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1
Non-Mendelian Patterns of Inheritance:Incomplete Dominance,
Codominance and Sex-Linked Traits
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Incomplete Dominance
F1 hybrids have an appearance somewhat in between the phenotypes of the two parental varieties.
Example: snapdragons (flower)red (RR) x white (rr)
RR = red flowerrr = white flower
R
R
r r
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Incomplete Dominance
Rr
Rr
Rr
Rr
R
R
r
All Rr = pink(heterozygous pink)
produces theF1 generation
r
4
Incomplete Dominance
Incomplete Dominance
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Blending of alleles to produce a different phenotype from either parent
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CodominanceTwo alleles are expressed
(multiple alleles) in heterozygous individuals.
Example: blood type
Phenotype Genotype1. type A = IAIA or IAi2. type B = IBIB or IBi3. type AB= IAIB
4. type O = ii
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Codominance Problem
Example:homozygous male Type B (IBIB)
x heterozygous female
Type A (IAi)
IAIB IBi
IAIB IBi
1/2 = IAIB
1/2 = IBi
IB
IA i
IB
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Another Codominance Problem
• Example: male Type O (ii) x female type AB (IAIB)
IAi IBi
IAi IBi
1/2 = IAi1/2 = IBi
i
IA IB
i
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Codominance
Question:If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents?
boy - type O (ii) X girl - type AB (IAIB)
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Codominance
Answer:
IAIB
ii
Parents:genotypes = IAi and IBiphenotypes = A and B
IB
IA i
i
Pattern of InheritanceComplete Dominance - 1 allele overshadows the other allele in a heterozygous individualIncomplete Dominance – blending of alleles to produce a different phenotype from either parent (Ex. Red rose X White rose = Pink rose)Codominance – both alleles of a gene are expressed(Ex. Red rose X White rose = red/white rose)
Incomplete or Codominance?
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Incomplete or Codominance?
More Patterns of Inheritance
• Autosomal inheritance – genes are located on the autosomes, same for both male and female
• Sex-linked inheritance – genes located on the sex chromosomes, different for male and female
• Sex-influenced traits – sex hormones create different phenotypes in males and females (Ex. Baldness)
•Multiple alleles – has more than 2 alleles for the same gene (Ex. blood types)
• Polygenic inheritance – coded for by many genes (skin color, hair color, height) 14
Blood Type
•Multiple Alleles – 3 or more alleles code for a trait
• Codominant Inheritance
• Blood types – A, B, AB, O are coded by 3 alleles (A, B, O)
• Universal Donor – O
• Universal Recipient – AB
• RH+ - antigen present
• RH- - no antigen present
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16
17
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Sex-linked Traits
Traits (genes) located on the sex chromosomes
Sex chromosomes are X and YXX genotype for femalesXY genotype for malesMany sex-linked traits carried
on X chromosome
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Sex-linked Traits
Sex Chromosomes
XX chromosome - female Xy chromosome - male
fruit flyeye color
Example: Eye color in fruit flies
Genes on the Male Sex Chromosomes
DOMINANT RECESSIVE
XHXH XHXh
XHY XhY
XH
Y
XH Xh
Sex-linked InheritanceColor Blindness – recessive, on X chromosomeNormal Vision is dominant
Genotypes:XCXC – normal female, non carrierXCXc’ – normal female, carrier (may pass recessive allele on to sons and/or daughters)Xc’Xc’ – colorblind female (will pass recessive allele to all childrenXCY – normal maleXc’Y – colorblind male (will pass recessive allele to daughters only)
~Hemophilia – bleeder’s disease, recessive, linked to the X chromosome
Protein Factor VIII or IX is missing but is necessary to clot bloodGenotypes:
XHXH – normal female, non carrierXHXh – normal female, carrierXhXh – hemophiliac femaleXHY – normal maleXhY – hemophiliac male
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Female Carriers
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Sex-linked Trait Problem
Example: Eye color in fruit flies (red-eyed male) x (white-eyed
female) XRY x XrXr
Remember: the Y chromosome in males does not carry traits.
RR = red eyedRr = red eyedrr = white eyedXY = maleXX = female
XR
Xr Xr
Y
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Sex-linked Trait Solution:
XR Xr
Xr Y
XR Xr
Xr Y
50% red eyed female
50% white eyed male
XR
Xr Xr
Y
Pedigrees
Making a Pedigree
A family tree traces a family name and various family members through successive generations.
Through a family tree, you can identify the relationships among your cousins, aunts, uncles, grandparents, and great-grandparents.
Pedigrees Illustrate Inheritance
A pedigree is a graphic representation of genetic inheritance.
It is a diagram made up of a set of symbols that identify males and females, individuals affected by the trait being studied, and family relationships.
Male
Female
Affected male
Affected female
Mating
Parents
Siblings
Known heterozygotes for recessive allele
Death
Pedigrees Illustrate Inheritance
Female Male
?
I
II
III
IV
1 2
1
1
1
32
2
2
4
3
3
5
4
4 5
Pedigrees Illustrate Inheritance
In a pedigree, a circle represents a female; a square represents a male. Highlighted
circles and squares represent individuals showing the trait being studied.
Circles and squares that are not highlighted designate individuals that do not show the trait.
Human Heredity
Pedigrees Illustrate Inheritance
A half-shaded circle or square represents a carrier, a heterozygous individual.
Human Heredity
Pedigrees Illustrate Inheritance
A horizontal line connecting a circle and a square indicates that the individuals are parents, and a vertical line connects parents with their offspring.
Each horizontal row of circles and squares in a pedigree designates a generation, with the most recent generation shown at the bottom.
The generations are identified in sequence by Roman numerals, and each individual is given an Arabic number.
1 2
1
1
1
32
2
2
4
3
3
5
4
4 5
?
I
II
III
IV
Human Heredity
dd dd
dd dd dd
dd
Dd
Dd
Dd Dd
Dd
DdDd
DD
DDDDDD
DDDDDD
Dd
DdDdDdDd
DD
DdDdDdDd
DdDd
dddddd
dddddddd
dddd
Hemophilia pedigree beginning with Queen Victoria
Genetic Rarities & Abnormalities
What can happen when meiosis goes awry…
I. Twinsa) Identical – develop from the same
fertilized egg (zygote), genetically identical, always same sex
b) Fraternal – 2 sperm fertilize 2 different eggs, genetically different
Conjoined Twins
Fusion OR fission in utero
Chromosome Theorya) Each gene occupies a specific place on
chromosomeb) Gene Mapping – locating and mapping
the position of a gene on the chromosome
c) Gene Linkage – some genes are linked together and are inherited together
d) Crossing Over – produces new allele combinations and increases variety
Types of Mutations – mistakesa) Germ mutations – occur in
gametes. Inheritable (colorblindness, hemophilia)
b) Somatic mutations – affect body cell, not inheritable (cancer)
c) Chromosomal mutations – most drastic, change in structure or # of chromosomes (Downs’ syndrome)
Point Mutationsa) Substitution – one base
exchanges for another, affects 1 amino acid(Ex. GCA-TCA GCT-TCA
b) Insertion (frame shift) – 1 base is inserted, affects several amino acidsEx. (GCA-TCA GCA-GTC-A
c) Deletion – base is removed, affects several amino acidsEx. (GCA-TCA GCT-CA
Point Mutation
Frameshift Mutation
Nondisjunction (Chromosomal mutation) – chromosomes do not separate during meiosis
a) Sex Chromosomesi. Turner’s Syndrome – XO – 45
chromosomes, female, sterileii. Kleinfelter’s syndrome – XXY – 47,
XXXY – 48, or XXXXY – 49 chromosomes, male, sterile
b) Autosomesi. Down’s syndrome (Trisomy 21)
extra 21st chromosomeii. Trisomy 8 and 13 – result in
miscarriages
Karyotypes
Nondisjunction
Patau’s Syndrome•1 out of 6,000 births
•Trisomy 13
•80-90% do not survive past 1 yr old
Edward’s Syndrome•Trisomy 18
•Second most common trisomy after down’s syndrome
•Only 5% live to age 1
•1 out of 8,000 births
•Severely retarded, many die from malformed heart
•Polydactyly or syndactyly
Cri du Chat•“ Cry of the Cat”
•Osteogenesis imperfecta
•Lobstein syndrome
•Brittle bone syndrome
•1 in 50,000 births
•Severe mental retardation
•Low mortality rate
Down’s Syndrome•Trisomy 21
•1 in 700 births
•Mental retardation
•Males are sterile but females are not
XYY-Jacob’s Syndrome a.k.a. “Super Males”
• 1 in 1,000 men
•Normal appearance, very tall
•Low IQ, prone to violence
Klinefelter’s Syndrome
•XXY
•1 in 1,000
•Usually sterile because of low sperm count
•Tall, sparse body hair
•Suffer from gynecomastia- male breast tissue
•Testosterone treatments
Turner’s Syndrome•XO genotype—Monosomy X
•1 in 2,500 births
•Short, sterile
•75% result in non-disjunction from the father
Other Diseasesa) Sickle Cell Anemia – codominant,
causes sickle cell shaped red cells in hemoglobin, common in people with African descent
b) Tay-Sachs – metabolic disorder, deteriorates brain, death by age 4, recessive is lethal
c) Cystic Fibrosis – thick mucus clogs, lungs, pancreas, liver. Death by age 20 without proper diet/medication
Sickle cell
How to know…
Family history (mostly probabilities) Genetic testing (ex: spit test) Karyotyping Amniocentesis
Amniocentesis – remove amniotic fluid to check for
genetic disorders