CHAPTER 9CHAPTER 9

Patterns of Inheritance

• Genetic testing

– Allows expectant parents to test for possibilities in their unborn child.

– Includes amniocentesis and CVS.

– Has risks associated with it.

Biology And Society: Testing Before Birth

• Gregor Mendel

– Was the first person to analyze patterns of inheritance.

– Deduced the fundamental principles of genetics.

Heritable Variation and Patterns of Inheritance

In an Abbey Garden

• Mendel studied garden peas because

– These plants are easily manipulated.

– These plants can self-fertilize.

• Mendel carried out some cross-fertilization

Figure 9.4

• He also created true-breeding varieties of plants.

• Mendel then crossed two different true-breeding varieties.

Mendel’s Law of Segregation

• Mendel performed many experiments.

– He tracked several characteristics in pea plants from which he formulated several hypotheses.

Figure 9.5

Monohybrid Crosses

• A monohybrid cross is a cross between parent plants that differ in only one characteristic.

• Mendel developed four hypotheses from the monohybrid cross:

– There are alternative forms of genes, called

alleles.

– For each characteristic, an organism inherits two alleles, one from each parent.

– Alleles can be dominant or recessive.

– Gametes carry only one allele for each inherited characteristic.

• Mendel’s law of segregation

– The two members of an allele pair segregate (separate) from each other during the production of gametes.

• An explanation of Mendel’s results, including a

Punnett square

Figure 9.6b

• Phenotype

– An organism’s physical traits

• Genotype

– An organism’s genetic makeup

Genetic Alleles and Homologous Chromosomes

• Homologous chromosomes

– Have genes at specific loci.

– Have alleles of a gene at the same locus.

Figure 9.7

• Homozygous– When an organism has identical alleles for a gene

• Heterozygous– When an organism has different alleles for a gene

Mendel’s Law of Independent Assortment

• A dihybrid cross

– Is the mating of parental varieties differing in two characteristics.

Figure 9.8

• Mendel’s law of independent assortment states that

– Each pair of alleles lines up without caring which side its on during gamete formation.

Using a Testcross to Determine an Unknown Genotype

• A testcross is a mating between

– An individual of unknown genotype and a homozygous recessive individual.

Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Figure 9.10

Figure 9.12

• A family pedigree

– Shows the history of a trait in a family.

– Allows geneticists to analyze human traits.

Figure 9.13

Human Disorders Controlled by a Single Gene

• Many human traits

– Show simple inheritance patterns.

– Are controlled by genes on autosomes.

Table 9.1

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.

Incomplete Dominance in Plants and People

• In incomplete dominance, F1 hybrids have an appearance in between the phenotypes of the two parents.

ABO Blood Type: An Example of Multiple Alleles and Codominance

• The ABO blood groups in humans are an example of multiple alleles.

Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Polygenic Inheritance

• Polygenic inheritance is the additive effects of two or more genes on a single phenotype.

Sex Chromosomes and Sex-Linked Genes

• Sex chromosomes

– Influence the inheritance of certain traits.

Sex Determination

• Sex chromosomes

– Are designated X and Y.

– Determine an individual’s sex.

Sex-Linked Genes

• Sex-linked genes

– Are any genes located on a sex chromosome.

– Were discovered during studies on fruit flies.

• Inheritance patterns of a sex-linked gene

Sex-Linked Disorders in Humans

• A number of human conditions result from sex-linked (X-linked) genes.

• Red-green color blindness

– Is characterized by a malfunction of light-sensitive cells in the eyes.

• Hemophilia

– Is a blood-clotting disease.