Patterns of InheritancePatterns of Inheritance

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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 CrossesAre Traits Linked?

Law of Independent AssortmentThe 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• 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