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Genetics Chapter 11, 12, & 14
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Gregor Mendel’s Peas Genetics is the scientific study of heredity.
Heredity is the passing on of traits from parents to offspring Gregor Mendel Nickname: Father of Genetics worked with pea plants work was important to the understanding of heredity Mendel carried out his work with ordinary garden peas. self-pollinating: when sperm and egg come from same plant inherit all of their characteristics from a single plant cross-pollination: having pollen from one plant fertilize the egg of another plant True-breeding: organisms that only have one version of each trait Made by allowing the plants to self-pollinate for many generations
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Gregor Mendel’s Peas Fertilization: when sperm and egg cells join
to make a new cell Gene: a section of DNA that codes for a specific trait. A trait is a specific characteristic that varies from one individual to another.
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F1 generation: offspring of the P generation
parental generation, or P generation: organisms that you originally cross F1 generation: offspring of the P generation F2 generation: offspring of the F1 generation Mendel then cross-pollinated two P generation plants that showed different forms for the same trait (ex: one produced white flowers and the other produced purple flowers). The offspring from the cross are called the F1, or “first filial,” generation. Filius is Latin for “son” The offspring of crosses between parents with different traits are called hybrids. The F1 hybrid plants all had the character of only one of the parents (ex: all the offspring only produced purple flowers)
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Mendel’s F1 Crosses on Pea Plants
Genes and Dominance Mendel’s F1 Crosses on Pea Plants What does the P stand for? Parental What are the traits? Seed shape, seed color, seed coat color, pod shape What are the alleles for seed shape? Round and wrinkled What are the alleles for see color? Yellow and green What are the alleles for seed coat color? Grey and white What are the alleles for pod shape? Smooth and constricted Why did only one version show in the offspring (F1 generation)? Because that allele is dominant. What is the dominant allele for seed shape? Round
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Rule of Inheritance Each person has 2 copies of each gene, 1 from each parent. Alleles: versions of a gene determine your physical characteristics
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Principle of Dominance
The principle of dominance states that some alleles are dominant and others are recessive. Dominant, if present, always masks recessive Each of the traits Mendel studied was controlled by one gene that occurred in two contrasting forms that produced different characters for each trait. An organism with a dominant allele for a trait will always exhibit that form of the trait. An organism with the recessive allele for a trait will exhibit that form only when the dominant allele for that trait is not present.
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Dominant: allele physically seen or expressed if present; represented by a capital letter (A, B, T)
Recessive: only expressed if both alleles are recessive; represented by a lower case letter (a, b, t) allele may be present but not physically seen if the dominant version is present
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Mendel’s F1 Crosses on Pea Plants
Genes and Dominance Mendel’s Seven F1 Crosses on Pea Plants Mendel’s F1 Crosses on Pea Plants When Mendel crossed plants with contrasting characters for the same trait, the resulting offspring had only one of the characters. From these experiments, Mendel concluded that some alleles are dominant and others are recessive.
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Homozygous: organisms that have two identical alleles for a particular trait, true-breeding for that trait Ex: TT (Tall), tt (short) Heterozygous: organisms that have two different alleles, hybrids for that trait Ex: Tt (Tall), Yy (Yellow Seed)
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Genetics Terms Homozygous Dominant: organisms that have both dominant alleles for a particular trait Ex: TT (Tall), YY (Yellow Seed), WW (Widow’s Peak) Homozygous Recessive: organisms that have both recessive alleles for a particular trait Ex: tt (Short), yy (Green Seed), ww (Straight Line)
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Genotype: genetic makeup
Ex: heterozygous, Tt Phenotype: physical characteristics, determined by alleles/genotype Ex: Tall
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Genes and Dominance
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The plants have different genotypes (TT and Tt), but they have the same phenotype (tall).
Although these plants have different genotypes (TT and Tt), they have the same phenotype (tall). TT Homozygous Tt Heterozygous
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P (tt: short) X P (TT: tall) = F1 (Tt: tall)
Mendel then cross-pollinated two P generation plants that showed different forms for the same trait (ex: one produced white flowers and the other produced purple flowers). The offspring from the cross are called the F1, or “first filial,” generation. Filius is Latin for “son” The offspring of crosses between parents with different traits are called hybrids. The F1 hybrid plants all had the character of only one of the parents (ex: all the offspring only produced purple flowers)
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Law of Segregation The Law of Segregation states that the two alleles responsible for a trait separate from each other when gametes, or sex cells, are formed (meiosis). A gamete will receive one allele or the other. Alleles for a trait are then "recombined" at fertilization, producing the genotype for the traits of the offspring.
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Applying the Law of Segregation
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Mendel’s Original Cross P (tt: short) X P (TT: tall) = F1 (Tt: tall)
Mendel’s Second Cross (Two heterozygous plants) F1 (Tt: tall) X F1 (Tt: tall) = F2 (TT; Tt; tt; some tall some short) Mendel crossed the F1 generation with itself to produce the F2 (second filial) generation. The traits controlled by recessive alleles reappeared in one fourth of the F2 plants. Mendel assumed that a dominant allele had masked the corresponding recessive allele in the F1 generation. The trait controlled by the recessive allele showed up in some of the F2 plants. This indicated that at some point the allele for shortness had been separated, or segregated, from the allele for tallness.
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Mendel's F2 Generation F1 Generation F2 Generation P Generation Tall
When Mendel allowed the F1 plants to reproduce by self-pollination, the traits controlled by recessive alleles reappeared in about one fourth of the F2 plants in each cross. Mendel crossed the F1 generation with itself to produce the F2 (second filial) generation. The traits controlled by recessive alleles reappeared in one fourth of the F2 plants. Tall Short Tall Tall Tall Tall Tall Short
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Segregation Alleles separate during gamete formation.
During gamete formation, alleles segregate from each other so that each gamete carries only a single copy of each gene. Each F1 plant produces two types of gametes—those with the allele for tallness and those with the allele for shortness. The alleles are paired up again when gametes fuse during fertilization. The TT and Tt allele combinations produce tall pea plants; tt is the only allele combination that produces a short pea plant. Each F1 plant produces two types of gametes—those with the allele for tallness, and those with the allele for shortness.
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Heterozygous Segregation
What is the probability that the parent will give the offspring the dominant allele? 50% Organisms that are heterozygous for a trait have a 50% chance of passing on either the dominate or recessive allele to their offspring Parent Aa A a 50% Chance 50% Chance
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Genetics and Probability
The likelihood that a particular event will occur is called probability. The principles of probability can be used to predict the outcomes of genetic crosses.
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Probabilities Predict Averages
Probabilities predict the average outcome of a large number of events. Probability cannot predict the precise outcome of an individual event. Just because it was predicted to happen does NOT mean it will happen In genetics, the larger the number of offspring, the closer the resulting numbers will get to expected values.
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Punnett Squares
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Punnett Squares Punnett square: diagram used to predict and compare the gene combinations that might result from a genetic cross
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Types of Genetic Crosses
Monohybrid cross - cross involving a single trait e.g. flower color Dihybrid cross - cross involving two traits e.g. flower color & plant height
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Monohybrid Crosses
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t t Tt Tt T T Tt Tt P1 Monohybrid Cross Trait: Plant Height
Alleles: T – Tall t – short Cross: Tall plant x Short plant TT x tt Genotype: Tt Phenotype: Tall Genotypic Ratio: All alike Phenotypic Ratio: All alike t t Tt Tt T T Tt Tt
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P1 Monohybrid Cross Review
Homozygous dominant x Homozygous recessive Offspring all Heterozygous (hybrids) Offspring called F1 generation Genotypic ratio is ALL ALIKE Phenotypic ratio is ALL ALIKE 4 tall plants
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T t TT Tt T t Tt tt F1 Monohybrid Cross Trait: Plant Height
Alleles: T – Tall t – short Cross: Tall plant x Tall plant Tt x Tt Genotype: TT, Tt, tt Phenotype: Tall & short G.Ratio: 1:2:1 P.Ratio: 3:1 T t TT Tt T t Tt tt
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F1 Monohybrid Cross Review
Heterozygous x heterozygous Offspring: 25% Homozygous dominant TT 50% Heterozygous Tt 25% Homozygous Recessive tt Offspring called F2 generation Genotypic ratio is 1:2:1 Phenotypic Ratio is 3:1 3 tall plants to 1 short plan
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Independent Assortment
The principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes. Just because you get the tall allele doesn’t mean you’ll get the purple flower allele Today we know that it is the chromosomes that segregate independently. Independent assortment helps account for the many genetic variations observed in plants, animals, and other organisms.
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Independent Assortment
To determine if the segregation of one trait affects the segregation of another trait, Mendel performed a two-factor cross. Mendel crossed true-breeding plants that produced round yellow peas (genotype RRYY) with true-breeding plants that produced wrinkled green peas (genotype rryy).
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All of the F1 offspring produced round yellow peas (RrYy).
The alleles for round (R) and yellow (Y) are dominant over the alleles for wrinkled (r) and green (y). When Mendel crossed plants that were heterozygous dominant for round yellow peas, he found that the alleles segregated independently to produce the F2 generation.
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The Two-Factor Cross: F2
Mendel crossed the heterozygous F1 plants (RrYy) with each other to determine if the alleles would segregate from each other in the F2 generation. RrYy × RrYy
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In Mendel’s experiment, the F2 generation produced the following:
some seeds that were round and yellow some seeds that were wrinkled and green some seeds that were round and green some seeds that were wrinkled and yellow The alleles for seed shape segregated independently of those for seed color. This principle is known as independent assortment. These genes do not influence each other's inheritance. (example: just because you have blue eyes does not mean you will inherit blonde hair too)
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Dihybrid Cross: Gamete Formation
Traits: Seed shape & Seed color Alleles: R round r wrinkled Y yellow y green FOIL!!!!!!!!! RrYy x RrYy RY Ry rY ry RY Ry rY ry All possible gamete combinations
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Two Factor Cross: RrYy x RrYy
The Punnett square predicts a 9 : 3 : 3 :1 ratio in the F2 generation. When Mendel crossed plants that were heterozygous dominant for round yellow peas, he found that the alleles segregated independently to produce the F2 generation.
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Two Factor Cross RRYY x rryy
When Mendel crossed plants that were heterozygous dominant for round yellow peas, he found that the alleles segregated independently to produce the F2 generation.
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FOIL!!!!!! Dihybrid Cross TtRr x TtRr
Traits: Plant height & Seed shape Alleles: T tall & t short R round & r wrinkled FOIL!!!!!! TtRr x TtRr TR Tr tR tr TR Tr tR tr All possible gamete combinations
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Dihybrid Cross TR Tr tR tr TR Tr tR tr
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Dihybrid Cross TR Tr tR tr Tall/Round: 9 Tall/wrinkled: 3
short/Round: 3 short/wrinkled: 1 9:3:3:1 phenotypic ratio TR Tr tR tr TTRR TTRr TtRR TtRr TTRr TTrr TtRr Ttrr TtRR TtRr ttRR ttRr TtRr Ttrr ttRr ttrr
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A Summary of Mendel's Principles
Genes are passed from parents to their offspring. (Rule of Inheritance) If two or more forms (alleles) of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive (Law of Dominance).
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In most sexually reproducing organisms, each adult has two copies of each gene. These genes are segregated from each other when gametes are formed. (Law of Segregation) The alleles for different genes usually segregate independently of one another. (Principle of Independent Assortment)
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Human Traits Pedigree Charts
shows the relationships within a family and follows how a particular trait is passed down
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Human Traits A square represents A circle represents
a male. A circle represents a female. A vertical line and a bracket connect the parents to their children. A horizontal line connecting a male and a female represents a marriage. A circle or square that is not shaded indicates that a person does not express the trait. A shaded circle or square indicates that a person expresses the trait. This drawing shows what the symbols in a pedigree represent. Active Art: PHSchool.com Cpb-4141
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Sometimes there can be a half-shaded circle or square
Half-shaded = heterozygous Carrier = has one allele for the trait but does not show the trait (heterozygous)
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Pedigree checklist 1. Is the trait dominant or recessive? 2. Fill in the genotypes of the recessive individuals. 3. Give the dominant individuals 1 dominant letter. 4. Look at the parents and/or kids of the dominant people to figure out their other letter.
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The following is a pedigree chart showing the inheritance of a rare lung disease in a family.
Individuals who are homozygous dominant (AA) do not have the disease. Individuals who are heterozygous (Aa) do not have the disease but are said to be carries of the disease. Individuals who are homozygous recessive (aa) have the lung disease. What are the genotypes of the lettered individuals? aa Aa Aa AA or Aa
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Human Chromosomes Karyotype: A picture of a full set of an organisms chromosomes arranged in pairs Cell biologists analyze chromosomes by looking at karyotypes. Cells are photographed during mitosis. Scientists then cut out the chromosomes from the photographs and group them together in pairs.
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Human Chromosomes sex chromosomes: a pair of human chromosomes that determine gender Females = XX Males = XY Autosomes, or autosomal chromosomes: remaining chromosomes that do not determine gender
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Pre-Sorted:
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Post-Sorted: It’s A Boy!
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Mutations 12-4 Mutations
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Mutations are changes in the genetic material.
Mutations that produce changes in a single gene are known as gene mutations. Mutations that produce changes in whole chromosomes are known as chromosomal mutations.
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Chromosomal Mutations
involve changes in the number or structure of chromosomes include deletions, duplications, inversions, and translocations
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Deletions involve the loss of all or part of a chromosome.
Chromosomal mutations involve changes in whole chromosomes.
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Duplications produce extra copies of parts of a chromosome.
Chromosomal mutations involve changes in whole chromosomes. Movie: Duplication
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Inversions reverse the direction of parts of chromosomes.
Chromosomal mutations involve changes in whole chromosomes.
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Translocations occurs when part of one chromosome breaks off and attaches to another.
Chromosomal mutations involve changes in whole chromosomes. Movie: Translocation and inversion
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Chromosomal Disorders
Nondisjunction chromosomes fail to separate can cause a person to have more than 2 or less than 2 of a chromosome Meiosis I: Nondisjunction Nondisjunction causes gametes to have abnormal numbers of chromosomes. The result of nondisjunction may be a chromosome disorder such as Down syndrome. Movie: Nondisjunction Meiosis II
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Chromosomal Disorders
Down Syndrome Karyotype Down syndrome involves three copies of chromosome 21. This karyotype is from a person with Down syndrome. Down syndrome causes mental retardation and various physical problems. People with Down syndrome can, however, lead active, happy lives. Photo credit: ©Dr. Dennis Kunkel/CNRI/Phototake
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Sex chromosomes abnormalities
Human development more tolerant of wrong numbers in sex chromosome But produces a variety of distinct syndromes in humans XXY = Klinefelter’s syndrome male XXX = Trisomy X female XYY = Jacob’s syndrome male XO = Turner syndrome female
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Klinefelter’s syndrome
XXY male one in every 2000 live births have male sex organs, but are sterile feminine characteristics some breast development lack of facial hair tall normal intelligence
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Jacob’s syndrome male XYY Males 1 in 1000 live male births
extra Y chromosome slightly taller than average more active normal intelligence, slight learning disabilities delayed emotional maturity normal sexual development
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Trisomy X XXX 1 in every 2000 live births produces healthy females
Why? Barr bodies all but one X chromosome is inactivated How many Barr bodies would you expect?
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Turner syndrome Monosomy X or X0 1 in every 5000 births
varied degree of effects webbed neck short stature sterile How many Barr bodies would you expect?
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