genetics introduction lec

8/11/2019 Genetics Introduction Lec

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LECTURES On GENETICS

Chapter 14: Mendel and the gene idea

I. the basic rules of inheritance were first demonstrated by MendelA. at the time of Mendels work, most thought that parental traits were fluids that blend in offspringB. Mendel recognized that this model did not explain what he obseredC. Mendel chose a model system and carefully established testing conditions

1. he used pea plants that he could outcross or allow to self!fertilize2. he chose traits that had two clear possible outcomes "yellow or green seeds, etc.#

3. he established true!breeding or pure lines to use for genetic crossesD. terminology for genetic crosses

1. P generation (or P1)$ parental generation2. F1generation$ first generation offspring "from filial#3. F2generation$ second generation offspring4. phenotype% appearance or characteristic of an organism5. genotype% genetic makeup of an organism, determines phenotype6. gene% unit of heredity& controls a trait that determines a phenotype. lo!"#% the location of a particular gene on a chromosome$. allele#% alternatie ersions of a gene%. do&inant% allele that dominates oer others in determining phenotype1'. re!e##ie% allele whose phenotypic expression is hidden when a dominant allele is present11. hyrid% offspring from a cross between two pure lines of different, competing phenotypes

II. rules and terminology for examination of genetic inheritance

A. Mendels la* o+ #egregation1. when Mendel crossed pure lines of different, competing phenotypes, he found that the '(generation was uniform and

matched one of the parents phenotypes

example) *(yellow seed + green seed all '(yellow seed

2. when '(plants were crossed or selfed, the 'plants had both *(phenotypes in a ratio of roughly -)(

using offspring from aboe '(+ '('- yellow seed) ( green seed

3. thus, contrary to the popular belief of the time, recessie traits are not lost in a mixing of parental phenotypes % they aremerely hidden in some carrier indiiduals

4. Mendel explained these ratios with what we now call his la* o+ #egregation& stated in modern terms) indiidualsnormally carry two alleles for each gene, these alleles must segregate in production of sex cells

5. later inestigations of cell diision reealed the mechanism for segregation) the pairing and subseuent separation ofhomologous chromosomes during meiosis

B. genotype vs.phenotype

1. phenotype is the actual appearance or characteristic, and is determined by genotype& knowing the phenotype will notalways directly reeal the genotype "recessie traits can be masked#2. genotype is the listing of the actual alleles present& if you know the genotype, you should be able to predict the

phenotype

genotypes are either ho&o,ygo"#or hetero,ygo"#

ho&o,ygo"#% the homologous chromosomes hae the same allele at the locus in uestion hetero,ygo"# - the homologous chromosomes hae different alleles at the locus& if there is a dominant allele

the trait of the dominant allele will be expressed

the same letter is used to indicate all alleles "superscripts or subscripts are sometimes needed, if there are more

than alleles known#

/0MI1213 2445456 275 82*I324I95/& recessie alleles are lowercase

C. r"le# o+ proailitygoern genetic inheritance1. the likelihood of a sex cell carrying a particular allele is determined by probability, its expected freuency of occurrence

"expressed in fractions, decimal fractions, percentages, or ratios#

2. the combination of sex cells to form a zygote is generally ruled by probability as well3. thus, the rules of probability goern genetics4. prod"!t r"le% when independent but not &"t"ally e!l"#ie eent# are combined, you &"ltiplytheir indiidual

probabilities to get the oerall probability of the result "genetic crosses, +, are multiplications of probabilities#5. #"& r"le% if there is more than one way to obtain a result "&"t"ally e!l"#ie eent##, you addtheir indiidual

probabilities to get the oerall probability of the result

the sum of allpossibilities is one "no more, no less#

D. P"nnett #/"are% way of diagramming genetic crosses that uses the laws of probability0. more terminology

1. te#t !ro##% mating an indiidual that has the dominant phenotype for a trait with an indiidual with the recessiephenotype& this often will reeal the genotype of the dominant parent, or at least gie some idea of the probablygenotype

2. &onohyrid !ro##% cross between indiiduals that are both heterozygous for the gene that you are following& note that

these gie a -)( phenotype ratioand a ())( genotype ratioF. practice applying the law of segregation) following one gene in a cross1. 2 pea plant with yellow seeds is crossed with a pea plant with green seeds "*(generation#. 2ll (-( offspring "'(

generation# hae yellow seeds. :hat are the likely genotypes of the *(plants;2. 3wo of the '(plants from aboe are crossed. :hat are the expected ratios of phenotypes and genotypes in the '

generation;3. be sure to work some examples on your own& the textbook and website hae plenty of genetics problems % note how

they are typically presented as word problems and expect that format on your testIII. expanding the rules and terminology to follow two "or more# genes in a cross

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A. la* o+ independent a##ort&ent1. dihyrid !ro##% cross between indiiduals that are both heterozygous for two different genes that you are following2. when Mendel performed dihybrid crosses he found phenotype ratios of


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examples include height and skin color in humans

most economically important traits are polygenic "cow milk production, cattle weight, corn crop yield, etc.#

polygeni! trait#dont fall easily into distinct categories& instead, they usually are measured traits "/"antitatie

trait##

when plotted out for a population, polygenic traits produce a nor&al di#tri"tion !"reif mating is random with

respect to the trait. also note that genotype is not the only basis for phenotype % enironment can hae a maCor impact on what phenotype is

seen for some traits. /o all of these exceptions inalidate Mendels laws;

1. 1o. Mendels laws explain the basic situation, and all of these exceptions are best understood in light of themechanisms that Mendel described. 6cientists generally try to understand simple cases before moing on to themore baffling ones, and often "as here# understanding the simpler cases helps form the basis for understanding themore complicated ones.

FoweerGit is important to know about these exceptions and apparent exceptions, because most genetic

inheritance has some aspect of at least one of these exceptions in it.?. 2utosomal recessie genetic disorders

A. most genetic disorders are inherited as autosomal recessie traitsB. the recessie allele is usually a nonfunctional "or poorly functional# copy of a gene whose product is needed in metabolismC. much genetic research with model organisms "mouse, fruit fly, etc.# uses such traits to determine gene identities and

functionsD. gene therapyis considered to be a promising possibility for treatment of many of these disorders

1. the idea usually is to put a functional copy of the gene into critical body cells2. the problem is how to get the gene deliered to the cells where it is needed % sometimes a irus is used to infect cells,with the irus actually carrying and expressing the desired gene

3. in some cases, particularly if blood is inoled, it appears that blood stem cells may be able to be remoed from thepatient, tran#+or&ed"hae new genetic material inserted#, and then returned to the patients body

4. the most promising transformation mechanism uses embryonic stem cells and cloning

take cells from a discarded embryo "relatiely common from in itro fertilization# and remoe the nucleus

replace the nucleus with one from a putatie gene therapy patient, and grow lots of cells in culture

perform a techniue to the gene you want into the cells, then select for the cells that do what you want

grow those cells in culture, treat them with hormones that cause them to differentiate into the cell type that you

want, and put those cells into the patient5xamples in humans5. phenyleton"ria"*H#

most common in those of western 5uropean descent& occurs in about ( in (,JJJ human births in the .6. phenylalanine "an amino acid# is not metabolized properly, leading to a buildup of a toxic compounds that can lead

to seere mental retardation

treated with a diet that dramatically reduces phenylalanine consumption& potential gene therapy target

6. #i!le !ell ane&ia

most common in those of 2frican descent& about ( in KJJ of 2frican!2mericans hae it

caused by a mutation in hemoglobin that makes it tend to crystallize when oxygen is not bound to it

makes red blood cells take on a sickle shape, which can slow or een block blood flow through eins andcapillaries

can damage tissues due to lack of oxygen and nutrients, and is ery painful shortens lifespan of red blood cells, leading to anemia "low red blood cell count#

treatments hae increased life expectancy, including stimulating fetal hemoglobin production and bone marrow

transplants& work continues on gene therapy

the heterozygous condition actually leads to increased resistance to malaria, and thus is faored when malaria ispresent % about ( in ( 2frican!2mericans are heterozygous and thus carriers for sickle cell anemia

. !y#ti! +iro#i#

most common in those of 5uropean descent "in this group, about ( in KJJ births, with about ( in J

phenotypically normal, heterozygous carriers for the trait#

abnormal mucus secretions, particularly in the lungs, due to a defect in 8l !ion transport

life expectancy short "about -J years#& treatments are limited % has been a target for gene therapy trials

heterozygous carriers may be less likely to die from diarrhea!inducing diseases "based on mouse model studies

inoling cholera#?I. 2utosomal dominant genetic disorders in humans

A. seere dominant genetic disorders are not common, because they are usually are not passed on to the next generation"affected indiiduals usually die before they hae children#

B. those that do exist typically hae late onset of disorder symptoms "late enough for those with the disorder to hae had

children#C. the best known autosomal dominant disorder is "ntington di#ea#e"2H2 Funtingtons chorea, or F/#

1. occurs in about ( in (J,JJJ human births in the .6. "no heterozygous carriers % it is a dominant disorder#2. affects central nerous system, leading to seere mental and physical deterioration3. onset of symptoms usually in -Js or =Js4. one of at least < known trin"!leotide repeat di#order# in humans

F/ is caused by a gene with a L82N repeat of -O!(JJx or more "normal allele has O!-K of these repeats#& more

repeats usually means earlier onset

fragile + syndrome and myotonic dystrophy are two other examples of trinucleotide repeat disorders

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D. hyper!hole#terole&iais the most common dominant genetic disorder known "estimates are that as many as ( in KJJ haeit#& generally causes high cholesterol leels in the blood, leading to heart disease

?II. Methods of studying human inheritanceA. ethics must be considered in studies of human genetics

1. most genetic research inoles producing inbred lines and controlled genetic crosses2. since we cant "or shouldnt# really do that with humans, we must use other means3. isolated populations with typically large families are often used because they proide much inbreeding and many data

points

B. family pedigree analysis1. pedigree% a chart summarizing phenotypes andDor genotypes within a family oer seeral generations2. standard symbols for pedigrees)

generations are designated with capital roman numerals, starting with the oldest generation at the top

each generation gets one row, and genetic parents are connected by a horizontal line

males are suare, females round

each indiidual gets a number, going from left to right for each generation

a ertical line connects parents to their offspring

coloring is used to indicate phenotype "and, sometimes, known genotypes#

3. pedigree analyses only work well when a single locus is inoled in determining a phenotype "so!called Mendeliantraits#& still, many disorder genes hae been identified and characterized with the help of pedigree analysis "somehuman genetic disorders will be discussed later in this unit#

4. you need to be able to analyze pedigrees and determine which is the most likely mode of inheritance for a single!gene

trait among these choices) autosomal dominant, autosomal recessie, +!linked dominant, +!linked recessieC. aryotyping

1. many genetic problems occur on the large!scale, chromosomal leel2. studies of karyotypes are often done to test for such problems3. a karyotype display reeals the composition of chromosomes for an indiidual

a cell sample is taken "white blood cells, amniocentesis, chorionic illus sampling, etc.#

cells are grown in culture, and eentually treated to make chromosomes easy to photograph

the chromosome images are then analyzed and used to create the karyotype display

4. chromosomes are identified by size, position of the centromeres, and staining patternsD. h"&an geno&e proe!t

1. seuencing the human genome proides a means to greatly accelerate studies of human genetics

the underlying genetic causes for gene!based traits can be studied more easily "including traits that inole

multiple genes#

seuence ariations can be readily analyzed

more sophisticated genetic testing can be performed, leading to the potential for genetically tailored medical

treatment. "a complete draft of the human genome seuence "P- billion basepairs# was made public in 2prilJJ- Lcoinciding with the KJ thanniersary of the :atson and 8rick paper announcing the structure of /12N %there are P-K,JJJ genes in the genome, based on current interpretations of the seuence#

?III. enetic testing and screening in humansA. conclusie tests for many genetic disorders are now aailableB. especially with the completing of the seuencing of the human genome, more sophisticated predictie probability tests are

aailable, such as for alleles that are associated with higher rates of breast cancerC. although testing gies more knowledge, it has limitations "there are often at best limited treatments for the disorder, and in

some cases the test only tells you if you are more or less likely to hae a problem#& testing leads to many ethical issuesand concerns that are still being addressed

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genetics introduction lec

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