Red X White flowers > Pink flowers
• With codominance, a cross between organisms
with two different phenotypes produces offspring
with phenotype in which both of the parental
traits appear together.
• Ex. Red x White flowers > Both colors seen
Which inheritance pattern does each cross represent?
Codominance or Incomplete Dominance
5/10/2017
X
=
100%
X
=
100%
Variation #3 Multiple Alleles
• A gene may have more than just two allele versions.
• Ex: Human Blood Type 3 alleles 4 phenotypes
• Type A
• Type B
• Type AB
• Type O
5/10/2017
Variation #4: Polygenic Inheritance
• Poly= Many
Genic= Genes
Polygenic inheritance is when many genes
work together to produce one trait.
Polygenic traits usually show a lot of variation.
Examples: Hair color, Skin color, Eye color
• Hair color is controlled by three sets of genes all
working together to create various hair colors.
• aabbcc
Blondest
hair color
AaBbCc
Medium Brown
hair color
(3 dominant/3 recessive)
AABBCC
Blackest
hair color
Skin tone is controlled by four sets of genes all working
together to create one shade of skin tone.
aabbccdd
palest skin
tone
AaBbCcDd
medium skin
tone
(4 dominant/4 recessive)
AABBCCDD
darkest skin
tone
AABbCc X AaBBCc
AABBCc
5/10/2017
AaBbcc
Example :
• In snapdragons color of the blossoms displays
polygenic inheritance.
How many genes control
AABB-red
the snapdragon color
AABb-peach
trait?
AaBb and AaBB –pink
Aabb – white
All other genotypes - yellow
5/10/2017
Variation #5: Sex Linked
• Sex linked traits are controlled by a gene located
only on the X or Y sex chromosomes
• These genes are “linked” on the sex chromosomes
• Most of these traits are found only on the X sex
chromosome which makes them specifically
“X linked” traits
5/10/2017
Females sex chromosomes: XX
• Males sex chromosomes: XY
• Females get sex linked diseases less often than
males because it is less likely to inherit two “bad”
genes than it is to inherit one.
• Females must inherit one gene from her father and
one from her mother.
• Males only inherit one X linked gene which comes
from his mother. (
5/10/2017
Ex. Sex linked traits – Red Green Colorblindess
5/10/2017
Ex. Sex linked trait –
Hemophilia Blood clotting disorder
5/10/2017
Ex of Sex linked trait Muscular Dystrophy
5/10/2017
:
• Carrier:
a person who is does not express a
phenotype but “carries” the allele for it and may
pass that gene on to their child. Is heterozygous.
5/10/2017
Pedigree Charts:
5/10/2017
Which inheritance pattern does each cross represent?
Codominance or Incomplete Dominance
5/10/2017
X
=
100%
X
=
100%