ap biology ch. 11 inheritance
TRANSCRIPT
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Warm up Match the items on the left with one item on
the right1. HH A. heterozygous2. Curly hair B. homozygous3. Hh C. phenotype4. Genotype D. tt
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Helpful
Crash Course Biology Hank Green
Bozeman Biology
Paul Anderson
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Inheritance
Ch. 11
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Main Topics Gregor Mendel’s work Mendel’s Laws Dominant/recessive Heterozygous/homozygous Alleles Codominance and incomplete dominance Epistasis, Pleiotropy, Multifactorial
Inheritance, Polygenic Traits
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The father of genetics
Gregor Mendel is considered the Father of Genetics
Born in 1822 Studied math &
physics at an Austrian university
He was the first person to study how traits are passed along from one generation to the next.
He did his work with the pea plant
Who’s your
daddy?
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Mendel’s Garden
Analyzed observable traits of peas growing in his monastery garden.
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Mendel’s Garden
Eight years & 20 volumes of data and analysis on 7 distinctive traits
Published in 1865
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Why peas?
The garden pea was a good choice for a variety of reasons. The garden pea: is easy to raise produces large numbers of offspring reproduces quickly has flowers which are self fertilizing
but can be easily crossed to other varieties
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Experimental Approach
Can also be cross-fertilized by human manipulation
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Mendel cross-fertilized true-breeding garden pea plants having clearly contrasting traits
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Allele for purple flowers
Homologouspair ofchromosomes
Allele for white flowers
Locus for flower-color gene
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Mendel's Theory of Segregation
Diploid organisms inherit two genes per trait
Each gene segregates from the other during meiosis so that each gamete will receive only one gene per trait
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How can the Chances of an Offspring’s Traits be Determined?
The chance of an offspring showing a certain trait can be determined by using the Punnett square.
The table contains spaces for the parent’s gametes and the possible offspring from that mating.
The alleles are represented by their letters. Genes come in pairs and must be separated during
gamete formation. These gametes (letter) of each pair are placed in
each of the outside spaces. They are then combined to form the possible
offspring.
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Punnett Square: Bb X Bb
bbBbb
BbBBB
bBGametes
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Monohybrid Crosses
Mendel's first experiments
One trait Monohybrid crosses
have two parents that are true-breeding for contrasting forms of a trait.
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All the offspring from the first cross showed only 1 form of the trait
This trait seemed “stronger” so he called it DOMINANT
When he crossed the offspring from the first cross, the other form of the trait reappeared, but only 1/4 of the time
This trait seemed “weaker” so he called it recessive
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Predicting the OutcomeWhy does one form of the trait
disappear in the first generation (F1 ),
only to show up in the second generation (F2 )??
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Artificial selection: populations could evolve (i.e. change) if members show variation in heritable traits
Variations that improved survival chances in the wild would be more common in each generation
This idea is known as natural selection
Prevailing Theories
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Mendel’s Experiments Natural selection did not fit with
prevailing view of inheritance-blending Blending would produce uniform
populations; such populations could not evolve
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Mendel’s Experiments Many observations did not fit
blending A white horse and a black horse did
not produce only gray horses
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Test (Back) Crosses To support his concept of
segregation, Mendel crossed F1 plants (Pp) BACK with homozygous recessives (pp)
What ratio would Mendel have gotten?He didn’t know the letter
combination of the F1 plants. The test (back) cross allowed him to figure it out
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Dominant phenotype,unknown genotype:
PP or Pp?
If PP,then all offspring
purple:
p p
P
P
Pp Pp
Pp Pp
If Pp,then 1
2 offspring purpleand 1
2 offspring white:
p p
P
Ppp pp
Pp Pp
Recessive phenotype,known genotype:
pp
His back crossed supported his idea of 2 “factors” for each individual, and the idea that those “factors” are segregated
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Dihybrid Crosses
Mendel also performed experiments involving two traits
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Predicting the OutcomeWhat is the predicted
PHENOTYPIC ratio
and the predicted
GENOTYPIC ratio that Mendel saw?
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Predicting the Outcome The F2 results showed
9/16 were tall and purple-flowered and 1/16 were dwarf and white-flowered-as were the original parents; however, there were 3/16 each of two new combinations: dwarf purple-flowered and tall white-flowered.
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OutcomesMonohybrid crosses
Both parents HETEROZYGOUS3:1 phenotype
Dihybrid crossesBoth parents HETEROZYGOUS
9:3:3:1 phenotype
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Theory of Independent Assortment
Each gene of a pair tends to assort into gametes independently of other gene pairs on non-homologous chromosomes
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Theory in Modern Form
Genes located on non-homologous chromosomes segregate independently of each other
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Practice with your neighbor
For the following questions Work with your neighbor to answer
the question. Answer the multiple choice
questionthen,
Use your notes to determine which one of Mendel’s principles it demonstrates
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1. A father carries 2 alleles for the gene for widow’s peak. He
carries one dominant allele and one recessive allele. His
gametes willa. All contain the dominant alleleb. All contain the recessive allelec. ½ will get the dominant allele and ½ will get
the recessive alleled. Each gamete will get both the dominant and
the recessive allele
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Which principle does question number one best
demonstrate? Principle of
Segregation
The dominant allele goes to one gamete and the recessive allele goes to another gamete
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2. A mother that is homozygous dominant for bushy eyebrows (BB) and heterozygous for round ears (Rr). The gametes she can make
will a. All have a B and a R in themb. ½ will have a B and ½ will have a R or
a r in themc. ½ will have a B and a R and ½ will
have b and rd. ½ will have B and R and ½ will have B
and r
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What principle does number 2 demonstrate?
The Principle of Independent Assortment
All gametes will have a B, since mom only has B.
The big B can be with the big R or the big B can be with the little r.
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3. In meiosis, a diploid cell divides twice to form 4 haploid
gametes. Each gamete contains:
a. A complete set of DNA identical to the parents
b. A ½ set of DNA, with just one copy of each chromosome
c. Homologous pairs of chromosomesd. Multiple copies of chromosomes,
depending on which ones moved during meiosis
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Which one of Mendel’s Principles does number 3
demonstrate? Principle of Segregation
All the homologous pairs of chromosomes separate so that there is just one of each pair in each gamete.
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4. When Mendel crossed a true breeding green pea plant (GG) with a true breeding yellow pea plant (gg), the offspring plants
werea. All greenb. All yellowc. ½ green and ½ yellowd. Green and yellow mixed
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Which one of Mendel’s principles does number 4
demonstrate? Principle of Complete Dominance
All offspring were Gg, and the dominant allele (G) masked the recessive allele (g)
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5. Mendel wanted to know if the color for pea seeds was linked to the shape of the pea seeds. He crossed a green, wrinkled seed
plant (Ggrr) with a yellow, smooth seed (ggRr) plant. The offspring
produced were:a. All green and wrinkledb. All yellow and wrinkledc. All green and smoothd. All yellow and smoothe. Some of each of the above
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Which one of Mendel’s Principles does number 5
demonstrate? Principle of Independent
Assortment
The green trait can go with the smooth or the wrinkled trait
The yellow trait can go with the smooth or the wrinkled trait
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Mendel’s Work
The work that Mendel did helped explain patterns of inheritance in eukaryotes.
But Mendel worked with traits that had a clear dominant/recessive pattern.
Also, the traits he worked with were all controlled by a single gene.
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Different Patterns of Inheritance As we now know,
many traits do not follow Mendelian Inheritance patterns.
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Degrees of Dominance Complete Dominance - BB and Bb =
same phenotype Incomplete Dominance - Bb has in-
between phenotype Codominance - Bb has both B and b
phenotype
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Co-dominance When both
alleles are expressed equally in the heterozygous individual.
A and B blood type alleles are co-dominant, because a person with AB genotype will have both A and B blood proteins.
Black and orange color in cats are co-dominant, because a heterozygous female will have both orange and black hair.
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Incomplete Dominance Both alleles are
blended together in the heterozygous individual.
Dominant allele cannot completely mask the expression of another
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Multiple Alleles
traits controlled by more that one gene (2 alleles) and so they have many different possible phenotypes.
These alleles can show dominant/recessive patterns or codominant patterns.
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Blood TypesGenotype of offspring
Phenotype of offspring
A
iAiB AB
iAi A
iAiA
iBiB B
iBi Bii o
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Rh factorRh factor Possible genotypes
Rh+
Rh-
+/+ or +/-
-/-
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So far we’ve only looked at how a single gene pair affects phenotype
More often - multiple genes involved 2 primary cases:
1. 2 or more genes affect a single trait 2. 1 gene affects the phenotype of
another gene
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Epistasis (standing upon)- 2 or more genes affect a single
trait Labs can be black, yellow, or chocolate
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Black is dominant to chocolate
BB and Bb = black bb = chocolate
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AND - another gene P codes for whether or not any pigment is put into the hair
PP and Pp = hair has pigment and dog will be black (BB or Bb) or brown (bb)
pp = no hair pigment and dog will be yellow, regardless of the “b” alleles
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So in this case, the P gene “stands upon” the B gene
P is epistatic to B We don’t get the classic 9:3:3:1 but some
other version of it
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Pleiotropy A single gene can
have multiple effects on phenotype
e.g. pleiotropic alleles --> multiple symptoms of sickle cell anemia (pain, jaundice, infections, fatigue, etc)
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Polygenic Inheritance 2 or more genes affect a single
phenotypic trait Eye color, skin color, height
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Skin color is controlled by at least 3 separate gene pairs
Genotype AABBCC would be very dark skin
Genotype aabbcc would be very light skin Any other combination would be
intermediate
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And, of course, skin color is also influenced by your environment - multifactorial inheritance
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X-linked traits
genes found on the X chromosome. show different inheritance patterns
in men than in women. X-linked traits may show
dominant/recessive or codominant patterns.
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X-linked traits Women have 2 X-
chromosomes, men have an X and a Y.
For women to express a recessive phenotype, they must inherit 2 X-chromosomes, both with the recessive allele.
For men to express the recessive phenotype, they need only 1 recessive X
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DNA in organelles DNA is also found in
mitochondria and chloroplasts.
Mitochondrial DNA is only passed from Mother to child.