Genetic Inheritance According to Gregor MendelCHAPTER 9 Overview of Inheritance Mendel's Pea Plants Monohybrid Crosses and Segregation Definitions of Genetic Terms Dihybrid Crosses and Independent Assortment Test Crosses The Role of Probability Pedigrees and Genetic Conditions Human Single-Gene Disorders
Heritable Variation and Patterns of Inheritance • Gregor Mendel • Was the first person to analyze patterns of inheritance. • Deduced the fundamental principles of genetics.
In an Abbey Garden • Mendel studied garden peas • Pea flowers have both male and female parts • The ovary of the carpel produces eggs by meiosis • The anthers of the stamens produce pollen (sperm equivalent) by meiosis • Pollen grains fly from the anthers of the same flower or from another plant to the carpel and fertilize the eggs in the ovary • Pea flowers are enclosed within petals such that only insect pollination can effectively transfer pollen from one plant to another
How Mendel Fertilized Pea Flower Eggs and Ensured a Single Pollen Source Figure 9.4
He also created true-breeding varieties of plants. • Mendel then crossed two different true-breeding varieties. • Mendel performed many experiments. • He tracked several characteristics in pea plants from which he formulated several hypotheses.
True Breeding Varieties Available to Mendel Figure 9.5
A Single-Factor (Monohybrid) Cross Figure 9.6a
The letters P and p were used to designate flower color: P = purple, p = white • Hypotheses From The Monohybrid Cross • There are alternative forms of genes called alleles • There are two gene forms (alleles) for every characteristic in the plant • Some alleles mask or hide the presence of other alleles; these are dominant alleles symbolized by a capital letter (eg. P). • Alleles that can be masked are called recessive alleles, symbolized by a lowercase letter (eg. p)
Allelic Combinations: Genotype • Every cell in an organism carries two alleles at a time for every characteristic • For a pea plant flower color, a plant could have cells with PP, Pp, or pp. • Genotype combinations • PP is homozygous dominant, yields purple flowers • pp is homozygous recessive, yields white flowers • Pp is heterozygous, yields purple flowers
Some Definitions • Phenotype • An organism’s physical traits • Genotype • An organism’s genetic makeup • Mendel’s law of segregation • The two members of an allele pair segregate (separate) from each other during the production of gametes.
Following Mendel’s True Breeding Cross True breeding varieties are homozygous PP pp Meiosis Meiosis p p p p Gametes: P P P P Or more simply: P P p p Genotype of the offspring are all Pp in the ratio: 4 Pp: 0 PP: 0 pp (Genotypic ratio) p p Next, determine all the possible ways the gametes of one parent can combine with the gametes of another: Fill out a Punnett Square P P Pp Pp Pp Pp Phenotypic ratio of the offspring: 4 purple: 0 white
Determining the F2 Offspring Pp x Pp
Genetic Alleles and Homologous Chromosomes • Homologous chromosomes • Have genes at specific loci. • Have alleles of a gene at the same locus.
Mendel’s Law of Independent Assortment • A dihybrid cross • Is the mating of parental varieties differing in two characteristics. • Two hypotheses for gene assortment in a dihybrid cross are • Dependent assortment. • Independent assortment • Mendel’s law of independent assortment states that • Each pair of alleles segregates independently of the other pairs during gamete formation.
Genes for Coat Color and Vision Sort Independently into Gametes Figure 9.9
Using a Testcross to Determine an Unknown Genotype • A testcross is a mating between • An individual of unknown genotype and a homozygous recessive individual.
The Rules of Probability • The rule of multiplication states that • The probability of a compound event is the product of the separate probabilities of the independent events.
Human Characteristics That Follow Mendel’s Laws Figure 9.12
Using a Pedigree to Follow an Autosomal Recessive Gene • A family pedigree • Shows the history of a trait in a family. • Allows geneticists to analyze human traits.
Human Disorders Controlled by a Single Gene • Many human traits • Show simple inheritance patterns. • Are controlled by genes on autosomes.
Recessive Disorders • Most human genetic disorders are recessive. • Individuals can be carriers of these diseases.
Dominant Disorders • Some human genetic disorders are dominant. • Achondroplasia is a form of dwarfism.