Incomplete Dominance, Codominance, Multiple Alleles,

and Sex-Linked Traits

AG Ag aG ag

AG Ag

Why are there only two possible combinations?

What is the probability of producing an offspring with the genotype AAGg?

AA -> ½ Gg -> ½ ½ x ½ = ¼

No, Mendel’s studies with peas applied to traits controlled by a single gene with two existing alleles where 1 allele is completely dominant to the other

This is not the case for MOST genes

Incomplete Dominance Codominance Multiple Alleles Sex-Linked Traits

Heterozygotes show a “blending” or intermediate form of the homozygous forms that exist – No true dominant allele

F2 genotypic ratio is 1:2:1 (HH: Hh: hh) Ex: Snap Dragons, Hypercholesterolemia

Two dominant alleles exist Heterozygotes express both alleles

EQUALLY Ex: Type AB Blood, Speckled Chickens,

Roan Cattle, Sickle-cell Anemia

More than two alleles exist for a particular gene, but every organism has only 2 of them

Ex: Blood Types (A B and O alleles) A and B are codominant A is dominant to O B is dominant to O O is recessive to A and B

Genes located on the sex chromosomes, mainly X-chromosome

Males XY; Females XX Males will show recessively inherited

characteristics more often than females. Ex: Hemophilia, colorblindness, white

eyes in flies

Genes are on sex chromosomes as opposed to autosomal chromosomes first discovered by T.H. Morgan at Columbia U. Drosophila breeding

good genetic subject prolific 2 week generations 4 pairs of chromosomes XX=female, XY=male

1910 | 1933

autosomalchromosomes

sexchromosomes

Huh!Sex matters?!

F2generation

100%red-eye female

50% red-eye male50% white eye male

P X

F1generation(hybrids)

100%red eye offspring

true-breeding white-eye male

true-breedingred-eye female

RR rr

x

r r

R

R

Rr

Rr Rr

Rr

100% red eyes

Rr Rr

x

R r

R

r

RR

Rr rr

Rr

3 red : 1 white

Doesn’t workthat way!

In humans & other mammals, there are 2 sex chromosomes: X & Y 2 X chromosomes

develop as a female: XX gene redundancy,

like autosomal chromosomes an X & Y chromosome

develop as a male: XY no redundancy

X Y

X

X

XX

XY

XY

50% female : 50% maleXX

XRXR XrY

x

Xr Y

XR

100% red eyes

XR

XRXr XRY

XRYXRXr

x

XRXr XRY

XR Y

XR

Xr

XRXr

XRYXRXR

XrY100% red females50% red males; 50% white males

BINGO!

Y chromosome few genes other than SRY

sex-determining region master regulator for maleness turns on genes for production of male hormones

X chromosome other genes/traits beyond sex determination

mutations:hemophiliaDuchenne muscular dystrophycolor-blindness

Sex-linked usually

means“X-linked”

more than 60 diseases traced to genes on X chromosome

Duchenne muscular dystrophyBecker muscular dystrophy

Ichthyosis, X-linkedPlacental steroid sulfatase deficiencyKallmann syndromeChondrodysplasia punctata, X-linked recessiveHypophosphatemiaAicardi syndromeHypomagnesemia, X-linkedOcular albinismRetinoschisis

Adrenal hypoplasiaGlycerol kinase deficiency

Incontinentia pigmentiWiskott-Aldrich syndromeMenkes syndrome

Charcot-Marie-Tooth neuropathyChoroideremiaCleft palate, X-linkedSpastic paraplegia, X-linked, uncomplicatedDeafness with stapes fixationPRPS-related goutLowe syndrome

Lesch-Nyhan syndromeHPRT-related goutHunter syndromeHemophilia BHemophilia AG6PD deficiency: favismDrug-sensitive anemiaChronic hemolytic anemiaManic-depressive illness, X-linkedColorblindness, (several forms)Dyskeratosis congenitaTKCR syndromeAdrenoleukodystrophyAdrenomyeloneuropathyEmery-Dreifuss muscular dystrophyDiabetes insipidus, renalMyotubular myopathy, X-linked

Androgen insensitivity

Chronic granulomatous diseaseRetinitis pigmentosa-3

Norrie diseaseRetinitis pigmentosa-2

Sideroblastic anemiaAarskog-Scott syndrome

PGK deficiency hemolytic anemia

Anhidrotic ectodermal dysplasia

AgammaglobulinemiaKennedy disease

Pelizaeus-Merzbacher diseaseAlport syndrome

Fabry disease

Albinism-deafness syndrome

Fragile-X syndrome

Immunodeficiency, X-linked,with hyper IgM

Lymphoproliferative syndrome

Ornithine transcarbamylase deficiency

< 30 genes on Y chromosome Sex-determining Region Y (SRY)

linked

Channel Flipping (FLP)Catching & Throwing (BLZ-1)Self confidence (BLZ-2)

note: not linked to ability geneDevotion to sports (BUD-E)Addiction to death &

destruction movies (SAW-2)

Scratching (ITCH-E)Spitting (P2E)

Inability to express affection over phone (ME-2) Selective hearing loss (HUH)

Total lack of recall for dates (OOPS)

Air guitar (RIF)