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Genetics & Human Inheritance

BIO 105

Chapter 20

Vocabulary

Alleles – different forms of a gene

Trait – some characteristic

Homozygous – individuals that contain two copies of the same allele

Heterozygous – individuals with different

alleles of the same gene

Vocabulary

Dominant allele – an allele whose effects

can be detected, regardless of the presence of other alleles (upper case)

Recessive allele – an allele whose effects are masked in the heterozygous

condition (lower case)

Genotype – set of alleles an individual

possesses for a particular trait

Phenotype – observable physical trait

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Law of Segregation

• Because the alleles for each gene

segregate during Meiosis, half the gametes bear one allele, and half bear the

other.

Law of Independent Assortment

• Alleles for genes on different

chromosomes segregate into gametes independently.

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Gregor MendelHow to predict the outcome of one-

trait crosses

1. Identify the possible gametes that each

parent can produce

2. Use a Punnett square to determine the

probable outcome of the genetic cross

– This diagram predicts the outcome of a

particular cross or breeding experiment

– With this you can generate a summary

of every possible combination of maternal and paternal alleles for a gene

Monohybrid

cross – a

cross in

which both

parents are

heterozygous

for one trait of

interest

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How to predict the outcome of two-trait crosses

1. Identify the possible gametes that each parent can produce

2. Use a Punnett square to determine the probable

outcome of the genetic cross

Marfan syndrome

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Cystic fibrosis

http://discovermagazine.com/2013/september/14-doorway-to-a-cure

Types of inheritance

1. Dominant vs. recessive

2. Codominance

3. Incomplete dominance

4. Polygenic inheritance

5. X-linked inheritance

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What makes an allele dominant or recessive?

A dominant allele codes for a normal, functional protein, whereas a recessive allele codes for

either a nonfunctional form of the protein or no protein at all.

i.e. tyrosinase necessary for melanin formation

Albinism

Autosomal Recessive Disorders

• These are disorders that are controlled by DNA on any of the 22 autosomes

• These are recessive disorders so they are only expressed if the person is homozygous for the recessive allele

• Examples include: cystic fibrosis, albinism, phenylketonuria

Cystic fibrosis pedigree

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Autosomal Dominant Disorders

• These disorders are also controlled by the non-sex chromosomes

• These disorders will be expressed if the person

has one dominant allele.

• Examples include: Huntington disorder, Marfan syndrome

Huntington’s Disease

It is a degenerative disease that affects the

cerebral cortex of the brain. Initial symptoms are abrupt, jerky movements

which typically develop in middle age.

Late in the disease dementia occurs.

Codominance

• Codominance occurs when more than one

allele produces a functional protein.

• In this case, the effects of both alleles are

separately apparent in a heterozygote.

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Blood Types

3 alleles:

IA

IB

i

Blood Types

Genotype Phenotype Blood Type

AB(IAIB)

A & B antigens AB

AA(IAIA)

A antigens A

BB(IBIB)

B antigens B

OO(ii)

No A or B antigens O

BO(IBi)

B antigens B

AO(IAi)

A antigens A

Incomplete Dominance: Sickle-cell hemoglobin

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Pleiotropy

Polygenic Inheritance

Multiple genes affect

expression of a certain trait.

X-Linked Inheritance in Humans

• X-linked genes have no homologous allele on the Y chromosome, so their inheritance is a bit different.

• Males only have one X chromosome, so if they inherit a recessive allele on the X chromosome they will express the trait.

• If females inherit one recessive allele, they have a chance to also inherit the dominant allele.

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Hemophilia

Prenatal testing for certain genetic

disorders

• Amniocentesis

• Chorionic villus sampling

Phenylketonuria - newborns

• Non-functional phenylalanine hydroxylase

• Microcephaly

• Intellectual disability

• Seizures

• Behavior

• Skin rashes