Chapter 5
Important characteristics of dominance
1. Dominance is an allelic interaction 2. Dominance doesn't alter the way that genes are transmitted; only the way that genes are expressed; Interaction between the products of the genes 3. Dominance is determined by the level that you look at it
How could a flower show codominance if it had a pink color?
A flower could show codominance if a red pigment and a white pigment were produced and combined to create a pink pigment; thus, both traits would be expressed
Allelic mutation
A mutation that occurs at the same locus
pleiotropy
A single genotype influences multiple phenotypes.
compound heterozygote
An individual with two different recessive alleles at a locus that results in a recessive phenotype. (Example: in cystic fibrosis, have a BUNCH of genes at the CTFR locus that control whether or not someone will have CF; if two of these different genes are defective, they are heterozygous genotypically, but show the phenotype of someone who is homozygous recessive)
How does cystic fibrosis prove that the level of phenotype that you observe effects the type of dominance that exists?
At the molecular level, people heterozygous for a defective cystic fibrosis transmembrane conductance regulator (CFTR) have both normal and defective CTFRs. Thus, at the molecular level, it displays codominance. However, at the physiological level, it displays complete dominance, because the normal CTFRs compensate for the fact that there's defective CTFRs
Why do Mexican hairless dogs who are hairless have a genotype that is homozygous?
Because the trait for being hairless is a recessive lethal allele, but it's dominant for hairless-ness (i.e. if you bred two heterozygotes together 1/4 of the offspring would die because they would possess two HH alleles; heterozygotes and homozygotes for the non-hairless allele would survive)
lethal allele
Causes the death of an individual organism, often early in development, and so the organism does not appear in the progeny of a genetic cross. Recessive lethal alleles kill individual organisms that are homozygous for the allele; dominant lethals kill both heterozygotes and homozygotes. Ratios that have three in the denominator are typically recessive, since 1/4 of the progeny in a cross between two heterozygotes with this trait usually die (assuming that the trait is recessive for lethality)
sex-limited characteristic
Characteristic encoded by autosomal genes and expressed in only one sex. Both males and females carry genes for sex-limited characteristics, but the characteristics appear in only one of the sexes. An extreme form of a sex-influenced characteristic. (Example: precocious puberty in humans; autosomal dominant characteristic that is shown only in males. While females can inherit this characteristic and pass it onto their offspring, they cannot display the traits of this; since it's a rare dominant disease, normal genotype is Pp)
sex-influenced characteristic
Characteristic encoded by autosomal genes that are more readily expressed in one sex. For example, an autosomal dominant gene may have higher penetrance in males than in females or an autosomal gene may be dominant in males but recessive in females. Inherited by Mendel's rules.
quantitative characteristic
Continuous characteristic; displays a large number of possible phenotypes, which must be described by a quantitative measurement.
expressivity
Degree to which a trait is expressed.
multifactorial characteristic
Determined by multiple genes and environmental factors.
genetic maternal effect
Determines the phenotype of an offspring. With genetic maternal effect, an offspring inherits genes for the characteristics from both parents, but the offspring's phenotype is determined not by its own genotype but by the nuclear genotype of its mother.
genomic imprinting
Differential expression of a gene that depends on the sex of the parent that transmitted the gene. If the gene is inherited from the father, its expression differs from that if the gene is inherited from the mother. (Example: Prader-willi vs Angelmann syndrome; both are the same deletion on the same chromosome, but when inherited from the father you get PW and when inherited from the mother you get Angelmann)
continuous characteristic
Displays a large number of possible phenotypes that are not easily distinguished, such as human height.
polygenic characteristic
Encoded by genes at many loci.
discontinuous characteristic
Exhibits only a few, easily distinguished phenotypes. An example is seed shape in which seeds are either round or wrinkled.
temperature-sensitive allele
Expressed only at certain temperatures. (Example: Rabbits that are brown tipped only at low temperatures, but completely white when reared at high temperatures)
hypostatic gene
Gene that is masked or suppressed by the action of a gene at a different locus.
How might you determine whether a particular trait is due to cytoplasmic inheritance or to genetic maternal effect?
If the father has an impact; with genetic maternal effect, it's the nuclear genotype of the mother that determines the phenotype of the offspring. On the other hand, cytoplasmic inheritance is based only in the cytoplasm of the mother; thus, only the mother plays a role in this kind of inheritance. To test for this, you could see if the F2 generation was impacted. If the results were seemingly random, then it could be cytoplasmic. If it was impacted by the parental group's genotype, then you could determine that it was genetic maternal effect.
Why is it that cytoplasmic inheritance can display so much variation?
If you have a cell with some mitochondria that displays mutant traits, and some that are wild type, when cytokinesis occurs, some of that mitochondria is going to be randomly segregated into different cells during cell division. Thus, you can end up with different traits depending upon how it segregates. This explains why all progeny of variegated plants aren't variegated; it depends upon where the different chloroplasts end up when the cells divide. This also explains why mitochondrial diseases are inherited only by the mother; since the egg cells house all of the mitochondria, the mitochondria is going to go into the offspring.
gene interaction
Interaction between genes at different loci that affect the same characteristic. (Example: in peppers, color is impacted at two loci (Y locus and C locus; if you have one loci that is dominant, and one that is recessive or both that are dominant and both that are recessive you get different phenotypes)
What is the difference between incomplete and complete dominance?
In incomplete dominance, the heterozygote has a different phenotype than the homozygotes. In complete dominance, the heterozygote has the same trait as the homozygote with the dominant trait.
anticipation
Increasing severity or earlier age of onset of a genetic trait in succeeding generations. For example, symptoms of a genetic disease may become more severe as the trait is passed from generation to generation.
cytoplasmic inheritance
Inheritance of characteristics encoded by genes located in the cytoplasm. Because the cytoplasm is usually contributed entirely by only one parent, most cytoplasmically inherited characteristics are inherited from a single parent.
epistatic gene
Masks or suppresses the effect of a gene at a different locus.
What type of epigenetic mark is responsible for genomic imprinting?
Methylation that is removed from the germ line cells but then replaced in the gametes
Dominant Epistasis
Only a single copy of an allele is required to inhibit the expression of the allele at a different locus. (12:3:1 phenotypic ratio) (Example: in squash, all you need is one W allele at the W locus to produce a plant that is white; to produce a green squash you need the plant to be homozygous recessive at both the W locus AND at the Y locus) (Phenotypic ratio - 12:3:1)
penetrance
Percentage of individuals with a particular genotype that express the phenotype expected of that genotype.
epigenetics
Phenomena due to alterations to DNA that do not include changes in the base sequence; often affect the way in which the DNA sequences are expressed. Such alterations are often stable and heritable in the sense that they are passed from one cell to another.
phenocopy
Phenotype that is produced by environmental effects and is the same as the phenotype produced by a genotype.
multiple alleles
Presence in a group of individuals of more than two alleles at a locus. However, each member of the group has only two of the possible alleles. (Example: in ducks have three different alleles for color; R = restricted M = mallard and d = dusky; in terms of dominance, you have R > M > d)
incomplete penetrance
Refers to a genotype that does not always express the expected phenotype. Some individuals possess the genotype for a trait but do not express the phenotype.
complete dominance
Refers to an allele or a phenotype that is expressed in homozygotes (AA) and in heterozygotes (Aa); only the dominant allele is expressed in a heterozygote phenotype. In a test cross with a heterozygote, we would expect a 3:1 phenotypic ratio and a 1:2:1 genotypic ratio
incomplete dominance
Refers to the phenotype of a heterozygote that is intermediate between the phenotypes of the two homozygotes. When a trait displays incomplete dominance, the genotype matches the phenotype
What is the difference between sex limited/sex influenced and sex linked characteristics?
Sex limited traits and sex influenced traits are found on autosomal chromosomes. They can be inherited by both men and women, but show decreased (either to very little or none at all) penetrance in one sex, and have a tendency to only show in one sex. They may be recessive in one sex and dominant in the other (example: the beard allele in goats is dominant for males but recessive for females). In the case of sex limited, there is no penetrance in one sex, although both sexes can carry the allele. (I.e. only one sex can display the trait). Sex linked characteristics, on the other hand, are found on sex chromosomes. They tend to be inherited more by men than by women, although women can inherit the traits as well. An example of this is red-green color blindness.
complementation test
Test designed to determine whether two different mutations are at the same locus (are allelic) or at different loci (are nonallelic). Two individuals that are homozygous for two independently derived mutations are crossed, producing F1 progeny that are heterozygous for the mutations. If the mutations are at the same locus, the F1 will have a mutant phenotype. If the mutations are at different loci, the F1 will have a wild-type phenotype. Note: this works for recessive mutations
Recessive epistasis
The presence of two recessive alleles (the homozygous genotype) inhibits the expression of an allele at a different locus (Example: labs will be yellow is the E locus is homozygous recessive; this is because the E locus determines pigment deposition) (9:3:4 phenotypic ratio)
What would it mean to say that something has a low penetrance but a high expressivity?
This would mean that not everyone who has the gene expresses the trait, but everyone who does express the trait tends to express it to its fullest degree
How does anticipation occur?
Trinucleotide repeats that slip off and form hairpin loops; eventually get straightened out, but as the DNA pol catches up, the repeats get longer and longer --> more severe/earlier onset of diseases
complementation
Two different mutations in the heterozygous condition are exhibited as the wild-type phenotype; indicates that the mutations are at different loci.
codominance
Type of allelic interaction in which the heterozygote simultaneously expresses traits of both homozygotes.
epistasis
Type of gene interaction in which a gene at one locus masks or suppresses the effects of a gene at a different locus.
Duplicate Recessive Epistasis
When two traits can mask the effect of the other; must be dominant at BOTH locations (i.e. can be Bb or BB; genotype could be written as B_A_) (9:7 ratio)