Incomplete Dominance

Complex Patterns of<br />

Heredity<br />

Ch. 11, 14<br />

p. 272-273, 350-351

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<strong>Incomplete</strong> <strong>Dominance</strong><br />

Codominance<br />

Summary<br />

Traits appear blended<br />

(red + white = pink)<br />

Both traits are<br />

expressed equally<br />

(A + B = AB)

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Multiple Alleles<br />

Polygenic Trait<br />

Summary<br />

More than two possible<br />

alleles, but only two<br />

present at a time.<br />

Single trait controlled<br />

by more than one gene.

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<strong>Incomplete</strong> <strong>Dominance</strong><br />

• Heterozygous offspring show a phenotype<br />

that is in-between the phenotypes of the<br />

homozygous parents<br />

• Neither allele is dominant<br />

Snapdragon<br />

Flowers<br />

RED + WHITE =<br />

PINK

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<strong>Incomplete</strong> <strong>Dominance</strong><br />

W<br />

W<br />

R R<br />

RW RW<br />

RW RW<br />

THE ALLELES REMAIN DISTINCT……ONLY THE<br />

PHENOTYPE APPEARS BLENDED.

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Codominance<br />

•2 dominant alleles are expressed<br />

equally at the same time<br />

Ex:<br />

Blood types<br />

“Roan” cattle<br />

Red + White = Red & White<br />

ROAN

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Homozygous<br />

Type A blood<br />

Codominance<br />

+<br />

Homozygous<br />

Type B blood<br />

=<br />

Type AB

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<strong>Incomplete</strong> <strong>Dominance</strong><br />

Codominance<br />

Summary<br />

Traits appear blended<br />

(red + white = pink)<br />

Both traits are<br />

expressed equally<br />

(A + B = AB)

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Multiple Alleles<br />

•Genes with 3 or more alleles<br />

•Only 2 alleles are present at a time<br />

Ex: Blood Types: A, B, AB, O

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Blood Types<br />

Phenotype Genotypes<br />

Type A I A I A or I A i<br />

Type B I B I B or I B i<br />

Type AB I A I B<br />

Type O ii

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Polygenic Traits<br />

• Traits controlled by two or<br />

more genes that interact<br />

• Ex: eye color, skin color,<br />

hair color, height

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Multiple Alleles<br />

Polygenic Trait<br />

Summary<br />

More than two possible<br />

alleles, but only two<br />

present at a time.<br />

Single trait controlled<br />

by more than one gene.

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Environmental Effects<br />

(copy this)<br />

• Phenotype is a combination of genetic<br />

and environmental influences<br />

–Example: Himalayan rabbit fur color<br />

depends on body temperature. The<br />

ears, nose, and feet are dark because<br />

they are cooler.

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Human Genetics

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Difficulties in Studying<br />

• Controlled<br />

experiments are<br />

not possible<br />

• Small numbers of<br />

offspring per<br />

generation<br />

• Long periods<br />

between<br />

generations<br />

Human Heredity

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Sex Determination<br />

• Autosomes – body chromosomes – first 22<br />

pair<br />

– Same in both male and female<br />

• Sex Chromosomes – 1 pair, last pair<br />

– XX – Female<br />

– XY - Male

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X X<br />

X XX XX<br />

Y<br />

XY XY<br />

XX = 50%<br />

XY = 50%<br />

You always have a 50% chance of having<br />

a girl and a 50% chance of having a boy!

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Sex-Linked Traits<br />

• Traits controlled by recessive genes located on<br />

sex chromosomes<br />

• Usually found on the X chromosome<br />

• More common in males<br />

• Ex:<br />

– Hemophilia – missing a blood clotting protein, 1<br />

in 10,000 males<br />

– Color blindness – red/green is the most<br />

common, 1 in 10 U.S. males<br />

– Muscular dystrophy – progressive weakening<br />

and loss of skeletal muscle, 1 in 3000 U.S. males

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Sex Linked Traits<br />

• Hemophilia – blood clotting<br />

enzyme<br />

is absent<br />

– Queen Victoria’s family<br />

affected

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Sex Linked Traits<br />

• Red – Green Colorblindness – individuals<br />

cannot distinguish between these two colors

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Sex Linked Traits<br />

• Duchene<br />

Muscular<br />

Dystrophy –<br />

wasting away of<br />

skeletal muscle

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X R<br />

Y<br />

X r<br />

Y<br />

X R<br />

X R X R<br />

X R Y<br />

X R<br />

X R X r<br />

X R Y<br />

X r<br />

X R X r<br />

X r Y<br />

X r<br />

X r X r<br />

X r Y<br />

Females<br />

•X R X R = normal<br />

•X R X r = carrier<br />

•X r X r = disease<br />

Males<br />

•X R Y = normal<br />

•X r Y = disease

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Sex – Limited Traits<br />

• Controlled by genes located in the<br />

autosomes<br />

• Only expressed in the presence of sex<br />

hormone<br />

• Only expressed in one gender<br />

• First seen at puberty (hormones<br />

produced in large enough quantities

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• Examples:<br />

Sex – Limited Traits<br />

– Male bird’s<br />

colorful plumage<br />

– Beard growth in<br />

males<br />

– Milk production<br />

in females

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Sex Influenced Traits<br />

• Traits that are caused by a gene whose<br />

expression differs in males and females<br />

• Also located on autosomes<br />

• Expressed in the presence of male or female<br />

hormones<br />

• Example: male pattern baldness<br />

– BB = normal<br />

– Bb = males bald, females normal<br />

– bb = males and females bald

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Hereditary Disorders<br />

• Techniques for detecting genetic disorders<br />

– Amniocentesis – technique used that<br />

removes and studies amniotic fluid<br />

during pregnancy;<br />

– identifies only<br />

chromosomal disorders

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Hereditary Disorders<br />

• Karyotyping – an<br />

enlarged photo of<br />

the chromosome<br />

pairs (map) to<br />

identify any<br />

abnormalities in the<br />

chromosomes

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Pedigree Charts<br />

• Shows how a trait and the genes that control it<br />

are inherited within a family<br />

• Identifies the presence or absence of particular<br />

trait in members of each generation

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Pedigree Charts<br />

• Males = Square<br />

• Females = Round<br />

• Generations = Roman Numerals (I, II, III, IV)<br />

• Individuals = Numbered or Named<br />

• Trait Expressed = Filled in<br />

• Normal = Empty<br />

• Carrier = Half filled