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Mendelian Genetics

Genetics Unit

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History of Genetics1. 1865: Gregor Mendel (Austrian monk)

presented results of 10 years of experimentation on pea plants.

2. Late 1800’s: increased research in microscopy and cytology (study of cells) recognized each chromosome transferred (contribution from parent).

3. 1903: chromosomes in gametes (sex cells) found to be involved in reproductive cycle.

Gregor Mendel studied genetics using pea plants.

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4. 1914: fruit fly began to be used for genetic research because of very short generation span.

5. 1940’s: chemical techniques developed to reveal association between genes in chromosomes and proteins made in the cell.

6. 1944: DNA’s responsibility is to code for production of proteins (acts like a recipe book) one gene codes for production of only one protein.

7. 1954: Watson and Crick proposed model of DNA structure to be coiled spiral ladder (double helix).

Drosophila used as experimental model in genetics.

In 1962 Watson and Crick win the Nobel Prize for discovering the structure of DNA.

DNA codes for protein.

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8. 1960’s: genetic code deciphered.9. 1970’s: description of how a gene

produces a protein (how nucleotide bases can create recipe for specific order of amino acids to build specific proteins.

10. 1990: scientists in United States organized Human Genome Project (HGP), an international effort to completely map and sequence the human genome

11. Feb. 2001 HGP published working draft of 3 billion base pairs of DNA.

DNA codes for protein.

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Gregor MendelModern genetics began in the mid-1800s in an abbey garden, where a monk named Gregor Mendel documented inheritance in peas.

Used experimental method. Collected data and counted

traits in peas.

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Pollen transferred from white flower to stigma of purple flower

anthersremoved

all purple flowers result

Mendel’s Work: Bred Pea Plants

F1

P

F2

self-pollinate

Cross-pollinated true breeding parents (P).

Raised seed and then observed traits (F1). F = filial (kids)

Allowed offspring to self-pollinate and observed next generation (F2- grandkids).

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F2generation

3:175%purple-flower peas

25%white-flower peas

Looking Closer at Mendel’s Work

P

100%F1generation(hybrids)

100%purple-flower peas

Xtrue-breedingpurple-flower peas

true-breeding white-flower peas

self-pollinate

Where didthe white

flowers go?

Whiteflowers came

back!

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What did Mendel’s findings mean? Traits come in alternative forms = alleles

Purple vs. white flower color

Different alleles vary in the sequence of nucleotides at the specific locus of a gene. Locus = gene location. Some difference in sequence of A, T, C, G

purple-flower allele & white-flower allele are two DNA variations at flower-color locus

different versions of gene at same location on homologous chromosomes

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What did Mendel’s findings mean? Some traits mask others.

Purple and white flower colors are separate traits that do not blend Purple x white ≠ light purple Purple masked white

Dominant Allele Functional protein. Masks other alleles. Represent with a capital letter (T).

Recessive Allele Allele makes a malfunctioning protein. Represent with a lower case letter (t).

homologouschromosomes

I ’ll speak for both of us!

wild typeallele producing

functional protein

mutantallele producingmalfunctioning

protein

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Genotypes Homozygous = same alleles = PP, pp Heterozygous = different alleles = Pp

homozygousdominant

homozygousrecessive

heterozygous

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Genotype vs. phenotypeDifference between how an organism “looks” & its genetics.

Phenotype Description of an organism’s trait. The “physical.”

Genotype Description of an organism’s genetic

makeup.

Explain Mendel’s results using…dominant & recessive …phenotype & genotype F1

P X

purple white

all purple

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Phenotype vs. Genotype2 organisms can have the same phenotype but have different genotypes.

homozygous dominantPPpurple

Pp heterozygouspurpleCan’t tellby lookin’

at ya!

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Mendel’s 1st Law of HeredityLaw of segregation.

During meiosis, alleles segregate. Homologous chromosomes separate.

Each allele for a trait is packaged into a separate gamete.

PP

P

P

pp

p

p

Pp

P

p

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Law of Segregation Which stage of

meiosis creates the law of segregation?

Whoa!And Mendel

didn’t even knowDNA or genes

existed!

Anaphase 1

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Monohybrid crossSome of Mendel’s experiments followed the inheritance of single characters.

Flower color. Seed color. Monohybrid crosses.

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Punnett squaresPp x Pp

P pmale / sperm

P

p

fem

ale

/ eg

gs

PP

75%

25%

3:1

25%

50%

25%

1:2:1

%genotype

%phenotype

PP Pp

Pp pp pp

Pp

Pp

F1generation(hybrids)

Aaaaah,phenotype & genotype

can have different ratios

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Monohybrid Cross PracticeIn corn, normal kernels are dominant to waxy kernels. If you crossed a waxy kernel plant to a heterozygous normal plant what type of seeds would be produced?