Patterns of Inheritance

Chapter 9

Gregor Mendel

• Austrian monk

• “Father of Modern Genetics”

• Famous for his work with peas

Mendel’s PeasMendel’s Peas

Cross-PollinatingCross-Pollinating

General Background General Background VocabularyVocabulary

Self-pollination: pollen from flower fertilizes the same plant

True-breeds: pure gene lines – offspring match parent due to self-fertilization)

Cross-pollination: pollen will fertilize a different plant

Hybrid: Offspring that result from a cross between organisms with different traits; express a combination of traits from

Character: heritable feature that varies among individuals (ex. seed color)

Trait: possible variations for a particular character (ex. yellow seeds vs. green seeds)

Mendel’s CrossesMendel’s Crosses

• Mendel noticed that one trait was always expressed over the other in the F1 offspring

• However, trait that disappeared always reappeared in about 25% of the F2 offspring.

Mendel’s Initial ConclusionMendel’s Initial Conclusion• Biological inheritance is determined by chemical factors Biological inheritance is determined by chemical factors

passed from one generation to the next passed from one generation to the next – Geneticists now refer to these heritable factors as Geneticists now refer to these heritable factors as genesgenes– Genes can come in more than one form, each form is an allele Genes can come in more than one form, each form is an allele

ex. B or b ex. B or b (The “B” gene w/ 2 alleles)(The “B” gene w/ 2 alleles)

• Additional relevant terminology:Additional relevant terminology:– Homozygous: two identical alleles (AA or aa)– Heterozygous: two different alleles (Aa)– Phenotype: Physical appearance – Genotype: Genetic make-up

• Homozygous dominant (AA)• Homozygous recessive (aa)• Heterozygous (Aa)

The Principles The Principles Dominance & SegregationDominance & Segregation

• DominanceDominance: Certain alleles will be : Certain alleles will be expressed over others, the expressed alleles expressed over others, the expressed alleles are are dominantdominant to the unexpressed to the unexpressed recessiverecessive allelesalleles

• SegregationSegregation: Each parent carries two : Each parent carries two alleles for each gene. During meiosis, the alleles for each gene. During meiosis, the pairs are separated to that only one allele is pairs are separated to that only one allele is sent to the offspring in the gamete from sent to the offspring in the gamete from each parent. each parent.

Genetics & Probability Genetics & Probability • Probability is the likelihood of

an event happening• Consider flipping a coin

– Likelihood of flipping heads = 50%

– (1 of 2 possibilities)– Likelihood of flipping heads

twice? •50% x 50% = 25%

• To predict outcomes of genetic crosses we use punnett squarespunnett squares

The Test Cross

Exploring Mendelian GeneticsExploring Mendelian Genetics• Does segregation of one set of alleles influence the

segregation of another pair of alleles?

• Mendel’s Two Factor (dihybrid) Crosses– Followed two traits at a time.– Same method as his original single-factor crosses– Cross-pollinated to produce the F1 and allowed them to

self-pollinate

• Mendel found that alleles for different traits did not influence each others segregation. This is referred to as the principle of independent assortment.

Pedigrees• Pedigrees are like genetic “family trees”. They are used

to show the inheritance of traits within families and to predict genotypes and/phenotypes of certain individuals.

• The following key explains the symbols and layout of a typical pedigree:

Autosomal Dominant Pedigree

Autosomal Recessive Pedigree