Chapter 4

MULTIPLE ALLELES• When a given gene has several

alleles, not just two• A diploid individual still has a

maximum of 2 alleles, one on each homologous chromosome

MULTIPLE ALLELES• ABO Blood Groups–Discovered in early 1900s–Important when considering

tranfusions–4 types; 3 alleles

MULTIPLE ALLELES• ABO Blood Groups

A: IAIA; IAiB: IBIB; IBiAB: IAIB

O: ii

MULTIPLE ALLELES• ABO Blood Groups–Antibody – protein molecule that

recognizes and binds to foreign material–Antigen – molecule that is recognized

as foreign and stimulates antibody production• Don’t stimulate antibody formation in

organism expressing them (exception: Autoimmune diseases)

MULTIPLE ALLELES• ABO Blood Groups–IA : specifies the ‘A’ antigen; antibodies

against ‘B’ and will clump onto the IB

–IB : specifies the ‘B’ antigen; antibodies against ‘A’ and will clump onto the IA

–AB : have both antigens, but no “anti-” antibodies–O (ii) : have no antigens and no “anti-”

antibodies

MULTIPLE ALLELES• ABO Blood Groups–Safe Transfusions:• A (IAIA /IAi) – can receive A or O–Can give to A or AB

• B (IBIB /IBi) – can receive B or O–Can give to B or AB

• AB (IAIB) – can receive AB or O–Can give to only AB

• O (ii) – can receive only O–Can give to any blood group, A, B or O

MULTIPLE ALLELES• What does this have to do with

molecular genetics?–The base pair sequence of a gene

specifies amino acid sequence of a protein…this protein function depends on the sequence of amino acids–So, a simple change in the base

sequence can drastically change the protein function

Modifications of Dominance• Complete Dominance–One allele is dominant to another, so

the heterozygous individual shows the dominant phenotype

• Incomplete Dominance–One allele is not completely dominant

to another (partial dominance), heterozygous individual shows a new intermediate phenotype

Modifications of Dominance• Incomplete Dominance–Ex. Plumage color in chickens• Cross a true-breeing black (CBCB) with a

true-breeding white (CWCW) and the heterozygous F1 offspring (CBCW) exhibits a bluish-grey plumage–C : color–B : black–W : white

• Can’t be true-breeders…why?

Modifications of Dominance• Incomplete Dominance– Explanation: believed to occur for this

reason…• CB : produces color gene expression• CW : produces no gene expression• So a heterozygous individual produces

“half” a dose of gene expression– Insufficient –Heterozygotes that produce a “normal”

dominant appearance are described as haplosufficient

Modifications of Dominance• Codominance– one allele is not dominant to another,

instead the phenotype produced exhibits both dominant phenotypes• Ex. ABO blood grouping (AB)

– Explanation…believed to occur because• Both alleles for competing phenotypes are expressed

In Review• Complete dominance– A/A & A/a produce the same phenotype and can

be written as A/- because the second allele does not change the expression of the gene

• Incomplete dominance & Codominance– A/A & A/a do not produce the same phenotype so

they must be written out as they appear

Modified Mendelian Ratios• Production of NEW Phenotypes

– Ex. Comb Shape in Chickens (may be true-breeders)• a) R/- p/p• b) R/- P/-• c) r/r P/-• d) r/r p/p

– Assuming recessives do not take any action,we can assume thesingle comb is aproduct of other geneswhile the others are dueto the activity of theR and P alleles

Modified Mendelian Ratios• Production of NEW

Phenotypes– Ex. Fruit Shape in Summer

Squash – available in long, sphere, and disk-shaped• Sphere – A dominant allele

of either gene and homozygous recessive of the other

• Disk-shaped – A dominant allele of both genes

• Long – double homozygous recessive

Modified Mendelian RatiosProduces NO new phenotype • Epistasis– Involves a gene masking or modifying the

phenotypic expression of another gene– Interaction between 2 or more genes to control a

single phenotype– Confined to dihybrid crosses where two pairs of

alleles assort independently – Does not produce a new phenotype, only masks• Epistatic gene – the gene that masks another• Hypostatic gene – the gene that is masked

Modified Mendelian Ratios• Epistasis– Recessive – must be homozygous• Ex: coat color in rodents – natural coat color in wild

rodents is a greyish color (produced by alternating bands of black and yellow – agouti pattern)– Aids in camouflage– Found in mice, squirrels, etc– Other colorations exist, but are recessive to agouti

» (A/– agouti; a/a nonagouti)» (C/– pigment; c/c albino)» (B/– black; b/b brown)» c is epistatic when homozygous (recessive gene)» A is hypostatic

Modified Mendelian Ratios• Epistasis– Recessive - must be homozygous• Ex: coat color in labrador retrievers – available in black,

yellow, & chocolate– One gene specifies black pigment (B/-) or brown (b/b)– An independent gene either allows (E/-) or hides (e/e) the

expression of the Black / brown gene» Black: B/- E/-» Chocolate: b/b E/-» Yellow: -/- e/e (B/- black noses; b/b brown noses)» e is epistatic when homozygous (recessive gene)» B is hypostatic

Modified Mendelian Ratios• Epistasis– Dominant• Ex: fruit color in summer squash – available in white,

yellow, or green– W/-, -/- white– w/w, Y/- yellow– w/w, y/y green

» W – epistatic (homozygous or heterozygous)» y - hypostatic

Modified Mendelian Ratios• Epistasis– Dominant• Ex: Greying in horses

– It doesn’t matter what color the horse’s base is (sorrel, black, bay, etc) over time the Grey gene will mask that phenotype

– It is a progressive process– Grey does not affect skin or eye color, only hair

Modified Mendelian Ratios• Epistasis– Duplicate Genes• When a gene at one locus produces a phenotype identical

to that produced at another locus• Ex: sweet peas flower colors

– C: colored– c: no color– P: purple– p: white

» Purple flowers: C/- P/-» White flowers: c/c -/- OR C/- p/p

(duplicate recessive epistasis OR complementary gene action)when 1 or both loci are homozygous recessive

Modified Mendelian Ratios• Essential & Lethal Genes– Mutations not only change phenotypes, they can also

cause death (which I guess technically does change the phenotype)

• Alleles resulting in death are lethal alleles, caused by essential genes (essential to the normal functioning of the organism)

• When caused by a dominant lethal allele both the heterozygous and homozygous individuals will show the lethal phenotype• When caused by a recessive lethal allele, only the

homozygous individual will show the lethal phenotype

Modified Mendelian Ratios• Essential & Lethal Genes– Lethal alleles• Ex: Yellow body color in mice

– Acts dominant in determining body color, but acts recessive in determining lethality (only heterozygotes survive to birth)

• Ex: Huntington’s disease in humans– Autosomal dominant (can’t be studied until reproductive age)– Onset doesn’t appear until early-thirties, and death in forites

• Ex: Hemophilia in humans– X-linked recessive

Modified Mendelian Ratios• Gene Expression– Penetrance – frequency with which a gene manifests

itself in individuals in the population• Depends on genotype and environment

– Expressivity – degree to which a gene or phenotype are expressed in an individual

– Environment – • Age of onset: creates internal environmental changes

– Genes are not “on” all the time; genes can be activated or deactivated over time» Pattern baldness» Muscular Dystrophy

Modified Mendelian Ratios• Gene Expression– Environment –

• Sex – expression of genes are influenced by gender– Sex-limited traits –autosomal genes that affect only 1 gender and not

the other» Ex: milk production» Ex: appearance of horns in some species» Ex: facial hair

• Temperature – Reactions are catalyzed by enzymes, which function in a certain range.– Ex: fur color in Himalayan rabbits

» >30 C – all white» <25 C – typical coloration (black paws, ears, nose tail) and

anywhere it is artificially cooled

Modified Mendelian Ratios• Gene Expression– Environment –

• Chemical – can have significant effect on an organism– EX: Phenylketonuria (PKU): autosomal recessive, trouble metabolizing

amino acid phenylalanine, diet determines severity (proteins)

– Nature vs Nurture• What are the relative contributions of genes and the

environment to the phenotype?– Ex: Height – influenced by genes (potential) and environment (diet,

overall health, hormones)– Ex: Alcoholism – influenced by genes (susceptibility) and environment

(choice)– Ex: Intelligence – influenced by genes (potential) and environment

(learning, challenges)