Genetic Inheritance According to Gregor Mendel CHAPTER 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 InheritanceGregor MendelWas the first person to analyze patterns of inheritance.Deduced the fundamental principles of genetics.
In an Abbey GardenMendel studied garden peas Pea flowers have both male and female partsThe ovary of the carpel produces eggs by meiosisThe anthers of the stamens produce pollen (sperm equivalent) by meiosisPollen grains fly from the anthers of the same flower or from another plant to the carpel and fertilize the eggs in the ovaryPea flowers are enclosed within petals such that only insect pollination can effectively transfer pollen from one plant to another
Figure 9.4How Mendel Fertilized Pea Flower Eggs and Ensured a Single Pollen Source
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.
Figure 9.5True Breeding Varieties Available to Mendel
Figure 9.6aA Single-Factor (Monohybrid) Cross
Hypotheses From The Monohybrid CrossSome alleles mask or hide the presence of other alleles; these are dominant alleles symbolized by a capital letter (eg. P). There are two gene forms (alleles) for every characteristic in the plant The letters P and p were used to designate flower color: P = purple, p = whiteThere are alternative forms of genes called allelesAlleles that can be masked are called recessive alleles, symbolized by a lowercase letter (eg. p)
Every cell in an organism carries two alleles at a time for every characteristicFor a pea plant flower color, a plant could have cells with PP, Pp, or pp.Allelic Combinations: GenotypeGenotype combinationsPP is homozygous dominant, yields purple flowerspp is homozygous recessive, yields white flowersPp is heterozygous, yields purple flowers
PhenotypeAn organisms physical traitsGenotypeAn organisms genetic makeupMendels law of segregationThe two members of an allele pair segregate (separate) from each other during the production of gametes.
Following Mendels True Breeding Cross PP ppP P P PMeiosisMeiosisp p p p Gametes:P P p pOr more simply:Next, determine all the possible ways the gametes of one parent can combine with the gametes of another:Fill out a Punnett SquareP
Pp pPp Pp
Genotype of the offspring are all Pp in the ratio:4 Pp: 0 PP: 0 pp (Genotypic ratio)Phenotypic ratio of the offspring:4 purple: 0 whiteTrue breeding varieties are homozygous
Determining the F2 Offspring Pp x Pp
Genetic Alleles and Homologous ChromosomesHomologous chromosomesHave genes at specific loci.Have alleles of a gene at the same locus.
Mendels Law of Independent AssortmentA dihybrid crossIs the mating of parental varieties differing in two characteristics.Two hypotheses for gene assortment in a dihybrid cross areDependent assortment.Independent assortmentMendels law of independent assortment states thatEach pair of alleles segregates independently of the other pairs during gamete formation.
Figure 9.9Genes for Coat Color and Vision Sort Independently into Gametes
Using a Testcross to Determine an Unknown GenotypeA testcross is a mating betweenAn individual of unknown genotype and a homozygous recessive individual.
The Rules of ProbabilityThe rule of multiplication states thatThe probability of a compound event is the product of the separate probabilities of the independent events.
Figure 9.12Human Characteristics That Follow Mendels Laws
A family pedigreeShows the history of a trait in a family.Allows geneticists to analyze human traits.Using a Pedigree to Follow an Autosomal Recessive Gene
Human Disorders Controlled by a Single GeneMany human traitsShow simple inheritance patterns.Are controlled by genes on autosomes.
Recessive DisordersMost human genetic disorders are recessive.Individuals can be carriers of these diseases.
Dominant DisordersSome human genetic disorders are dominant.Achondroplasia is a form of dwarfism.