CHAPTER 14CHAPTER 14MENDEL & the GENE IDEAMENDEL & the GENE IDEA

14.1 - Mendel’s 14.1 - Mendel’s DiscoveryDiscovery

The Law of Segregation The 2 alleles of a gene separate (segregate) during gamete formation, so that a sperm or egg only carries 1 allele of each pair Explains 3:1 ratio found in hybrid crosses

The Law of Independent Assortment Each pair of alleles segregates into gametes independently of other pairs Explains 9:3:3:1 ratio found in hybrid crosses

14.1 – Mendel’s 14.1 – Mendel’s DiscoveryDiscovery

Vocabulary Character – heritable feature Trait – variant of a character (heritable feature) True Breed – Whatever traits the parent has are expressed in ALL subsequent populations

For example, self-pollinating a purple flowered plant produces a generation of only purple flowered plants. Typically, need to do this for at least two (2) generations to ensure that the parent is a true-breed

Hybrid – Mating (crossing) of two (2) true-breeding varieties of true-breeds

14.1 – Vocabulary 14.1 – Vocabulary (Continued)(Continued)

P Generation – Parent generation Two (2) true-breeding parents being crossed

F1 – First filial (child or son) generation Hybrids

F2 – Second filial generation Each member of F1 self-pollinates Hybrids again 3:1 ratio and 9:3:3:1 ratio when 2 characters are considered

Dominant vs. RecessiveDominant vs. Recessive Dominant trait

One that will mask the recessive trait if found together Recessive trait

Trait that will be masked if found with the dominant trait

What Mendel Found…What Mendel Found…(stretched the truth about)(stretched the truth about)

Only looked at “all-or-nothing” traits Sometimes called binary traits – “yes” vs. “no”

Mendel took true breeding for 1 trait, and pollinated it with a true breed for another trait What is the difference between trait & character? Purple flowered + White flowered

F1 = All colored purple (all expressed only one trait) F2 = 3:1 ratio of one trait to the other

Quick ReviewQuick Review___ GenerationWhat type of plants?____________

___ GenerationWhat type of plants?____________

___ GenerationWhat type of plants?____________

Mendel’s ModelMendel’s Model Particulate theory of inheritance There are dominant and recessive forms of a particular character In F1 the dominant character masked the recessive character In F2 the recessive character reappeared, unaltered, from F1 Hence, no blending was occurring, just one trait (dominant) was masking the effects of the other (recessive).

Mendel’s Model (Page 2)Mendel’s Model (Page 2) 1. Alternative versions of genes account for variations in inherited characters

The alternative versions are called alleles One plant had the allele for purple flower color while the other had alleles for white flower color

Chromosomal Relationship Each gene resides on a chromosome DNA at that locus, however, varies in its nucleotide sequence (C, G, T, or A [U for RNA]) at that locus So the alleles are different nucleotide sequences on a chromosome

Even though the sequences are found @ the same locus

Alleles – Alternative Versions of a geneAlleles – Alternative Versions of a gene

Mendel’s Model (Page 3)Mendel’s Model (Page 3) 2. For each character, an organism inherits two (2) alleles, one from each parent

A locus is represented twice in a diploid cell Once on the chromosome from Mom, Once on the chromosome from Dad

The two alleles may be the same or different If the 2 alleles are the same = Homozygous Different = Heterozygous

If the alleles differ, then the dominant allele determines the organism’s appearance. If the other allele, recessive allele, has no noticeable effect on appearance

Law of SegregationLaw of Segregation The 2 alleles for a heritable character separate (segregate) during gamete formation & end up in different gametes This makes sense in terms of chromosome, we already know that homologous chromosomes assort independently during meiosis into gametes However, Mendel did his research far before the discovery or even the mere conception of chromosomes

Some more vocab…Some more vocab… Homozygous Dominant

2 copies of dominant allele Homozygous Recessive

2 copies of recessive allele Heterozygous

One copy of dominant allele, one recessive Genotype

Genetic Makeup Phenotype

Appearance

Phenotype vs. GenotypePhenotype vs. Genotype

Punnett SquaresPunnett Squares You know ‘em, & you love them!! Put one parent along the top, the other parent along the left side: Let us assume that we cross a homozygous dominant (PP) & homozygous recessive (pp):

P P

p Pp Pp

p Pp Pp

Figure 14.5 (Pp. 255)

Your Turn…. Your Turn…. Cross all 6 combinations:

Cross Genotype Phenotype

HomoD x HomoD

HomoR x HomoR

Heter x Heter

HomoD x HomoR

HomoD x Heter

HomoR x Heter

Pattern recognition…Pattern recognition… What pattern did you find in phenotypes for each of the crosses? What about the genotypes? What must be the parental genotypes?

If all have recessive phenotype If there is a 3:1 ratio of phenotype If you get half with dominant, and half with recessive If you get all dominant (careful with this one!)

Test CrossTest Cross

Def - Cross a recessive phenotype & Dominant phenotype We know the genotype of the recessive phenotype (must be HomoR) but we don’t know the genotype of the Dominant. If the cross produces:

ALL Dominant = ??? Dominant, Recessive =???½ ½

Monohybrid vs. Dihybrid CrossMonohybrid vs. Dihybrid Cross

Monohybrid Cross – Take pure breeds for 1 character and cross (AA x aa) Dihybrid Cross – Take pure breeds for 2 characters and cross (AABB x aabb) Dihybrid crosses =

F1 all hetero for both characters F2 9:3:3:1 for phenotype

Dihybrid CrossDihybrid Cross

How to explain the How to explain the results?results?

We already know that the alleles contained on individual chromosomes segregate from each other in meiosis BUT what about the traits themselves?

Do the traits segregate independently of each other or does their distribution depend on the other trait being crossed? The answer now is complicated, but Mendel chose (lied about them but who cares? ) traits that behaved independently

Law of Independent Law of Independent AssortmentAssortment

States that each pair of alleles segregates independently of other pairs of alleles during gamete formation This rule really only pertains to genes (allele pairs) on different chromosomes

That is, on non-homologous chromosomes If they are on homologous chromosomes, they are considered to be linked genes Cannot do Mendelian genetics, so we save this for next chapter (CH 15)

14.2 – Laws of Probability govern Mendelian 14.2 – Laws of Probability govern Mendelian GeneticsGenetics

The Multiplication & Addition rules apply to monohybrid crosses Multiplication rule – probability of a compound event is equal to the product of the individual probabilities of the independent events Addition rule – If an event that can occur in two or more independent, mutually exclusive ways, its probability is the sum of the individual probabilities

In multi-character crosses, consider each character separately and then multiply the probabilities together

14.2 – Laws of 14.2 – Laws of ProbabilityProbability Probability Basics

Probabilities range from 0 to 1 0 = NOT gonna happen 1 = It will happen

P(event) = Probability of an event occurring P(Tails on a fair coin) = ½ P(king of spades) = 1/52 P(king) = 4/52 = 1/23

Sum of probabilities of all possible outcomes must sum to 1 P(heads) + P(tails) = 1

Multiplication RuleMultiplication Rule If two (2) events occur but are independent of each other, then we can multiply their individual probabilities to get the compound probability P(both events). Example: P(Dolphins will beat Bills) = 0.5 & P(Jets will beat Steelers) = 0.5, THENP(both events) = 0.5x0.5 = 0.25

*** NOTE: these events MUST be independent

Multiplication Rule (AND Multiplication Rule (AND rule)rule)

P(head) = ½ So if I flip a coin twice, what is the P(heads on the first flip AND the second)? Are coin flips independent of each other?

In other words, does getting heads on the first flip affect the other coin flip? P(Both heads) = P(heads) x P(heads) = x ½ ½P(Both heads) = ¼

Addition Rule (OR rule)Addition Rule (OR rule) When there are 2 or more ways independent & mutually exclusive ways to get some result, you can add up the individual probabilities for the compound result. Example: P(Pats win Superbowl) = 0.5 & P(Fins win Superbowl) = 0.1, THEN

(Pats OR Fins win Superbowl) = 0.5 + 0.1= 0.6

Find P(aa) & P(Aa)Find P(aa) & P(Aa) P(aa) = P(“a” from mom) * P(“a” from dad)= * ½ ½= ¼

P(Aa) = P(Aa) + P(aA)= ( x ) + ( x ) ½ ½ ½ ½= + ¼ ¼= ½

Solving Complex Probability Questions with Solving Complex Probability Questions with these rulesthese rules

What is the probability of only two recessive phenotypes in a trihybrid cross of One parent is heterozygous for all 3 characters Other is hetero for flower color, but HomoR for the other 2 characters

Your Turn…Your Turn… What is the probability of getting 1 recessive phenotype in a dihybrid cross between 2 heterozygotes (hetero for both characters)

Characters: Y = yellow seed color, y = green seed color R = round seed shape, r = wrinkled seed shapeAnswer the problem both ways:1. Use compound probabilities to calculate this value2. Use Punnett square to determine the number directly

AaBbCc x AabbccP(Recessive Phenotype for at least two traits)?

P(aabbCc) = ¼ * ½ * ½ = 1/16P(aabbcc) = ¼ * ½ * ½ = 1/16P(aaBbcc) = ¼ * ½ * ½ = 1/16P(Aabbcc) = ½ * ½ * ½ = 2/16P(AAbbcc) = ¼ * ½ * ½ = 1/16

6/16 = 3/8

DiscussDiscuss

Detail at least 2 limitations of Mendelian Inheritance Give an example of each of these limitations How are these limitations accommodated or accounted for? Format

Use complete sentences Type up in Word or other Word processing program Send it to your email

14.3 – Extensions of Mendelian 14.3 – Extensions of Mendelian GeneticsGenetics

Mendelian genetics seems to be relevant to only a small set of heritable features For only a few characters there are…

Only 2 versions of an allele (green or yellow) 1 gene codes for a single external character 1 allele is completely dominant to the other

The basic patterns of segregation & independent assortment apply to more complex patterns of inheritance

Incomplete Dominance vs. CodominanceIncomplete Dominance vs. Codominance

Complete dominance Phenotype of heterozygote & HomoD are indistinguishable

Incomplete dominance Phenotype of heterozygote is in between the 2 Homo phenotypesExample: pink snapdragons

Codominance Phenotype of heterozygote is separate & distinguishable from HomoD & HomoR. Example: AB blood type

Example of Incomplete DominanceExample of Incomplete Dominance

Doesn’t incomplete dominance provide Doesn’t incomplete dominance provide evidence for “blending” theories?evidence for “blending” theories?

So incomplete dominance does NOT provide evidence for “blending” theories

Dominance & PhenotypeDominance & Phenotype The observed dominance/recessiveness of alleles depends on the level of the investigation Consider Tay-Sachs disease

Brain cells of the baby do not metabolize certain lipids As lipids accumulate, seizures, blindness, and mental degeneration Death occurs within a few years of conception

Tay-Sachs disease example (Page 2)Tay-Sachs disease example (Page 2)

@ the Organismal level, the disease is recessive Only children with 2 copies of the recessive trait will have the malady

Heterozygote is not afflicted – they produce some lipid-metabolizing enzyme, though not as much as in HomoD So intermediate enzyme production

This suggests that @ the biochemical level, the disease is an example of incomplete dominance Which is Tay-Sachs: dominance or incomplete dominance?

How common are dominant alleles?How common are dominant alleles?

Polydactyly Extra fingers or toes

1 of 400 in the US The allele for polydactyly is dominant, but rarely present Recessive homozygotes (HomoR) are found 399 out of 400 instances

Multiple AllelesMultiple Alleles Only 2 alleles existed for Mendel’s peas, but this is not typical for most traits Consider ABO blood group in humans

A refers to the “A” carbohydrate & type A blood B refers to (seriously, I’m not writing this down) O means neither A or B carbohydrate is found AB means both A & B are found

BUT the A and B alleles are codominant and are both expressed if an individual inherits both alleles

Blood Types Blood Types

ClumpingClumping

Nobel Prize, so it has to be good!?!Nobel Prize, so it has to be good!?!

http://nobelprize.org/educational_games/medicine/landsteiner/readmore.htmlThe Blood Type Game – Will u kill the patient?http://nobelprize.org/educational_games/medicine/landsteiner/index.html

PleiotrophyPleiotrophy Single gene has multiple effects Should be unsurprising given intricate molecular and cellular interactions for development of an organism

EpistasisEpistasis A gene at one locus alters the phenotypic expression of a gene at a second locus Example: Mouse fur color

Bb or BB = Black bb = brown If HomoR for (C) gene [cc], then no fur color (albino or white)

Regardless of fur color specified by brown-black gene If NOT HomoR for (c) gene [Cc or CC], then can be brown (bb) or black (Bb or BB)

Epistasis ContinuedEpistasis ContinuedWhat is the phenotype of BBcc? BbCc? bbCC? Bbcc? BBCC? bbcc?

Epistasis - Punnett Epistasis - Punnett SquareSquare

Polygenic InheritancePolygenic Inheritance The additive effect of 2 or more genes on 1 phenotypic character Called quantitative characters since there is a continuum of gradations Normal curve of phenotypes Example: human skin pigmentation is determined by at least 3 separately inherited genes AABBCC = Dark skin colored aabbcc = Light skin colored AaBbCc = Intermediate skin colored

Polygenic Trait – Skin Polygenic Trait – Skin ColorColor

Environmental ImpactsEnvironmental Impacts Skin pigmentation, and every other character, is affected by environmental factors as well

For example, sun tanning Hence, we say that a genotype is associated with (or codes for) a range of phenotypic possibilities

The phenotypic range is called the norm of reaction for a genotype For example, red blood cell count (part of standard screening protocol - CBC, Chem7[BMP7], UA)

Exercise = higher RBC count Altitude, presence of infectious agents, race, age = differential RBC

14.4 – Humans & Mendelian Inheritance14.4 – Humans & Mendelian Inheritance

Pedigree Analysis Family tree describing the interrelationships of parents & children across the generations

Pedigree AnalysisPedigree Analysis

Genetic DisordersGenetic Disorders Due Friday (11/13/09) For each of the following disorders1. What are the major characteristics of this disorder?2. What is the genetic basis of the disorder?3. Relate disorderly structure to dysfunction

Sickle Cell Anemia Tay-Sachs Huntington’s disease SCID Trisomy 211

Mendelian Genetics?Mendelian Genetics? Due Wednesday (11/11/09) For each of the following inheritance patterns1. How does each differ from typical Mendelian inheritance?2. How does Mendelian genetics explain this pattern of inheritance?3. Give an example 4. Relate genetic structure to the phenotype for each

Epistasis Codominance Incomplete Dominance Multiple alleles

ENDEND

Recessively Inherited Recessively Inherited TraitsTraits

Albinism to Cystic Fibrosis Only show up in individuals who have 2 copies of the recessive trait Heterozygotes are called carriers

Normal phenotype, but may transmit disease to offspring

Cystic FibrosisCystic Fibrosis Lethal disease Common for those of European descent 1 of 2,500 affected 1 of 25 are carriers Affects Chloride ion transport between a cell and extracellular fluid If untreated, most die before 5th birthday

Typically, patients live until their 20s or 30s with efficacious treatment

Sickle-Cell DiseaseSickle-Cell Disease Recessive autosomal disease Affects African-Americans and those of African descent Affects Hemoglobin protein in RBCs Low blood oxygen = hemoglobin molecules clump together forming sickle shaped RBCs Sickled RBCs may clump together creating chronic vascular occlusion of small vessels Example of incomplete dominance

Heterozygotes are usually normal but will show some symptoms during prolonged periods of reduced blood oxygenation

Consanguineous MatingsConsanguineous Matings Mating of close relatives Dramatically increases the probability of passing on recessive traits Most deleterious alleles have such severe effects that spontaneous abortion occurs long b4 birth

Dominantly Inherited Dominantly Inherited DisorderDisorder Most harmful alleles are recessive, but some human diseases are due to dominant alleles

Achondroplasia – form of dwarfism Heterozygous individual = dwarf

1 in 25,000 have achondroplasia, so 99% of the population are HomoR Hypothesis: if there is a lethal disease carried on a dominant allele, it would have burned out its carriers by now. UNLESS, the lethal disease carried by a dominant allele is one that affects organisms of advanced age

Like Huntington’s disease