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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
• 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. Self-crossed flowers to make sure white/purple flowered plants were true-true-breedingbreeding
2. Crossed true-breeding plants (white X purple)
(X means “crossed with”)
3. 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 re-appeared 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 offspring2. Each individual receives 2 factors which code
for the same trait3. Not all factors are identical – alternative gene
forms are called alleles4. Alleles do not influence each other as alleles
separate independently into gametes5. 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), PLUSprobability 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 = ¼probability of obtaining yy = ¼probability 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