Transcript Mendelian Genetics 2014

GENETICS
Gregor Mendel’s Discoveries
Complex Inheritance
Patterns of Inheritance
Gregor Mendel
• Gregor Mendel: 1843 - Augustinian monastery.
• University of Vienna - 1851 to 1853
• Experimentation in causes of variation in plants.
• 1857 - breeding garden peas to study inheritance.
• Many varieties, distinct heritable features (characters)
with different variants (traits).
• Mendel brought an experimental and
quantitative approach to genetics.
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• Hybrids
• The true-breeding parents - P generation
• hybrid offspring - F1 generation.
• Pollinate F1 hybrids - F2 generation.
• law of segregation
• law of independent
assortment.
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Law of segregation - the two alleles
for a characteristics are packaged
into separate gametes
• 705 purple-flowered F2 plants
• 224 white-flowered F2
• 3-1 ratio
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• Dominant
• Recessive
Table 14.1
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Mendel’s hypothesis to explain the results.
1. Alternative version of
genes (different alleles) account for
variations in inherited characters.
• Different alleles vary somewhat in the sequence
of nucleotides at the specific locus of a gene.
• The purple-flower
allele and white-flower
allele are two DNA
variations at the
flower-color locus.
Fig. 14.3
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2. For each character, an organism
inherits two alleles, one from each
parent.
• A diploid organism inherits one set of chromosomes
from each parent. - Each diploid organism has a pair
of homologous chromosomes (two copies of each).
• These homologous loci may be identical,
homozygous for that character (PP or pp)
• Alternatively, the two alleles may differ,
heterozygous for that character (Pp).
• Ex. - a plant can inherit a purple-flower allele from
one parent and a white-flower allele from the other.
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3. If two alleles differ,
• The dominant allele, is fully expressed in
the the organism’s appearance.
• The other, the recessive allele, has no
noticeable effect on the organism’s
appearance.
4. The two alleles for each character
segregate (separate) during gamete
production. (law of segregation)
• Distribution of homologous chromosomes to
gametes in meiosis.
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Review Questions
• Define P, F1, F2 generation.
• What does Mendel’s law of segregation
state?
• What are the four parts of Mendel’s law of
segregation?
• Define Homozygous and Heterozygous.
What is another way to say homozygous?
• Define Dominant and Recessive
Punnett square
•Predicts the outcome of a
cross between two
organisms.
•law of segregation - 3:1
ratio in F2 generation.
• F1 - two gametes:
• 1/2 purple-flower
allele
• 1/2 white-flower allele
• gametes unite randomly
• four combinations
Fig. 14.4
• Some vocab:
• description of traits - phenotype.
• The words that describe the character (Purple or
White)
• genetic makeup - genotype.
• The letters we assign to the character (PP or Pp
or pp)
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• For flower color in peas, both PP and Pp
plants have the same phenotype (purple)
but different genotypes (homozygous and
heterozygous).
• The only way to
produce a white
phenotype is to
be homozygous
recessive (pp)
for the flowercolor gene.
Fig. 14.5
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Test Cross
• It is not possible
to predict the
genotype of an
organism with a
dominant
phenotype.
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• Dominance does not mean more common in a
population.
• Polydactyly is due to an allele dominant to the
recessive allele for five digits per appendage.
• The recessive allele is far more prevalent than
the dominant allele in the population.
• 399 individuals out of 400 have five digits per
appendage.
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Law of independent assortment
• Single character cross - monohybrid cross.
• Two different characters - dihybrid cross.
• Mendel crossed true-breeding plants that had
yellow, round seeds (YYRR) with true-breeding
plants that has green, wrinkled seeds (yyrr).
• The allele for yellow seeds (Y) is dominant to the allele for
green seeds (y).
• The allele for round seeds (R) is dominant to the allele for
wrinkled seeds (r).
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• two characters are not transmitted from
parents to offspring as a package.
• The Y and R alleles and y and r alleles do not
stay together.
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• Two pairs of alleles segregate independently of
each other.
• The presence of one specific allele for one trait has
no impact on the presence of a specific allele for the
second trait.
• Four distinct
phenotypes in a
9:3:3:1 ratio.
• law of independent
assortment - alleles
must be on different
chromosomes
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Fig. 15.1
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Complete Dominance (recap)
• Mendel used traits that were classified as
complete dominance.
• Each character (but one) is controlled by a single
gene.
• Each gene has only two alleles, one of which is
completely dominant to the other.
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Check in?
• Define Genotype and Phenotype
• What do we use a Punnett square for?
• Define Mendel’s Law of Independent
Assortment
Beyond Mendel
• We know that the relationship between
genotype and phenotype is rarely as simple
as dominant and recessive.
• Mendel was never able to prove these…
• Incomplete dominance
• Codominance
• Multiple Alleles
• Polygenic inheritance
• Sex-linked traits
Incomplete Dominance
• Heterozygotes show a distinct phenotype,
not seen in homozygotes.
• The dominant trait is not
completely dominant over
the recessive.
• Result is a mixed phenotype.
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Codominance
• Two alleles affect the phenotype in
separate, distinguishable ways.
• There are two dominant alleles that both
contribute to phenotype.
• MN blood groups - due to the presence of two
specific molecules on the surface of red blood
cells.
• (genotype MM) have one type of molecule on
their red blood cells, (genotype NN) have the
other type, MN (genotype MN) have both
molecules.
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Multiple Alleles (and codominance)
• There are more than 2 alleles that
contribute to genotype.
• The ABO blood groups in humans are
determined by three alleles, IA, IB, and i.
• Both the IA and IB alleles are DOMINANT to
the i allele
• The IA and IB alleles are codominant to each
other.
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• Because each individual carries two alleles, there are six possible
genotypes and four possible blood types.
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RBC Surface Antigens
• Antigens are proteins found on the surface of a cell
Rh Factor with blood typing
• What about the + and – blood types?
• Rh factor is what determines the + or – blood
typing.
• Alleles= Rh+ and Rh-
Stop and Review
• A woman with type A+, whose dad was O-, has a
child with a man who is AB-. What are the
potential blood types of their child
• A cat with a long tail is crossed with a cat who
has a short tail. All of their offspring have
medium length tails. Why? What alleles would
you use?
Polygenic Inheritance
• Additive effects of two
or more genes on a
single phenotypic
character.
• skin color in humans.
• An AABBCC individual is
dark and aabbcc is light.
•
AaBbCc X AaBbCc
(intermediate skin shades)
produce offspring with a
range of shades.
Fig. 14.12
Sex-linked traits
• In addition to their role in determining sex, the
sex chromosomes, especially the X chromosome,
have genes for many characters.
• These traits typically follow the complete
dominance principles discussed previously, but
are linked to the X chromosome, as shown below.
• Men are affected more often by a sex linked trait.
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• Duchenne muscular dystrophy
• Absence of normal X-linked gene for a key
muscle protein, called dystrophin.
• Progressive weakening of the muscles and loss
of coordination.
• Hemophilia - absence of one or more
clotting factors.
• Normal proteins slow and stop bleeding.
• Individuals with hemophilia have prolonged
bleeding because a firm clot forms slowly.
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Pedigree analysis reveals Mendelian
patterns in human inheritance
heterozygotes are carriers - may transmit a
recessive allele to their offspring, but do not
suffer from the disease.
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Recap and review?
• Distinguish between Incomplete dominance
and Codominance.
• What are the three different alleles for
human blood typing. How many geneotypes
can be made with these alleles?
• Define Polygenic Inheritance.
• Why are men more susceptible to a sexlinked disease?
• What is a pedigree used for?