biology regents periods 2, 4 and 7. what is genetics? genetics is the scientific study of heredity
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Biology RegentsPeriods 2, 4 and 7
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What is Genetics?Genetics is the scientific study of heredity
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What is a Trait?A trait is a specific characteristic
that varies from one individual to another.Examples: Brown hair, blue eyes, tall,
curly
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What is an Allele?Alleles are the
different possibilities for a given trait.Every trait has at least
two alleles (one from the mother and one from the father)
Example: Eye color – Brown, blue, green, hazel
Examples of Alleles:A = Brown Eyesa = Blue EyesB = Green Eyesb = Hazel Eyes
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What are Genes?Genes are the sequence of DNA that codes for a protein and thus determines a trait.
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Gregor MendelFather of Genetics1st important studies of
heredityIdentified specific traits in the
garden pea and studied them from one generation to another
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Mendel’s Conclusions
1.Law of Segregation – Two alleles for each trait separate when gametes form; Parents pass only one allele for each trait to each offspring
2.Law of Independent Assortment – Genes for different traits are inherited independently of each other
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Dominant vs. RecessiveDominant - Masks the other trait; the trait
that shows if presentRepresented by a capital letter
Recessive – An organism with a recessive allele for a particular trait will only exhibit that trait when the dominant allele is not present; Will only show if both alleles are presentRepresented by a lower case letter
R
r
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Dominant & Recessive Practice
TT - Represent offspring with straight hair
Tt - Represent offspring with straight hair
tt - Represents offspring with curly hair
T – straight hairt - curly hair
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Genotype vs. PhenotypeGenotype – The genetic makeup of an
organism; The gene (or allele) combination an organism has. Example: Tt, ss, GG, Ww
Phenotype – The physical characteristics of an organism; The way an organism looksExample: Curly hair,
straight hair, blue eyes, tall, green
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Homozygous vs. HeterozygousHomozygous – Term used
to refer to an organism that has two identical alleles for a particular trait (TT or tt)
Heterozygous - Term used to refer to an organism that has two different alleles for the same trait (Tt)
RR
Rr
rr
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Punnett SquaresPunnett Square – Diagram
showing the gene combinations that might result from a genetic cross
Used to calculate the probability of inheriting a particular traitProbability – The chance
that a given event will occur
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Punnett SquareParent
Parent Offspring
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How to Complete a Punnett Square
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Y-Yellowy-white
Genotype:1:2:1
(YY:Yy:yy)
Phenotype:3 Yellow1 White
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You Try It Now!Give the genotype and phenotype for the
following cross: TT x tt (T = Tall and t = Short)
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TT x ttStep One: Set Up Punnett Square (put one parent
on the top and the other along the side)
T T t t
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TT x ttStep Two: Complete the Punnett Square
T T t t
Tt Tt
Tt Tt
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TT x ttStep Three: Write the genotype and phenotype
T T t t
Tt Tt
Tt Tt
Genotype: 4 - Tt
Phenotype:100% Tall
Remember: Each box is 25%
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You Try It Now!Give the genotype and phenotype for the
following cross: Tt x tt
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Tt x ttStep One: Set Up Punnett Square (put one parent
on the top and the other along the side)
T t t t
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Tt x ttStep Two: Complete the Punnett Square
T t t t
Tt tt
Tt tt
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Tt x ttStep Two: Complete the Punnett Square
T t t t
Tt tt
Tt tt
Genotype: Tt - 2 (50%)tt - 2 (50%)
Phenotype:50% Tall50% Short
Remember: Each box is 25%
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Some Terminology P1 – Original parents
F1 – First generation
F2 – Second generation
P1 X P1 = F1
F1 X F1 = F2
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Incomplete DominanceIncomplete Dominance - Situation
in which one allele is not completely dominant over another.Example – Red and
white flowers are crossed and pink flowers are produced.
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CodominanceCodominance - Situation in which
both alleles of a gene contribute to the phenotype of the organism.Example – A solid white cow is crossed
with a solid brown cow and the resulting offspring are spotted brown and white (called roan).
+
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Multiple AllelesMultiple Alleles- Three or more
alleles of the same gene.Even though three or more alleles
exist for a particular trait, an individual can only have two alleles - one from the mother and one from the father.
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Examples of Multiple Alleles1. Coat color in rabbits is determined
by a single gene that has at least four different alleles. Different combinations of alleles result in the four colors you see here.
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Examples of Multiple Alleles2. Blood Type – 3 alleles
exist (IA, IB, and i), which results in four different possible blood types
3. Hair Color – Too many
alleles exist to countThere are over 20
different shades of hair color.
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Multiple AllelesThere Are Always Multiple Alleles!
Genetic inheritance is often presented with straightforward examples involving only two alleles with clear-cut dominance. This makes inheritance patterns easy to see.
But very few traits actually only have two alleles with clear-cut dominance. As we learn more about genetics, we have found that there are often hundreds of alleles for any particular gene. We probably know this already - as we look around
at other people, we see infinite variation.
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Polygenic TraitPolygenic Trait - Trait
controlled by two or more genes.Polygenic traits often show
a wide range of phenotypes. Example: The wide range of
skin color in humans comes about partly because more than four different genes probably control this trait.