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Chapter 14: Mendel and the Gene Idea
Mendel used peas because
 Short generation time
 Can cross or self pollinate
 Clear cut/discrete traits (not continuous)
 Easy to control mating
 Can breed a large number of plants at one time
The Process
 Start with a true breeding plant
o Self pollinate until all offspring are of same variety
o Eliminate all but one characteristic.
o cross two true breeding plants.
o Hybrid: offspring of genetically dissimilar parents (not a clone)
P Generation: Parental
F1 Generation: First Filial (hybrid offspring)
F2 Generation -2nd Filial (offspring of F1)
Vocabulary to Know
Gene: region of DNA that controls a given trait
Each gene has variants called alleles
Allele: variant of a gene.
Locus: site on chromosome where specific gene located.
Mendel’s terminology
 Character (heritable feature)
 Trait: variant of character (white/purple)
Law of Segregation
Allele pairs separate during gamete formation and re-pair during fertilization.
1. Alternate versions of genes (alleles) account for variations in inherited characteristics
• Determined by nucleotide sequence on DNA
• Found at specific locus (spot) on chromosome.
2. For each character, an organism inherits two alleles, one from each parent.
• Each genetic locus represented twice in diploid cell, on homologous chromosomes.
• We know now--not always true
3. If two alleles differ, then one, the dominant allele, is fully expressed in an organism’s appearance. The
other, the recessive allele, has no noticeable effect on the organisms appearance.
• Not always the case, applies only to situation with complete dominance
4. The two alleles for each character separate during gamete production
Law of Independent Assortment
 Not only do alleles separate during gamete formation, but alleles for each trait separate independently of
others located on different pairs of homologous chromosomes.
 It is a matter of chance as to which member of a homologous pair of chromosomes goes to which daughter
cell during Meiosis I.
Punnett Squares
 Use to predict results of a cross between individuals of known genotype
 Capital = dominant, lower case = recessive
 “monohybrid cross”: looking at one trait.
 Mate two heterozygotes in a monohybrid cross  3:1 phenotypic ratio.
Homozygous: identical alleles
 Homozygous dominant: two dominant alleles
 Homozygous recessive: two recessive alleles
Heterozygous: two different alleles
Genotype: genetic makeup
Phenotype: how the genes are expressed
 Appearance, physiology
Dihybrid cross: looking at two traits.
 Law of Independent Assortment
o Each allele segregates independently during gamete formation.
o Exception is linked genes, will look at this in chapter 15.
 A dihybrid cross between two organisms that are heterozygous for two traits, each of which has two forms -dominant or recessive - always produces a phenotypic ratio of o 9 dominant for traits A and B: 3 dominant for trait A but recessive for trait B: 3 recessive for trait A
but dominant for trait B: 3 recessive for traits A and B
o 9:3:3:1
Practice Problems
Genetic Crosses
• What would be the genotypic and
phenotypic ratios of the offspring of two
individuals, one being homozygous
dominant and the other being heterozygous
for a trait?
Genetic Crosses
• John and his wife are both heterozygous for the
traits of tongue rolling and freckles.
– If John and his wife have 16 children, what
combinations of phenotypes would you observe, and in
what ratios would they occur.
– A little more realistically, if John and his wife have two
children, what is the probability that one will be a
tongue roller (dominant) but not have freckles
– What is the probability that both of their children will
be unable to roll their tongues (recessive) and not have
freckles (recessive)?
Probability to Solve Genetics
• YyRr x Yyrr
• What is the chance that the offspring will have each
of the following Genotypes
Probability Practice
• P: PpyyRr x PPYyrr
• What is the probability that F1 will be PPYyrr?
• What is the probability that F1 will be either PPYyrr
or Ppyyrr?
• What is the probability of having an offspring with a
dominant phenotype for each trait?
• What is the probability of having an offspring with a
recessive phenotype for each trait?
Other Forms of Inheritance
Incomplete Dominance
Multiple Alleles
Sex Linked Inheritance
Polygenic Inheritance
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