COMPLEX PATTERNS OF INHERITANCE

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COMPLEX

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RR

Rr

rr

R r

R

r

1 homozygous dominant

2 heterozygous

1 homozygous recessive

Simple Dominant Traits

Not a complex pattern –

(3)

Simple Dominant Traits

•Simple dominant

traits need

only 1 allele in order to be

expressed – RR or Rr

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Simple Recessive Disorders

• Recessive Disorders affect

offspring with 2 recessive alleles – (homozygous recessive)

•Most genetic disorders are

caused by

recessive alleles

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Recessive Disorders

• Cystic Fibrosis

– 1 in 28 white Americans carry recessive allele

– Defective PROTEIN in plasma membrane

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• Tay-Sachs

– Absence of ENZYME that breaks down LIPIDS

–Lipids accumulate in cells

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Recessive Disorders

• Phenylketonuria (PKU)

–Absence of ENZYME to convert phenylalanine to tyrosine

–Phenylalanine cannot be broken down

–Accumulates in cells –Damages CNS

(8)

COMPLEX

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INCOMPLETE DOMINANCE

•Heterozygote is a

mix

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Snapdragon plant

RED PINK WHITE

R R´ R’ R´

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• Flowers contain enzymes that control pigment production

• The R allele makes red pigment

• R´ defective enzyme that makes no

pigment

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• RR plenty red pigment = red

• RR´ has one R allele and

can produce half the amount of red pigment = pink

• R’R’ can produce no pigment =

INCOMPLETE DOMINANCE

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INCOMPLETE DOMINANCE

•A pink carnation is crossed

with a white carnation

1. Write the genotypes of the two parent carnations.

2. Use a Punnett square to

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INCOMPLETE DOMINANCE

Parents = RR' x R'R'

R R' R'

R'

R R' R' R'

R R' R'R'

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CODOMINANCE

• Heterozygote has both alleles • Use BOTH uppercase letters • Both alleles show equally

• What would happen if you bred…

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CODOMINANCE

Black rooster White hen

Black & White Chicken

BB WW

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Black rooster

BB WW

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SICKLE-CELL ANEMIA

• (CODOMINANCE)

• Hemoglobin, the oxygen-carrying protein, differs by one amino acid, which changes shape of red blood cell

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SICKLE-CELL ANEMIA

• Sickled cells have shorter life span anemia

(low # of red blood cells)

• Causes tissue damage

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SICKLE-CELL ANEMIA

• Individual who are heterozygous for the allele produce both normal and

sickled red blood cells (codominance) • Able to produce enough hemoglobin to

have no serious health problems • “have the sickle-cell trait”

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SICKLE-CELL ANEMIA

• A boy is born with sickle-cell anemia. Neither parent has the disease.

1. Write the boy’s genotype for this disease.

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SICKLE-CELL ANEMIA

N ? N

?

NN N__

N__ _SS

Parents’ genotypes = NS x NS

Boy’s genotype = SS

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Why does the sickle cell allele persist? (must be some advantage)

Carriers (with one sickle gene and

one normal hemoglobin gene) have some protective advantage against

malaria

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People who have sickle cell anemia are less likely to die from malaria. The parasite is carried by the

Anopheles mosquito.

Cells that sickle make a poor home for this parasite. The body can eliminate it before the parasite spreads too

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MULTIPLE ALLELES

• Traits controlled by 2 or more alleles

• Pigeon:

Ash-red (BA) = dominant to all

Blue (B) = dominant to b, but recessive to BA

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• ABO - Human Blood Groups

• Video introduction…

• I is gene for blood type

• I codes for a molecule that

attaches to a membrane protein on the surface of red blood cells

• Each allele codes for a different surface molecule

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BLOOD TYPE & MULTIPLE ALLELES

•Blood Groups:

A, B, AB,

&

O

•3 alleles:

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BLOOD TYPE & MULTIPLE ALLELES

Genotype Surface Molecule

Phenotype

IAIA and IAi A A

IBIB and IBi B B

IAIB A & B AB

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Let’s look!

What do red blood cells look like?

Find the antigens on the red blood cells.

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• Incompatible blood types will

clump together – death

• Your immune system recognizes red blood cells as belonging to

you. If cells with different surface molecules enter your body, your immune system will attack them.

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• Humans have 23 pairs of chromosomes:

22 pairs of autosomes

1 pair of sex chromosomes

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• Females: sex chromosomes are

XX

• Males – sex chromosomes are

XY

• Male determines sex of offspring • At fertilization, sex is determined

by laws of probability

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• The X and Y chromosomes are not

homologous - the Y chromosome has

no corresponding alleles to those on the X chromosome.

• For males,

recessive alleles on

X chromosome will NOT

have another allele to “mask” it on the Y

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SEX-LINKED INHERITANCE

• Traits are controlled by genes on sex

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COLORBLINDNESS: AN X-LINKED DISORDER

• Colorblindness – caused by a common X-linked recessive gene

• lack of one or more color receptors

• Rods – night vision; cones - pigments

• Colorblindness screening

• Usually genetic

• no treatment, but most people adjust

• http://www.colorvisiontesting.com/online%20test.htm#Test%20Card%20Number%203%20an

(47)

•Condition in which blood

does not properly clot

•1 / 10,000 males

•1 / 1,000,000 females

HEMOPHILIA :

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HEMOPHILIA : AN X-LINKED DISORDER

• Males inherit the allele for

hemophilia on the X chromosome from mother

•A

single

recessive

allele

will

cause the disease

(females would need

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POLYGENIC INHERITANCE

• Trait is controlled by more than one gene

• All heterozygotes are intermediate phenotype

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POLYGENIC INHERITANCE

• Skin color

• Eye color

• height and intelligence in

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Changes in

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• In sex chromosomes:

• If X is missing = XO

• An extra chromosome: XXX or XXY

• Any Y = male; No Y = female

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What if an entire chromosome was missing, or an extra one was

present?

Usually due to accident in meiosis (nondisjunction)

• Trisomy – extra – 47 chromosomes

• Trisomy 21 = Down Syndrome Changes in Chromosome

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• The genetic make up at

fertilization is only the organism’s

potential to develop and function • Other factors can influence how

gene is expressed or if it is expressed

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• External environment –

temperature, nutrition, light, chemicals, infection agents

• Leaf size, thickness and shape depend on amount of light

received

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• Internal environment - Hormones

structural differences

–horns in sheep are different in male and female

–Male pattern baldness

–Peacock/peahen feathers • Age can affect gene function

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Quick Check

1. In what type of inheritance does the heterozygote show BOTH alleles?

2. What type of inheritance is sickle cell anemia? 3. What is the advantage of having the sickle trait? 4. What type of inheritance can produce a pink

flower from a cross between red and white flower?

5. What controls flower pigment?

6. What type of molecule is hemoglobin? 7. Polygenic inheritance produces a __ of

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