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HEREDITY

“The Code of Life”

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What is Genetics?(Chapter 1)

• Genetics is the study of heredity

• Genetics is the study of how traits are passed from an organism to its offspring

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Genetics

Genetics helps to explain why living organisms:

• Resemble their parents

• Each have unique characteristics from any other living organism

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History of Genetics

Began with Gregor Mendel

• in mid-1800’s

• working with pea plants– looking for a pattern in the way

certain characteristics (or traits) are handed down from one generation to the next.

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Pea Plants

Why were pea plants chosen?

1. Grow and reproduce quickly

2. Have a variety of characteristics (or traits)

3. Many traits (list) could be studied in one experiment

4. Can be crossed, or bred, easily

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Reproductive Structures of Flowers

• Stamen = Male Sex Structure (pollen)

• Pistil = Female Sex Structure (seeds)

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Self-Pollination

Pollen from the stamen (male) of one flower lands on the pistil (female) of the same flower, or a different flower on the same plant.

Since only one plant has been involved , it has pollinated itself.

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Self-Pollination(Pure Tall x Pure Tall)

X =

Tall Tall Tall Tall Tall Tall

Purebred Purebred

Parents F1 Generation

(TT) (TT) (TT) (TT) (TT) (TT)

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Self-Pollination(Pure Short x Pure Short)

X =

Short Short Short Short Short Short

Purebred Purebred

Parents F1 Generation

tt tt tt tt tt tt

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Cross-Pollination

Pollen from the stamen (male) of one flower lands on the pistil (female) of the flower of a different plant.

The pollen has crossed between two plants, hence cross-pollination.

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Cross-Pollination(Pure Tall x Pure Short)

X =

Tall Short Tall Tall Tall Tall

HybridsPurebred

Parents F1 Generation

TT tt Tt Tt Tt Tt

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Cross-Pollination(Hybrid Tall x Hybrid Tall)

X =

Tall Tall Tall Tall Tall Short

HybridsHybrids

F1 Generation F2 Generation

Purebred Purebred

Tt Tt TT Tt Tt tt

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“Father of Genetics”

• Gregor Mendel became known as the “Father of Genetics”.

• He identified these traits as “characters” which we know call genes.

• Genes are the units of heredity.• Genetics is the study of heredity, or the

passing of traits from an organism to its offspring.

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Dominant and Recessive

• Dominant means “STRONG”• Recessive means “weak”• Mendel found that some traits were

stronger (dominant) while others were weaker (recessive).

• Generally, letters are used to represent the gene– Capital letter is used for the Dominant gene– Small letter is used for the Recessive gene

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Purebred vs. Hybrid

• Purebred: If genes are alike for a particular trait– Example: YY or yy

• Hybrid: If genes are different for a trait – Example: Yy– NOTE: Hybrids are produced by crossing two

purebred organisms with opposite traits

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Mendel’s Hypothesis

• Mendel made a hypothesis (suggested explanation) for the way traits were passed to the next generation– Each trait must have a pair of factors, or

genes, for each trait– Each plant in next generation also had a pair

of genes for each trait, so must have received one from each parent.

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The Law of Segregation

• During the creation of the sex cells (sperm for the male, eggs for the female), the parent’s gene pairs must segregate (or separate). This is the Law of Segregation.

• Sex cells carry half the gene pair for the new generation. So that after fertilization (union of sperm and egg), the new individual has the completed pair of genes.

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Law of Segregation

Yy Parent

Y y Sex cell

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Law of Independent Assortment

• This law states the each gene pair for a trait is inherited independently of the gene pairs for all other traits.

Yy Tt Rr Ss

Y y T t R r S s

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Incomplete Dominance

• Sometimes, in the gene pairs, neither gene is dominant nor recessive. Neither gene has the ability to mask (hide) the other. As a result, the traits carried by the two genes appear to be blended. This is Incomplete Dominance.

X =

RR WW RW(red) (white) (pink)

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Six Genetic Principles

1. Traits, or characteristics, are passed on from one generation of organisms to the next generation.

2. The traits of an organism are controlled by genes.

3. Organisms inherit genes in pairs, one gene from each parent.

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Six Genetic Principles(continued)

4. Some genes are dominant, whereas other genes are recessive.

5. Dominant genes hide recessive genes when both are inherited by an organism.

6. Some genes are neither dominant nor recessive. These genes show incomplete dominance.

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Genetics and Probability

• Probability is the possibility, or likelihood, that a particular event will occur.

• Probability won’t tell us what WILL happen, BUT…

• Probability can be used to predict the results of genetic crosses.

• A probability is usually written as a fraction or a percentage.

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Punnett Squares

• In addition to probability, a special chart called a Punnett square is used to show the possible gene combinations in a cross between two organisms.

• The chart was developed by Reginald C. Punnett, an English geneticist.

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Phenotype and Genotype

• The Phenotype refers to the physical appearance of an organism.

• The Genotype is the actual gene makeup of the organism.

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Phenotype and Genotypes

BB bb

Black White

X

Black BlackBlack Black

Bb BbBb Bb

Parents

F1 Generation

(Purebred) (Purebred)

Hybrid HybridHybrid Hybrid

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Phenotype and Genotypes(continued)

Bb Bb

Black Black

X

Black BlackBlack White

BB BbBb bb

F1 Generation

F2 Generation

Purebred PurebredHybridHybrid

(Hybrid) (Hybrid)

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Pea Plant Characteristics

(or Traits)• Tall vs. Short (T and t)• Round Seeds vs. Wrinkled Seeds (R and r)• Yellow Seeds vs. Green Seeds (Y and y)• Colored Seed Coat vs. White Seed Coat (C and c)• Full Pod vs. Pinched Pod (F and f)• Green Pod vs. Yellow Pod (G and g)• Side Flowers vs. End Flowers (S and s)

•(return)

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Additional Links

• http://www.brooklyn.cuny.edu/bc/ahp/MGInv/MGI.Intro.html

• http://anthro.palomar.edu/mendel/mendel_1.htm

• http://www.brooklyn.cuny.edu/bc/ahp/BioInfo/GP/Definition.html

• http://nces.ed.gov/nceskids/probability/

• http://nces.ed.gov/nceskids/Games.asp