MBG 2040 Study Guide - Midterm Guide: Lethal Allele, Sickle-Cell Disease, Gregor Mendel
18 Oct 2016
School
Department
Course
Professor
MBG 2040
SLG TUES THURS 4-5:30 LIB 061
REVIEW
- Gene is the fundamental unit of heredity
- Come in multiple forms called alleles
- Genes are located on chromosomes
- DNA carries genetic information
- Genetic information is transferred from DNA to RNA to protein
Definitions
Gene: a region of DNA that helps determine
a characteristic
Allele: one of two or more alternative forms
of a gene
Locus: specific place on a chromosome
occupied by an allele
Genotype: set of alleles possessed by an
individual organism
Heterozygote: individual possessing two
different alleles at a locus
Homozygote: individual possessing two of
the same alleles at a locus
Phenotype: appearance or manifestation of
a character
Gregor Mendel
- 3:1 ratio in monohybrid crosses
- 9:3:3:1 ratio in dihybrid crosses
Pedigree Assumptions
- trait is rare and autosomal recessive, individuals who marry into the pedigree are not
carriers but are homozygous for the normal allele
- trait is rare and dominant, then affected individuals are usually heterozygous
Recessive Mutations
- often involve loss of gene function
Null/Amorphic Alleles
- a non-functional protein is produced or
- no protein is produced
Hypomorphic Alleles
- a poorly functioning protein is produced or
- reduced amounts of a normal functioning protein is produced
find more resources at oneclass.com
find more resources at oneclass.com
Autosomal Dominant Traits
- involve gain/change of gene function
- traits that are rare and dominant, affected individuals are most likely heterozygous (Bb)
- atig’s / B idiiduals ould e rare, geerall see B rosses
Dominant Hypermorphic Alleles
- negative phenotypic outcome due to over-production of a normal protein or
- negative phenotypic outcome due to the production of a protein with increased activity
levels
Neomorphic Alleles
- negative phenotypic outcome bc of the presence of an altered protein that has a new
function
- negative phenotypic outcome when the altered protein interferes with the wildtype
protein (dominant-negative alleles)
Complete Dominance/Complete Recessiveness
- extremes of a range (BB and bb)
Incomplete Dominance
- BB, Bb and bb all differ phenotypically
- Bb is intermediate between homozygous phenotypes (red, white, pink)
Codominance
- BB, Bb and bb all differ phenotypically
- Bb exhibits phenotypes of both homozygotes (spots, stripes, both)
Multiple Alleles Often Exist for a Gene at a Particular Locus
- Some genes have many alleles
- Some of these alleles are common, others rare
- Some alleles are associated with certain breeds or varieties of plants and animals
- Frequency of a particular allele varies from population to population
Allelic Series
- Describes the dominance hierarchy of multiple alleles
- > is dominance = is incomplete or codominance
Blood Types
- Three alleles (IA IB i) four blood types
- IA
o Encodes a transferase which adds
acetylgalactosamine
o Sits on the surface of red blood cells
- IB
o Encodes a transferase which adds galactose
o Sits on the surface of red blood cells
- I
o Encodes a non-functional transferase
o Nothing sits on the surface of the red blood cells
- IAIB
o has acetylgalactosamine and galactose on surface of red blood cells
o universal acceptor
find more resources at oneclass.com
find more resources at oneclass.com
Types of Alleles Based on Phenotypes
- Wildtype Allele: a functional enzyme or other protein is produced (typically the most
common)
o dominant over the loss of function allele
o half as much protein is synthesized but this is often enough to achieve wildtype
phenotype (haplosufficiency)
- Loss of Function Allele: an enzyme or other protein is no longer being produced,
produced at lower levels or is non functional
Exceptions To The Rule
- Dominant alleles can be gain of function mutations
o The mutant allele produces a protein that has increased (detrimental) function
Hutigto’s Disease
- Dominant allele can be a loss of function allele
o in the heterozygote, half as much protein is synthesized and this is not sufficient
for a normal phenotype (haploinsuffiecient)
Recessive Lethal Alleles
- essential genes when mutated lead to lethal phenotype
- lethal allele (Y) is a recessive allele
o recessive bc it only causes death in homozygotes
- the effect of the allele on colour is dominant (Yy is yellow)
- Eg. mm = normal tail, Mm = no tail, MM = lethal
- Dominant lethal genes can be expressed in both the heterozygote and the homozygote
- Recessive lethal genes are only expressed in the homozygote
Genes function to Produce Polypeptides
- Genes often have multiple alleles
- Mutant alleles may be dominant, recessive, incompletely dominant or codominant
- In homozygous condition, recessive mutations often abolish or diminish poly peptide
activity
- Some dominant mutations produce a polypeptide that interferes with the activity of the
polypeptide produced by the wild-type allele of a gene
Penetrance and Expressivity
- Penetrance: genotype same in all of the population but only some express the
phenotype
o variation in the population
o either express the epeted pheotpe or do’t
o Example: polydactyly
- Expressivity: the degree to which a phenotype is expressed (mild-severe)
o not all or nothing, gradient
o Example: split hand-foot sdroe, piealdis, Hutigto’s disease
What Causes Incomplete Penetrance and Expressivity?
- Due to the effects of other genes and to environmental factors that can alter or
completely suppress the effect of a particular gene
Environmental Factors that Can Affect Phenotypic Expression
- Age, sex, temp, chemicals
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Gene is the fundamental unit of heredity. Genetic information is transferred from dna to rna to protein. Gene: a region of dna that helps determine a characteristic. Allele: one of two or more alternative forms of a gene. Locus: specific place on a chromosome occupied by an allele. Genotype: set of alleles possessed by an individual organism. Heterozygote: individual possessing two different alleles at a locus. Homozygote: individual possessing two of the same alleles at a locus. Pedigree assumptions trait is rare and autosomal recessive, individuals who marry into the pedigree are not carriers but are homozygous for the normal allele trait is rare and dominant, then affected individuals are usually heterozygous. A poorly functioning protein is produced or reduced amounts of a normal functioning protein is produced. Autosomal dominant traits involve gain/change of gene function traits that are rare and dominant, affected individuals are most likely heterozygous (bb)
