Chapter 03.2: Extensions To Mendel: Complexities In Relating Genotype To Phenotype

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Are most traits determined by the allelic variations of a single gene or are they multifactorial?

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multifactorial, arising from the action of two or more genes, or from interaction between genes and the environment

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environment

Back 2

In genetics, the encompasses all aspect of the outside world an organism comes into contact

e.g., temperature, diet, exercise, uterine environment, psychological environment

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How common is the sickle cell allele?

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1 in 13 African-Americans are a carrier of the allele

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transgene

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a gene or genetic material that has been transferred naturally or by any of a number of genetic engineering techniques from one organism to another.

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AIDS

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acquired immune deficiency syndrome

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hematopoietic

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the formation of blood cellular components

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genotypic class

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a grouping defined by a set of related genotpes that will produce a particular phenotype. The term is most useful in describing progeny of dihybrid or multihybrid crosses involving complete dominance; for example, in a cross between Aa Bb individuals, the genotypic classes are A- B-, A- bb, aa B-, and aa bb

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complementary gene action

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genes working in tandem to produce a particular trait

e.g., AA bb (white flowers) x aa BB (white flowers) producing Aa Bb (purple flowers)

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What's the explanation for complementary gene action ?

(note: answer refers to the flower example)

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It takes two enzymes catalyzing two separate biochemical reactions to change a colorless precursor into a colorful pigment.

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epistasis

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a gene interaction in which the effects of an allele at one gene hide the effects of alleles at another gene

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epistatic

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the allele that is doing the masking (in epistasis)

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hypostatic

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the gene that is being masked (in epistasis)

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recessive epistasis

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where homozygosity for a recessive allele of a second gene is required to hide the effects of another gene

e.g., in labradors, where --/ee results in a yellow coat

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A 9:3:4 ratio might indicate:

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recessive epistasis

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Bombaby blood is an example of:

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recessive epistasis

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12:3:1 or 13:3 ratios may imply:

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dominant epistasis

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dominant epistasis

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epistasis in which the dominant allele of one gene hides the effects of another gene

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heterogeneous trait

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a mutation at any one of a number of genes can give rise to the same phenotype

e.g., any one of 50 mutations for ear development all cause deafness

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complementation test

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method for discovering whether two mutations are in the same or separate genes

the method of discovering whether a particular phenotype arises from mutations in the same or separate genes

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the occurence of complementation reveals:

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genetic heterogeneity

Front 21

With incomplete dominance, the interaction of two genes can produce ____ different phenotypes for a single trait

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nine different


AA/BB, Aa/Bb, aa/bb, et cetera

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Does the same genotype always produce the same phenotype?

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No!

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retinoblastoma

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the most malignant form of eye cancer; dominant mutation, yet only 75% of carriers develope the disease

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penetrance

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how many members of a population with a particular genotype show the expected phenotype

can be complete (100%) or incomplete

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expressivity

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refers to the degree or intensity with which a particular genotype is expressed in a phenotype

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modifier genes

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have a more subtle, secondary effect; alter the phenotypes produced by the alleles of other genes

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expressivity

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refers to the degree or intensity with which a particular genotype is expressed in a phenotype

can be variable or unvarying

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What leads to the visible effect of the coat on Siamese cats?

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temperature; light areas where the temperature is warm, dark where the temperature is cool

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conditional lethal

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an allele that's lethal only under certain conditions

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permissive

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the range of condition (e.g., temperature) under which the allele remains viable

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restrictive

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the allele that's lethal under certain conditions (e.g., under temperature)

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phenocopy

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a change in phenotype arising from such environmental agents that mimics the effects of a mutation in a gene

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homozygosity for the mutant PKU allele:

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eliminates the activity of a gene encoding the enzyme phenylalanine hydroxylate

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phenylalanine hydroxylate

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converts the amino acid phenylalanine to the amino acid tyrosine

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absence of phenylalanine hydroxylate causes:

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buildup of phenylalanine, resulting in neurological problems

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penetrance

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the occurrence of a phenotype (e.g., heart disease from lack of exercise)

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expressivity

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the seriousness of a phenotype
(e.g., the degree of damage from heart disease)

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century

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a period of 100 years

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quandary

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a state of perplexity or uncertainty, esp. as to what to do; dilemma.

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discontinuous trait

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phenotypes that resulted from alternative alleles that were clear-cut, e.g., either short or tall

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continuous trait

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segregating allels of many different genes whose interaction with each other and the environment produces phenotypes showing continuous variation

e.g., height, weight, skin color in humans

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quantitative traits

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continuous traits

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polygenic

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controlled by multiple genes

they are, by definition, multifactorial
(note that not all multifactorial genes are polgenic)