01 mendelian genetics class - wordpress.com...gregor (johann) mendel 1822 –1884. augustinian abbey...
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MendelianGenetics
Oral HistologyKristine Krafts, M.D.
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• Describe (in simple terms) Mendel’s basic research method, and discuss the results in the F1 and F2 generations. • List Mendel’s laws and explain what they mean in plain
English.• Define gene, allele, genotype, phenotype, homozygosity,
and heterozygosity.• Explain how autosomal dominant, autosomal recessive, and
X-linked recessive inheritance patterns work.• Using Punnett squares, show the percentages of affected,
unaffected, and carrier offspring for each of the above patterns.• Know how each inheritance pattern appears on a pedigree
(and be able to tell, if given a pedigree, which pattern of inheritance is present).
Objectives
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Mendelian Genetics
• Mendel, peas, and genes
• Walt, Teddy, and chromosomes
• Mendelian patterns of disease
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Mendelian Genetics
• Mendel, peas, and genes
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Gregor (Johann) Mendel1822 – 1884
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Augustinian Abbey of St. Thomas in Brno, Moravia, 1896.
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The Abbey today.
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In 1856, Mendel began investigating patterns of inheritance in garden peas.
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First, he established pea lines with two different forms of a feature – like tall height and short height.
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He grew each line for many generations until the line
was pure-breeding.
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Result:
One form of a feature always concealed the other form.
He called the visible form a dominant trait, and the hidden form a recessive trait.
Then, he bred the two lines with each other.
P (parental) generation
F1 (filial) generation(from Latin filius,
or son)
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Then, he allowed the plants to self-fertilize.
Result:
The hidden form reappeared in a small
number of plants –always in a 3:1 ratio!
F2 (filial) generation
F1 (filial) generation
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The results were always the same:F1 = 100% dominant, F2 = 75% dominant, 25% recessive
He studied seven plant traits using this method.
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In 1865, after experiments with almost 30,000 pea plants, Mendel presented his results to the Natural Science Society in Brno.
They published his findings as a paper, “Experiments in Plant Hybridization,” in their Transactions of the Natural Science Society in Brno journal.
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1. Features like flower color are controlled by pairs of heritable factors that come in different versions.
Mendel’s Model of Inheritance
genes
Segments of DNA that carry the
information for specific features
(like flower color).
alleles
Different versions of the same gene (one allele
carries the info for white flowers, another allele
carries the info for purple flowers).
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1. Features like flower color are controlled by pairs of heritable factors that come in different versions.
2. One version (the dominant form) can mask the other (the recessive form).
Mendel’s Model of Inheritance
That’s why this happens! Tall is dominant.
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So far so good!
F2
F1
P
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But what about this?
F2
F1
P
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1. Features like flower color are controlled by pairs of heritable factors that came in different versions.
2. One version (the dominant form) can mask the other (the recessive form).
3. The two paired factors separate during gamete production, and one gamete randomly receives one factor.
Mendel’s Model of Inheritance
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T t
T tT t
T tT
t
T
t
T T
t t
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T T
t tT t
T tT
T
t
t
T t
T t
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Mendel’s Laws
Law of SegregationWhen an organism makes gametes, each gamete randomly receives one of that organism’s two alleles.
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1. Features like flower color are controlled by pairs of heritable factors that come in different versions.
2. One version (the dominant form) can mask the other (the recessive form).
3. The two paired factors separate during gamete production, and one gamete randomly receives one factor.
4. Factors controlling different features are inherited independently of each other.
Mendel’s Model of Inheritance
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One feature (like seed color) did not affect the inheritance pattern of another feature (like seed shape).
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Mendel’s Laws
Law of Independent AssortmentThe alleles of two different genes get sorted into gametes independently of one another.
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Q. Then what happened?
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Q. Then what happened?A. Nothing! Complete silence.
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Q. Then what happened?A. Nothing! Complete silence.
Q. WTF?! Why?
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Q. Then what happened?A. Nothing! Complete silence.
Q. WTF?! Why?A. Because the “blending model” of
inheritance was widely accepted. Also, ego.
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• Mendel, peas, and genes
• Walt, Teddy, and chromosomes
Mendelian Genetics
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1902 paper 1903 paper
Chromosome Theory of Inheritance
“Genes are found on chromosomes. Chromosomes come in pairs,
one from mom and one from dad.”
“The way chromosomes behave during meiosis explains why genes are inherited according to
Mendel’s laws.”
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Mendel said genes come in pairs – one from mom, one
from dad.
Walt and Teddy said genes are located on
chromosomes, which also come in pairs – one from
mom, one from dad.
Mendel was right!
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Mendel’s law of segregation, explained!
Mendel:each gamete
gets one allele.
Walt and Teddy: homologous
chromosomes separate in meiosis, so each gamete gets
one chromosome (and one allele).
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Walt and Teddy: In meiosis, the members of different chromosome pairs are sorted into
gametes independently of one another (and therefore, so are their different alleles).
Mendel’s law of independent assortment, explained!
Mendel:different
alleles are sorted
independently.
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Quick Summary of Mendelian Genetics
• Each trait is encoded by a single gene.• Each trait has two forms, each encoded by a
separate allele.• One allele is always dominant, and the other is
always recessive.• The phenotypes are always predictable.
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Genotype Phenotype
Quick Summary of Mendelian Genetics
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• Mendel, peas, and genes
• Walt, Teddy, and chromosomes
• Mendelian patterns of disease
Mendelian Genetics
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Many diseases show Mendelian inheritance.
Assume that:
• The disease is caused by a single mutated gene.• The mutated allele (B) is dominant, and the
normal allele (b) is recessive.
Let’s look at how this works with a pretend disease that makes your skin blue.
Diseaseallele
(B)
Normalallele
(b)
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Let’s say there’s a guy with the disease, and we know he’s heterozygous.
His wife doesn’t have the disease.
How many of their kids will have the disease?
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B
Mom
Dad
b
b b
Bb Bb
bb bb
According to Mendel’s laws, 50% of the children will inherit the
dominant allele, and will have the disease.
Punnett square
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Bb
Bb Bbbbbb
bb
Autosomal dominant inheritance(same family as previous slide)
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PedigreeMale Female
Affectedmale
Affectedfemale
Autosomal dominant inheritance(same family as previous slide)
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Kindred = the patient’s extended family
Proband = the patient(also called propositus,
or index case)
Founders = the first mother and father of the kindred
Quick Review: How to Read a Pedigree
I
II
III
1 2
1 2 3 4
1 2 3 4
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Female
Quick Review: How to Read a Pedigree
Male
Fraternal twins
Identical twins
Sex unspecified Siblings
Mating
Consanguineous mating
Divorced
Dead
Aborted or stillborn
Affected
Heterozygous
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Mendelian Inheritance Patterns
Dominant Recessive
Autosomal Autosomal dominant Autosomal recessive
X-linked X-linked dominant X-linked recessive
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B
b
b b
Bb Bb
bb bb
Autosomal Dominant Inheritance Pattern
Dad(affected)
Mom(unaffected)
50% of children will be affected
Alleles:B = diseaseb = normal
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Affected person
Normalallele
Diseaseallele
Unaffected person
Autosomal Dominant Inheritance Pattern
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Autosomal Recessive Inheritance Pattern
B
b
B b
BB Bb
Bb bb
Dad(carrier)
Mom(carrier)
25% of children will be affected50% will be unaffected carriers
25% will be unaffected non-carriers
Alleles:B = normalb = disease
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Affected person
Normalallele
Diseaseallele
Unaffected person
Autosomal Recessive Inheritance Pattern
Carrier
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X-linked Recessive Inheritance PatternUnaffected Dad, Carrier Mom
X1
Y
X1 X2
X1X1
X1Y X2Y
Dad(unaffected)
Mom(carrier)
Alleles:X1 = normalX2 = disease
Daughters: 50% unaffected, 50% carriersSons: 50% unaffected, 50% affected
X1X2
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Diseaseallele
Affected personUnaffected person
Carrier
X-linked Recessive Inheritance PatternUnaffected Dad, Carrier Mom
Normalallele
![Page 57: 01 Mendelian genetics class - WordPress.com...Gregor (Johann) Mendel 1822 –1884. Augustinian Abbey of St. Thomas in Brno, Moravia, 1896. The Abbey today. In 1856, Mendel began investigating](https://reader034.vdocuments.mx/reader034/viewer/2022050109/5f46c0f57e706d4d8904aa98/html5/thumbnails/57.jpg)
Wait, why doesn’t a double dose of the X chromosome cause problems in women?
Or, put another way, why doesn’t it cause problems for men to just have one copy of the X chromosome?
![Page 58: 01 Mendelian genetics class - WordPress.com...Gregor (Johann) Mendel 1822 –1884. Augustinian Abbey of St. Thomas in Brno, Moravia, 1896. The Abbey today. In 1856, Mendel began investigating](https://reader034.vdocuments.mx/reader034/viewer/2022050109/5f46c0f57e706d4d8904aa98/html5/thumbnails/58.jpg)
Because X-inactivation!
46, In females, one X
chromosome is inactivated (it gets crumpled up into
a Barr body).
Early in embryonic development, each cell
randomly inactivates one X.
All the cells descending from each of these early cells
maintain the same pattern of X-inactivation.
46,
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= inactivated X chromosome (Barr body)
Genotype
Active chromosomesin somatic cells
Phenotype
Males Females
X-inactivation is easy to see in tortoiseshell cats
![Page 60: 01 Mendelian genetics class - WordPress.com...Gregor (Johann) Mendel 1822 –1884. Augustinian Abbey of St. Thomas in Brno, Moravia, 1896. The Abbey today. In 1856, Mendel began investigating](https://reader034.vdocuments.mx/reader034/viewer/2022050109/5f46c0f57e706d4d8904aa98/html5/thumbnails/60.jpg)
Can be used as a quick determination of sex
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X-inactivation is often called “lyonization” in honor of
Mary F. Lyon,the British geneticist who
discovered the process in 1961.
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X-linked Recessive Inheritance PatternAffected Dad, Unaffected Mom
X2
Y
X1 X1
X1X2
X1Y X1Y
Dad(Affected)
Mom(Unaffected)
Alleles:X1 = normalX2 = disease
Daughters: 100% carriersSons: 100% unaffected
X1X2
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Diseaseallele
Affected personUnaffected person
Carrier
X-linked Recessive Inheritance PatternAffected Dad, Unaffected Mom
Normalallele