Chapter 5: HereditySection 2: Genetics Since Mendel

Minersville Area School DistrictMr. MotukRoom 124

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A. Incomplete Dominance1. Neither allele for a trait is dominant.2. The phenotype produced is

intermediate between the two homozygous parents.

3. This results in a blending of the two traits.

a. Example:RR = Red Flowersb. WW= White Flowersc. RW = Pink flowers

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B= Allele for Black FurW= Allele for White Fur

So if we had incomplete dominance: 1. BB would be homozygous Black Fur2. WW would be homozygous White Fur3. BW would be heterozygous GREY Fur

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Incomplete Dominance

B. Multiple Alleles1. More than two alleles that control a

trait are called multiple alleles. 2. Traits controlled by multiple alleles

produce more than three phenotypes. 3. An excellent example of multiple

alleles is human blood type. a. Blood types exist in four possible

phenotypes: A, B, AB, and O.

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IA= type A bloodIB= Type B bloodi= Type “O” bloodA Blood is dominant over “O” blood (i).B blood is dominant over “O” blood (i). A blood is codominant with B blood. (both

show up)

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Blood Type Alleles

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Blood Type GenotypesGenotype Phenotypic Result

IAIA Type A bloodIAi Type A BloodIAIB Type AB Blood (Codominance)IBIB Type B BloodIBi Type B Bloodii Type O Blood

C. Polygenic inheritance1. A group of gene pairs acts together

to produce a trait, which creates more variety in phenotypes.

2. Many human traits are controlled by polygenic inheritance, such as hair and eye color.

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D. Mutations1. Mutations are genes that are

altered or copied incorrectly. 2. A mutation can cause harmful,

beneficial, or have no effect. 3. Chromosome disorders

a. Caused by more or fewer chromosomes than normal.

b. Down Syndrome is caused by an extra copy of chromosome # 21.

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Down’s Sydrome9

E. Recessive Genetic Disorders

1. Both parents have a recessive allele responsible for the disorder and pass it to their child.

2. Because both parents are heterozygous they DON’T show any symptoms.

3. Cystic fibrosis is a homozygous disorder.

a. What is Cystic Fibrosis?(Journal Entry)

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F. Sex Determination1. Chromosomes that determine the

sex of an organism are XX in females and XY in males.

a. Females produce eggs with X chromosomes only.

b. Males produce sperm with either an X or a Y chromosome.

2. The male determines the sex of the offspring.

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G. Sex-linked Disorders1. An allele inherited on an X or Y

chromosome is a sex linked gene. 2. Color blindness is a sex linked

disorder caused by a recessive allele on the X chromosome.

a. Color blindness is more common in men than women. Why?

3. A pedigree follows a trait through generations of a family.

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H. How to use a Pedigree1. Males are represented by squares. 2. Females are represented by circles. 3. Completely filled in circle or square

shows that the trait can be seen in that person.

4. A half colored in circle or square indicates a carrier.

a. Carriers are heterozygous for the trait, and the trait is not seen in that individual.

5. People represented by empty circles or squares do not have the trait and are not carriers.

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I. Example of a Pedigree(Pedigree worksheet to follow)

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