Transcript Slide 1

CHAPTER 10: Introduction to Genetics
This is the study of how traits or characteristics
are passed from parent to offspring
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Mendel:
•Was interested in how organisms passed traits
(characteristics) from parent to offspring
•Decided to work with garden peas because of several
characteristics
•Pollination (fertilization, sexual reproduction) could
easily be controlled
•Peas produce lots of offspring (seeds)
•A generation only takes 1 year to produce
•Peas have many traits with only 2 phenotypes
(appearance) and no intermediate phenotypes
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He chose 7 traits to study
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His first step was to develop true
breeding lines for each of the traits
he studied:
•There were 14 true breeding lines in all
•This took several years
•True breeding means that if 2 true breeding
individuals are crossed (mated) that all the
offspring will have the parents trait…in other
words – if 2 individuals with yellow seeds are
crossed, all the offspring will have yellow seeds
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His next step was to artificially cross individuals from 2
true breeding lines with opposite traits (ex: yellow seeds
x green seeds), producing a hybrid line:
•Grow the true breeding plants
•Cross individuals with opposite traits by hand
•Harvest seeds
•Plant seeds the following spring
•Grow hybrid plants (F1 hybrids)
•Observe the traits of the offspring
FOR ALL 7 traits the F1 individuals all showed
only one of the parental traits ( ex: for a yellow
seed x green seed cross, all the F1 seeds were
yellow)
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The chart below show Mendel’s results for all
7 traits he studied.
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Mendel called the trait that appeared in the
F1 generation dominant, and the trait that
was not visible recessive.
His next question: had the recessive trait
disappeared, or was it masked by the
dominant trait?
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PART 1 QUESTIONS:
1. Why did Mendel choose to work with peas?
2. What is a true breeding line of a species?
3. What is a hybrid line?
4. What did Mendel call the trait that appeared in the
F1 generation?
5. What did Mendel call the trait that was not visible in
the F1 generation?
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To find the answer, he
allowed the plants of
the F1 generation to
self pollinate:
•Saved the seeds of the F1
generation
•Planted the following spring
•Observed the
characteristics of the F2
plants
•Found the about ¼ showed
the recessive trait
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PART 2 QUESTIONS:
1.What did Mendel observe about the recessive
trait in the F2 generation?
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MENDEL CONCLUDED THAT:
•The pea plants contained 2 copies of a ‘factor’ that
controlled each trait
•Factors occurred in different varieties (ex: tall and short)
•A parent gave one copy of its 2 factors to its offspring
•If plants contained 1 or 2 copies of the dominant factor,
the plant had the dominant trait
•2 copies of the recessive factor were needed to produce
the recessive trait
•The probability that a parent will provide each factor is
50%
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How do Mendel’s results from the 19th century
(1800’s) fit with what was discovered about DNA in
the 20th century (1900’s)?
•Mendel’s traits are determined by genes which are located on
chromosomes
•Most genes have 2 or more alleles, variations of genes that
produce variations of a trait (FOR EXAMPLE: an allele for black fur
and an allele for brown fur in mice – both affect fur color, but
produce different colors)
•Most organisms have 2 copies of each chromosome (one from the
mother and the other from the father). Organisms with 2 copies of
each chromosome are called diploid. Egg and sperm cells have 1
copy of each chromosome and are called haploid
•These chromosomes have the same genes, in the same order, but
may have different alleles for a gene. They are called homologous.
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The human genome has 46 chromosomes:
•22 pairs of autosomes, each member of a pair contains
the same genes
•1 pair of sex chromosomes (XX = female, XY = male
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These images for all the human chromosomes can be found at:
https://public.ornl.gov/site/gallery/gallery.cfm?topic=47&restsection=HGPArchive
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PART 3 QUESTIONS:
1. What is the modern term used for what Mendel called
factors?
2. What is the relationship between the terms gene and
allele?
3. How many copies of each chromosome do most
sexually reproducing organisms have?
4. What vocabulary term describes the above condition?
5. Describe the arrangement of genes on homologous
chromosomes.
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The cellular basis of reproduction:
•A new individual is formed when an egg and
sperm fuse
•Each egg or sperm contains 1 copy of each
chromosome and are described as haploid
•Eggs and sperm are produced by a special type
of cell division called meiosis, which cuts the
number of chromosomes in half (Haploid)
•Special sequence of steps
•Assures each egg or sperm has one copy of each
chromosome, although which copy ends up in which
gamete is random (independent assortment)
•After egg and sperm fuse, the new organism has
one copy of each chromosome from each parent,
for a total of 2 copies of each chromosome
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MEIOSIS:
•There are 2 cell divisions
•The first provides each
new cell with 1 duplicated
chromosome (2 sister
chromatids attached by a
centromere)
•Duringthe2nd division, the
centromere disappears and
each cell ends up with one
copy of each chromosome
•After meiosis has ended, there
are 4 cells, each with one copy
of each chromosome (these
cells are called haploid)
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There are
differences in
meiosis between
males and females
•Male meiosis produces 4
equal sperm cells
•Female meiosis produces 1
egg cell and 3 polar bodies
•Most of the contents of
the cell (cytoplasm and
organelles) end up in the
egg
•The polar bodies remove
the extra DNA
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During meiosis, homologous chromosome exchange pieces of
DNA.
This process is called crossing over
The farther apart 2 genes are on a chromosome, the more likely
they will be separated during meiosis by crossing over
Genes close together usually don’t separate and are called linked
genes
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PART 4 QUESTIONS:
1. How many copies of a homologous chromosome does
an egg or sperm contain?
2. What is the type of cell division that produces eggs
and sperm called?
3. How does the process named above affect the
number of chromosomes in a cell?
4. What is crossing over and when does it occur during
meiosis?
5. Describe how meiosis is different for males and
females.
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Biologists often want to determine the probable outcome of a cross
between 2 individuals
This is done by modeling the cross using a Punnett square
First – some vocabulary:
•Individuals with 2 identical alleles for a trait are called homozygous
•Individuals with 2 different alleles for a trait are called heterozygous
•Dominant alleles are denoted with capital letters
•Recessive alleles are denoted with lowercase letters
FOR EXAMPLE: a pea plant heterozygous for height would be
identified as
height
Tt, this is defined as the plant’s genotype for
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MOST GENES SEPARATE
INDEPENDENTLY DURING MEIOSIS:
• the probability of a particular homologous
chromosome ending up in a specific gamete is 0.5
•Therefore the probability of an allele ending up in a
specific gamete is 0.5
•The probability of 2 different homologous
chromosomes ending up in the same gamete is equal to
the product of the probability of the 2 events occurring
independently
•The number of genetically different gametes an
organism can produce can be determined by 2n, where n
= the number of homologous chromosomes.
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PART 5 QUESTIONS:
1. Use the vocabulary word allele to define homozygous.
2. Use the vocabulary word allele to define heterozygous.
3. What do you know about an individual cow with the
genotype Rr, when ‘R’ denotes an allele for a red coat
and ‘r’ denotes an allele for a white coat.
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The genetic contribution of each parent is determined by meiosis
The allele that ends up in an egg or a sperm is random
Heterozygotes will produce 50% of the eggs or sperm with each allele
FOR EXAMPLE:
A pea plant heterozygous for height (Tt) will produce 50% T alleles and 50% t alleles
Cross 2 pea plants heterozygous for height:
Tt x Tt
Genetic contribution of 1 parent
(gametes)
Genetic
contribution
of 1 parent
(gametes)
T
t
T
TT
Tt
t
Tt
tt
The offspring from this cross will be:
•75% tall (25% homozygous dominant (TT) and 50% heterozygous (Tt)
•25% short (homozygous recessive (tt)
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STEPS TO WORKING GENETICS PROBLEMS:
1. Determine the genotypes of the parents
2. Determine the alleles in the gametes the parents will produce
3. Write the gametes at the top and down the side of the
Punnett square
4. Fill in the boxes for the progeny
5. Determine the genotypic ratio of the progeny
6. Determine the phenotypic ratio of the progeny
This information can be used to answer questions about a
genetic cross
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GENETICS PROBLEM EXAMPLE:
In mice black fur (B) is dominant over brown fur (b). A female
brown mouse is mated with a heterozygous black male mouse.
Show the results of the cross. Genotype
Phenotype
Alleles
Mother
and
Father
and
1. How many genotypes will be present in the progeny?
2. How many phenotypes will be present in the progeny?
3. What fraction of the offspring will have each genotype and phenotype?
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Incomplete Dominance
•Heterozygotes look
different from a
homozygotes
•Blending of
phenotypes
RR x RIRI
RRI
http://chsweb.lr.k12.nj.us/psidelsky/Chapter10(Gen).htm
Codominance: Both alleles of a gene pair are
expressed.
Example: Human Blood Types
• Human blood type is determined by codominant
alleles. There are three different alleles: IA, IB, and i.
The IA and IB alleles are codominant, and the i allele
is recessive.
• The possible human phenotypes for blood group are
type A, type B, type AB, and type O.
• Type A and B individuals can be either homozygous
(IAIA or IBIB, respectively), or heterozygous (IAi or
IBi, respectively). Type O is IOIO. ;)
S-B-8-1_Non-Mendelian Heredity
PowerPoint
Codominance: More Examples
Rhododendron
coloration
http://upload.wikimedia.org/wikipedia/commons/f/f3/Codominance_Rhododendron.jpg
Roan coloration in cows (red
coat with white blotches)
http://en.wikipedia.org/wiki/File:Light_Roan_Shorthorn_Heifer_DSCN1872b.j
pg
S-B-8-1_Non-Mendelian Heredity
PowerPoint
1.
Distinguish between incomplete dominance and
codominance.
2.
How many phenotypes are seen when codominance is
possible?
3.
Red color (R) in snapdragons shows incomplete dominance
to white snapdragons (RI). The heterozygotes are pink. If
pink snap dragons are crossed with a white snapdragon,
what are the genotypic/ phenotypic ratios?
Multiple Alleles
• When 4 or more possible
phenotypes exist within a
population
– Blood type
– Eye & hair color
– Fur in mammals
http://www.learner.org/interactives/dna/genetics8.html
Polygenic Inheritance
http://www.nature.com/nrg/journal/v10/n12/box/nrg2670_BX1.html
Poly-many genic-gene
• More than one gene on
more than one loci or
on more than one
chromosome.
• Give the appearance of
gradation
– Hair & skin color
– Height
– Body size
http://scienceblogs.com/geneticfuture/a_ruler_beats_genetics_in_pred/tall_short1.jpg
EPISTASIS
When a gene alters the
expression of a
different gene.
EXAMPLE: albanism (albino)
•Albino individuals have a
normal homozygous or
heterozygous genotype for
pigmentation.
•A recessive allele for a
different gene prevents the
formation of the pigment
melanin
Sex-Linked Traits
• Genes located on a sex
(X or Y) chromosome
♀XX & ♂XY
– Some forms of
hemophilia
– Red-green
colorblindness
http://www.ccs.k12.in.us/chsteachers/Amayhew/Biology%20Notes/sexlinked%20notes.htm
INDEPENDENT ASSORTMENT: genes on different chromosomes and
genes far away on the same chromosome are inherited
independently. This happens during meiosis.
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Independent Assortment produces a wide variety of gametes.
There is more than one way to determine the gametes produced by
an individual during meiosis.
Consider an individual with the genotype AaBb
Technique 1:
AaBb
GAMETES:
AB
aB
Ab
ab
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