1. Beyond Mendel’s Laws of Inheritance

2. 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

3. Incomplete dominance P F1 100% 1:2:1 F2 X true-breeding red flowers
white flowers
100%
100% pink flowers F1
generation
(hybrids)
self-pollinate
25%
white F2
generation
25%
red
1:2:1
50%
pink

4. Codominance
There are two or more alleles that are dominant in a phenotype.
Both alleles are expressed in heterozygous condition.
Ex: Red – RR, white – WW, red and white - RW

5. Multiple Alleles 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

7. Polygenic inheritance
Some phenotypes determined by additive effects of 2 or more genes on a single characteristic
phenotypes on a continuum
human traits
skin color
height
weight
intelligence
behaviors

8. Sex linked traits 1910 | 1933 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

9. Discovery of sex linkage
true-breeding
red-eye female
true-breeding
white-eye male X P
Huh! Sex matters?!
100%
red eye offspring F1
generation
(hybrids)
100%
red-eye female
50% red-eye male
50% white eye male F2
generation

10. Genetics of Sex X Y X XX XY X XX XY
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 XX XY X XX XY
50% female : 50% male

11. Morgan’s flies… x x XRXR XrY XRXr XRY Xr Y XR Y XR XR XRXr XRY XRXR
100% red females
50% red males; 50% white males
100% red eyes

12. 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
X chromosome
traits other than sex determination
mutations: (all are recessive)
hemophilia
Duchenne muscular dystrophy
color-blindness
Duchenne muscular dystrophy affects one in 3,500 males born in the United States.
Affected individuals rarely live past their early 20s.
This disorder is due to the absence of an X-linked gene for a key muscle protein, called dystrophin.
The disease is characterized by a progressive weakening of the muscles and loss of coordination.

13. Hemophilia XHXh XHY Hh x HH XH XHXh XH Y Xh XHXH XHXH XHY XHY XH Xh XH
sex-linked recessive
Hemophilia
XHXh
XHY
Hh x HH XH
XHXh XH Y
male / sperm Xh
XHXH
XHXH
XHY
XHY XH Xh
female / eggs XH
XHY
XHXh
XHXh
XhY
XhY Y
carrier
disease

14. X-inactivation Female mammals inherit 2 X chromosomes XH XHXh Xh
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
XHXh
Xh
tricolor cats can only be female
patches of orange

15. 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
The relative importance of genes & the environment in influencing human characteristics is a very old & hotly contested debate
a single tree has leaves that vary in size, shape & color, depending on exposure to wind & sun
for humans, nutrition influences height, exercise alters build, sun-tanning darkens the skin, and experience improves performance on intelligence tests
even identical twins — genetic equals — accumulate phenotypic differences as a result of their unique experiences
Color of Hydrangea flowers is influenced by soil pH

16. Pedigree Analysis
A pedigree is a family tree that describes the interrelationships of parents and children across generations
Inheritance patterns of particular traits can be traced and described using pedigrees

17. (a) Is a widow’s peak a dominant or recessive trait?
Fig b
1st generation
(grandparents)
2nd generation
(parents, aunts,
and uncles)
3rd generation
(two sisters)
Figure 14.15a Pedigree analysis
Widow’s peak
No widow’s peak
(a) Is a widow’s peak a dominant or recessive trait?

18. (b) Is an attached earlobe a dominant or recessive trait?
Fig c
1st generation
(grandparents)
2nd generation
(parents, aunts,
and uncles)
3rd generation
(two sisters)
Figure 14.15b Pedigree analysis
Attached earlobe
Free earlobe
(b) Is an attached earlobe a dominant or recessive trait?