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Intro to Genetics and Mendel Honors Biology Ms. Kim. Transmission (passing down) of Traits. How? One possible explanation of heredity is a “blending” hypothesis genetic material contributed by two parents mixes. Another Hypothesis. - PowerPoint PPT Presentation
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+Intro to Genetics and MendelHonors Biology
Ms. Kim
+ Transmission (passing down) of Traits
How?One possible explanation of heredity is a “blending” hypothesisgenetic material contributed by two parents mixes
+ Another HypothesisAn alternative to the blending model is the hypothesis of inheritance (genes) Parents pass on discrete heritable units (factors) called genes
http://wps.aw.com/bc_campbell_biology_7/29/7523/1925929.cw/nav_and_content/index.html The Novelty Gene Video
+ Gregor Johann Mendel (1843)
Austrian Monk- “Father of Modern Genetics”
Documented a mechanism of inheritance through his experiments with garden peas
The scientific study of heredity is called GENETICS!
Worked with pea plants in his monastery
Correctly believed that heritable factors (genes) retain their individuality from generation to generation i.e. – marbles (no blending of colors!)
Figure 14.1
+Gregor Johann Mendel Mendel used the scientific method to identify two laws of inheritance
Mendel discovered the basic principles of heredityBy breeding garden peas in carefully planned (CONTROLLED) experiments
+ Mendel’s Experimental Method Why did Mendel choose pea plants? available in many varieties of traits
They have seven distinct & observable traitseasy to gethe could strictly control which plants
mated with which Grow quickly
They reproduce quickly & have a short life cycleThey produce many offspring in one cross
Mendel’s Experimental CrossPurebred white
and purple flowers
White flowers reappear in
some offspring
Offspring were allowed to self
pollinate
What did Mendel notice? Did the trait for white flowers disappear in F1 generation?
A. All PurpleB. All WhiteC. Both
White/Purple
+Some genetic vocabulary
Character: a heritable feature, such as flower color
Trait: a variant of a character, such as purple or white flowers
+ Mendel
observed the same patternIn many
other pea plant characters
+ Pea Plant FertilizationSelf fertilization : mate with self produce identical offspringTRUE or PURE breeds
Cross fertilization : mate with another can produce different offspringHYBRIDS http://wps.aw.com/bc_campbell_biology_7/29/7523/1925929.cw/nav_and_content/index.htm
l Colored Cotton Video
+ Mendel’s ExperimentMendel only looked at “either-or” charactersEx: Purple OR white flowers
Mendel started his experiments with “true-breeding”Made through self fertilization so plants are “TRUE” for only 1 traitKnown as HOMOZYGOUS for trait
+ What was Mendel’s Procedure?
1. He made 14 “TRUE BREEDS” 1 for EACH trait
he looked at These are the
original parents Are called the
P generation
+ What was Mendel’s Procedure?2. He used cross fertilization to mate 2
true breeds for same gene Ex: Purple vs white flower color
3. He collected the offspring (progeny)
The hybrid (mixed) offspring of the P generation
Are called the F1 generation
+ What was Mendel’s Procedure?
4. He crossed (using cross fertilization) male and female from F1 progeny
When F1 individuals are mated together
The F2 generation is produced
+What did Mendel Discover?
A 3:1 ratio, purple to white flowers, in the F2 generationP Generation
(true-breeding parents)
Purpleflowers
Whiteflowers
F1 Generation (hybrids)
All plants hadpurple flowers
F2 Generation
Where did the white color go?
P F1 F2
+
17+Genetics Vocab
Mendel worked with his pea plants until he was sure that all were true-breeding varieties (pure bred)P generation parental generation have offspring called the F1 generation (hybrids)Hybrid (F1) the offspring of two true breeding varietiesIf F1 generations self-fertilize/cross, their offspring are called the F2 generation
+ What are Mendel’s factors?
Mendel’s “factors” are now called allelesAlternative version or form of a gene
Figure 14.4
Allele for purple flowers
Locus for flower-color gene Homologouspair ofchromosomes
Allele for white flowers
F
f
19+Mendel’s ExperimentsAfter studying pea plants, Mendel concluded that:
Traits are passed from one generation to the next through genes.
Each trait is controlled by a different form of a gene called an allele
Some alleles are dominant to others called recessive traits
New question: Have the recessive alleles disappeared or are they still present in the parents?
+ What did Mendel Conclude?
Mendel reasoned thatIn the F1 plants, only 1 factor (ex: purple flower) was affecting physical outcome color in hybrids
This factor was dominant and the hidden factor was recessive
+ Recessive is…Represented by a lowercase letter (it is
NOT the letter itself, though)an allele that does NOT produce a
characteristic effect when present with a dominant allele
only expressed when present with another (identical) recessive allele This is known as the homozygous
conditionaa or hh
+ Dominant is…Represented by a uppercase letter an allele that produces the same trait whether inherited with a another dominant allele (homozygous) or with a recessive allele (heterozygous)Aa or AA
The allele that is expressed if present
+ Frequency of Dominant Alleles
Dominant allelesNot necessarily better, stronger, etc. than recessive alleles
Ex: Polydactyl
24+Mendel’s Experiments Mendel crossed the first generation and saw that the
recessive trait showed up in about 1 of 4 plants. Conclusion: Law of Segregation! What is the Law of Segregation?!
Organisms inherit two copies of each gene (one from each parent) Organisms donate only one copy of each gene in their gametes (sex cells)
Therefore, the two copies of each gene segregate (separate) during gamete formation (meiosis)
+The Law of Segregation: Mendel’s 1st Law
Each gamete ONLY gets 1 allele
+ More about Alleles…
Each individual has 2 alleles for the same gene located on homologous chromosomes
Each parent passes 1 allele for each gene to his/her offspringIn sperm or egg
What stage of meiosis are alleles segregated into gametes?Meiosis Anaphase I
+
a A
28+Mendel’s Observations
Used pea plants to see patterns in the way various traits were inherited
Using his data, he saw that different traits are inherited separately Example: Green pea color isn’t always inherited
with wrinkled pea shape Green peas can be smooth and round too!
This is called the Law of Independent Assortment!
29+What is the Law of Independent Assortment?
Allele pairs (traits) separate independently of each other during gamete formation (meiosis) Different traits are inherited
separately Example – peas can be green and
wrinkled OR green and roundThis explains genetic diversity among organisms
+ Law of Independent Assortment:Mendel’s 2nd Law
Says genes are inherited independently of other genesGenes are not linked unless on the same chromosome!
Mendel assumed traits occur on different chromosomes!
Occurs during Metaphase I
+ Useful Genetic Vocabulary Homozygous
A pair of IDENTICAL (same) alleles for that gene
Exhibits true-breedingaa = homozygous recessive (or just recessive) HH = homozygous dominant
Heterozygous Pair of alleles that are different for that gene
Aa or Hh
+More Genetic Vocabulary
An organism’s genotypeIs its genetic (DNA) makeupA.k.a.-the allele combination (includes 2 alleles)
An organism’s phenotypeIs its physical outcome of the genotypeEx: blue eyes or AB blood type
+ Mendelian Genetics…aka- COMPLETE DOMINANCE
If an organism is heterozygous (Hh),The effect of the recessive allele is HIDDEN
Heterozygous and homozygous dominant have SAME phenotype
The 1st allele is “completely dominant” over the 2nd allele
+
Figure 14.6
3
Phenotype
Purple
White
GenotypePP
(homozygous)
Pp(heterozygous)
pp(homozygous)
Ratio 3:1 Ratio 1:2:1
Purple
Purple
1
1
1
2Pp
(heterozygous)
+Why Did Mendel Keep Getting the SAME results?
We can answer this question using a Punnett squarea diagram (box) used to predict probabilities of possible outcomes for offspring that will result from a cross between 2 parentsSHOWS EXPECTED RESULTS (not necessarily actual)
+Practice
PURPLE X WHITE
PURPLE
PURPLE & WHITE
Which flower color is recessive?A. WhiteB. PurpleC. NeitherD. Both
+Practice
PURPLE X WHITE
PURPLE
PURPLE & WHITE
Which flower color is recessive? WHITE
What would the genotype be for the recessive flower?
A. PP homozygous dominantB. pp homozygous recessiveC. Pp Heterozygous
+Practice
PURPLE X WHITE
PURPLE
PURPLE & WHITE
Which flower color is recessive? WHITE
What would the genotype be for the recessive flower? pp homozygous recessive
Which flower color is dominant?A. WhiteB. PurpleC. NeitherD. Both
+Practice
PURPLE X WHITE
PURPLE
PURPLE & WHITE
Which flower color is recessive? WHITE
What would the genotype be for the recessive flower? pp homozygous recessive
Which flower color is dominant? PURPLE
What would the genotype be for the dominant flower color?
A. PP homozygous dominantB. Pp heterozygous C. pp homozygous recessiveD. Both A & B
40+Genetics Vocab (pt 2)
Monohybrid cross cross where parents differ in only one trait (Rr x rr)
Dihybrid cross cross where parents differ in two traits (RrHh x rrHH)
Punnett square – a diagram that shows the gene combinations that might result from a genetic cross of two parents
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