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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
autosomal
chromosomes
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 recessive
HypophosphatemiaAicardi 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 fixation
PRPS-related gout
Lowe syndrome
Lesch-Nyhan syndromeHPRT-related gout
Hunter 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 gene
Devotion 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)
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