GENETICSUnit 7

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allele

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How we inherit our genesEach human offspring inherits one maternal

set of 23 chromosomes and one paternal set.

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Unlucky catch A baby inheriting two copies of the defective FMO3 gene will develop fish odor syndrome.

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Homozygous vs. Heterozygous

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Homozygous vs. Heterozygous

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Family resemblance: your mother and father each contribute to your genetic makeup.

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TAKE-HOME MESSAGE • Offspring resemble their parents because

they inherit genes—instruction sets for biochemical, physical, and behavioral traits, some of which are responsible for diseases—from their parents.

Challenge Question• How many alleles of each gene do you

have? Where did you get them?

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hereditythe passing on of physical or

mental characteristics genetically from one generation

to another

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HEREDITY

I AM the family face;Flesh perishes, I live on,Projecting trait and traceThrough time to times anon,And leaping from place to placeOver oblivion.

The ears-heired feature that canIn curve and voice and eyeDespise the human spanOf durance—that is I;The eternal think in man,That heeds no call to die. —Thomas Hardy, Moments of Vision and Miscellaneous Verses,

1917

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HISTORY

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History of Genetics Timeline

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Some traits are controlledby a single gene

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TAKE-HOME MESSAGE• Many human traits are determined by

the instructions a person carries on a single gene, and the traits exhibit straightforward patterns of inheritance.

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Another ideaoffspring reflect a simple blending of their two parents’ traits via the

blood

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Charles Darwin pangenesis

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LamarckInheritance of Acquired

Characteristics

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Gregor Mendel: Mini-Biography

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Mendel didn’t do anything radically new, but simply applied methodical

experimentation and scientific thinking

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Mendel chose a good organism to study: the garden pea.

• relatively easy to fertilize manually by “pollen dusting”

• easy to collect dozens or even hundreds of offspring from a single cross

• pea plants are fast enough breeders that Mendel could conduct experiments that lasted for multiple generations

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Mendel chose to focus on seven easily categorized traits

a. flower color is purple or white b. seed color is yellow or green c. flower position is axial or terminald. pod shape is inflated or

constrictede. stem length is long or shortf. pod color is yellow or greeng. seed shape is round or wrinkled

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Mendel began his studies by first repeatedly breeding together similar plants until he had many distinct populations, each of which was unvarying

for a particular trait

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TAKE-HOME MESSAGE• In the mid-1800s, Gregor Mendel

conducted studies that helped us understand how traits are inherited.

• He applied methodical experimentation and rigorous hypothesis testing, focusing on easily observed and categorized traits in garden peas.

Challenge Question• Why was the pea plant a good

organism with which to study heredity? 28

Punnett Squares

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Mendel’s conclusions:• that the inheritance of each trait

is determined by "units" or "factors" that are passed on to descendants unchanged (these units are now called genes)

• that an individual inherits one such unit from each parent for each trait

• that a trait may not show up in an individual but can still be passed on to the next generation 30

Mendel's observations from these experiments can be summarized in two principles:

• the principle of segregation • the principle of independent

assortment

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Principle (Law) of Segregation:• for any particular trait, the pair of

alleles of each parent separate • only one allele passes from each

parent on to an offspring• which allele in a parent's pair of

alleles is inherited is a matter of chance

• segregation of alleles occurs during the process of sex cell formation (meiosis)

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Principle (Law) of Independent Assortment:

• different pairs of alleles are passed to offspring independently of each other

• result - new combinations of genes present in neither parent are possible

• today, we know this is because the genes for independently assorted traits are located on different chromosomes 34

But wait…there’s more!

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Phenotype vs. Genotype

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Phenotypeset of observable

characteristics of an individual resulting from the interaction

of its genotype with the environment

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Genotypegenetic constitution

(“formula”) of an individual organism

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Chance is important in genetics for two reasons:

• consequence of segregation• fertilization is a chance event

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Test Cross

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Test cross

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• In this test-cross, a homozygous white female alligator is bred with a normally colored male of unknown genotype. • The color of their

offspring will help identify whether the male is homozygous dominant or heterozygous.

Test CrossYou mate a pigmented male alligator to a female albino alligator. The clutch of baby alligators includes both pigmented and albino individuals. What is the genotype of the father?1. MM2. Mm3. mm4. 1 and 2 are equally possible. 45

TAKE-HOME MESSAGE

• In a test-cross, an individual with a dominant phenotype and an unknown genotype is mated with a homozygous recessive individual.

• The phenotypes of the offspring reveal the unknown genotype.

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Pedigrees• In cross-pollinating plants that either produce

yellow or green pea seeds exclusively, Mendel found that the first offspring generation (f1) always has yellow seeds.   However, the following generation (f2) consistently has a 3:1 ratio of yellow to green.

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Pedigrees• This 3:1 ratio occurs in later generations as well.

  Mendel realized that this was the key to understanding the basic mechanisms of inheritance.

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What is the genotype of the paternal grandmother?

1. Homozygous recessive

2. Heterozygous3. Homozygous

dominant4. Cannot be

determined

What is the probability that you (“Me” in the diagram) will be a carrier for this disease?1. 1/42. 1/33. 1/24. 2/35. 3/4

Sometimes it’s hard to figure out the pattern…

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Exceptions to the Rule…

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Incomplete dominance and codominance: the effects of both alleles in a genotype can show up in the phenotype.Incomplete dominance: the heterozygote appears to be intermediate between the two homozygotesCodominance: the heterozygote displays characteristics of both homozygotes

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Inheritance of the ABO Blood Groups

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The gene for aldehyde dehydrogenase (ADH) is incompletely dominant. A woman who is heterozygous for this gene (ADH+/ADH) marries a man who is homozygous for the normal allele (ADH+/ ADH+). What is the probability that they will have heterozygous children?

1. 1/42. 1/23. 3/44. 2/3

An individual with type O blood marries an individual with type AB blood. Which of the following would not be a possible offspring blood type?1. A2. B3. O4. All of the above are

possible.

Multiple AllelesGene that has three or more alleles

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Polygenic TraitsControlled by two or more than two

genes

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Genes & the Environment• Drinking diet soda can

be deadly if you carry a single bad gene

• Ice pack on the back of rabbits & cats

• Alcoholism• Weight

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