Download Observing Patterns in Inherited Traits

Observing Patterns in
Inherited Traits
Mendelian Genetics
Mendel’s Insight into Inheritance Patterns
• More than a century ago people knew that traits were inherited, but
didn’t know that the information for these traits was contained in
organized units called genes
• People thought that the traits from the mother and father were
“blended” at fertilization
• Blending could not explain the obvious variation in traits that
people could observe with their own eyes
• “Blending” proponents dismissed Charles Darwin’s theory of
natural selection
• Theory said that individuals of a population vary in the details
of the traits they have in common
• Over generations, variations that help an individual survive
and reproduce show up among more offspring than
variations that do not
• Less helpful variations might persist, but among fewer
individuals – they may even disappear
Mendel’s Insight into Inheritance Patterns
• Gregor Mendel had guesses
that sperm and eggs carry
distinct “Units” of information
about heritable traits
• By studying pea plants for
generation after generation,
Mendel found indirect
observable evidence of how
parents transmit genes to
offspring
Mendel’s Experimental Approach
• Studies the garden pea plant
• Self-fertilizing
• Male and female gametes originate in the same
plant
• They can “true breed” for certain trains
• This means that successive generation will be
just like parents in one or more trait.
• All offspring grown from seeds of self-fertilized
white-flowered parent plants will also have
white-flowers
• Pea plants can also cross-fertilize
Mendel’s Experiments
Terms used in modern genetics
• Remember that in Mendel’s time, no one knew about
genes, meiosis, or chromosomes
• Genes – units of information about heritable traits
transmitted from parents to offspring. Each gene has a
specific location (locus) on a chromosome
• Cells with a diploid chromosome number (2n) have
pairs of genes, on pairs of homologous chromosomes
• Mutation alters a gene’s molecular structure and its
message about a trait. It may cause a trait to change, as
when one gene for flower color specifies white and a
mutant form specifies yellow.
• Alleles – all molecular forms of the same gene
Terms used in modern genetics
• When offspring inherit a pair of identical alleles for a trait
generation after generation, we expect them to be a truebreeding lineage. Offspring of a cross between two
individual that breed true for different forms of a trait are
hybrids; each one has inherited nonidentical alleles for
that trait
• When a pair of alleles on homologous chromosomes are
identical, this is a homozygous condition
• When the two alleles are not identical, this is called the
heterozygous condition
Terms used in modern genetics
• An allele is dominant when its effect on a trait masks
that of any recessive allele pairs with it. We use capital
letters to signify dominant alleles and lowercase letters
for recessive ones. A and a are examples.
• A homozygous dominant individual has a pair of
dominant alleles (AA) for the trait being studied
• A homozygous recessive individual has a pair of
recessive alleles (aa).
• A heterozygous individual has a pair of nonidential
alleles (Aa)
Terms used in modern genetics
• Genotype refers to the particular alleles that an
individual carries
• Phenotype refers to an individual’s observable traits.
• P stand for parents
• F1 refers to the first-generation offspring
• F2 refers to the second-generation offspring
Mendel’s Theory of Segregation
• Mendel used something called a monohybrid cross to
test his hypothesis that pea plants inherit two “units” of
information for a trait, one from each parent.
• Monohybrid crosses use F1 offspring of parents that
breed true for different forms of a trait
• Parents are AA crossed with aa (homozygous
dominant cross with homozygous recessive)
• Offspring will all be heterozygous (the
monohybrids)
• From meiosis these parents would produce A
alleles and a alleles
Mendel’s Theory of Segregation
AA = purple
A
Aa = purple
A
Aa
Aa
Aa
Aa
a
a
A = dominant allele for purple flowers
A = recessive allele for white flowers
All F1 offspring would
be purple
Mendel’s Theory of Segregation – Crossing
AA = purple
the F1 Generation
Aa = purple
A
a
A
a
AA
Aa
Aa
aa
aa = white
Results of this cross are 3 purple flowered plants and 1
white flowered plant a 3:1 ratio
Mendel’s Theory of Segregation
• This 3:1 ratio indicated to Mendel that fertilization is a chance event
having a number of possible outcomes
• Probability – that chance that each outcome of an even will occur is
proportional to the number of ways in which the event can be
reached
• These squares are called Punnett-square method
Possible Events
Probable Outcome
Sperm A meets egg A
¼ AA offspring (Purple)
Sperm A meets egg a
¼ Aa offspring (Purple)
Sperm a meets egg A
¼ Aa offspring (Purple)
Sperm a meets egg a
¼ aa (white)
Mendel’s Theory of Segregation
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• Mendel used rules of probability and counted many
offspring
• This minimized sampling error deviations in the
predicted results
Test Crosses
• Testcrosses supported Mendel’s prediction
• Take an organism that shows dominance for a specific trait, but
its genotype is unknown
• Cross it with a known homozygous recessive individual in a
number of matings
• Results will show whether it is homozygous dominant or
homozygous recessive
• Mendel crossed F1 breeding plants with true-breeding white
flowered plants (homozygous recessive)
• If the F1 plants were homozygous dominant then all of
the offspring would be purple
• If the F1 plants were heterozygous then only about half
the offspring would be purple
Testcrosses
A
A
a
Aa
Aa
a
Aa
Aa
All Purple
Testcrosses
A
a
a
a
Aa
aa
Aa
aa
Half Purple Half White
Mendel’s Theory of Segregation
• Diploid cells have pairs of genes, on pairs of
homologous chromosomes. The two genes of each pair
are separated from each other during meiosis, so they
end up in different gametes
Mendel’s Theory of Independent Assortment
• Used dihybrid crosses to explain how two pair of genes assort into
gametes
• We are now looking at two traits at a time
• An experimental intercross between F1 dihybrids (two traits)
that are identically heterozygous for two pairs of genes
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46.cw/index.html
• Theory became known as the theory of independent
assortment
• As meiosis ends, genes on pairs of homologous
chromosomes have been sorted out for distribution into one
gamete or another, independently of gene pairs of other
chromosomes
More Patterns than Mendel Thought
• Mendel was lucky, he focused (likely by accident) on
traits that were clearly dominant and recessive forms
• Sometimes gene expression is not as straightforward
• ABO blood types
• A case of codominance
• A pair of nonidentical alleles affecting two
phenotypes are both expressed at the same time
in heterozygotes
• ABO blood typing refers to the kind of glycolipid on
the surface of the red blood cells
More Patterns than Mendel Thought
• ABO blood types
• Three alleles IA, IB, i
• IA and IB are codominant when paired together
• i is recessive
• If the letter for an allele has a superscript it show a lack of
dominance
• Type A blood types
• IAIA, IAi
• Type B blood types
• IBIB, IBi
• Type AB blood types
• IAIB
• Type O blood types
• ii
Blood Types Continued
• http://en.wikipedia.org/wiki/Blood_type
Incomplete dominance
• One allele of a pair is not fully dominant over its partner
• Heterozygous phenotype is somewhere between the
two homozygotes
• Snapdragons – Cross true-breeding red with truebreeding white and you get pink flowers
• Red snapdragons have two alleles that let them
make a lot of red pigment
• White snapdragons have two mutant alleles
that produce no pigment
• If you get one red and one white allele some
red pigment gets produced resulting in a pink
flower
Incomplete dominance
• Same thing applies in chickens for their combs
• R and P alleles
• Rose = RRpp
• Pea = rrPP
• Walnut = RrPp
• Single = rrpp
Walnut
Single genes with a wide reach
• Pleiotropy – alleles at one locus on a chromosome that
may affect two or more traits in good or bad ways
• Typically seen in genetic disorders
• Cystic fibrosis
• Sickle-cell anemia
• Marfan Syndrome
When products of gene pairs interact
• Fur color in Labrador retrievers – depends on the amount of a
protein called melanin that gets deposited in the hair
• Black
• Yellow
• Brown
• Allele B (black) is dominant to b (brown)
• Allele E permits full melanin deposition
• Two ee alleles reduces it so the fur is yellow
• Allele C may override the first two
• CC or Cc do make an enzyme that allows melanin production
• cc does not allow for production and albinism or lack of
melanin is the result
Complex variation in traits
• Generally individuals of a population display a range of
small differences in most traits
• Called continuous variation
• Depends mainly on how many gene products affect a
given trait and on how many environmental factors
impact them.
• Greater number of genes and environmental
factors, the more continuous is the distribution of
all versions of the trait.
• Eye color
• Height
Genes and the Environment
•
Himalayan rabbit
• Homozygous for the ch allele
of the gene specifying an
enzyme involved in melanin
production
• It is a heat sensitive
enzyme that is active only
when the temperature is
below 33ºC or 91ºF.
• When or where it is warm
the rabbit is grows white
hair
• Where it is cold or cooler
the dark hair appears