Presentation on theme: "Introduction to Mendelian genetics"— Presentation transcript:
1Introduction to Mendelian genetics Who is Gregor Mendel?Introduction to Mendelian genetics
2TRUE OR FALSE?Girls inherit more traits from their mother than their fatherYou have inherited traits that are not apparentColor blindness is more common in males than femalesIdentical twins are ALWAYS the same sexA person can transmit genetic traits to their offspring which they themselves DO NOT showThe father determines the sex of a childThe total number of male births exceeds female births each yearAcquired characteristics, like mathematical skills, can be inheritedFraternal twins are more closely relates to each other than to other siblings
4GeneticsThe field of Biology devoted to understanding how characteristics are passed from parents to offspring
5Gregor MendelIn the 19thcentury, Mendel studied heredity-which is the transmission of characteristics from parent to offspringMendel is most famous for studying pea plantsHe studied what he called “factors” in pea plantsFactors would be things like tall or short (height), or yellow or green (pod color)
7Gregor Mendel First, Mendel grew true-breeding plants According to Mendel, true-breeding plants are plants that will always produce offspring with the same traitsSo a true-bred pea plant with purple flowers will only produce plants with purple flowers because it only has the “factors” for purple (not white).
8Gregor Mendel: P generation Mendel bred two opposite true-breeding plantsFor example, he bred a true-breeding purple flower pea plant and a true-breeding white flower pea plantHe called this his P generation – parent generation
9Gregor Mendel: F1 generation All of the offspring of the P generation (which he called the F1 generation) turned out purpleMendel called purple flower color the dominant factorHe hypothesized that when the dominant factor was present, the recessive factor(white color) did not show.
10Gregor Mendel: F2 generation Next, Mendel crossed the offspring from the F1 generation (he called this the F2 generation)–He observed that about 75% of the flowers were purple and about 25% were white–This is equal to about a 3:1 ratio
11Mendel P Generation(true-breeding parents) Purple White flowers F1 Generation(hybrids)All plants hadpurple flowersF2 Generation
13Mendel’s LawsKeep in mind that Mendel knew nothing of Punnett squares, genes, alleles, or even DNA!!!All he could do was observe phenotypes and record ratios and other statisticsHe came up with 2 important laws as a result of his observations.
14Mendel’s Laws: Law of Segregation Mendel concluded that each plant gets two factors (alleles) for a characteristic and when the plant reproduces, these two factors separate or segregate. So…Each gamete (sex cell) gives one factor (allele) AND therefore…Each offspring gets one factor from each parent
15Law of Segregation: Punnett Squares Alleles separateAlleles separate
16Mendel’s Laws: Law of Independent Assortment Mendel did experiments using more than one trait (like height and seed color)He noticed that one trait did not influence the inheritance of another traitIn other words, different factors separate independently of each other during the formation of gametes
17Mendel’s Laws: Law of Independent Assortment Examples:Pea plants can be short or tallTheir seeds can be green or yellowShort plants can have green or yellow seedsTall plants can have green or yellow seedsSo the inheritance of one does not affect the inheritance of the other.Mendel noticed this with all the traits he studied
19Mendel’s Laws Independent Assortment is not always true- If different genes are located on the same chromosome, then they will most likely be inherited togetherThese are called Linked Genes
20What were Mendel’s factors in reality? We call these alleles today–Alleles are alternative forms of a geneAlleles for flower color were purple and whiteThe characteristics (like height) are caused by genes on DNAGenes are segments of DNA that code for one proteinEach gene has 2 alleles, or versions (1 from mom and one from dad)
21What were Mendel’s factors in reality? The reason alleles come in pairs is because chromosomes come in pairs (homologous pairs)!!One allele on each chromosome!WHAT A COINCIDENCE!!!
22The Genetics of Mendel’s Experiments Some VocabDominant trait-masks the recessiveShown with capital lettersRecessive trait-only shows if dominant is not presentShown with lower case lettersPhenotype-physical appearanceFor example purple, wrinkled, tall, etc
23The Genetics of Mendel’s Experiments Some VocabGenotype-genetic makeupThis is usually abbreviated with letters like Gg, FF, or hhGenotypes for a trait are usually2 letters because you get 2 alleles (1 from mom and 1 from dad)Homozygous-two of the same alleles (like HH or hh)Heterozygous-two different alleles (like Hh)
24The Genetics of Mendel’s Experiments Mendel’s P generation had the genotypes FF (for purple) and ff (for white)True breeding is also homozygousFF is homozygous dominantff is homozygous recessive
25The Genetics of Mendel’s Experiments We can show the results Mendel observed using a Punnett Square:A Punnett Square shows possible genetic combinations in the zygotesMendel crossed his true breeding purple and white flower pea plantsWe write this as FF x ffLET’S DO THIS ON THE BOARD
27The Genetics of Mendel’s Experiments What Mendel did not know:All of F1 pea plant flowers heterozygous (two different alleles), or FfThat is why they were all purpleRemember dominant alleles mask recessive allelesSo with one purple allele present and one white, only purple would show as it is dominant
28F2 generation LET’S EXAMINE EACH RATIO FOR EACH CROSS: F2 Generation What genotypes do you start with?How are they crossed?What are your results?What is the genotypic ratio (genes)?What is the phenotypic ratio of purple (F) to white (f)?
29Punnett Square Examples Let’s do a Punnett square for BB x BbB= black fur in bunniesb= white fur in bunniesBlack fur is dominantWhat is the genotypic ratio?What is the phenotypic ratio?What are the chances for a white bunny?
30Punnett Square Examples Let’s look at a heterozygous crossBb x BbWhat is the genotypic ratio?What is the phenotypic ratio?What are the chances for a white or black bunny?
31Predicting the Results of Heredity What do these ratios and percents mean?If we flip a coin, there is a 50% chance that it will land on heads. But it is still possible to get 5 tails in a row (although it is highly UNLIKELY!)The more times you flip it, the more likely your results will be 50:50If Bb and Bb bunnies mate, there is a 1:4 chance the offspring will be white (this does NOT mean that they will or will not have white bunnies)If they have LOTS of children, about 25% of them will be white
32REMEMBER… Homozygous dominant means 2 BIG letters Heterozygous means one big one littleHomozygous recessive means 2 little lettersIf an organism shows the dominant trait, then the can be either heterozygous OR homozygous dominant
33Test CrossWhen genotypes are not known, a test cross can be performed to figure it outThe organism with an unknown genotype is crossed with a homozygous recessive individual.Test crosses are often used in breeding (like dog breeding) to determine is organisms are really “pure bred” (homozygous) for desired characteristics
34Test CrossProblem:Let’s say you want to breed black bunnies and you do not want any white bunniesWhat would be the only parents’ genotypes to produce black bunnies?BB x BBThere are 2 ways to know for sure which black bunnies are homozygous and which are heterozygous: expensive genetic testing, or test crosses
35Test CrossSolution:We take some black bunnies and mate them with white bunnies (homozygous recessive)Let’s look at the Punnett Square results to see the possible resultsRemember, black bunnies can be either BB or Bb
36Test Cross – Punnett Squares (try each cross) If a BB is crossed with bb, no white bunnies are producedIf a Bb is crossed with bb, then white bunnies may be producedIf a test cross produces white bunnies, we know the unknown genotype is Bb; if not the genotype is BBThe cross would be performed multiple times to be sure of the results
38Predicting Dihybrid Crosses When 2 traits are being looked at…Let’s do a cross between two heterozygous tall, heterozygous purple flowered pea plantsSo, TtFfx TtFfFor each plant, we now look at genotype for color and height
39Predicting Dihybrid Crosses Instead of 2 possible gametes, there will be 4So, the Punnett Square will be 4 x 4Phenotypic RatiosTall, purple : tall, white : short, purple : short, whiteKeep same letters together, capitals 1stYou will not be asked for genotypic ratios for dihybrid crossesWhat are the phenotypic ratios?LET’S DO IT ON THE BOARD
41Complex InheritanceMendel observed monogenic traits and no linked genes…It’s not usually that simple….
42Other Types of Inheritance Incomplete DominanceThe phenotype of the heterozygote is intermediate between phenotypes of the dominant and recessive traitsExample: when a homozygous red carnation is crossed with a homozygous white carnations, then pink carnations are producedWe usually don’t use lower case letters in this type of inheritance because nothing is really dominant
43Incomplete Dominance Let’s look at the cross on the board RR = Red RW= pinkWW= whiteLet’s look at the cross on the board
44Other Types of Inheritance CodominanceOccurs when both alleles for a trait are expressed in heterozygous offspringCodominant alleles are often symbolized with different letters
45Codominance BB = Brown BW= Roan WW= White Notice both brown and white are present in the heterozygous genotype
46Codominance LET’S EXAMINE THE PUNNETT SQUARE ON THE BOARD Roan x Roan BW x BWWhat are the ratios for each phenotype?
47Other Types of Inheritance Multiple Alleles:Genes with 3 or more alleles (or variations)Human blood type shows codominance and it has multiple alleles-A, B, and O
48Blood Type Human blood types have 3 alleles A, B, and O. Each person still only gets 2 alleles, but there are 3 possibilitiesO is recessive to A and B,A and B are codominant:Genotype AO or AA = A bloodGenotype BO or BB = B bloodGenotype OO = O bloodGenotype AB = AB blood (both alleles expressed)
50Terminology Genotype Heterozygous B BO Heterozygous A AO Homozygous recessive OOHomozygous A AAHomozygous B BBAB(technically heterozygous) AB
51Codominance Punnett Square LET’S EXAMINE THE PUNNETT SQUARE ON THE BOARDHeterozygous A with Heterozygous BAO x BOWhat are the ratios for each phenotype?
52Other Types of Inheritance Sex-Linked Genes and TraitsRemember sex chromosomes are the chromosomes that determine the sex of an organismSo these are traits/genes carried on sex chromosomesThese traits are symbolized using a superscript on the X or Y, such as Xr or XR
53Other Types of Inheritance Sex-Linked Genes and Traits Examples:In fruit flies, the gene for eye color is on the X chromosome. Red (XR) is dominant, white (Xr) is recessive.To have white eyes, females must have the genotype XrXr, or in other words TWO white allelesTo have white eyes, males must have the genotype XrY, or in other words ONE white alleleThis is why X chromosome sex-linked traits are more common in males
54Try the Punnett Square Homozygous red eyed female x white eyed male XRXR x XrYWhat are the ratios for each phenotype?
55Other Types of Inheritance Polygenic Inheritance:Traits that are controlled by more than one geneMost human traits are polygenicExamples are height, skin color, eye color, and hair color
57Other Types of Inheritance Complex Characters:Characters that are influenced by genetics AND the environmentSkin color and height are examples
58Other Types of Inheritance Sex-Influenced Traits:Traits in which males and females show different phenotypes even though they have the same genotypesBaldness is an example- it is dominant in men, but recessive in womenThe differences are mainly due to males and females producing different hormones (chemical signals)
59Other Types of Inheritance Single Allele TraitsTraits where there is only one alleleIf you have the allele you have the trait-there is no recessiveHuntington’s disease is an example
60Another way to show heredity…. PedigreesAnother way to show heredity….
61Pedigree is a chart or “family tree” that tracks which members of a family have a particular trait.
62Pedigrees In pedigrees, carriers have one copy of the recessive allele So they CARRY the trait, but they do not show itPedigrees can be used to make predictions aboutFuture offspringThe genotype of individuals in the pedigree
63PedigreesThe first pedigree tracks the widow’s peak, so the filled in shapes have a widow’s peakWidow’s peak is a dominant traitCarriers are not always shown on pedigreesThink about what alleles their parents can give them
64First generation(grandparents) WwwwwwWwSecond generation(parents plus aunts and uncles)WwwwwwWwWwwwThirdgeneration(two sisters)WWwworWwDominant trait (widow’s peak)ww= no widow’s peakWW= widow’s peak
65Chromosome MutationsChromosome mutations involve changes in the structure of a chromosome or the loss or gain of a chromosome.–Deletion: The loss of a piece of chromosome due to breakage–Inversion: A chromosomal segment breaks off, flips around, and reattaches-Missence: A change in chromosomal arrangement by insertion of DNA segment
66Translocation-A piece of chromosome breaks off and reattached to a nonhomologous chromosome
67Chromosome MutationsNondisjunction-When a chromosome fails to detach from its homologue during meiosis, so one gamete gets an extra chromosomeInstead of a haploid number (n) or diploid (2n), the gamete has 3 chromosomes (3n)
68Chromosome Mutations (Examples) Down’s syndromeNondisjunction of chromosome 21 in the egg cell produces 3 copies of chromosome 21Symptoms include: heart defects, stunted growth, mental retardationCystic fibrosisCan be caused by several mutations on chromosome 7 (insertion, missence)Symptoms include: problems with respiratory and digestive systems
69Chromosome Mutations (Examples) Klinefelter’s syndromeA male receives an extra X chromosome (XXY) because of nondisjunction of egg cellSymptoms: Boyish, rounded look (despite age) and often infertilityTrisomy 18 (Edward’s syndrome)Nondisjunction of chromosome 18 results in 3 copies of chromosome 18Much more severe problems than Down’s syndrome. Only 10% of births survive to their first birthday