Unit 2: Biological basis of life, heredity, and genetics

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Summary1. Quiz info - Quiz next Wednesday, 3-22

2. Wrap-up Ch 3 - Cell Division

3. Abduction

4. Mendel's Principles of Inheritance

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Mitosis and Meiosis - types of cell division RecapMitosis-Somatic cells (i.e., body cells - skin, hair, muscle, etc.) are duplicated-ONE division produces cells with all 46 chromosomesResults: two daughter cells, genetically identical to parents and siblings

Meiosis-Gametes (sex cells like sperm and ova in humans)-TWO divisions produces cells with only 23 chromosomesResults: four daughter cells, not genetically identical

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Unlike mitosis, meiosis involves...-Gametes transmit genetic info from parent to offspring

Crossing over: chromosomes break and reconnect onto different chromosomes which results in...

Recombination: new combinations of genetic information is created-Every generation has new genetic combinations = additional variation

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NEW genetic information?Recombination makes it so new combinations of genetic information appears from generation to generation

But…

Mutations - Changes in the nucleotide sequence of DNA-Only way NEW genetic variation is introduced

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Natural selection and geneticsNatural selection - Traits making reproductive success more likely given environmental pressures will appear in higher frequencies from generation to generation.

Sources of Variation*Mutations - new genetic information*Meiosis (recombination) - new combinations of genetic information

Result: new variation is found in every generation of sexually reproducing populations

Now we know how the variation NS needs to act on is created. (Thanks science!)

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Mendel - 1860s Monk experimenting with peasRecall: Cross-breeding - artificial selection, farmers...metaphor for natural selection

Missing in Darwin's theory of NS: a mechanism governing how traits were inherited

Background

-Mendel cross-bred pea plants-Observed the traits individual plants possessed over thousands of generations

Inferred the Principles of Inheritance

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Mendel - ExperimentsExperiment: Selectively cross-breed common pea plants over many generations.

Parental generation: Mendel cross-breed purebreds

F1 generation: first offspring generationLater generations self-pollinated

Mendel observed: Some traits seen in offspring w/out blending of parent traitsE.g., Petals = either white or purple; seeds = either yellow or green - no inbetween

Observation: Cross-breeding plants w/ yellow seeds and plants w/ green seeds ALWAYS produced offspring with yellow seeds for F1.

BUT F2 = 3:1 ratio of yellow to green8

Mendel - Cross-breeding pea plantsMendel inferred:There must be a regularity governed by a mechanism of inheritance; specifically,

*Inheritance of each trait is determined by a 'unit' (gene) offspring receive from their parents unchanged

*Individuals inherit one 'unit' from each parent for each trait

*Traits might not be expressed in an individual but can still be passed on to the next generation

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Mendel - more inferencesParental generation = Homozygous yellow seeds + homozygous green seedsF1 = Heterozygous yellow seedsI.e., Each offspring inherited two different alleles (one from each parent)

Genotype - genetic makeup of an individual (e.g., YY, GG, YG, etc)

Phenotype - physical expression of an individual's genotype (e.g., yellow, green, tall, short, smooth, wrinkled, etc.)

Mendel inferred (some more): Whenever F1 breed each plant will have an equal chance of passing on either Y or G alleles

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Mendel - overview-Cross-bred pea plants for thousands of generations-different trait expressions controlled by discrete units (genes)

alleles - the different expressions of a gene

The principles of inheritance

1. Segregation - for a trait, the pair of expressions from each parent separate and only one passes from parent to offspring.Meiosis - NOW we know this principle is Meiosis

2. Independent Assortment - different pairs of alleles are inherited by offspring independent from one another.

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Mendel - inferences from observationsNote: Parent plants = pureblood = homozygous for seed color

I.e., each parent had identical expressions of the 'unit' (now gene) for this trait

Allele - Alternative forms/expressions of a gene

E.g., trait: seed color; expression: yellow or green. Y = yellow allele and G = green allele.

Parent 1 = YY

Parent 2 = GG

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Mendel's InferencesGenotype - genetic makeup of an individual (e.g., YY, GG, YG, etc)Phenotype - physical expression of an individual's genotype (e.g., yellow, green, tall, short, smooth, wrinkled, etc.)

Mendel observed: Some allele expressions dominated others.

E.g., Pea seed genotype = YG resulted in phenotype yellow so the dominant expression/form/allele = yellow (green is recessive)

E.g., Trait: height; alleles: tall T, short t

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Dominance and recessivenessRecessive - traits that are not expressed in heterozygotes

Dominance - traits that are expressed in heterozygotes AND homozygotes-these traits prevent the expression of recessive alleles in heterozygotes.

Alleles - the different expressions of a gene-genes = segments of DNA -> direct protein synthesis->found at different locus or loci of a chromosomeSince they are paired the dominant allele will be expressedHeight example

H = tall = dominant allele and h = short = recessive allele 14

Mendelian traits-discrete traits determined by alleles at a single genetic locus-they're either present or absent-allele frequencies of a trait in a given population

~20,000 Mendelian traits in humans - most biochemical

Dominant Mendelian traits = cleft chin, dwarfism; Recessive = Tay-Sachs disease, Phenylketonuria (PKU), albinism, sickle-cell anemiaRecessive disorders manifest if homozygous - if heterozygous, a person = unaffected but carrier

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Mendelian traitsMendelian traits - discrete traits determined by alleles at a single genetic locus

Dominant traits = cleft chin, dwarfism;

Recessive = Phenylketonuria (PKU), albinism, sickle-cell anemiaRecessive disorders manifest if homozygous - if heterozygous, a person = unaffected but carrier

More clear with discrete Mendelian traits but gets hairy when we look at the next trait type

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Polygenic TraitsPolygenic - traits influenced by genes at 2 or more loci

E.g., stature, skin, eye, and hair color

Continuous traits - gradiation of difference in several expressions

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Mendelian Traits vs Polygenic TraitsMendelian = discrete categories of variationPolygenic = continuous

Both -determined by Mendelian principles at specific loci-Dominance and recessiveness still a factor

NOTE: Mendelian traits = less likely affected by environmental factorsEx: ABO determined at fertilization and stays constant irrespective of environmental factors.

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Modern Evolutionary TheoryModern synthesis in the later 1920s-early 30s.

Evolution now defined in two stages

1. Variation - inherited differences among organisms is produced and redistributed through various processes

2. Natural selection acts on variation resulting in differential reproductive success (85p).

-Both mutations and natural selection contribute to evolution

Current definition of Evolution - Change in allele frequency from one generation to the next.

Allele frequencies = indicators of a group/population's genetic composition -Described as proportions or percentages of a total

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Things that produce and redistribute variation1. Mutation - any change in DNA - bases, chromosome number &/or structure-Also any alteration of an allele into another/of the a gene

-Random

Ex. HbS is a different allele of hemoglobin-Only affect evolution if they occur in sex cells - mutations lead to change only if they're inherited

Mutations are "the only way to produce new genes (that is, variation)" (86p).

Evolution solely due to mutation = rareOnly when combined with natural selection do we get significant/rapid evolutionary change

Ex. Point mutations - substitution of one base for another-disrupt protein production or cause production of defective proteins 20

Things that produce and redistribute variation2. Recombination - exchange of DNA segments b/w chrom. pairs during meiosis

Like mutations - Doesn't cause change in allele frequencies/evolution alone

BUT some genes are influenced by the alleles they're close to and recombination changes the composition of chromosome parts which further influences the ways certain genes function

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Mendelian traits in humans - ABO blood systemA, B, and O = alleles at the ABO locus on chromosome 9

Antigens - what the A/B/O alleles code for

Antigen A = genotype; blood type = phenotypeOnly B, then blood type = B

The O allele is recessive to both A and B-If type-O blood received - two copies of the allele from each parent (homozygous recessive)

If blood type A then either genotype AA or AOIf blood type B then either genotype BB or BO

Type AB = codominance - two different alleles present and both expressed on the surface of red blood cellsIn this situation, both alleles influence the phenotype

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Mendel's Experiments and observationsExperiments: Cross bred pea plants with different physical traits and then let them generate many populationsE.g., The trait of height of a pea plant can be expressed two ways: tall and shortCross breeding produced

F1 - all plants = tall

F2 = 3/4 tall and 1/4 short 3:1 ratio of tall:short, respectively.

Mendel's inferences from the datadifferent trait expressions controlled by discrete units(genes)

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Dominance and recessivenessRecessive - traits that are not expressed in heterozygotesDominance - traits that are expressed in heterozygotes AND homozygotes-these traits prevent the expression of recessive alleles in heterozygotes.

Alleles - the different expressions of a gene->found at different locus or loci of a chromosomeSince they are paired the dominant allele will be expressed

Height exampleH = tall = dominant allele and h = short = recessive allele

Occur in pairs and Mendel realized that this explained the pattern of inheritance from generation to generation

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Dominance and recessivenessRecessive - traits that are not expressed in heterozygotes-only expressed in homozygotesDominance - traits that are expressed in heterozygotes AND homozygotes-these traits prevent the expression of recessive alleles in heterozygotes.Alleles - the different expressions of a gene

Height exampleH = tall = dominant allele and h = short = recessive allele Occur in pairs and Mendel realized that this explained the pattern of inheritance from generation to generation

Genotype = HH, Hh, hh = an individual's combination of alleles

Phenotype = observable characteristic dictated by the genotypeHH, Hh = tallhh = short

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Mendel's Principles of Inheritance: Independent Assortment

Principle of independent assortment - The distribution of one allele pair into a gamete does not influence the distribution of another pair-New combinations of genes not seen in the parents are possible in the offspring

E.g., A pea plant's inheritance of the ability to produce yellow seeds over green seeds doesn't make the inheritance of white petals over purple petals more likely.

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Dominance and recessivenessRecessive - traits that are not expressed in heterozygotes-only expressed in homozygotesDominance - traits that are expressed in heterozygotes AND homozygotes-these traits prevent the expression of recessive alleles in heterozygotes.

Note: Dominant alleles are not "stronger" than recessive ones nor are they more common b/c natural selection favors them

Alleles - the different expressions of a gene

Height exampleH = tall = dominant allele and h = short = recessive allele

Occur in pairs and Mendel realized that this explained the pattern of inheritance from generation to generation

Genotype = HH, Hh, hh = an individual's combination of alleles

Phenotype = observable characteristic dictated by the genotypeHH, Hh = tallhh = short

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Cell Division - Complications in meiosis and Sex Chromosomes

Complications with meiosis

98% of newborns have correct numbers of chromosomes50% of pregnancies end in miscarriages. 70% of those miscarriages result from abnormal chromosome numbers.

-nondisjunction - homologous chromosomes [strands] fail to separate during meiosis - results: monosomy or trisomy Ex. Down syndrome (trisomy 21) occurs when chromosome 21 is copied three times in an individual, a phenomenon occurring 1/1000 births.

Sex chromosomes-nondisjunctions yield karyotypes XXY, XO, XXX, and XYY. Results in mental dysfunction, sterility, or lethal because - impossible to survive without an X chromosome.

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Mendel's Principles of InheritanceRecapitulate Mendel's inferences - two important principles

1. The principle of segregation

-Alleles occur in pairs because chromosomes occur in pairs-during gamete formation, members of each pair of alleles separate so each gamete contains one member of each pair-Only ONE allele is inherited - which one is inherited is due to chance

Recall: We now know segregation happens during meiosis

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Principles of Inheritance: Segregation

Principle of Segregation - for a given trait, allele pairs from each parent separate and one allele from each parent is inherited by the offspring.

Determined by chance.

Meiosis - NOW we know this principle is Meiosis

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Anthropology connectionsThe Human Genome Project was made possible. -sequenced all 30,000 genes humans have in their genome.

The Neandertal Genome has also been sequenced and same with the Chimpanzee genome...600 other species as well. -compare and contrast the characteristics found among the genomes in order to better understand our own evolutionary history.

Stem cells - Undifferentiated cells > able to divide and differentiate into other cell types (e.g., a cell that could become a blood, liver, or kidney cell)

Need more background but here’s a non-terrible video.https://youtu.be/evH0I7Coc54 31