Transcript C-13 Mendelian Genetics

Mendelian Genetics
The term ‘Mendelian genetics’
typically relates to the outcomes of
simple dominant and recessive
gene pairings
Shows specific ratios or patterns
of inheritance within a lineage of
offspring generations (e.g., F1 and
F2 generations)
Early ideas of heredity
1) Constancy of species
– heredity occurs
within the boundary
of the species; not so
prior to the Middle
ages
(Ex: giraffe and
minotaur)
View held thru time of Darwin
• Direct transmission of
traits – child is formed
after hereditary
material from all parts
of parent’s body come
together – blending
occurs
Gregor Mendel (1822 – 1884)
• Studied garden peas
• 1st to use mathematics to examine
outcomes of crosses
• Large # of pea varieties with at least
7 easily distinguished traits
• Peas are small, easy to grow, short
generation time
• Peas can self-fertilize; bisexual
Some definitions for tracking traits via Mendelian
inheritance
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Genotype/Phenotype
Gene/allele
Dominant/Recessive alleles
Homozygous/Heterozygous
P/F1/F2 generations
Genotypic ratio/Phenotypic ratio
Monohybrid cross/Dihybrid cross
Mendel conducted studies in 3 stages
1.
2.
3.
Self-crossed flowers to make
sure white/purple flowered
plants were true-breeding
Crossed true-breeding plants
(white X purple)
(X means “crossed with”)
Crossed F1 plants to see
traits in future generation
(F2 generation)
Mendel came to understand….
• Plant progeny (offspring)
did not show blending of
traits
• For each pair of alternative
traits, 1 was not expressed
in F1 generation, but reappeared in F2 generation
• Traits segregate among the
progeny
• Alt, traits are expressed in
3:1 ratio in F2
Punnett squares allow analysis using symbols for
gametes and genotypes
Outcome of crossing true
breeding purple-flowered and
white-flowered pea plants
F1 progeny: All purple flowered
F2 progeny: 3 purple to 1 white
Self cross each
of the F2’s
The Mendelian ratio
• Phenotypic ratio of 3:1
yet,
• Genotypic ratio of
1:2:1
When crossing heterozygous
individuals of trait controlled by
simple dominant/recessive alleles
Mendel proposed a simple model
of heredity – 5 parts:
1. Parents transmit “factors’ to offspring
2. Each individual receives 2 factors which code
for the same trait
3. Not all factors are identical – alternative gene
forms are called alleles
4. Alleles do not influence each other as alleles
separate independently into gametes
5. The presence of an allele does not insure that its
trait will be expressed
Monohybrid Crosses
genotype: total set of alleles of an individual
PP = homozygous dominant
Pp = heterozygous
pp = homozygous recessive
phenotype: outward appearance of an
individual
Monohybrid Crosses
Principle of Segregation – Mendel’s first
Law of Heredity
Two alleles for a gene segregate during
gamete formation and are rejoined at
random, one from each parent, during
fertilization.
Dihybrid Crosses
Dihybrid cross: examination of 2 separate
traits in a single cross
-for example: RR YY x rryy
The F1 generation of a dihybrid cross (RrYy)
shows only the dominant phenotypes for
each trait.
Dihybrid cross
between two
heterozygous parents
Instead of 4 possible
outcomes, there are now
16!!
Dihybrid Crosses
Principle of Independent Assortment:
Mendel’s 2nd Law.
In a dihybrid cross, the alleles of each gene
assort independently.
Probability – Predicting Results
Rule of addition: the probability of 2
mutually exclusive events occurring
simultaneously is the sum of their
individual probabilities.
When crossing Pp x Pp, the probability of
producing Pp offspring is
probability of obtaining Pp (1/4), PLUS
probability of obtaining pP (1/4)
¼ + ¼ = ½
Probability – Predicting Results
Rule of multiplication: the probability of 2
independent events occurring simultaneously is
the PRODUCT of their individual probabilities.
When crossing Rr Yy x RrYy, the probability of
obtaining rr yy offspring is:
probability of obtaiing rr = ¼
probability of obtaining yy = ¼
probability of rr yy = ¼ x ¼ = 1/16
Testcross
Testcross: a cross used to determine the genotype
of an individual with dominant phenotype
-cross the individual with unknown genotype (e.g.
P_) with a homozygous recessive (pp)
-the phenotypic ratios among offspring are
different, depending on the genotype of the
unknown parent