mendel’s experiments heredity x genetics punnett...
TRANSCRIPT
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Chapter 2: Traits and How They Change
Section 2: Genetics
Heredity x GeneticsMendel’s experimentsPunnett Square
For the test: study the powerpoint and textbook
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1)What is Heredity? 2)What is genetics?
Heredity is the passing of traits from parents to offspring.
** Genetics is the branch of Biology that studies genes, heredity, and the variation of inherited traits in living organisms.
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Mendel – Austrian monk, lived in the 1800s – The Father of Genetics
▶ He was the first to describe the inheritance of traits in an offspring, by studying pea plants.
▶ Why pea plants?
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Pea plants reproduce quickly, sothe inheritance of traits can be observed in a short period of time
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▶ As a result of his observations, Mendel developed 3 basic principles of genetics
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Read pages 45-46 Mendel’s Principles of Heredity
▶ 1st Principle of Dominance▶ 2nd Principle of Segregation▶ 3rd Principle of Independent
Assortment
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Traits are determined by different factors.Each trait is determined by at least two factors. One factor is dominant over the other
Today we call factors genes and alleles
1st PrincipleMendel’s Principle of Dominance
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What are dominant and recessive alleles?
Dominant alleles will show their effect on the phenotype whenever they are present in the genotype. Recessive alleles will show their effect on the phenotype only when two of them are present in the genotype.
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2nd Principle:Mendel’s Principle of Segregation Mendel also concluded that for each trait: Organisms receive ONE allele from the father and ONE from the mother.
This allows for many different allele combinations AND explains why variation exists among the offspring of the same parents.
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Mendel’s 3rd principle :Law of Independent assortment
▶ Alleles for one trait have no effect on the inheritance of alleles of another trait
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Vocabulary not in the book!
Homozygous or Purebred – an organism that has two identical alleles for a trait
Heterozygous or Hybrid – an organism that have two different alleles for a trait
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**Representing alleles▶ Letters are used to represent alleles▶ Capital letters are used for dominant alleles and
lowercase letters are used for recessive alleles▶ Ex: short and tall – tall is dominant and short is
recessive▶ we always use the first letter of the dominant trait
TT homozygous or purebred = organism is tall tt homozygous or purebred = organism is shortTt heterozygous or hybrid = organism is tall (it has
the dominant allele, that will always show in the phenotype)
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Punnett Square
▶ Tool to predict the genotype and phenotype of an offspring
Ex: cross a pink flower with a white flower. Pink is dominant
▶ Steps:1) Represent alleles2) Do the Punnet square3) Interpret the results in % for the
genotypes and phenotypes
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More examples: Mendel’s experiments :
Mendel named: P - Parental generation F1 generation - the result of 1st crossing F2 generation - result of the 2nd crossing
****IMP: No matter what trait he was working with, Mendel observed that there was a pattern in the inheritance of traits.
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Discovering the Patterns:EX: Tall and short plants – tall is dominant
-Cross two homozygous for the dominant trait -Cross two homozygous for the recessive trait -Cross two heterozygous
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Discovering the Patterns: RESULTS BELOW ARE ALWAYS THE SAME NO MATTER THE TRAIT
-Cross two homozygous for the dominant trait = 100% with the dominant trait
-Cross two homozygous for the recessive trait = 100% with the recessive trait
-Cross two heterozygous = 75% with the dominant trait and 25% with the recessive trait
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Practicing Punnett Square
- Pea plants with different seed colorDominant – yellow seed – what is the genotype?Recessive – green seed – what is the genotype? 1)Homozygous yellow x homozygous green2)Heterozygous yellow x heterozygous yellow3)green x green4)Heterozygous yellow x green5)Homozygous yellow x homozygous yellowIdentify Genotypes and phenotypes for all crossesPage 47 – Applying math
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Non-Mendelian Inheritance – not in the book – use powerpoint to study
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▶ The inheritance of traits not always follows Mendel's rules
▶ Mendel studied traits determined by one gene with 2 pair of alleles and one allele is always completely dominant over the other
▶ This type of inheritance is called Complete Dominance
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▶ Incomplete Dominance ▶ Codominance ▶ Multiple Alleles▶ Polygenic Inheritance▶ Sex-Linked Traits
Nowadays we know that there are other types of inheritance:
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▶ traits determined by genes with alleles that are neither dominant nor recessive .
▶ The offspring show a phenotype that is a blend of the parents phenotype (pink flower)
Incomplete dominance- Snapdragon
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▶ Snapdragons:
A plant that is homozygous for the red allele will have red flowers
A plant that is homozygous for the white allele will have white flowers
A plant that is heterozygous will have pink flowers
Neither the red nor the white alleles are dominant, so the phenotype of the offspring is a blend of the two parents.
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Incomplete dominance- Snapdragon
Representing the alleles: use the upper case letter of the trait = colorCR CR – redCW CW – whiteCR CW – pink
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Incomplete dominance- Snapdragon
Punnett Square : CR CR – red versus CW CW – whiteCR CW – pink versus pink
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Incomplete Dominance in horses - color of the fur
CCCC
CWCW
CCCW
- Chestnut – homozygous- White – homozygous - Palomino – heterozygous
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Incomplete Dominance in horses
CCCC
CWCW
CCCW
- Chestnut - homozygous- White – homozygous - Palomino – heterozygous
Do a Punnett Square to cross: Palomino with PalominoPalomino with Chestnut
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Incomplete dominance in humansWavy hair – inherit curly hair from a parent and straight hair from another parent
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▶ Codominance is a type of incomplete dominance.
▶ Codominance would show both alleles equally. It is not a blend of the traits as is seen in incomplete dominance.
▶ alleles are represented as in Incomplete Dominance
Codominance
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Blood Type AB in humans is a case of CodominanceBlood types are: A,B,AB,O
Blood AB:A and B are proteins called antigens, present in the blood In Blood AB - both proteins are present in the phenotype. (not a blend)
worksheet
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▶ The trait is determined by a gene that has more than 2 alleles.
▶ Blood type is an example▶ There are:2 dominant alleles: alleles for type A and
alleles for type BOne recessive for type O
Multiple Alleles: another type of inheritance
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Blood type alleles and how to represent them:
i – recessiveIA and IB are Codominant(when present,both will show in the phenotype – Type AB)
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Cross type AB with Type O
50% A heterozygous50% B heterozygous
Cross heterozygous A with heterozygous BCross heterozygous A with recessive O
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▶ When traits are determined by more than one gene.
▶ EX: height, eye color, skin color ▶ More difficult to study because of the
wide variety of phenotypes (range) that the different gene combinations can produce
Polygenic Inheritance
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SKIN COLOR: 3 Genes and 6 Allelesthe skin gets lighter when more recessive alleles are present in the genotype
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Different genotypes and phenotypes
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▶ X and Y chromosomes determine the sex▶ Remember, if you have: ▶ 2 chromosomes X – female (XX)▶ 1 chromosome X and one Y – male (XY)▶
Sex-Liked traits - Remember
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▶ are traits linked with the X or Y chromosomes
▶ linked means that the chromosome has the allele for that trait, so the trait is inherited with the X or Y chromosome
▶ Sex linked traits are usually X-linked ▶ Y-linked traits – very rare▶ Ex of sex-linked traits: Color-blindness,
hemophilia – both recessive traits
What are Sex-Liked traits?
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▶ Conditions for the recessive trait to show in the phenotype:
Females need to be homozygous to have the disease, males have to have only one chromosome to be affected by the disease.
Sex-Linked traits to the X chromosome:
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The alleles in the Sex-linked traits ▶ EX: Hemophilia –bleeding disorder that slows
the blood clotting process – it is recessive
▶ To be hemophilic – women need two lower case h and man needs one lower case h
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The alleles in the Sex-linked traits ▶ EX: Colorblindness - it is recessive
▶ to be color blind – woman needs 2 lower case c and male needs 1 lower case c.
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What is a carrier?
Is an organism that carries the allele for a certain trait but does not have the trait expressed in the phenotype.
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