Chapter 4_Extensions Of Mendelian Inheritance Flashcards
Mendelian inheritance
Describes inheritance patterns that obey two laws: the law of segregation and the law of independent assortment.
Simple Mendelian Inheritance
One allele is dominant over the other.
Describe Simple Mendelian on an inheritance and molecular level.
- Inheritance: This term is commonly applied to the inheritance of alleles that obey Mendel’s laws and follow a strict dominant/recessive relationship.
- Molecular: 50% of the protein, produced by a single copy of the dominant (functional) allele in the heterozygote, is sufficient to produce the dominant trait.
Describe Incomplete Dominance on an inheritance and molecular level.
- Inheritance: This pattern occurs when the heterozygote has a phenotype that is intermediate between either corresponding homozygote.
- Molecular: 50% of the protein, produced by a single copy of the functional allele in the heterozygote, is not sufficient to produce the same trait as the homozygote making 100%.
Describe Incomplete Penetrance on an inheritance and molecular level.
- Inheritance: This pattern occurs when a dominant phenotype, is not expressed even though an individual carries a dominant allele.
- Molecular: Even though a dominant gene may be present, the protein encoded by the gene may not exert its effects. This can be due to environmental influences or due to other genes that may encode proteins that counteract the effects of the protein encoded by the dominant allele.
Describe Overdominance on an inheritance and molecular level.
- Inheritance: This pattern occurs when the heterozygote has a trait that is more beneficial than either homozygote.
- Molecular: Three common ways that heterozygotes gain benefits:
- 1.) Their cells may have increased resistance to infection by microorganisms.
- 2.) They may produce more forms of protein dimers, with enhanced function.
- 3.) They may produce proteins that function under a wider range of conditions.
Describe Codominance on an inheritance and molecular level.
- Inheritance: This pattern occurs when the heterozygote expresses both alleles simultaneously.
- Molecular: The codominant alleles encode proteins that function slightly differently from each other, and the function of each protein in the heterozygote affects the phenotype uniquely.
Describe X-Linked on an inheritance and molecular level.
- Inheritance: This pattern involves the inheritance of genes that are located on the X chromosome.
- Molecular: If a pair of X-linked alleles shows a simple dominant/recessive relationship, 50% of the protein, produced by a single copy of the dominant allele in a heterozygous female, is sufficient to produce the dominant trait (in the female).
Describe Sex-Influenced Inheritance on an inheritance and molecular level.
- Inheritance: This pattern refers to the effect of sex on the phenotype of the individual. Some alleles are recessive in one sex and dominant in the opposite sex.
- Molecular: Sex hormones may regulate the molecular expression of genes. This can influence the phenotypic effects of alleles.
Describe Sex-Limited Inheritance on an inheritance and molecular level.
- Inheritance: This refers to traits that occur in only one of the two sexes.
- Molecular: Sex hormones may regulate the molecular expression of genes. This can influence the phenotypic effects of alleles. In this case, sex hormones that are primarily produced in only one sex are essential to produce a particular phenotype.
Describe Lethal Alleles on an inheritance and molecular level.
- Inheritance: An allele that has the potential of causing the death of an organism.
- Molecular: Lethal alleles are most commonly loss-of-function alleles that encode proteins that are necessary for survival. In some cases, the allele may be due to a mutation in a nonessential gene that changes a protein to functino with abnormal and detrimental consequences.
Wild-Type Alleles
Prevalent alleles in a natural population. At the molecular level, a wild-type allele typically encodes a protein that is made in the proper amount and functions normally.
Can more than one wild type allele occur in large populations?
Yes, this is referred to as genetic polymorphism.
Mutant Alleles
Random mutations that occur in populations and alter preexisting alleles.
Are random mutations more often helpful or disruptive?
More disruptive to gene function. This means that mutant alleles are often defective in their ability to express a functional protein.
What is a big difference between wild-type and mutant alleles?
The recessive allele usually contains a mutation that causes a defect in the synthesis of a fully functional protein.
Gain-Of-Function Mutations
Change the gene or the protein encoded by a gene so that it gains a new or abnormal function.
How can a mutant allele be dominant over a wild-type allele?
Gain of function mutation, dominant negative mutation, or haploinsufficiency.
Dominant-Negative Mutations
The protein encoded by the mutant gene acts antagonistically to the normal protein. In a heterozygote, the mutant protein counteracts the effects of the normal protein and thereby alters the phenotype.
Haploinsufficiency
The mutant allele is a loss of functino allele. It is used to describe patterns of inheritance in which a heterozygote (with one functional allele and one inactive allele) exhibits an abnormal or disease phenotype.
What is polydactyly, and what causes it?
Polydactyly is a trait that causes the affected individual to have additional fingers or toes (or both). It is due to an autosomal dominant allele (the allele is found in a gene located on an autosome (not a sex chromosome) and a single copy of this allele is sufficient to cause this condition.
How do recessive traits effect incomplete penetrance?
For recessive traits, incomplete pentrance would occur if a homozygote carrying the recessive allele did not exhibit the recessive trait.
Expressivity
The degree to which the trait is expressed.
How do we explain incomplete penetrance and variable expressivity?
The range of phenotypes is often due to environmental influences and/or due to effects of another gene.
Give an example of the relationship between environment and phenotype (a DRAMATIC one).
Penylketonuria (PKU) is an autosomal recessive disease is caused by a defect in a gene that encodes the enzyme phenylalanine hydroxylase. Homozygous individuals with this defective allele are unable to metabolize the amino acid phenylalanine properly. PKU individuals manifest a variety of detrimental traits including mental impairment, underdeveloped teeth, and foul-smelling urine. If diagnosed early and treated, they develop normally.
Norm of Reaction
The effects of environmental variation on a phenotype. Specifically, it is the the phenotypic range seen in individuals with a particular genotype.
