what is genetics? - wordpress.com€¦ · what is an allele? allelesare the different possibilities...
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WhatisGenetics?�Genetics is the scientific study of
heredity (i.e. how heritable traits are passed on from generations to generations)
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WhatisaTrait?�A trait is a specific characteristic that varies
from one individual to another.� Examples: Brown hair, blue eyes, tall, curly
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WhatisanAllele?�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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WhatareGenes?�Genes are the
sequence of DNA that codes for a protein and thus determines a trait.
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GregorMendel�Father of Genetics� 1st important studies of
heredity� Identified specific traits in the garden pea
and studied them from one generation to another
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Mendel’sConclusions
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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Dominantvs.Recessive�Dominant - Masks the other trait; the trait that
shows if present� Represented 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 present� Represented by a lower case letter
R
r
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Dominant&RecessivePractice
�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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Homozygousvs.Heterozygous�Homozygous – 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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Genotypevs.Phenotype� Genotype – 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 looks� Example: Curly hair,
straight hair, blue eyes, tall, green
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PunnettSquares�Punnett Square – Diagram showing the
gene combinations that might result from a genetic cross
�Used to calculate the probability of inheriting a particular trait� Probability – The chance
that a given event will occur
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PunnettSquareParent
Parent Offspring
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HowtoCompleteaPunnettSquare
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Y-Yellowy-white
Genotype:1:2:1
(YY:Yy:yy)
Phenotype:3 Yellow1 White
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YouTryItNow!� Give the genotype and phenotype for the following
cross: TT x tt (T = Tall and t = Short)
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TTxttStep One: Set Up Punnett Square (put one parent on the top
and the other along the side)
T Tt
t
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TTxttStep Two: Complete the Punnett Square
T Tt
t
Tt Tt
Tt Tt
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TTxttStep Three: Write the genotype and phenotype
T Tt
t
Tt Tt
Tt Tt
Genotype: 4 - Tt
Phenotype:100% Tall
Remember: Each box is 25%
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YouTryItNow!� Give the genotype and phenotype for the following
cross: Tt x tt
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TtxttStep One: Set Up Punnett Square (put one parent on the top
and the other along the side)
T tt
t
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TtxttStep Two: Complete the Punnett Square
T tt
t
Tt tt
Tt tt
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TtxttStep Two: Complete the Punnett Square
T tt
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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SomeTerminology�P1 – Original parents�F1 – First generation�F2 – Second generation
�P1 X P1 = F1
�F1 X F1 = F2
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IncompleteDominance� Incomplete 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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Codominance� Codominance - 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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MultipleAlleles�Multiple 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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ExamplesofMultipleAlleles1. 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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ExamplesofMultipleAlleles2. 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 count� There are over 20
different shades of hair color.
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MultipleAlleles� There 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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PolygenicTrait� Polygenic 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.