DNA, Traits, and Heredity

DNA, Traits, and Heredity

Genetics Unit

Grade 8 Physical Science

Mr. Reid

1Mr. Reid


Objectives

• Learn about DNA, Traits, and Heredity. This lecture will lead into why structural changes to genes (mutations) located on chromosomes may affect proteins and may result in harmful, beneficial, or neutral effects to the structure and function of the organism. MS-LS3-1

• Determine the meaning of symbols, key terms, and other domain-specific words and phrases as they are used in a specific scientific or technical context relevant to grades 6–8 texts and topics. RST.6–8.4

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Warm up Questions:

• How can we have different traits than our parents?

• What is DNA?

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What is DNA?

• Genetic material that directs all the cell’s activities, including the making of new cells.

DNA is short for Deoxyribonucleic acid. It is a self-replicating material which is present in nearly all living organisms as the main constituent of chromosomes. It is the carrier of genetic information.

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DNA Structure

• DNA is made of:

o Sugar called deoxyribose

o Phosphate Group

oNitrogen bases

• Double helix shape or a twisted ladder

• Nitrogen bases are like sentences, they make the codes for all organisms

DNA is different for every person (except for identical twins).The nitrogen bases are Guanine, cytosine, adenine, and thymine. GCAT, G binds with C, A binds with T.

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Chromatid (or sister chromatids)

• One half of a chromosome

• Sister chromatids are connected by a centromere at the center

The Centromere is what joins the chromosome and keeps it together.A chromatid is a threadlike structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes

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Chromosome

• Condensed form of DNA

• Chromatin condenses into a chromosome during cell division

• 2 sister chromatids form 1 chromosome

a threadlike structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genesChromatin condenses into a chromosome during cell division2 sister chromatids form 1 chromosome

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Fun Facts about DNA

• If you unravel all the DNA from all your cells, you could reach the moon 6000 times!

• Humans and chimps share 94-99% of their DNA

• It would take a person typing 60 wpm, 8 hours a day, around 50 years to type out the entire human genome

We’re all 99.9 percent alike.

Of the 3 billion base pairs in the human genome, only 0.1% are unique to us. While that 0.1% is still what makes us unique, it means we’re all more similar than we are different.

Genes make up only about 3 percent of your DNA.

Genes are short segments of DNA, but not all DNA is genes. All told, genes are only about 1-3% of your DNA. The rest of your DNA controls the activity of your genes.

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What is a gene?

• A factor that controls a trait

• Located on DNA

• Examples:• gene for plant height• gene for eye color

• Two copies of genes (exists in

pairs) because you get one set

from your mother and one

from your father

• Fill in the blanks in the picture

A gene is a unit of heredity which is transferred from a parent to offspring and is held to determine some characteristic of the offspring.

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What is a trait?

• Physical characteristic

• How you look

Examples of traits:brown eyes, blue eyesstraight hair, curly hairblood type – A, B, AB, 0

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Traits

• Now take a few minutes to think of some traits that people have and share them with your table group.

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What is heredity?

• The passing of traits from parent to offspring

• Example: Parent with blue eyes has a child with blue eyes

• Do you think the sickle cell trait is passed on from both parents or only one parent? Look at the diagram to answer.

Heredity is the passing on of physical or mental characteristics genetically from one generation to another.Do you think the sickle cell trait is passed on from both parents or only one parent? Look at the diagram to answer.

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What is genetics?

• Study of how traits are inherited

Genetics is the genetic properties or features of an organism, characteristic, etc.

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What are alleles?

• Different forms of a gene

• Example: For the gene of height, you have an allele for tall and an allele for short

Most genes exist in many forms called alleles. An allele is any of the alternative forms of a gene that may occur at a specific locus. Out cells have two alleles for each gene, one from each parent.

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What controls the inheritance of traits?

• Traits are determined by the alleles inherited from parents

• Alleles can be dominant or recessive

The Alleles are either dominant or recessive and come from both parents. Parents pass on either the dominant or the recessive allele.

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Dominant Allele

• Trait that always appears when the allele is present

• Written with a CAPITAL letter

• Examples:

• Tall plants = T

• Brown eyes = B

Brown eyes (B) are dominant to recessive eye colors, Tall plants (T) are dominant to recessive plant heights.

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Recessive Alleles

• Traits that can be masked/covered up when the dominant allele is present

• Written with a lowercase letter

Blue eyes (b) are a recessive to brown eyes, and short plants (t) are recessive to tall plants

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Now You Try

• If tall plants were T and brown eyes were B, what do you think short plants and blue eyes would be?

Short plants:t

Blue eyes: b

Tall plants = TBrown eyes = B

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How to write alleles for a single trait

• Use only 1 letter to represent a trait, then use upper- and lowercase to show dominant and recessive alleles

• Write them side-by-side (BB, Bb, bb)

• Example: Plant height

• Correct!

T = tall

t = short

Wrong! T = talls = short

1 letter represents a trait, and two traits are received, one from each parent. The diagram on the right is a Punnett square.

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Comparing purebreds and hybrids

Purebred

• Alleles are identical (both dominant or both recessive)

• Trait appearing can be either dominant or recessive

• Examples: TT, BB, bb, tt

Hybrid

Has one dominant allele and one recessive alleleTrait appearing will always be dominantExamples: Tt, Bb

Purebreds have two identical alleles, either dominant or recessive. Hybrids have on dominant allele and one recessive allele.

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Discussion Question

• Are traits controlled by dominant alleles more common than traits controlled by recessive alleles?

Cheek dimples vs no cheek dimples

Widows peak vs no widows peak

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Analysis Question

• Write a brief 3-5 sentence explanation as to why people look so different from one another.

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DNA, Traits, and Heredity

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