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When heterozygote falls in between the phenotypes of two homozygotes
It affects the phenotype that genes produce but not the way in which genes are inherited
Phenotype of the heterozygote is the same as the phenotype of one of the homozygotes
The result of interactions between genes at the same locus
Does not alter the way in which genes are inherited only influences the way in which they are expressed as phenotype
Phenotype of the heterozygote is not intermediate between the phenotypes of the homozygotes; rather the heterozygote simultaneously expresses the phenotypes of both homozygotes.
The genotype does not always produce the expected phenotype.
Ex. Polydactyl - ppl may still have a normal number of fingers and toes
The percentage of individual organisms having a particular genotype that express the expected phenotype.
The degree to which a character is expressed.
ex. Polydactyl - ppl may have a stump instead of an entire finger. Thus only a certain degree of expressivity
Incomplete Penetrance and variable expressivity are due:
To effect of other genes and environmental factors that can alter or completely suppress the effect of a particular gene.
The mere presence of a gene does not guarantee it's expression
Causes death at an early stage of development:
Usually 2:1 ratio for recessive lethal allele
For some loci, more than two alleles are present within a group of orgasims, the locus has multiple alleles.
ABO Blood groups.
The three common alleles for the ABO blood group locus are: IA
, which encodes the A antigen; IB
, which encodes the B antigen; and i, which encodes no antigen.
O blood type
AB Blood Type
The products of genes at different loci combine to produce new phenotypes that are not predictable from the single-locus effects alone.
When one gene masks the effect of another gene at different locus
Similar to dominance, except dominance entails the masking of genes at the same locus.
The gene that does the masking
The gene whose effect is masked
Two recessive alleles required to mask other gene.
Only a single copy of an allele is required to inhibit the expression of the allele at a different locus
Duplicate recessive epistasis
Two recessive alleles at either of two loci are capable of suppresing a phenotype. Either or.
Dihybrid cross produces:
Trihybrid cross produces:
Used to determine whether two mutations occur at the same locus or occur at different loci.
Parents are homozygous for different mutations are crossed. If an individual organism possessing two recessive mutations has a wild type phenotype, indicates that mutations are non allelic genes.
X linked traits, passed from father to daughter
Y linked traits are passed from father to all sons
Are determined by autosomal genes but they are expressed differently in males and females.
Are encoded by autosomal genes that are more readily expressed in one sex.
Sex limited characteristics
Are encoded by autosomal genes whose expression is limited to one sex.
Some characteristics encoded by genes located in the cytoplasm.
A zygote inherits nuclear genes from both parents; but typically, all its cytoplasmic organelles, and thus all its cytoplasmic genes, come from one of the gametes, usually the egg.
No mechanism analogous to mitosis or meiosis ensures that cytoplasmic genes are evenly distributed during cell division
Genetic maternal effect
The phenotype of the offspring is determined by the genotype of the mother.
The genes are inherited from both parents, but the offspring's phenotype is determined not by its own genotype but by the genotype of its mother.
The phenotype of the progeny is NOT necessarily the same as the phenotype of the mother, because the progeny's phenotype is determined by the mother's genotype, NOT her phenotype.
The differential expression of genetic material depending on whether it is inherited from the male or female parent.
The expression of some genes is significantly affected by their parental origin.
The key to whether the gene is expressed is the sex of the parent transmitting the gene
Imprinting and genetic conflict:
The male parent passes on alleles that promote maximum fetal growth of their offspring.
Maternal alleles that cause more-limited fetal growth are favored because committing too many of the femal parent's nutrients to any one fetus may limit her ability to reproduce in the future and because giving birth to very large babies is difficult and risky for the mother
Reversible changes to DNA that influence do not alter the base sequence but may affect how a gene is expressed.
A genetic trait becomes more strongly expressed or is expressed at an earlier age as it is passed from generation to generation. It is caused by an unstable region of DNA that increases in size from generation to generation.
Temperature sensitive allele
An allele whose product is functinoal only certain temperatures.
Ex. Bunny with dark ears and legs at lower temperatures
Environmental factors alone that can produce a phenotype that is the same as the phenotype produced by a genotype.
Ex. Flies exposed to a chemical can also produce white eyes.
Characteristics which have a few easily distinguised phenotypes.
Ex. Coats of dogs are black, brown, or yellow. Blood types are A, B, AB, or O.
Characteristics that exhibit continuous distribution of phenotypes. They have many possible phenotypes.
AKA. Quantitative characteristics.
Characteristics encoded by genes at many loci
One gene affects multiple characteristics
Many continuous characteristics that are both polygenic and influenced by environmental factors. Many factors help determine phenotype.
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