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Beyond Mendel’s Laws of Inheritance

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Beyond Mendel’s Laws of Inheritance. Extending Mendelian genetics. Simple system Peas are genetically simple Traits controlled by a single gene gene has 2 alleles Complete dominance Rarely that simple. Incomplete dominance. Heterozygote shows an intermediate, blended phenotype example:

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extending mendelian genetics
Extending Mendelian genetics
  • Simple system
    • Peas are genetically simple
    • Traits controlled by a single gene
    • gene has 2 alleles
    • Complete dominance
  • Rarely that simple
incomplete dominance
Incomplete dominance
  • Heterozygote shows an intermediate, blended phenotype
    • example:
      • RR = red flowers
      • rr = white flowers
      • Rr = pink flowers
        • make 50% less color

RR

WW

RW

RR

RW

WW

incomplete dominance1
100% pink flowers

F1

generation

(hybrids)

100%

25%

red

50%

pink

25%

white

1:2:1

F2

generation

Incomplete dominance

X

true-breeding

red flowers

true-breeding

white flowers

P

self-pollinate

co dominance
Co-dominance
  • 2 alleles affect the phenotype equally & separately
    • human ABO blood groups
    • 3 alleles
      • IA, IB, i
      • IA & IB alleles are co-dominant
        • glycoprotein antigens on RBC
        • IAIB = both antigens are produced
      • i allele recessive to both
inheritance pattern of achondroplasia
Inheritance pattern of Achondroplasia

Aa x aa

Aa x Aa

dominantinheritance

a

a

A

a

Aa

Aa

AA

Aa

A

A

dwarf

dwarf

lethal

a

a

aa

aa

Aa

aa

50% dwarf:50% normal or1:1

67% dwarf:33%normalor2:1

pleiotropy
Pleiotropy
  • one gene affects more than one phenotypic character
      • 1 gene affects more than 1 trait
      • dwarfism (achondroplasia)
      • gigantism (acromegaly)
epistasis
Epistasis
  • One gene completely masks another gene
    • coat color in mice = 2 separate genes
      • C,c:pigment (C) or no pigment (c)
      • B,b:more pigment (black=B) or less (brown=b)
      • cc = albino, no matter B allele
      • 9:3:3:1 becomes 9:3:4

B_C_

B_C_

bbC_

bbC_

_ _cc

_ _cc

epistasis in labrador retrievers
Epistasis in Labrador retrievers
  • 2 genes: (E,e) & (B,b)
    • pigment (E) or no pigment (e)
    • pigment concentration: black (B) to brown(b)

eebb

eeB–

E–bb

E–B–

polygenic inheritance
Polygenic inheritance
  • Some phenotypes determined by additive effects of 2 or more genes on a single character
    • phenotypes on a continuum
    • human traits
      • skin color
      • height
      • weight
      • intelligence
      • behaviors
skin color albinism
albinismSkin color: Albinism
  • However albinism can be inherited as a single gene trait
    • aa = albino

albinoAfricans

melanin = universal brown color

enzyme

melanin

tyrosine

sex linked traits
Sex linked traits
  • 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
discovery of sex linkage
Discovery of sex linkage

true-breeding

red-eye female

true-breeding

white-eye male

X

P

100%

red eye offspring

F1

generation

(hybrids)

100%

red-eye female

50% red-eye male

50% white eye male

F2

generation

morgan s flies
Morgan’s flies…

x

x

XRXR

XrY

XRXr

XRY

Xr

Y

XR

Y

XR

XR

XRXr

XRY

XRXR

XRY

XR

Xr

XRXr

XRY

XRXr

XrY

100% red females

50% red males; 50% white males

100% red eyes

genes on sex chromosomes
Genes on sex chromosomes
  • Y chromosome
    • few genes other than SRY
      • sex-determining region
      • production of male hormones
  • X chromosome
    • other genes/traits beyond sex determination
      • hemophilia
      • Duchenne muscular dystrophy
      • color-blindness
human x chromosome
Ichthyosis, X-linked

Placental steroid sulfatase deficiency

Kallmann syndrome

Chondrodysplasia punctata,

X-linked recessive

Hypophosphatemia

Aicardi syndrome

Hypomagnesemia, X-linked

Ocular albinism

Retinoschisis

Duchenne muscular dystrophy

Becker muscular dystrophy

Chronic granulomatous disease

Retinitis pigmentosa-3

Adrenal hypoplasia

Glycerol kinase deficiency

Norrie disease

Retinitis pigmentosa-2

Ornithine transcarbamylase

deficiency

Incontinentia pigmenti

Wiskott-Aldrich syndrome

Menkes syndrome

Androgen insensitivity

Sideroblastic anemia

Aarskog-Scott syndrome

PGK deficiency hemolytic anemia

Charcot-Marie-Tooth neuropathy

Choroideremia

Cleft palate, X-linked

Spastic paraplegia, X-linked,

uncomplicated

Deafness with stapes fixation

Anhidrotic ectodermal dysplasia

Agammaglobulinemia

Kennedy disease

PRPS-related gout

Lowe syndrome

Pelizaeus-Merzbacher disease

Alport syndrome

Fabry disease

Lesch-Nyhan syndrome

HPRT-related gout

Immunodeficiency, X-linked,

with hyper IgM

Lymphoproliferative syndrome

Hunter syndrome

Hemophilia B

Hemophilia A

G6PD deficiency: favism

Drug-sensitive anemia

Chronic hemolytic anemia

Manic-depressive illness, X-linked

Colorblindness, (several forms)

Dyskeratosis congenita

TKCR syndrome

Adrenoleukodystrophy

Adrenomyeloneuropathy

Emery-Dreifuss muscular dystrophy

Diabetes insipidus, renal

Myotubular myopathy, X-linked

Albinism-deafness syndrome

Fragile-X syndrome

Human X chromosome
  • Sex-linked
    • “X-linked”
    • more than 60 diseases traced to genes on X chromosome
hemophilia
XHXH

XHXh

XHXh

XHXh

XH

XHY

XHY

XhY

Xh

male / sperm

XH

Y

XHXH

XHY

XHY

XH

female / eggs

Xh

XhY

XHXh

Hemophilia

sex-linked recessive

Hh x HH

XH

Y

carrier

disease

environmental effects
Environmental effects
  • Phenotype is controlled by both environment & genes

Human skin color is influenced by both genetics & environmental conditions

Coat color in arctic fox influenced by heat sensitive alleles

Color of Hydrangea flowers is influenced by soil pH

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