Journal What is genetics? What is heredity?. Mendelian Genetics.
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Transcript of Journal What is genetics? What is heredity?. Mendelian Genetics.
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Journal•What is genetics? What is heredity?
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Mendelian Genetics
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Where does an organism get its unique characteristics?
• An individual’s characteristics are determined by factors that are passed from one parental generation to the next.
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• Every living thing has a set of characteristics inherited from its parent or parents.
• The delivery of characteristics from parent to offspring is called heredity.
• The scientific study of heredity, known as genetics, is the key to understanding what makes each organism unique.
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Genetics•The science of
heredity
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Heredity• The set of characteristics an
organism receives from its parents
• Not only the science of humans but also of many other organisms
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Gregor Mendel•Studied heredity carefully & objectively using pea plants
“Father of Genetics”
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• The modern science of genetics was founded by an Austrian monk named Gregor Mendel.
• Mendel was in charge of the monastery garden, where he was able to do the work that changed biology forever.
•
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– Mendel carried out his work with ordinary garden peas, partly because peas are small and easy to grow. A single pea plant can produce hundreds of offspring.
– Today we call peas a “model system.”
• Scientists use model systems because they are convenient to study and may tell us how other organisms, including humans, actually function.
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What did Mendel know?• Mendel knew that the male part of each
flower makes pollen, which contains sperm—the plant’s male reproductive cells.
• Similarly, Mendel knew that the female portion of each flower produces reproductive cells called eggs.
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–During sexual reproduction, male and female reproductive cells join in a process known as fertilization to produce a new cell.
– In peas, this new cell develops into a tiny embryo encased within a seed
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Pea Plants can reproduce through:
1. Cross-pollination
2. Self-pollination
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Self-Pollination• Pollen from a flower fertilizes the
egg cells on the same flower• A plant grown from a seed
produced by self-pollination inherits all of its characteristics from the single plant that bore it. In effect, it has a single parent.
• Offspring have one parent
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True-Breeding Stock• Always passes its characteristics
to the next generation• Produce offspring with identical
traits to themselves• Ex. True-breeding stock of pea
plants always produces tall plants with green pods
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Cross-Pollination• Pollen from one plant fertilizes eggs
from another plant
• Offspring have 2 parents
a.k.a. crossThis process, known as cross-
pollination, produces a plant that has two different parents.
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Trait• A specific characteristic of an
individual and may vary from one individual to another
• A characteristic that distinguishes one individual from another
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Mendel experimented using cross-pollination.
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Mendel Studied 7 Traits
• Seed shape
• Seed color• Seed coat
color
• Pod shape• Pod color• Flower
position• Plant height
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Hybrid• When two plants cross or
cross pollinate the offspring is called a hybrid
• HybridOffspring of parents with different characteristics
• Ex. Tall plant X short plant
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F1 Generation•The hybrids•F stands for filius meaning son in latin
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P Generation•True breeding generation
•“Parents”
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•Mendel crossed the F1 generation among themselves
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F2 Generation•Result of a cross between two F1 hybrids
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•From the F1 X F1 crosses Mendel discovered several rules concerning how traits were transferred from generation to generation
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Genes• The name given to different
traits of an organism• The unit that determines traits;
the traits of an organism
• Ex. If a plant is tall, it has a gene for tallness.
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•Mendel concluded from each of the traits that each pea plant must contain two traits—one from each parent
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Alleles•Different forms of a gene for a specific trait
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•Ex. The gene that determines height in pea plants has 2 alleles; one that produces a tall plant & one that produces a short plant
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• Some genes have only 2 alleles while others have dozens of different alleles
• Alleles are represented by either uppercase or lowercase letters
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Two Types of Alleles•Dominant Allele•Recessive Allele
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Dominant Allele
•Allele that will be expressed
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• Ex. If a plant has one allele for tallness & one for shortness, then the plant will be tall because the dominant allele is the allele for tallness & that will be expressed by the plant
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•Expressed by Uppercase Letters–Ex. Dominant allele for tallness: “T”
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Recessive Allele• Allele that will not be expressed• An organism with a recessive
allele for a trait will exhibit that form only when the dominant allele for that trait is not present.
• Represented by lowercase letters
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• If a plant has a tallness allele & an allele for shortness, the plant will be tall, the shortness allele would be recessive.
• The plant would be tall not short.
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•Example: Recessive Allele for tallness “t”
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Homozygous Alleles• Have 2 alleles for the same trait
• The organism will have a pair of identical alleles—either two dominant or two recessive
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•Homozygous Tall—TT •Homozygous short—tt •TT—homozygous dominant
• tt—homozygous recessive
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Heterozygous Alleles
•Have 2 opposite alleles•The organism will have one dominant & one recessive allele
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•Tt—heterozygous tall; the plant will be tall, but will carry one dominant & one recessive allele
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• Naming & labeling all the different kinds of alleles & gametes allowed Mendel to do something called a test-cross to determine how organisms would look after mating—called the crosses Punnett Squares
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Punnett Squares
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Punnett Square• A chart that illustrates Mendel’s test-crosses
between organisms• Punnett squares allow you to predict the
genotype and phenotype combinations in genetic crosses using mathematical probability.
• Allows one to determine how gene traits will result after two parent alleles have crossed
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Phenotype• The form of the trait that an
organism displays• A plant can express a phenotype
for either tallness or shortness no matter what pair of alleles it has. It may be homozygous tall ( TT ) or heterozygous tall ( Tt ), but the phenotype will still be “tall”
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•Think P for physical—what you see the organism has
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Genotype•An organism’s genetic composition
• It will give the actual gametes that make up the genetic trait.
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• A plant can express a phenotype for either tallness or shortness
• More specific than the phenotype
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•Tells which type of tall or short plant
• It will determine if a plant is homozygous tall or heterozygous tall
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–The genotype of an organism is inherited, whereas the phenotype is formed as a result of both the environment and the genotype.
– Two organisms may have the same phenotype but different genotypes.
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Mendel’s Laws (acquired from Test-crossing)
• Law of Unit Characteristics• Law of Segregation• Law of Independent Assortment
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Law of Unit Characteristics
• There are units in a cell that are responsible for traits, and these units come in pairs (otherwise known as alleles)
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•Each offspring he believed received one allele from each parent.
•Each gamete carries an allele.
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Law of Segregation
• States that the alleles for a trait separate when gametes are formed. The allele pairs are then randomly united at fertilization
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Law of Independent Assortment
• The principle of independent assortment states that genes for different traits can segregate independently during gamete formation.
• Genes for different traits can segregate independently during the formation of gametes
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•Helps account for the many genetic variations observed in plants, animals, and other organisms
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Monohybrid Cross
• a genetic cross containing ONE TRAIT
• (ex. The trait for tallness)
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How To Make a Punnett Square for a One-Factor Cross
– Write the genotypes of the two organisms that will serve as parents in a cross.
– In this example we will cross a male and female birds that are heterozygous for large beaks. They each have genotypes of Bb.
– Bb and Bb
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Determine what alleles would be found in all of the possible gametes that each parent could produce.
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– Draw a table with enough spaces for each pair of gametes from each parent.
– Enter the genotypes of the gametes produced by both parents on the top and left sides of the table.
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Fill in the table by combining the gametes’ genotypes.
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– Determine the genotypes and phenotypes of each offspring.
– Calculate the percentage of each. In this example, three fourths of the chicks will have large beaks, but only one in two will be heterozygous
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Example of a Monohybrid Cross
• In pea plants, round seeds ( R ) are dominant to wrinkled ( r ). In a genetic cross of two plants that are heterozygous for the seed shape trait, what are the phenotypic and genotypic ratios?
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Parents: Rr X Rr
RR Rr
Rr rr
R rR
r
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Genotypes:
¼ RR, 2/4 Rr, ¼ rr
Phenotypes:
¾ Round, ¼ wrinkled
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Dihybrid Cross•Crosses with 2 traits•From these crosses Mendel came up with the Law of Independent Assortment
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Example • In pea plants, round ( R )are dominant
to wrinkled(r). Also tall plants (T) are dominant to short (t). In a genetic cross of 2 plants in which one plant is heterozygous round, heterozygous tall (RrTt) and a second plant is homozygous recessive wrinkled, homozygous recessive short (rrtt), what are the phenotypic and genotypic ratio?
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Journal
In humans widow’s peak (W) is dominant over straight hairline (w). A heterozygous man for this trait marries a woman who is also heterozygous.
• a. List possible genotypes of their offspring.• b. List the phenotypic ratio for their
children.
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Exceptions to the Rules
• Incomplete Dominance• Codominance• Pleiotropy• Epistasis• Polygenic Traits
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Incomplete Dominance
•The appearance of F1 hybrids that appear half way in between the two parents
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Example• If a red flowering plant were
mated to a white flowering plant one would expect red or white to be the colors of the offspring. In this case all of the offspring were pink in color.
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• A cross between red-flowered snapdragons and white-flowered snapdragons produces offspring with pink flowers. Let R = red and W = white flowers. Suppose a pink-flowered plant is crossed with a pink-flowered plant, what are the genotypes and phenotypes of the offspring?
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Codominance•Both alleles of a gene are expressed without a recessive
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Example• If white feathers & black
feathers were both dominant traits in a chicken, then a heterozygous chicken (one with an allele for white & black feathers) will have both feather colors
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Pleiotropy• When a single gene influences
multiple phenotypic traits
• A new mutation in a gene will have an effect on all traits simultaneously
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Epistasis• The interaction between two or
more genes to control a single phenotype
• Occurs when more than one gene is needed to control one trait
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Polygenic Traits• Traits determined by multiple
alleles for a characteristic
• Ex. Skin color—there are 6 genes responsible