Beyond Mendel’s Laws
of Inheritance
AP Biology
2006-2007
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
AP Biology
Discovery of sex linkage
P
F1
true-breeding
red-eye female
X
true-breeding
white-eye male
100%
red eye offspring
Huh!
Sex matters?!
generation
(hybrids)
F2
generation
AP Biology
100%
red-eye female
50% red-eye male
50% white eye male
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
X
XX
XY
X
XX
XY
 develop as a male: XY
 no redundancy
AP Biology
50% female : 50% male
Morgan’s flies…
x
XR XR
Xr
XR
XR
AP Biology
XR Xr
XR Xr
x
XrY
XR Xr
Y
XRY
XRY
100% red eyes
XR
Xr
XRY
XR
Y
XR XR
XRY
XR Xr
X rY
100% red females
50% red males; 50% white males
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
AP Biology
sex-linked recessive
Hemophilia
H Xh x X
HY
HH
XHh
XH
female / eggs
male / sperm
XH
XH
Y
XH XH
XH Y
XH Xh
Xh
XH
Xh
AP Biology
XH Xh
XhY
carrier
disease
XHY
Y
Sex-linked disorders
 Most sex-linked disorders are found on
the X chromosome.
 Boys are more affected than girls.
Duchenne muscular dystrophy
 Hemophlia
 Colored blindness

AP Biology
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 Xh
tricolor cats
can only be
AP Biology
female
Xh
patches of orange
Linked Genes
 Genes located near each other on the
same chromosome tend to be inherited
together.
AP Biology
Linked genes
 Linked genes can be separated by
crossing over of homologous
chromosomes during prophase I of
meiosis.
 These offspring are known as
recombinants or recombinant types.
AP Biology
Mapping the distance between genes
 A genetic map based on recombination
frequencies is called a linkage map.
AP Biology
Errors in Meiosis
 Caused by nondisjunction in meiosis I
or meiosis II.
AP Biology
Errors in Meiosis
 Aneuploidy – having an abnormal
number of chromosomes
 Monosomic – having a missing
chromosome
 Trisomic – having one extra
chromosome.
 Polyploidy – having one or more extra
sets of chromosomes
AP Biology
Alterations of chromosome structure
AP Biology
Human disorders due to chromosomal alterations









Down Syndrome (trisomy 21)
Edwards Syndrome (trisomy 18)
Patau Syndrome (Trisomy 13)
XXY – Klinefelter syndrome
XO – Turner Syndrome
XXX – no problems
XYY – no problems
Philadelphia Chromosomes
Cri du chat
AP Biology
Genomic Imprinting
 Variations in phenotype depending on
whether an allele is inherited from the
male or female parents.
 Happens during gamete formation and
silences a particular allele of certain
genes.
 Angelman Syndrome
 Prader-Willi Syndrome
AP Biology
Inheritance of organelle genes
 Some organelles have small circular
DNA. These organelles reproduce
themselves and transmit their genes to
daughter organelles.
 Mitochondria – organism’s inherit their
mother’s mitochondrial DNA
 Chloroplasts
AP Biology