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MENDELS LAWS, MONOHYBRID AND DIHYBRID CROSSES Mrs. Stewart Honors Biology

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Bell work Describe the relationship between genotype and phenotype

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STANDARDS CLE 3210.4.1Investigate how genetic information is encoded in nucleic acids. CLE 3210.4.3 Predict the outcome of monohybrid and dihybrid crosses.

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OBJECTIVES Analyze the law of segregation Create a punnett square using the genotypes of parents Predict the outcome and probability of monohybrid crosses

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DECIDE WITH YOUR PARTNER

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REVIEW: WHAT IS HEREDITY? Why do children look like their parents? Why do brothers and sisters resemble each other? We inherit traits from our parents Heredity = the passing of genetic traits from parents to offspring

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TRAITS AND GENES Genes carry the instructions that define our traits Genes = segments of the DNA sequence that code for a particular trait Traits = genetically determined variations of characteristics (qualities) Example: natural hair color, eye color, skin tone, etc. The environment we live in can also help define our traits Example: a persons genes may code for a certain hair color, but exposure to dyes, chemicals, sunlight, etc can change that color Characteristic = can be altered by the environment

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DOGS Tell your CAT how to differentiate between a characteristic and a trait

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HOW DO WE INHERIT TRAITS FROM OUR PARENTS? Remember Meiosis?

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HOW DO WE INHERIT TRAITS FROM OUR PARENTS? Human body cells (somatic cells) have 2 complete sets of 23 chromosomes 2 x 23 = 46 chromosomes One set of 23 comes from sperm (Dad) One set of 23 comes from egg (Mom) Each parent contributes one complete set to the child, giving the child a mix of genes

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FERTILIZATION Fertilization one sperm fuses with an egg to form a zygote The zygote now has 2 sets of 23 chromosomes (46 total) This cell will begin dividing and will ultimately become a child. Zygote

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CATS Tell your DOG how we inherit traits from our parents

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PARENTS CONTRIBUTE ONE OF EACH CHROMOSOME PAIR TO THE CHILD On a karyotype, there are 2 chromosomes at each site. These represents the 2 chromosomes received from the parents. One from mom, one from dad. During meiosis, these will separate into different gametes (sex cells).

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MENDEL PROPOSED TWO LAWS These laws explain how the homologous chromosome pairs for each parent will separate into the gametes during meiosis. Law of segregation Law of Independent Assortment

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LAW OF SEGREGATION Homologous chromosomes separate during the formation of gametes

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DOGS Explain the law of segregation to your CAT

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LAW OF INDEPENDENT ASSORTMENT Allele pairs separate independently during gamete formation -which means that the transmission of traits to offspring are independent to one another.

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CATS Explain the law of independent assortment to your DOG

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SIBLINGS Since parents contribute chromosomes randomly, every child inherits a unique combination of traits. Some may resemble mom; some may resemble dad; others will be completely unique They may be resemble each other or be totally different.

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THINK PAIR SHARE CATS AND DOGS How can we predict the inheritance of traits? Punnett Squares

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PUNNETT SQUARES? Punnett Squares use genotypes to predict inheritance Punnett Squares show the law of segregation in action MomMom Dad What do the letters on the outside of the punnett square represent? What do each of the squares inside the punnett square represent?

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DOES IT MATTER WHICH SIDE YOU PUT THE PARENTS GENOTYPE ON? bb BBb B No, the results are the same.

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MONOHYBRID CROSS Monohybrid = a cross between two organisms that predicts the inheritance pattern/probability of only one characteristic at a time

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PRACTICE TOGETHER fur color B = brown fur b = white fur Which trait is dominant? Cross a homozygous dominant with a heterozygous dominant What are the genotypes of the parents? What percentage of the offspring will have white fur? Brown Fur = B BB and Bb BbBb B B BB Bb 0

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CHECK FOR UNDERSTANDING A one-eyed purple people eater is crossed with a two- eyed purple people eater. All of their offspring have two eyes. Which trait is dominant? Use the letter E or e to represent the alleles (variations) for this gene. What is the genotype of the offspring if you cross a purebred one-eyed purple people eater with a homozygous two-eyed purple people eater? What generation are the offspring of this cross part of? If you crossed the offspring with each other, how many of the resulting offspring would have two eyes? Two-eyed Ee F 1 3 out of 4 or 75 %

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RATIOS Ratio how much of one thing there is in comparison to another Genotypic ratio = ratio of possible genotypes in offspring of a cross Phenotypic ratio = ration of possible phenotypes in offspring of a cross

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KNOWN SHORTCUTS Homozygous dominant crossed with a homozygous recessive 100% of offspring will be heterozygous 4:0 ratio of heterozygous to other genotypes 100% of offspring will show dominant trait 4:0 ratio of dominant to recessive Example: Cross a BB x bb B = black fur; b = white fur Bb bb BBBB

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HETEROZYGOUS X HETEROZYGOUS Complete this cross - Bb x Bb Genotypic ratio: BB : Bb: bb = 1:2:1 Always! Phenotypic ratio: Dominant trait shown: Recessive Trait shown = 3:1 Always! BbBb Bb BB bb

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TEST CROSS Guinea Pig Fur ColorB = Black fur b = White fur What are the possible genotypes for a Guinea Pig with black fur? How can we determine which genotype is correct? Test Cross = cross an individual that expresses the dominant trait with a homozygous recessive individual to determine the genotype

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TEST CROSS If a black guinea pig is crossed with a homozgous recessive white guinea pig and even one of the offspring is white, what is the only possible genotype for the black guinea pig? Bb bb BBBB Bb bb b BbBb

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THREE TYPES OF DOMINANT RELATIONSHIPS FOR ALLELES Complete dominance Incomplete dominance Codominance

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COMPLETE DOMINANCE Standard dominant vs recessive relationship

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INCOMPLETE DOMINANCE One is not completely dominant over the other Instead, heterozygotes will have a blending of the dominant and recessive traits

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CODOMINANCE Both alleles for a gene are expressed in the heterozygotes simultaneously Neither allele is dominant or recessive, nor do they blend together

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DIHYBRID CROSSES A cross in which TWO characteristics are tracked

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USE F.O.I.L. METHOD TO DETERMINE GAMETES

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F pair the first two alleles of each characteristic O pair the outer two alleles I pair the inner two alleles L Pair the last two alleles for each characteristic F BLO Bl I bLL bl You do the next one on your worksheet

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HETEROZYGOUS X HETEROZYGOUS What are the gamete combinations for each guinea pig? Offspring Shortcut:9:3:3:1 phenotypic ratio Always! 9 Two dominant traits 3 One dominant one recessive trait 3 The other dominant and recessive combo 1 both recessive traits