Genetics Chapter 4: Extensions Of Mendelian Ratios

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These two postulates are the basic principles of gene transmission from parent to offspring:

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that genes are present on homologous chromosomes and that these chromosomes segregate from each other and assort independently from other segregating chromosomes during gamete formation.

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allele

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One of the possible mutational forms of a gene, often distinguished from other alleles by phenotypic effects.
An allele is an alternative form of a gene.

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gene interaction

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Production of novel phenotypes by the interaction of alleles of different genes. a situation in which a single phenotype is affected by more than one set of genes.

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X-linkage

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The pattern of inheritance resulting from genes located on the X chromosome.

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neo-Mendelian genetics

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The course of investigation, stemming from Mendel's findings and following the rediscovery of Mendel's work in the early 1900s, which focuses on the many ways in which genes influence an individual's phenotype.

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wild type

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The most commonly observed phenotype or genotype, designated as the norm or standard.
This is often, but not always, dominant. Wild-type alleles are responsible for the corresponding wild-type phenotype and are the standards against which all other mutations occurring at a particular locus are compared.

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mutant allele

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Contains modified genetic information and often specifies an altered gene product.

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The source of alleles:

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mutation

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For a new allele to be recognized by observation of an organism, what must occur?

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the allele must cause a change in the phenotype.

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loss of function mutation

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Mutations that produce alleles with reduced or no function.

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null allele

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A mutant allele that produces no functional gene product. Usually inherited as a recessive trait.

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gain of function mutation

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A mutation that produces a phenotype different from that of the normal allele and from any loss of function alleles. generally result in dominant alleles, since one copy of the mutation in a diploid organism is sufficient to alter the normal phenotype. Examples of gain-of-function mutations include the genetic conversion of proto-oncogenes, which regulate the cell cycle, to oncogenes, where regulation is overridden by excess gene product. The result is the creation of a cancerous cell.

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New phenotypes result from:

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changes in functional activity of gene product

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neutral mutation

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A mutation with no immediate adaptive significance or phenotypic effect. The gene product presents no change to either the phenotype or to the evolutionary fitness of the organism.

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wild-type trait is denoted by:

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superscript +

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leu

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System of Genetic Nomenclature for bacteria with mutation blocking leucine biosynthesis.

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BRCA1

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System of Genetic Nomenclature for a human gene associated with inherited risk of breast cancer.

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How Alleles Affect Phenotype?

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Not always simple dominant / recessive issue

Gene interaction
Phenotype controlled by more than one gene

Sex-linked genes
(X-linkage in X/Y organisms)

Phenotype can depend on genotype and environmental effects.

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incomplete dominance

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Expressing a heterozygous phenotype that is distinct from the phenotype of either homozygous parent. Also called partial dominance.

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Tay–Sachs disease

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human biochemical disorder in which homozygous recessive individuals are severely affected with a fatal lipid-storage disorder and neonates die during their first one to three years of life. Heterozygotes, with only a single copy of the mutant gene, are phenotypically normal, but with only about 50 percent of the enzyme activity found in homozygous normal individuals.

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threshold effect

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whereby normal phenotypic expression occurs anytime a certain level of gene product is attained. Most often, and in particular in Tay–Sachs disease, the threshold is less than 50 percent.

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codominance

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Condition in which the phenotypic effects of a gene’s alleles are fully and simultaneously expressed in the heterozygote.
The MN blood group in humans illustrates this phenomenon as well as the ABO blood-type system.

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Codominant inheritance is characterized by:

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istinct expression of the gene products of both alleles. This characteristic distinguishes codominance from incomplete dominance, where heterozygotes express an intermediate, blended, phenotype. For codominance to be studied, both products must be phenotypically detectable.

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How can we spot incomplete dominance?

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In the case of red and white snapdragons, A cross of two pink individuals will produce red, white and pink individuals.
The phenotypic ratio will be 1:2:1, like the genotypic ratio.
Heterozygotes show a distinct intermediate phenotype- different from homozygous genotypes
Neither trait is dominant

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T/F: Clear-cut [visual] examples of incomplete dominance are common.

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False. Clear-cut [visual] examples of incomplete dominance are relatively rare. Individuals may appear completely dominant until viewed at the molecular level.

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Number of different alleles for Hb?

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36

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multiple alleles

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In a population of organisms, three or more alleles of the same gene.

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T/F: Multiple alleles can be studied only in populations.

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True. Any individual diploid organism has, at most, two homologous gene loci that may be occupied by different alleles of the same gene. However, among members of a species, numerous alternative forms of the same gene can exist.

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ABO blood groups

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The ABO system, like the MN blood types, is characterized by the presence of antigens on the surface of red blood cells. The A and B antigens are distinct from the MN antigens and are under the control of a different gene, located on chromosome 9. As in the MN system, one combination of alleles in the ABO system exhibits a codominant mode of inheritance.

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How is the ABO phenotype of any individual ascertained?

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by mixing a blood sample with an antiserum containing type A or type B antibodies. If an antigen is present on the surface of the person's red blood cells, it will react with the corresponding antibody and cause clumping, or agglutination, of the red blood cells. When an individual is tested in this way, one of four phenotypes may be revealed. Each individual has either the A antigen (A phenotype), the B antigen (B phenotype), the A and B antigens (AB phenotype), or neither antigen (O phenotype).

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isoagglutinogen

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An antigenic factor or substance present on the surface of cells that is capable of inducing the formation of an antibody (e.g., the A and B antigens on the surface of human red blood cells).

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Describe the Bombay Phenotype:

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Woman typed as type O, but
One parent has type AB blood and
She is an obvious IB allele donor to two children…

Woman subsequently found to be homozygous FUT1 at the fucosyl transferase locus

No fucose on H substance, no substrate to make A or B antigens
Example of epistasis (more later)

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he specificity of the A and B antigens is based on:

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the terminal sugar of the carbohydrate group.

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H substance

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The carbohydrate group present on the surface of red blood cells to which the A and/or B antigen may be added. When unmodified, it results in blood type O.

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wild-type FUT1 allele

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present in almost all humans, directs the conversion of a precursormolecule to the H substance by adding a molecule of fucose to it. The I A and I Balleles are then able to direct the addition of terminal sugar residues to the Hsubstance. The I O allele is unable to direct either of these terminal additions. Failure to produce the H substance results in the Bombay phenotype