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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 ProblemExample: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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CodominanceQuestion:

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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CodominanceAnswer:

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 InheritanceA 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