Mendel and Heredity

(Chapter 8)

Mendel and Heredity

(Chapter 8)D. Blanck

PLHS Biology

D. BlanckPLHS Biology

I. Orgins of Genetics:I. Orgins of Genetics:

A. Heredity: the passing of traits from parents to offspring

(Characteristic=Trait)

before DNA and chromosomes were discovered, heredity was a great mystery

A. Heredity: the passing of traits from parents to offspring

(Characteristic=Trait)

before DNA and chromosomes were discovered, heredity was a great mystery

B. Gregor Johann Mendel: 1) Austrian monk that

is credited as the “father” of the scientific study of heredity

2) Experimented with different varieties of garden peas

a) 1st to develop rules to predict patterns of heredity

B. Gregor Johann Mendel: 1) Austrian monk that

is credited as the “father” of the scientific study of heredity

2) Experimented with different varieties of garden peas

a) 1st to develop rules to predict patterns of heredity

b) heredity provides the basis for: Genetics –the study of

“genes”

c) repeated experiments of T.A. Knight

Mendel extended Knight’s work by applying math!(ratios and proportions)

b) heredity provides the basis for: Genetics –the study of

“genes”

c) repeated experiments of T.A. Knight

Mendel extended Knight’s work by applying math!(ratios and proportions)

3. Why did Mendel use garden peas? a) Peas have clear traits - easy to

tell apart (see table 8-1, pg 163) b) Easy to control pollination (male

and female parts are in same flower) 1) self-fertilization – flower

fertilizes itself 2) cross-pollination –

transfer of pollen between plants

c) Easy to grow

3. Why did Mendel use garden peas? a) Peas have clear traits - easy to

tell apart (see table 8-1, pg 163) b) Easy to control pollination (male

and female parts are in same flower) 1) self-fertilization – flower

fertilizes itself 2) cross-pollination –

transfer of pollen between plants

c) Easy to grow

C) Mendel’s work with ratios: 1. Mendel’s Experiment

monohybrid cross – only 1 trait

Step 1: Make sure plant is true-breeding

allow plant line to self-pollinate for many generations

results in no variation in traits

C) Mendel’s work with ratios: 1. Mendel’s Experiment

monohybrid cross – only 1 trait

Step 1: Make sure plant is true-breeding

allow plant line to self-pollinate for many generations

results in no variation in traits

Step 2: Cross two “P” generation (parental generations) plants with contrasting traits

observe offspring (F1 generation)

record # of F1 plants with each trait

Step 2: Cross two “P” generation (parental generations) plants with contrasting traits

observe offspring (F1 generation)

record # of F1 plants with each trait

Step 3: Allow F1 plants to self-pollinate

observe and count this second

generation of offspring = F2 generation

Step 3: Allow F1 plants to self-pollinate

observe and count this second

generation of offspring = F2 generation

2. Mendel’s results: a) F1 gen. – showed only one form of trait

(ex: purple flowers) b) F2 gen. – showed both forms of trait

(ex: 705 purple: 224 white)

c) For each of the 7 traits, he found the same 3:1 ratio!

2. Mendel’s results: a) F1 gen. – showed only one form of trait

(ex: purple flowers) b) F2 gen. – showed both forms of trait

(ex: 705 purple: 224 white)

c) For each of the 7 traits, he found the same 3:1 ratio!

Mendel’s results for other traitsMendel’s results for other traits

results2results2

II. Gene Theory:

A. Mendel’s Hypothesis - “foundation of genetics”

1. For each trait, an individual has 2 copies of the gene, one from each parent

2. There are alternative versions of genes

II. Gene Theory:

A. Mendel’s Hypothesis - “foundation of genetics”

1. For each trait, an individual has 2 copies of the gene, one from each parent

2. There are alternative versions of genes

Alleles = alternative forms of a gene (green seed vs yellow seed)

1 allele for each gene comes from each parent

Genotype = set of 2 allelesex: GG or gg (G = green and g= yellow)

Phenotype = observable characteristicex: pea appears green or

yellow

Alleles = alternative forms of a gene (green seed vs yellow seed)

1 allele for each gene comes from each parent

Genotype = set of 2 allelesex: GG or gg (G = green and g= yellow)

Phenotype = observable characteristicex: pea appears green or

yellow

Homozygous = 2 identical alleles for a trait

(ex: GG and gg)

Heterozygous = 2 different alleles for a trait

(ex: Gg or gG)

Homozygous = 2 identical alleles for a trait

(ex: GG and gg)

Heterozygous = 2 different alleles for a trait

(ex: Gg or gG)

3. When 2 different alleles occur together, one may be completely expressed. The other may have no observable effect on phenotype

a) Dominant = allele exclusively expressed:

PP = purple Pp = purple

b) Recessive = allele NOT expressed

when dominant form is present: Pp = Purple pp = white

3. When 2 different alleles occur together, one may be completely expressed. The other may have no observable effect on phenotype

a) Dominant = allele exclusively expressed:

PP = purple Pp = purple

b) Recessive = allele NOT expressed

when dominant form is present: Pp = Purple pp = white

III. Studying Heredity: III. Studying Heredity:

A. Punnett Square – predicts the expected genotypes of a cross.

A. Punnett Square – predicts the expected genotypes of a cross.

• Punnett squares can also deal with multiple or complex traits.

A = Greena = absence of Green

(blue)B = Brownb = absence of Brown

(blue)

• Punnett squares can also deal with multiple or complex traits.

A = Greena = absence of Green

(blue)B = Brownb = absence of Brown

(blue)

Color BlindnessColor Blindness• An X-Linked Trait• An X-Linked Trait

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Codominance or incomplete dominance

Codominance or incomplete dominance