Introduction to Mendelian Genetics

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• Genetics:– The study of heredity

• Heredity:– How traits are passed down generation to

generation• History:

– Gregor Mendel is known as the father of genetics

• Gregor Mendel/1850’s:– Austrian monk– Study mathematics and botany at the

University of Vienna– Use both of these areas of study to analyze

genetic crosses using pea plants.

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Mendel worked with 7 pea plant traits:

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Mendel’s experiments:

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Important terms to know:• Gene:

– A segment of DNA that codes for a specific protein/trait.

• Allele:– A specific form of a gene

• Dominant:– The overpowering form of a gene– Occurs most often in a population– Represented by a capital letter

• Recessive:– The weaker form of a gene– Occurs less often in a population– Represented by a lower case letter

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Important terms to know:

• Phenotype:– A physical trait that you can see– Examples in peas: tall, short, wrinkled,

smooth• Genotype:

– The genetic make up /gene combination– Three types of genotypes:

• RR…..Homozygous dominant• Rr……Heterozygous• rr……. Homozygous recessive

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The law of dominance

• Simply stated: – Anytime a dominant allele is present the

dominant trait will be seen, it overpowers the recessive allele.

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• The ability to roll your tongue is dominant over the inability to roll your tongue.– Tongue roller …. T– Non tongue roller…t– Homozygous Dominant:

• TT• Tongue roller

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-Heterozygous•Tt•Tongue roller

-Homozygous Recessive•tt•Nontongue roller

Karyotype

- A diagram created from a picture of a person’s actual chromosomes.

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Use of the karyotype:

• Used to determine if a person inherited a certain gene or genetic disorder

• Used to determine the chance of passing a gene or genetic disorder on to their child

• Used to determine the sex of a child.

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It’s a male:

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Why?

XY

It’s a female:

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Why?

XX

Down Syndrome

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Edward’s Syndrome

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Creating a karyotype:

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-Look at the length of the chromosome

-Look for the position of the centromere

-Look at the repeating pattern of light and dark bands representing genes

-Look at the specific genes which have been identified and mapped/labeled through the Human Genome Project.

Punnett Squares

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A mathematical method to determine the chance of producing an offspring with a specific trait.

1 Trait Crosses

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Purple pea blossoms are dominant over white blossoms

Heterozygous purple flower crossed with another Heterozygous purple flower

Results:

Genotypic ratio:

- 25% BB - 50% Bb - 25% bb

Phenotypic ratio:

-75% purple -25% white

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In rabbits black fur is dominant over tan fur.

Cross a Homozygous dominant black fur male with a Homozygous recessive tan fur female.

2 trait crosses

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Yellow peas (Y) are dominant over green peas (y). Green pea pods (G) are dominant over yellow pods (g).

Cross a Heterozygous green pea pod, Heterozygous yellow pea with another Heterozygous green pea pod, Heterozygous yellow pea

GgYy x GgYy

2 trait cross

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In guinea pigs black fur is dominant over white fur and smooth fur is dominant over rough fur.

Cross a Homozygous recessive white, Heterozygous smooth fur female with a Heterozygous black fur, Homozygous dominant smooth fur male.

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