Genetics Chapter 4: Extensions of Mendelian Inheritance

What is most important about gene expression and protein production?

the right protein is produced in the right concentration at the right time

What theories explain why a heterozygous individual displays the dominant phenotype (haplosufficiency)?

• 50% of protein is sufficient to show dominant phenotype

• cell identifies there is only one dominant allele and upregulates gene expression for that trait

What is a dominant mutant?

only one copy needed to be expressed (NOT good)

What are the types of dominant mutants?

1. gain of function

2. dominant-negative

3. haploinsufficiency

What is a gain of function mutation?

• activating mutation

• one allele is sufficient to produce a new characteristic that negatively affects cell

What is an example of a gain of function mutation?

p53 mutation causes cell to divide uncontrollably and makes the cell drug resistant (harmful new functions)

What is a dominant-negative mutation?

• mutation in one allele prevents cell from recognizing function of gene

• protein produced by mutant interferes with normal protein

What is an example of a dominant-negative mutation?

RAS mutated so it cannot release GDP and bind to GTP (cell does not process signal)

What is haploinsufficiency?

• one copy cannot make enough protein to show dominant phenotype

• heterozygote does not have the same phenotype as homozygous dominant

What is incomplete dominance?

• one copy of dominant trait is insufficient to produce dominant phenotype

• third phenotype that is intermediate (blended)

What are examples of incomplete dominance?

flower color in snap dragons and four o’clock flowers

Is incomplete dominance an example of haploinsufficiency?

no

What is the difference between incomplete dominance and codominance?

incomplete dominance results in a third, blended phenotype while codominance shows patches of both phenotypes

What is the genotypic ratio of an incomplete dominance monohybrid cross?

1:2:1

What is the phenotypic ratio of an incomplete dominance monohybrid cross?

1:2:1

What is the phenotypic ratio of an incomplete dominance dihybrid cross?

• 6:3:3:2:1:1

• 3 dominant, dominant

• 1 dominant, recessive

• 6 incomplete (blended), dominant

• 2 incomplete (blended), recessive

• 3 recessive, dominant

• 1 recessive, recessive

Looking at a cell, what is the protein production of a homozygous dominant individual?

100% protein production

Looking at a cell, what is the protein production of a heterozygous dominant individual?

• less than 100% protein production but not distinguishable to the naked eye

• example: pea plants with heterozygous for pea shape will be smooth to the naked eye but will have some small wrinkles

Looking at a cell, what is the protein production of a homozygous recessive individual?

so much less protein that you can see it with a naked eye

Phenylketonuria results in the inability to create the enzyme that catabolizes phenylalanine. How much phenylalanine is found in a cell that is PP for the gene that determines phenylketonuria?

1-2 mg

Phenylketonuria results in the inability to create the enzyme that catabolizes phenylalanine. How much phenylalanine is found in a cell that is Pp for the gene that determines phenylketonuria?

3-5 mg (a little bit more than PP individuals)

Phenylketonuria results in the inability to create the enzyme that catabolizes phenylalanine. How much phenylalanine is found in a cell that is pp for the gene that determines phenylketonuria?

600 mg (significantly more than both PP and Pp individuals)

What is incomplete penetrance?

a heterozygous individual may or may not express trait

What is penetrance?

how many people in the heterozygous population show the trait

What is expressivity?

how much of a gene is expressed in a heterozygous individual

What is an example of incomplete penetrance?

• polydactyly

• some people with dominant allele have extra digits

• all people with two dominant alleles (homozygous dominant) have extra digits

What is an example of expressivity?

• low expressivity = one extra digit

• high expressivity = one extra digit on two limbs

• very high expressivity = one extra digit on all four limbs

Is the environment nature or nurture?

nurture

How does the environment affect gene expression?

• influences expression of trait

• ex. in a nourished environment, genes can be expressed to their full extent

What is a polygenic trait?

trait controlled by multiple genes

What types of traits are greatly affected by the environment?

polygenic traits

What is an example of environmental influence on polygenic traits?

drosophila: number of facets in eye decrease as temperature increases

What is overdominance?

heterozygote better suited to survival than homozygous dominant

What is an example of overdominance?

• sickle cell anemia

• heterozygote is normal

• has 50% donut-shaped and 50% sickle-shaped cells

• the presence of the sickle-shaped cells protect against malaria

What does overdominance in sickle cell anemia protect against?

malaria

What does overdominance in tey sachs disease protect against?

tuberculosis

What does overdominance in PKU protects against?

fungal toxin (nephrotoxin)

What is the mechanism by which overdominance for sickle cell anemia protects against malaria?

plasmodium enters the body through a mosquito bite —> target red blood cells —> need cells to divide in order to reproduce —> sickle cells don’t divide —> protect against malaria

What is a heterodimer?

protein made up of more than one type of polypeptide

What is a monodimer?

protein made up of only one type of popypeptide

How do having both the dominant and recessive alleles benefit heterozygotes in the case of overdominance?

they have more enzymes that can work in a wide range of temperatures

Which studies two variables of one gene: overdominance or heterosis?

overdominance

Which studies many different genes: overdominance or heterosis?

heterosis

What is heterosis?

hybrid plants with many different genes have better survival than pure bred

What is an example of heterosis?

golden rice have genes from multiple species

What is another name for heterosis?

hybrid vigor

What is a polymorphic trait?

trait that has multiple wild type alleles caused by multiple alleles

What is an example of a polymorphic trait?

type A and type B blood exist as polymorphic traits for one gene

If two colors come together to produce a lighter color, is it incomplete dominance or codominance?

incomplete dominance (red + white become pink)

If two colors come together to produce a darker color, is it incomplete dominance or codominance?

codominance (grey + tan become dark brown)

What is a monomorphic trait?

have wild type and mutant allele only

What is the series of dominance?

polymorphism where dominant alleles fall in a heirarchy

What is an example of a series of dominance?

• fur color in rabbits

• lentils

What is the series of dominance in rabbit fur color?

• agouti > chinchilla > himalayan > albino

• C > Cch > Ch > cc

What does the C allele code for in rabbits?

• eumelanin vs pheomelanin

• C (agouti) have normal production of both eumelanin and pheomelanin

• Cch (chinchilla) have less production of pheomelanin (partial defect)

• Ch (himalayan) enzyme only works in cool temperatures (feet, ears, nose)

• cc (albino) have no pigmentation

What is the series of dominance for lentils?

marbled 1 > marbled 2 > spotted = dotted (codominant) > clear

What is a temperature sensitive conditional allele?

enzyme produced by allele only functions in certain temperatures

What is an example of a temperature sensitive conditional allele?

• enzyme works where temperature is low (himalayan bunnies and siamese cats)

• enzyme works where temperature is high (cows)

What is codominance?

equal expression of both alleles

What is an example of codominance?

• type AB blood

• black and white cows

• M and N blood group

What could be a factor in the M and N blood group?

stress

What are X-linked genes?

present along X-chromosome

What is an example of X-linked genes?

• color blindness

• hereditary enamel hypoplasia

What do X-linked genes change?

monohybrid cross ratios

What are holandric genes?

Y-linked genes

What is the sex determining gene?

SRY

Where is the SRY gene found?

Y-chromosome

What are pseudo-autosomal genes?

genes present in both X and Y chromosomes

What are allosomes?

X and Y (sex) chromosomes

What are sex-influenced traits?

• concentration of hormones influence expression in heterozygous individuals

• depending on hormones the trait may or may not show in heterozygous individuals

What is an example of sex-influenced traits?

balding patterns and cow horns

What type of gene affects balding patterns?

autosomal gene (on chromosome 3)

What chromosome is the ABO blood group gene on?

9

What does the ABO blood group code for?

glycosyltransferase will either attach an A antigen or a B antigen (or neither)

What determines blood type?

glycosyltransferase

What chromosome is the FUT1 gene on?

19

What does the FUT1 gene code for?

H antigen added to glycoproteins by fucosyltransferase

What happens if someone has two recessive alleles for the FUT1 gene?

H antigen will not be added to glycoproteins and A and B antigens will be unable to be added (individual will have type O blood)

What is the H antigen?

precursor that allows glycosyltransferase to transfer A or B antigen

What type of antibodies are formed in a person with type A blood?

B antibodies

What type of antibodies are formed in a person with type B blood?

A antibodies

What type of antibodies are formed in a person with type AB blood?

no antibodies

What type of antibodies are formed in a person with type O blood?

A and B antibodies

What is the “Bombay” genotype?

having two recessive alleles for the FUT1 gene (hh)

How are sex influenced traits seen in balding patterns?

• male heterozygote shows balding

• female heterozygote does not have balding

• homozygous dominant females will bald but might have few strands of hair

• homozygous dominant males will bald

How does hormones affect balding patterns?

enzyme responsible for balding binds to testosterone and blocks hair follicles from growing (therefore balding is only seen in male heterozygotes not female)

What is the difference between horns and scurs?

horns have nerves while scurs do not

How are sex influenced traits seen in cow horns?

• heterozygous males have scurs

• heterozygous females do not have scurs

What are sex limited traits?

• traits restricted to only one sex

• determined by sex chromosomes, sex organs, and sex hormones

What is an example of sex limited traits?

rooster vs hen feathering

What are lethal alleles?

can cause death of organism

What are essential genes?

genes that must be present for the survival of an organism

What type of genes are most lethal alleles?

essential genes that control housekeeping functions

What are conditional lethal alleles?

only result in death in certain environmental conditions

What are semi-lethal alleles?

some individuals will die as a result of the allele while others will not

What is an example of a lethal allele that has different levels of penetrance?

Huntington disease

What is Huntington disease?

• dominant pattern of inheritance

• too many repeated CAG exon segments

What are the different levels of penetrance in Huntington disease?

• <26 CAG repeats —> no disease (normal)

• 27-35 CAG repeats —> no disease but can pass on disease (intermediate penetrance)

• 36-39 CAG repeats —> may have symptoms or may have symptoms in old age (reduced penetrance)

• >40 CAG repeats —> have disease and will pass on disease (full penetrance)

What is the maximum number of CAG repeats that result in a negligible chance of being passed on?

<35

What is anticipation?

offspring show signs of disease sooner than parents or could have more severe symptoms than parents