Extensions of Mendelian inheritance

Molecular Genetics Le

what are the five major extensions of mendelian inheritance

1. variable expressivity

2. incomplete penetrance

3. multiple alleles

4. dosage compensation

5. gene dosage effect

what is penetrance

presence or absence of an associated trait

what is incomplete penetrance

genotype is present but the expected phenotype is not always expressed

what is expressivity

degree or severity with which a genotype is expressed in the phenotype

what is variable expressivity

individuals with the same genotype show different phenotypic severities

incomplete penetrance: ON/OFF or severity

ON/OFF: the trait is either expressed or not

expressivity: ON/OFF or severity

severity: trait is expressed to different degrees

what can affect phenotype besides genotype

penetrance, expressivity, modifier genes, genetic background, and environment

what types of genes can contribute to incomplete penetrance

epistatic genes and pleiotropic genes

what are epistatic genes

genes that mask or modify the effects of another gene

what are pleiotropic genes

genes that affect multiple traits

what does the phrase ‘no gene is an island’ mean

a gene’s effect can be modified by other genes and the environment

human example of incomplete penetrance

BRCA1 mutations

which fly mutant demonstrates variable expressivity

eyeless

which fly mutant demonstrates both variable expressivity and incomplete penetrance

antennapedia

what is the eyeless mutation

recessive autosomal mutation causing reduced eye size and facet number

why is eyeless an example of variable expressivity

identical genotypes can have different degrees of eye reduction

what color eyes do eyeless flies have

red

what is antennapedia

dominant gain of function mutation

what phenotype does antennapedia cause

legs grow where antennae should develop

why is antennapedia a gain of function mutation

causes a new developmental activity in the wrong body region

what is a gain of function mutation

mutation that gives a protein a new or enhanced function

what is neomorph

a gain of function mutation with a new activity

examples of neomorphs

antennapedia and Bar eye

what is an amorph mutation

knockout mutation in which the allele is inactive

what is a hypomorph mutation

mutation that retains reduced activity

which allele is an amorph

white eye (w1)

which allele is a hypomorph

apricot

what gene controls drosophila eye pigmentation

white (W) gene

what does W⁺ produce

normal red eye pigment

what does w1 produce

no pigment = white eyes

what are multiple alleles

three or more alleles existing for the same gene

what three white gene alleles are used in this lab

wild type (w⁺) apricot (wa) white (w1)

dominance hierarchy of white alleles

w⁺ > wa > w1

why is wa dominant to w1

produces more pigment than w1

why is wa recessive to w+

produces less pigment than wild type

can an allele be dominant in one pairing and recessive in another

yes: dominance depends on the interacting alleles

what eye color does apricot produce

peach or light orange eye color

what is dosage compensation

mechanisms that equalize X linked gene expression between sexes

why is dosage compensation needed

males and females have different numbers of X chromosomes

how do drosophila achieve dosage compensation

increasing transcription from the males single X chromosome

what is a hemizygous male

male with only one copy of an X linked gene

what is the gene dosage effect

phenotypic intensity changes with the number of gene copies present

protein production with 0 alleles

0%

protein production with 1 allele

50%

protein production with 2 alleles

100%

what mutation demonstrates gene dosage effect in flies

Bar eye

what type of mutation is Bar eye

dominant X lined gain of function

how does Bar affect the eye

reduces number of eye facets

relationship between Bar copy number and facets

more Bar alleles = fewer eye facets

how do homozygous Bar females compare with hemizygous Bar males

females have fewer facets and smaller eyes

what is wild type fly eye morphology

red, smooth, spherical eyes with about 800 facets

which mutation affect eye color

white and apricot

which mutations affect eye shape

eyeless and Bar

which lab mutation are autosomal

eyeless and antennapedia

which lab mutations are X linked

white, apricot, and Bar

why do apricot males and homozygous apricot females look similar

dosage compensation equalizes expression

how does an apricot/white heterozygous female differ from apricot females

eye color is about half as intense

why can apricot show both dosage compensation and gene dosage effect

expression is equalized between sexes, yet phenotype still depends on allele number in some genotypes

what is codominance

both alleles are fully expressed simultaneously

what protein does the ABO gene encode

cell surface antigen protein

which ABO allele is recessive

i

genotypes for type A blood

I^AI^A and I^Ai

genotypes for type B blood

I^BI^B and I^Bi

genotype for tyep AB blood

I^AI^B

genotype for type O blood

ii

what antibodies does type A blood have

anti B

what antibodies does Type B blood have

anti A

what antibodies does type AB blood have

neither anti A or anti B

what antibodies does type O blood have

both anti A and anti B

what determines a persons ABO blood type

antigens present on red blood cells

what is the Rh factor

blood group system based primarily on the D antigen

what does Rh+ mean

Rh antigen is present

what does Rh- mean

Rh antigen is absent

is Rh+ dominant or recessive

dominant

possible Rh+ genotypes

Rh⁺Rh⁺ or RhRh^-

genotype of Rh-

Rh^-Rh^-

can Rh antibodies be present naturally

no, develop after exposure

how can Rh sensitization occur

blood transfusions or pregnancy involving Rh incompatibility

what is erythroblastosis fetalis

hemolytic disease of the newborn caused by maternal anti Rh antibodies attacking fetal red blood cells