Transcript Mendelian Genetics

Mendelian Genetics
1860's: Gregor Mendel
described the
fundamental principles of
inheritance.
Mendel discovered that
certain traits show up in
offspring plants without
any blending of parent
characteristics.
For example, the pea flowers
are either purple or white:
intermediate colors do not
appear in the offspring of
cross-pollinated pea plants.
Terms to Know
• Genetics – study of heredity
• Fertilization – process of combining haploid sex cells
(egg and sperm)
• true-breeding (pure bred) - organisms that produce
offspring identical to themselves
• Trait - specific characteristic EX: eye color
• Gene - a functional unit that controls an inherited trait
• Allele – alternative form that a single gene may have for
a particular trait. EX: pink or white flowers
• Gamete – haploid sex cell as egg or sperm
More Terms to Know
• Homozygous – organism with two of the same alleles for
a specific trait. EX: BB
• Heterozygous - organism with two different alleles for a
specific trait. EX: Bb
• Hybrid – organism that is heterozygous for a specific trait
EX: Bb
• Genotype –an organism’s allele pairs. EXs: Bb, BB, bb
• Phenotype – observable characteristic that is expressed
as a result of an allele pair. EX: purple flower color
• Dominant allele – an expressed allele EX: B
• Recessive allele – a hidden or masked allele EX: b
Some Terms Used in Genetics
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Genes
Locus
Diploid cells
alleles
homozygous
heterozygous
dominant allele
recessive allele
phenotype
genotype
Probability
Probability is the likelihood that a specific event will occur
or is the likely outcome a given event will occur from
random chance.
A Probability may be expressed as a Decimal (0.75), a
Percentage (75%), or a Fraction (3/4).
Probability is determined by the following Equation:
PROBABILITY = Number of times an event is expected
to happen
Probability Examples
With each coin flip there is a 50% chance of heads
and 50% chance of tails.
Chance of inheriting one of two alleles from a
parent is also 50%.
Each coin toss is an independent event.
Therefore, the probability of flipping three heads
in a row is:
½ X ½ X ½ = 1/8
Rules of Probability
Offspring resemble their parents
Why ?
• What is transmitted from parent to offspring ?
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Offspring are not all identical
Why ?
Inheritance is often discrete, not blending
Why ?
Acquired characters are not inherited
Why ?
Mendel said that Blending
Inheritance does not occur
• Blending implies that offspring have a
simple mixture of the parent’s
characteristics
• Mendel showed that characteristics from
each parent are separate and that
offspring inherit the characteristics of
one or other parent depending on
certain rules of inheritance
Mendel’s experimental design
1) Produced true-bred pea plants
by self-fertilization for several
generations. Thus, he assured that
traits were constant, and transmitted unchanged
from generation to generation.
2) He then performed crosses between varieties
exhibiting alternative forms of traits. e.g. crossed
white flower on a plant with a plant that produced
purple flowers.
3) He permitted the hybrid offspring to self-fertilize
for several generations. Thus, he allowed the
alternative forms of a character to segregate
among the offspring (progeny).
What Mendel found
The F1 generation
True-breeding parents with
contrasting forms of a trait
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When Mendel crossed 2
contrasting varieties of
peas, the hybrid
offspring did not have
flowers of intermediate
color as the theory of
blending inheritance
would predict.
Instead, the flower color
always resembled one of
the parents.
F1 or first filial generation
• It is customary to refer to these
offspring as the F1 or first filial (filius is
Latin for son) generation.
• Mendel referred to the trait expressed
in the F1 plants as dominant and the
alternative form as recessive.
• We usually indicate the dominant allele
with an upper case character (e.g. A)
and the recessive with a lower case
character (e.g. a)
The F2 generation
• He allowed individual F1 plants to selffertilize, he found that 705 F2 plants (75.9%
had purple flowers and 224 (24.1%) had
white flowers.
• Approximately, 1/4 exhibited the recessive
form.
• In other words, the dominant: recessive
ratio was 3 purple:1 white.
Punnett Squares
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R.C. Punnett developed a simple diagram (now
called a Punnett Square) for visualizing the
possible genotypic combinations of F2 individuals
F2 generation
• The heterozygous Aa plants look just like the
homozygous AA plants (i.e. have the same
phenotype)
• Thus, the 3:1 phenotype ratio is a disguised
1AA:2Aa:1aa = 1:2:1 genotype ratio
Mendel’s results
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No blending
3:1 phenotypic ratio
1:2:1 genotypic ratio
Punnett square shows that each sperm has
equal probability of fertilizing an egg.
Therefore, each plant has a 3/4 (75%)
chance of inheriting one dominant allele
Monohybrid
Cross
Mendel’s First Law of
Heredity: Segregation
• Each organism contains
two factors (alleles) for
each trait. These factors
segregate, or separate,
in the formation of
gametes (eggs and
sperm).
• This segregational behavior
has a simple physical basis:
the alignment of
chromosomes are random
on the metaphase plate
during meiosis
Mendel’s Second Law of Heredity:
Independent Assortment
• Mendel then set out to test whether different
genes segregate independently.
• He tested a Dihybrid case (RrYy X RrYy):
• In a cross involving different seed shape alleles
(round R and wrinkled r) and different seed
color alleles (yellow Y and green y), all the F1
individuals were identical, each one
heterozygous for both seed shape (Rr) and
seed color (Yy).
Dihybrid Cross
What did Mendel actually observe ?
• From a total of 556 seeds from dihybrid
plants that he allowed to self-fertilize he
observed:
• 315 round yellow (R_Y_), 108 round green
(R_yy), 101 wrinkled yellow (rrY_) and 32
wrinkled green (rryy).
• These results are very close to a 9:3:3:1 ratio
(would be 313:104:104:35).
• Consequently, the 2 genes appeared to
assort independently of each other.
Mendel’s Second Law of
Heredity:
Independent Assortment
• A modern re-statement of this 2nd law is:
Genes that are located on different
chromosomes assort independently during
meiosis.
The mechanism of Independent
Assortment
Because…….
It is a fundamental law of
probability that the probability of
two independent events
occurring together is the product
of their individual probabilities
This is the
Law of Multiplicative Probabilities
Multihybrid = n loci
• For example, cross 5 independent loci:
• Aa bb Cc Dd Ee X
Aa Bb Cc dd Ee
• What is the proportion of homozygous recessives
(aa bb cc dd ee) ?
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Aa X Aa: ½ a X ½ a = ¼ aa
bb X Bb: 1/1 b X ½ b = ½ bb
Cc X Cc: ½ c X ½ c = ¼ cc
Dd X dd: ½ d X 1/1 d = ½ dd
Ee X Ee: ½ e X ½ e = ¼ ee
• Multiply probabilities: ¼ X ½ X ¼ X ½ X ¼ = 1/256
(=0.39%)
Gene Recombination
• Now we know that a gene codes for a protein
(enzyme)
• Genetic Recombination – when there is a new
combination of genes produced by crossing
over.
– Linked genes usually travel together during gamete
formation. This is an exception to Mendel’s law of
segregation.
– Crossing over is more frequent between genes that
are far apart than close together. This information can
be used to develop a chromosome map which maps
the sequence of genes on a chromosome
• Polyploidy – one or more extra set of
chromosomes (not diploid)
– Example: Triploid(3N)- found in goldfish and
earthworms; lethal in humans
Exceptions
1. incomplete dominance – occurs when Two or More Alleles Influence the
Phenotype, resulting in a Phenotype Intermediate between the Dominant
Trait and Recessive
Example red X white 4 o’clock flowers = pink flowers
2. codominance – occurs when Both Alleles for a gene are Expressed in a
Heterozygous offspring
Examples: certain varieties of chicken where black X white = speckled
chicken
Sickle Cell Anemia
3. multiple alleles – more than 2 possible alleles for a gene.
Examples: blood type in humans, rabbit coat color
4. polygenic traits – traits controlled by 2 or more genes.
Examples: eye color of fruit flies, human skin color
5. Sex Determination – sex chromosomes (X and Y) vs autosomes
Sex-Linked Traits – recessive X linked trait
Examples: color blindness, hemophilia
X-Linked Color Blindness
Polygenic
Inheritance