1. Review Chromosomes 1-22 are Humans have 46 chromosomes (23 pairs).
autosomes- “self-body”
All traits we have discussed
have been autosomal traits
Males and females have an equal chance of inheriting autosomal traits
F f F f
FF Ff
Ff ff
Ex: Freckles: Autosomal Trait

2. Sex Chromosomes Last pair of chromosomes XX (homologous)= Female
XY(non-homologous)= Male
Sex-Linked Traits are traits coded by genes found on either the X or Y chromosome.

3. Boy or Girl?
For any new offspring there is a 50/50 chance of producing a boy or a girl…
X X
Since the genes are on the X or Y; with sex linked traits sometimes whether or not a trait is inherited (good or bad) depends upon sex of the child
X X
X Y X Y

4. Sex-Linked Traits Referred to as being X- “linked” or Y- “linked”
Normal traits:
hormones
2nd sex characteristics
Abnormal traits:
hemophilia
color blindness
muscular dystrophy

5. X-linked Punnett Squares
XR Xr XR Y
XRXR
XRXr
Possible Genotypes: Female XRXR XRXr XrXr Male XR Y Xr Y
XRY
XrY
50% normal boy/girl
25% carrier girl
25% affected male
(Assumes trait is recessive)
Xr Xr XR Y
XR XR Xr Y
XRXr
XRXr
100% girls carriers
100% boys affected
(Assumes trait is recessive)
XRXr
XRXr
100% girls carriers
100% males normal
(Assumes trait is recessive)
XrY
XrY
XRY
XRY

6. X-Linked Recessive Inheritance
Passed from mothers to sons
Always expressed in males
Females are ‘carriers’ if heterozygous
Genetic disorders like hemophilia
muscular dystrophy
color-blindness
are all X-linked recessive

8. 250

9. H h h H h h h H h 50%/2:2 H h H 25% or 1:4 Sex male 50% or 2:2
All Male h h

10. Sex-Linked Traits and Pedigrees

11. MALE???

12. B R
25% B 0%
B B
B R
25% B R
B B
50%
BLACK R
B R
B R B B

13. Tracking Inherited Traits: Pedigrees
Diagram used to visually determine patterns of inheritance.
Tracks occurrence/frequency of genetic traits through generations of related organisms.
Can be used to determine whether a gene is autosomal or sex-linked, and whether it is dominant or recessive.
Can Track:
Autosomal Dominant (RR, Rr)
Autosomal Recessive (rr)
X-Linked Recessive (XrXr, XrY)
X-Linked Dominant (XRXR; XRXr)
Y- Linked Dominant (XYR)
Y-Linked Recessive (XYr)

14. What do you notice about this pedigree?
Kate
George

15. I II III IV ALBINISM: Inheritance? Key: Female Expressing
Autosomal Recessive
Key:
Female Expressing
Trait
Male
Heterozygous (If trait
is recessive: “carrier”)
Marriage P A S T G E N R I O I
Children II
Children are
listed by birth order
III
Is II-7 affected?
What sex is III-6?
What are the
genotypes of II-3 and II-4?
How is albinism inherited?
Yes IV F
Heterozygous
Present 

16. What do you notice about this pedigree?
Aa Aa aa aa aa AA W
What do you notice about this pedigree?
Aa AA Aa Aa
aa aa A_ A_ aa
aa aa aa aa
It represents an X-linked
recessive inheritance

17. Autosomal recessive 4 4
Male parent for all future generations
It is shaded because he is carrier for short-legged allele
II-1
All 4 carry, 2 express

18. XBY XBXb XbY XBXb XB Y Xb Y XBX_ XBY Xb Xb XbY XbXb
XBX_ XbY XBXb XbY XBXb XBY
XBXb XBY XBY XbY XBY XBX_

19. I-1, II-2, II-3, II-7, III-3 DOMINANT 6 2 1 NIECE-UNCLE
GRAMMA-GRANDSON

20. BOTH SIBLINGS EXPRESS. 50% CHANCE
RECESSIVE
PARENTS DO NOT EXPRESS,
BUT, CHILDREN DO
CONSANGUINE COUSINS
IV-1, IV-3 I
III-1, III-2
YES
BOTH SIBLINGS EXPRESS. 50% CHANCE
FEMALES
X-LINKED RECESSIVE
YES I
DAD HAS RECESSIVE X
YES

21. X-Linked traits are more commonly expressed by males. Why?
Males inherit only 1 X chromosome, so the trait is expressed regardless of whether it is dominant or recessive