Biology 130 October 27, 2000

Mendel and the Gene Idea

Reading Assignment: Chapter 15, pp. 261-269 (Note that you should have read Chapter 14 by now)

Homework Assignment: Drosophila crosses using the Virtual Fly Lab. Due in class on Monday, October 30.

Check the web site for BioCoach exercises and links to information about various human diseases.

Extending Mendelian Genetics, continued:

• Pleiotropy: ability of a gene to affect an organism in more than one way (i.e. more than one phenotypic manifestation) example of sickle cell anemia; and other diseases
• Polygenic inheritance: additive effect of two or more genes on a single phenotypic character (the opposite of pleiotropy) example in book is skin color
• Environmental impact on phenotype: Environment can have an impact as well as genotype. flower color is one example; height that you reach, body development; development of certain diseases with genetic components is another. For example: diabetes, heart disease,

Mendelian Inheritance in Humans:

• Humans aren’t great experimental system
• Long reproductive cycle
• You can’t do controlled breeding.
• Instead use pedigree analysis
• Circles indicate females, squares indicate males
• Filled symbols indicate individuals expressing trait being followed (i.e. widow’s peak)
• Usefulness of genetically isolated populations

Recessively Inherited Disorders:

• Traits inherited in a recessive manner are indicated by half filled squares or circles

• Individuals who have one recessive allele are called carriers, because they may transmit the disease to their offspring.
• Examples include cystic fibrosis, Tay-Sachs, sickle cell anemia

Dominantly Inherited Disorders

• One dominant allele causes the disease
• Examples include Huntington’s disease and achondroplasia

Lethal Genes

• Alleles or pair of alleles that adversely affect viability
• Dominant mutations die out instantly
• Recessive alleles cause lethality when homozygous
• Example: yellow coat color allele in mice

Multifactoral Disorders

• Combination of certain alleles at a particular genetic locus plus environmental influence lead to some diseases
• Some diseases may be affected by one gene; while others may be affected by multiple genes
• Examples include heart disease, diabetes, some forms of cancer

Genetic Testing and Counseling

• Diagnosis of carriers
• Biochemical (i.e. Tay-Sachs)
• Genetic (i.e. Huntington’s disease)
• Ethical considerations
• Fetal diagnosis-
• Amniocentesis
• Chorionic villi sampling

Chapter 15: The Chromosomal Basis of Inheritance

The Chromosomal Basis of Mendel’s Laws

• Chromosome Theory of Inheritance: Mendelian genes have specific loci on chromosomes, and it is the chromosome that undergoes segregation and independent assortment
• Figure 15-1

Thomas Hunt Morgan and Drosophila

• Used Drosophila melanogaster as his experimental organism
• Nomenclature should be familiar now
• Discovery of white eye mutation
• Linked to X chromosome
• Genes located on sex chromosomes are called sex-linked genes
• Many more genes on X chromosomes than Y
• Sex-linked inheritance Figure 15-3

Linked genes tend to be inherited together because they are on the same chromosome

• Genes located on the same chromosome tend to be inherited together in genetic crosses.
• Such genes are termed linked genes
• Drosphila example of linked genes
• Body color (gray = wild type b⁺; black = mutant b)
• Note that in contrast to the b gene, the body color mutation used in lab, ebony (eb), is located on a different chromosome than the vg gene.
• Wing shape (normal = wild type vg⁺; vestigial = mutant vg)
• Do a “test cross” of an F₁ female to a homozygous recessive male (Figure 15.4)
• Do not see a 1:1:1:1 ratio of offspring phenotypes
• “Parental” phenotypes majority of offspring
• Genes for wild type/black body color and normal/vestigial wings must be located on same chromosome
• These genes are inherited together unless something happens to separate them
• New phenotypes result from genetic recombination

Independent assortment of chromosomes and crossing over produce genetic recominants

• Parental phenotypes: offspring showing same phenotype as one of the parents in a test cross, example of pea plant YyRr (F₁ of YYRR x yyrr) crossed to yyrr plant. YR and yr are parental phenotypes
• Recombinant phenotypes: offspring showing different combinations of traits than parents. In above example, Yr and yR would be recombinant phenotypes
• Frequency of recombinant phenotypes is the % recombination
• Unlinked genes (i.e. genes on different chromosomes) show 50% recombination
• Separation of unlinked genes occurs at metaphase I of meiosis

Recombination of linked genes

• Recombinant phenotypes are seen between linked genes due to crossing over
• Figure 15-5

Recombination data can be used to generated genetic maps

• First developed by Alfred Sturtevant
• The greater the distance between two genes, the greater the percentage of recombinant phenotypes
• Figure 15-6
• 1% recombination = 1 map unit = 1 centimorgan
• Linkage maps are relative maps of chromosomes; they give the order of the gene, not the absolute distance between them Figure 15-7