Unit ThreeUnit Three“Cell Proliferation and “Cell Proliferation and

Genetics”Genetics”““Foundations of Genetics”Foundations of Genetics”

Gregor Mendel and the PeaGregor Mendel and the Pea Gregor Mendel, a Catholic monk, was the first Gregor Mendel, a Catholic monk, was the first

to focus on using Quantitative observations to focus on using Quantitative observations (numbers) to better understand the (numbers) to better understand the mechanisms of hereditymechanisms of heredity

Prior to his initial research in the mid 1800’s Prior to his initial research in the mid 1800’s there were others before him who carried out there were others before him who carried out similar crossessimilar crosses

Because Mendel used numbers and kept very Because Mendel used numbers and kept very good records, his results were more reliable good records, his results were more reliable and less likely to be refutedand less likely to be refuted

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Gregor MendelGregor Mendel

Mendel’s Experimental Mendel’s Experimental SystemSystem

1. Mendel selected seven pairs of lines that 1. Mendel selected seven pairs of lines that differed in easily distinguishable traits differed in easily distinguishable traits (smooth versus wrinkled, green versus yellow (smooth versus wrinkled, green versus yellow etc.)etc.)

2. He knew from the anecdotal findings of 2. He knew from the anecdotal findings of previous researchers, he would be able to previous researchers, he would be able to quantify his resultsquantify his results

3. Pea plants are small, easy to grow, produce 3. Pea plants are small, easy to grow, produce many offspring, and mature quicklymany offspring, and mature quickly

4. Mendel carried out the fertilization process 4. Mendel carried out the fertilization process in a controlled environment (stop uncontrolled in a controlled environment (stop uncontrolled variables from negatively affecting results)variables from negatively affecting results)

Mendel’s Experimental Mendel’s Experimental DesignDesign

1. Mendel began by letting each variety self-1. Mendel began by letting each variety self-fertilize for several generations; this ensured fertilize for several generations; this ensured “True-Breeding”…producing same expression “True-Breeding”…producing same expression of trait repeatedly; these lines were called of trait repeatedly; these lines were called the “P Generation” (P = Parental)the “P Generation” (P = Parental)

2. Mendel then conducted his experiment: 2. Mendel then conducted his experiment: crossed two pea varieties exhibiting different crossed two pea varieties exhibiting different expression of a trait: green versus yellow expression of a trait: green versus yellow pea; the offspring that resulted were called pea; the offspring that resulted were called “F“F11 Generation” (First Filial) Generation” (First Filial)

3. Lastly, Mendel allowed the plants 3. Lastly, Mendel allowed the plants produced in the previous cross to self-produced in the previous cross to self-fertilize; the offspring that resulted were fertilize; the offspring that resulted were called “Fcalled “F22 Generation” (Second Filial) Generation” (Second Filial)

Mendel’s ObservationsMendel’s Observations For each pair of contrasting expressions For each pair of contrasting expressions

of a trait that Mendel crossed, he of a trait that Mendel crossed, he observed the same result: an expression observed the same result: an expression of a trait disappeared in the First Filial of a trait disappeared in the First Filial generation, but then reappeared in the generation, but then reappeared in the Second Filial generationSecond Filial generation

In the case of flower color, when he In the case of flower color, when he crossed purple and white flowers, all the crossed purple and white flowers, all the First Filial plants he observed were First Filial plants he observed were purple….Mendel called this “Dominance”, purple….Mendel called this “Dominance”, and he called the disappearance of the and he called the disappearance of the white color, “Recessive”white color, “Recessive”

Mendel’s ObservationsMendel’s Observations In the Second Filial generation, he observed In the Second Filial generation, he observed

there were some white flowers along with the there were some white flowers along with the predominant color of purplepredominant color of purple

The white color (Recessive) physically The white color (Recessive) physically disappeared from the First Filial generation, and disappeared from the First Filial generation, and then reappeared in the Second Filial generationthen reappeared in the Second Filial generation

Mendel determined the Recessive white flower Mendel determined the Recessive white flower color did not really disappear, but rather, was color did not really disappear, but rather, was not expressed in the First Filial generation not expressed in the First Filial generation

Mendel was beginning to realize the difference Mendel was beginning to realize the difference between “Phenotype” and “Genotype”between “Phenotype” and “Genotype”PhenotypePhenotype 3:13:1 Second Filial GenerationSecond Filial GenerationGenotypeGenotype 1:2:11:2:1

Mendel’s Mendel’s Hypotheses….TheoryHypotheses….Theory

Mendel proposed a series of hypotheses Mendel proposed a series of hypotheses that later came to be known as the that later came to be known as the “Theory of Heredity” (now Laws)“Theory of Heredity” (now Laws)– Hypothesis 1: Parents do not transmit traits Hypothesis 1: Parents do not transmit traits

directly but rather transmit coded information directly but rather transmit coded information that generates the expression of traitsthat generates the expression of traits

– Hypothesis 2: Each parent contains two Hypothesis 2: Each parent contains two copies of the trait, both can either be the copies of the trait, both can either be the same expression of the trait in question or same expression of the trait in question or differing expressionsdiffering expressions

Mendel’s Mendel’s Hypotheses….TheoryHypotheses….Theory

Hypothesis 3: Alternate expressions of a trait are Hypothesis 3: Alternate expressions of a trait are called “Alleles”; “Phenotype” is the physical called “Alleles”; “Phenotype” is the physical expression of the trait, “Genotype” is the allelic expression of the trait, “Genotype” is the allelic composition present which is not always the composition present which is not always the same as what is physically expressedsame as what is physically expressed

Hypothesis 4: The two alleles that an individual Hypothesis 4: The two alleles that an individual possesses do not affect each other/change possesses do not affect each other/change each othereach other

Hypothesis 5: The presence of an allele does not Hypothesis 5: The presence of an allele does not mean that allele will necessarily be expressed mean that allele will necessarily be expressed (remember Phenotype versus Genotype)(remember Phenotype versus Genotype)

Analyzing Mendel’s ResultsAnalyzing Mendel’s Results An individual receives one allele for An individual receives one allele for

each trait from each parent as a each trait from each parent as a result of Meiosisresult of Meiosis

Dominant alleles are designated with Dominant alleles are designated with capital letters, and recessive alleles capital letters, and recessive alleles are designated with lowercase lettersare designated with lowercase letters– Example:Example: Purple flower – “A”Purple flower – “A”

White flower – “a”White flower – “a”

Analyzing Mendel’s ResultsAnalyzing Mendel’s Results AA = Homozygous Dominant – PurpleAA = Homozygous Dominant – Purple

Aa = Heterozygous – PurpleAa = Heterozygous – Purple

aa = Homozygous Recessive - Whiteaa = Homozygous Recessive - White

Punnett SquaresPunnett Squares In a Punnett Square, the possible In a Punnett Square, the possible

gametes of one individual are listed gametes of one individual are listed along the horizontal side of the square, along the horizontal side of the square, while the gametes of the other individual while the gametes of the other individual are listed along the vertical sideare listed along the vertical side

Two types of crosses are completed Two types of crosses are completed using Punnett Squares: using Punnett Squares: – Monohybrid Cross – one trait focusMonohybrid Cross – one trait focus– Dihybrid Cross – two trait focusDihybrid Cross – two trait focus

Monohybrid CrossMonohybrid Cross

Dihybrid CrossDihybrid Cross

TestcrossTestcross To determine the genotype of To determine the genotype of

individuals, a “Testcross” is individuals, a “Testcross” is conducted:conducted:

Mendel’s LawsMendel’s Laws First Law = Segregation = two alleles First Law = Segregation = two alleles

of a trait separate from each other of a trait separate from each other during the formation of gametes, so during the formation of gametes, so that half of the gametes will carry one that half of the gametes will carry one copy and half will carry the other copycopy and half will carry the other copy

Second Law = Independent Second Law = Independent Assortment – genes located on Assortment – genes located on different chromosomes are inherited different chromosomes are inherited independently of one anotherindependently of one another

Mendel’s LawsMendel’s Laws

Facts about DNA & Traits Facts about DNA & Traits DNA makes RNA makes ProteinDNA makes RNA makes Protein Genes influence the phenotype (physical Genes influence the phenotype (physical

expression of trait) by specifying the types expression of trait) by specifying the types of proteins found in the body, which greatly of proteins found in the body, which greatly influences how well the body functionsinfluences how well the body functions

Mutations (changes) alter the DNA, which Mutations (changes) alter the DNA, which in turn, alters the expression of traitsin turn, alters the expression of traits

Mutations cause Evolution to occur, which Mutations cause Evolution to occur, which then leads to Natural Selection, then then leads to Natural Selection, then finally, Adaptationsfinally, Adaptations

The Exceptions to Mendelian The Exceptions to Mendelian Inheritance Inheritance

Polygenic Inheritance – expression of Polygenic Inheritance – expression of trait is determined by several genes trait is determined by several genes (segments of DNA), therefore, there (segments of DNA), therefore, there can be many variations of said trait; can be many variations of said trait; many genes affect one traitmany genes affect one trait

Pleiotropic Effects – an individual Pleiotropic Effects – an individual allele (one section of DNA on one allele (one section of DNA on one chromosome) has more than one chromosome) has more than one effect on phenotypeeffect on phenotype

The Exceptions to Mendelian The Exceptions to Mendelian InheritanceInheritance

Incomplete Dominance – alleles are Incomplete Dominance – alleles are neither dominant nor recessive and as neither dominant nor recessive and as a result, produce a heterozygote that a result, produce a heterozygote that is intermediate between both parents; is intermediate between both parents; example is a red flower and white example is a red flower and white flower produce a pink flower progenyflower produce a pink flower progeny

Codominance – often in heterozygotes Codominance – often in heterozygotes there is not a dominant allele; instead there is not a dominant allele; instead the effects of both alleles are the effects of both alleles are expressed; example is blood type: A, expressed; example is blood type: A, B, OB, O

Chromosomes are the vehicles of Chromosomes are the vehicles of Mendelian InheritanceMendelian Inheritance

Sex Linkage – a trait determined by a Sex Linkage – a trait determined by a gene on the sex chromosome; recall the gene on the sex chromosome; recall the sex chromosomes are the last two, and for sex chromosomes are the last two, and for humans that means they are the 45humans that means they are the 45thth and and 4646thth

A commonly used example that A commonly used example that demonstrates sex linkage is “Hemophilia”demonstrates sex linkage is “Hemophilia”

Hemophilia PedigreeHemophilia Pedigree

Recessive Chromosomal Recessive Chromosomal DisordersDisorders

Often, disorders/diseases manifest Often, disorders/diseases manifest themselves only when a person is themselves only when a person is homozygous recessivehomozygous recessive

A commonly used example is Sickle-A commonly used example is Sickle-cell Anemiacell Anemia

Dominant Chromosomal Dominant Chromosomal DisordersDisorders

Disorders/diseases that only manifest Disorders/diseases that only manifest themselves when someone is themselves when someone is homozygous dominant or homozygous dominant or heterozygousheterozygous

A commonly used example is A commonly used example is Huntington’s DiseaseHuntington’s Disease

Genetic Counseling and Genetic Counseling and TherapyTherapy

The older a person is, the higher the chances The older a person is, the higher the chances his/her children could have a genetic disorderhis/her children could have a genetic disorder

Also, a propensity toward a certain Also, a propensity toward a certain disease/disorder can “run in the family”disease/disorder can “run in the family”

For the above two reasons, it is recommended For the above two reasons, it is recommended that couples who are preparing to have that couples who are preparing to have children get genetic counselingchildren get genetic counseling

Once a woman is pregnant, ultrasounds and Once a woman is pregnant, ultrasounds and amniocenteses can detect genetic disordersamniocenteses can detect genetic disorders

DNA Screening with IVF patients….ethical DNA Screening with IVF patients….ethical questions!?!questions!?!