This document discusses extensions to Mendel's laws of inheritance, including incomplete dominance, co-dominance, pleiotropy, epistasis, polygenic inheritance, sex linkage, and environmental influences on phenotypes. It provides examples like flower color in plants displaying incomplete dominance, blood types showing co-dominance, diseases like dwarfism influenced by pleiotropic genes, coat color in mice impacted by epistatic gene interactions, traits like height determined by multiple genes, and sex-linked traits in Drosophila first observed by Thomas Hunt Morgan.

1. Beyond Mendel’s Laws
of Inheritance
AP Biology 2006-2007

2. Extending Mendelian genetics
 Mendel worked with a simple system
 peas are genetically simple
 most traits are controlled by a single gene
 each gene has only 2 alleles, 1 of which
is completely dominant to the other
 The relationship between
genotype & phenotype
is rarely that simple
AP Biology

3. Incomplete dominance
 Heterozygote shows an intermediate,
blended phenotype
 example:
 RR = red flowers →RR
 rr = white flowers →WW
 Rr = pink flowers →RW
 make 50% less color
AP Biology RR RW WW

4. Incomplete dominance
true-breeding X true-breeding
P red flowers white flowers
100% pink flowers
F1
generation 100%
(hybrids)
It’s like
self-pollinate flipping 2
pennies!
25% 50% 25%
red pink white 1:2:1
F2
generation
AP Biology

5. Co-dominance
 2 alleles affect the phenotype equally &
separately
 not blended phenotype
 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
AP Biology

6. Genetics of Blood type
pheno- antigen antibodies donation
genotype
type on RBC in blood status
type A antigens
A I I
A A or A
I i on surface
of RBC
anti-B antibodies __
type B antigens
B IB IB or IB i on surface
of RBC
anti-A antibodies __
both type A &
type B antigens universal
AB IA IB on surface
no antibodies
recipient
of RBC
no antigens universal
anti-A & anti-B
O ii on surface
of RBC
antibodies donor
AP Biology

7. Pleiotropy
 Most genes are pleiotropic
 one gene affects more than one
phenotypic character
 1 gene affects more than 1 trait
 dwarfism (achondroplasia)
 gigantism (acromegaly)
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8. Acromegaly: André the Giant
AP Biology

9. Inheritance pattern of Achondroplasia
Aa x aa Aa x Aa
dominant
inheritance
a a A a
A Aa Aa A AA Aa
dwarf dwarf lethal
a aa aa a Aa aa
50% dwarf:50%
AP Biology normal or 1:1 67% dwarf:33% normal or 2:1

10. Epistasis
 One gene completely masks another gene
 coat color in mice = 2 separate genes
 C,c:
pigment (C) or
B_C_ no pigment (c)
 B,b:
bbC_ more pigment (black=B)
or less (brown=b)
_ _cc
 cc = albino,
no matter B allele
 9:3:3:1 becomes 9:3:4
How would you know that
difference wasn’t random chance?
AP Biology Chi-square test!

11. 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–
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12. 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
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13. Johnny & Edgar Winter
Skin color: Albinism
 However albinism can be
inherited as a single gene trait
 aa = albino
albino
Africans
melanin = universal brown color
enzyme
tyrosine
AP Biology
melanin
albinism

14. OCA1 albino Bianca Knowlton
AP Biology

15. 1910 | 1933
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
AP Biology

16. Classes of chromosomes
autosomal
chromosomes
sex
chromosomes
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17. Discovery of sex linkage
true-breeding true-breeding
X
P red-eye female white-eye male
Huh!
Sex matters?!
100%
F1 red eye offspring
generation
(hybrids)
100% 50% red-eye male
red-eye female 50% white eye male
F2
generation
AP Biology

18. What’s up with Morgan’s flies?
x x
RR rr Rr Rr
r r R r
R Rr Rr R RR Rr
Doesn’t work
that way!
R Rr Rr r Rr rr
AP Biology 100% red eyes 3 red : 1 white

19. Genetics of Sex
 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 X Y
 develop as a male: XY
 no redundancy X XX XY
X XX XY
AP Biology
50% female : 50% male

20. Let’s reconsider Morgan’s flies…
x x
XR X R XrY X R Xr XR Y
Xr Y XR Y
XR XR
XR Xr XR Y XR X R XR Y
BINGO!
XR Xr
XR Xr XR Y XR Xr XrY
100% red females
AP Biology 100% red eyes 50% red males; 50% white males

21. Genes on sex chromosomes
 Y chromosome
 few genes other than SRY
 sex-determining region
 master regulator for maleness
 turns on genes for production of male hormones
 many effects = pleiotropy!
 X chromosome
 other genes/traits beyond sex determination
 mutations:
 hemophilia
 Duchenne muscular dystrophy
AP Biology  color-blindness

22. Ichthyosis, X-linked
Placental steroid sulfatase deficiency
Human X chromosome Kallmann syndrome
Chondrodysplasia punctata,
X-linked recessive
Hypophosphatemia
 Sex-linked
Aicardi syndrome
Duchenne muscular dystrophy Hypomagnesemia, X-linked
Becker muscular dystrophy Ocular albinism
Retinoschisis
Chronic granulomatous disease
usually
Retinitis pigmentosa-3 Adrenal hypoplasia
 Glycerol kinase deficiency
Norrie disease
Retinitis pigmentosa-2 Ornithine transcarbamylase
means
deficiency
Incontinentia pigmenti
Wiskott-Aldrich syndrome
“X-linked” Menkes syndrome
Androgen insensitivity
Sideroblastic anemia Charcot-Marie-Tooth neuropathy
 more than Aarskog-Scott syndrome
PGK deficiency hemolytic anemia
Choroideremia
Cleft palate, X-linked
Spastic paraplegia, X-linked,
60 diseases
Anhidrotic ectodermal dysplasia uncomplicated
Deafness with stapes fixation
Agammaglobulinemia
Kennedy disease PRPS-related gout
traced to Pelizaeus-Merzbacher disease
Alport syndrome
Lowe syndrome
Lesch-Nyhan syndrome
genes on X
Fabry disease HPRT-related gout
Immunodeficiency, X-linked, Hunter syndrome
with hyper IgM Hemophilia B
chromosome Lymphoproliferative syndrome Hemophilia A
G6PD deficiency: favism
Drug-sensitive anemia
Albinism-deafness syndrome Chronic hemolytic anemia
Manic-depressive illness, X-linked
Fragile-X syndrome Colorblindness, (several forms)
Dyskeratosis congenita
TKCR syndrome
Adrenoleukodystrophy
Adrenomyeloneuropathy
Emery-Dreifuss muscular dystrophy
AP Biology Diabetes insipidus, renal
Myotubular myopathy, X-linked

23. Map of Human Y chromosome?
< 30 genes on
Y chromosome Sex-determining Region Y (SRY)
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) Air guitar (RIF)
Scratching (ITCH-E)
Spitting (P2E) linked
Inability to express
affection over phone (ME-2) Selective hearing loss (HUH)
Total lack of recall for dates (OOPS)
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24. AP Biology

25. sex-linked recessive
Hemophilia
XHhh x XHY
H
X HH
XH
male / sperm XH X h
XH Y Xh
X HY
female / eggs
X H XH X H
XH
X HY
Xh X H Xh Xh Y
Y
AP Biology carrier disease

26. X-inactivation
 Female mammals inherit 2 X chromosomes
 one X becomes inactivated during
embryonic development
 condenses into compact object = Barr body
 which X becomes Barr body is random
 patchwork trait = “mosaic”
patches of black
XH 
XH X h
tricolor cats Xh
can only be
female
AP Biology
patches of orange

27. Male pattern baldness
 Sex influenced trait
 autosomal trait influenced by sex hormones
 age effect as well = onset after 30 years old
 dominant in males & recessive in females
 B_ = bald in males; bb = bald in females
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28. 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
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is influenced by soil pH

29. Any Questions?
AP Biology 2006-2007