1. Chromosomal Basis of Inheritance & Sex-Linked Genes
Chapter 15
Section 1-2
Pgs
Objective: I can describe how scientists proved genes are physically on chromosomes and use those ideas to solve sex-linked genetics problems.

2. Chromosome Theory of Inheritance
Genes exist on chromosomes
Behavior of genes (determined by Mendel in 1850’s) similar to behavior of chromosomes during mitosis/meiosis (seen in 1900’s), specifically…(2 concepts)
Segregation (of alleles & homologs: disjunction)
Independent assortment (of characters & non-homologous chromosomes) + crossing over (if on same chromosome)
Theory was supported by Sutton, et al (read book!)
But there was no definitive proof!

4. Proving Genes on Chromosomes
Thomas Hunt Morgan & Fruit Flies
Determined wild types for many characters
Wild type = most prevalent (abundant) trait
Is it AUTOMATICALLY dominant?
Is it likely dominant?
Others are “mutant” types
A different “standard” to symbolize allele
Wild-type = red = w+
Mutant = white = w

5. Proving Genes on Chromosomes
Morgan’s Experiment
This proves that wild-type red is
3:1 ratio still proved red was dominant, but only MALES were white-eyed!
WHY?!?
dominant

6. Proving Genes on Chromosomes
Since sex of fruit fly involved, had to look at sex chromosomes (XX = female, XY = male)
The only way for F2 sons to get white eyes is
if gene for white eyes on X chromosome…
This proves gene linkage
What is “this”?
Sex-linked traits (type of)
Gene linkage
!!!
Wild-type = red = w+
Mutant = white = w

7. Sex Chromosomes
A set of chromosomes that determines (genetic) sex of the offspring
Other chrmsmes. = autosomes (determine “body”)
Different organisms have different “systems”
X-O system
Z-W system
X-Y system
Diploid/Haploid system

8. Human Sex Chromosomes Women = XX Men = XY
Which is more important for survival? X – Why?
Who determines sex? Men with Y
Punnett Squares can be for just 1 gene on chromosome or entire chromosome itself! A B C D E F a b c d e f A B C d E F a b c d e f X Y
X X
X Y D d
D d
d d

9. Inheritance of Sex Chromosomes
Can treat sex as a totally different character!
2 traits: male / female
Gene = entire chromosome (“alleles” = X or Y)
Genetics Problem: The gene for eye color (found on autosome #10 in rabbits) is controlled by simple dominance with red eyes dominant over white.
What are the chances that a heterozygous red-eye male and a white-eye female will have red-eye kids? 50%
What are the chances that a heterozygous red-eye male and white-eye female will have a red-eye boy?

10. 1 1 1 1 Looking @ 2 characters (each with own alleles)
Eye Color: Red = White = R r
Sex: Female-ness = Male-ness = X Y
Because looking at 2 characters at same time…
heterozygous red-eye male x white-eye female
rrXX
RrXY
…must do…
Genotype Count
RrXX = RrXY = rrXX = rrXY = 1 1 1 1
Phenotype Count
Red-eye male =
Red-eye female=
White-eye male =
White-eye female= RX RY rX rY
RrXX
RrXY
rrXX
rrXY 1 1 1
Chance of having red-eye boy? 1
25%
Chance that a son will have red eyes?
50%

11. Sexual Development Everyone is conceived as a “female” offspring
when examine fetus under 4 months, genital area looks identical between males and females
At 2 months (embryo), IF a Y chromosome is present, SRY gene activates
Sex Determining Region on Y chromosome (sorry)
Trigger that causes testes to develop
Testes start producing testosterone which changes body chemistry and body parts
Other genes involved too!
At 4 months (fetus), can see physical difference

12. Sex-Linked Genes
X chromosome carries genes that are NOT involved in sex determination (still vital to survival)
Will be inherited WITH the X chromosome (linked!)
Traits determined by these genes are sex-linked!
Y chrmsme mostly determines if male  why?
Size difference! X bigger than Y  X carries more genes!
Types of Sex-Linked
X-linked genes
Y-linked genes
(most of problems are
X-linked, because…)

13. Sex-Linked Traits – Geno./Pheno.
Eye color is a sex-linked trait for fruit flies Red eyes are dominant over white.
Key: Red = White =
Add as superscripts to X chromosome only! R r
Females (♀)
Males (♂)
Red Eyes (Homozygous)
X X
X Y
Red Eyes (Heterozygous)
White eyes
Only X chromosome can hold alleles
XRY
Red eye male
No homozygous or heterozygous (hemizygous) R R R R r R r r r

14. Sex-Linked Traits – Punnett Sq.
Cross a female fly that has homozygous red eyes with a white-eyed male fly
CROSS: _______ x _______
♀ ♂
What are the chances
of having a red-eyed
female?
What % of daughters
have red eyes?
What % of sons will
XR XR
Xr Y Xr Y XR
XR Xr
XR Y
50%
100%
100%

15. Sex-Linked Traits – Punnett Sq.
Cross a white-eyed female fly with a red-eyed male fly
CROSS: _______ x _______
♀ ♂
What are the chances
of having a red-eyed
male?
What % of daughters
have red eyes?
What % of sons will
have white eyes?
Xr Xr
XR Y XR Y Xr
XR Xr
Xr Y 0%
100%
100%

16. Sex-Linked Traits Conclusions
Explains why sons tend to take after mothers
Sons get only Y chromosome from dad
Any traits on X can only come from mother
Explains why daughters tend to take after fathers
Fathers can contribute genes on X chromosome
Daughters still get an X from mom, too
Daughters/fathers seems similar when compared to sons/mothers (more relative feel)
Sex-Linked truly proved that chromosomes hold genes (X NEVER crosses over with Y)

17. Solving Pedigree Charts

18. Human Sex-Linked Disorders
Red-Green color-blindness
X-linked recessive
Duchenne muscular dystrophy 20)
Hemophilia (blood cannot clot)
Hairy Ears
Y-linked recessive
Incontinentia Pigmenti (skin, eyes, nerves  lethal)
X-linked dominant

19. X Inactivation in Female Mammals
X-Inactive Specific Transcript
Men have only 1 X chromosome
Implies need only 1 X to survive normally
Women have 2 X chromosomes (don’t need both)
One of them inactivates (DNA methylation / XIST)
Condenses into a compact object: Barr body
If a woman is heterozygous, then which X will be inactivated? it’s random!
X inactivation is random in EACH cell
Thus, a female body is composed of some cells with XA and some cells with Xa

20. X Inactivation in Female Mammals
Implications
If each cell has XA and Xa
Then XA expression dominates of Xa
But if some cells have only 1 XA and other cells have 1 Xa (each cell sort of becomes hemizygous)
Then some cells express dominant phenotype and others express recessive phenotype
So, body becomes a mosaic!
Human female sweat glands