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Patterns of Inheritance
Ancestry and Obesity
In The News
Historical Views of Inheritance
• Hippocrates (~ 400 B.C.)
– Particles given off from bodies
of father and mother
– Offspring was mixture
• Homunculus (pre-1900)
– Each sperm contained a tiny
preformed human
Gregor Mendel
• Austrian Monk
• Worked with garden peas
• Discovered process of heredity
Suitability for Using Peas
• Each flower has male & female parts
• Sex organs are enclosed & protected
• Pollination can be controlled
True-breeding
• Generations of self-fertilization produce
consistent offspring
• Mendel used true-breeding varieties
Mendel’s Experiments
• Artificially crossed true-breeding plants
• Recorded offspring from cross-fertilization
Characters Used by Mendel
• 7 true-breeding traits
• Monohybrid crosses
Mendel’s Crosses
• Example:
– Tall plant crossed with short plant
– Hybrid offspring (F1) were not intermediate
– Resembled only one parent
F1 of Hybrid Cross
• Dominant – form expressed in F1
• Recessive – form not expressed in F1
Mendel’s Next Step
• Allowed F1 to self fertilize
• F2 expressed dominant & recessive forms
• Ratio in F2 = 3:1 (dominant:recessive)
Mendel’s Monohybrid Crosses
Conclusions of Mendel’s Work
• Traits are inherited as genes
• Alleles are alternate forms of genes
• Gametes receive only 1 allele of each pair
• Alleles may differ or may be identical
Mendel’s Law of Segregation
Each gamete receives only one of an organisms pair of
alleles, and which one it receives is determined by chance
Alleles of an Individual
• Homozygous – identical alleles for a trait
• Heterozygous – different alleles for a trait
Genotype
• An organisms allelic makeup
• Dominant designated by capital letter
• Recessive designated by small letter
Phenotype
• Expression of alleles
• Dominant gene is expressed if present
Punnet Square
• Diagram to predict possible combinations
Test Cross
Determining Genotype
Dihybrid Crosses
Are Traits Linked?
Law of Independent Assortment
The transmission of alleles for one trait into gametes does
not affect the transmission of alleles for other traits
Location of Mendel’s Factors
• Mendel did not know where traits were located
• Chromosomes were discovered 22 years after
Mendel’s work was published
• Walter Sutton suggested that Mendel’s factors
were on chromosomes
Thomas Hunt Morgan
• Studied fruit flies, Drosophila melanogaster
• Discovered different traits in flies
• Experiments to test Mendelian inheritance
Morgan’s Cross
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White-eyed male
Red-eyed female (normal eye color)
Eye color followed Mendel’s 3:1 ratio
However, all white-eyed F2 were males
Conclusion of Morgan’s
Experiment
• White eyes were linked to males
• Trait was found on X chromosome
• Females have two X chromosomes
Conclusion of Morgan’s
Experiment
• Traits are located on chromosomes
• Some traits are sex-linked
End chapter 13