1. Alterations to Mendel Incomplete or partial dominance Codominance
Multiple alleles and Lethal alleles
Gene interactions & multiple genes
Epistasis and complementation
Effect of environment
Extranuclear inheritance
Sex-linked, sex-limited, & sex-influenced
Sex determination and Gene dosage
Polygenics

2. The sex chromosomes The rest of the chromosomes are autosomes.
Sex chromosomes determine sex of individual.
X and Y differ in size, X much bigger in humans
But still synapse during meiosis, so still a pair
In humans, fruit flies, XX = female; XY = male.
In several other organisms, other combinations of sex chromosomes determine sex.
Because there are genes on sex chromosomes, inheritance of certain traits can be sex-linked.

3. Sex inheritance In humans, fruit flies, XX = female; XY = male.
Inheritance of sex is just like any other trait, except it involve inheritance of an entire chromosome.
Because there are genes on sex chromosomes, inheritance of certain traits can be sex-linked.

4. Sex Linkage and Determination
Early 1900s, Thomas Hunt Morgan was doing classical genetics on fruit flies, looking for mutants and checking out the patterns of inheritance.
He studied the white eye phenotype and discovered something odd…

5. What Morgan saw Reciprocal cross produced a different result:
Inheritance of eye color depended on sex of the fly.

6. Morgan and the F2 generation
When the all red eyed F1 heterozygotes were crossed, close to a 3:1 ratio was observed, but the traits were not evenly divided between the sexes.

7. Explanation
The traits of sex and eye color did not assort independently (as the traits in peas did).
The traits are linked.
The gene is NOT PRESENT on the Y chromosome.
R = red eye
R = white eye
XX = female
XY = male

8. significance
With regard to X linked traits, males have only one allele, not two. They are said to be hemizygous.
Morgan’s work led to the understanding that genes are located on chromosome’s because inheritance of certain traits corresponded to inheritance of a visibly different chromosome.
Inheritance of X-linked traits results in typical crisscross inheritance: mother to son.

9. Crisscross inheritance
Carrier mother passes allele to son who expresses it, passes allele to daughter who carries it, etc.
Hemophilia & color blindness: examples in humans.

10. Sex linked, limited, & influenced
Sex linked inheritance is when the allele is present on a sex chromosome (usually X).
Sex limited: when other genetic factors restrict expression to one sex
Bulls don’t give milk.
Sex influenced: other genetic factors modulate expression. Example: pattern baldness
Females must be homozygous recessive for trait
Even then, hormonal factors
restrict expression.
imsc.usc.edu/.../contemporary_sb_3.html

11. Sex determination
Different organisms have different chromosomal mechanisms for determining sex.
XX/XO: typically, the male has one copy.
Nematodes, e.g. C. elegans
XX/XY: as in humans, fruit flies, XX = female; XY = male.
Heterogametic sex is the one that produces a mixture of gametes. Usually the male but:
Female can be heterogametic in some species
Designation is ZZ/ZW where female is ZW

12. More on sex determination
Temperature affects sex determination in many reptile species
Females result from low, high, or extremes of temperature.
Hypothesis:

13. History Sex determination studies began in late 1800’s
Work in humans started around 1912, but didn’t get it right until 1956.
Keys to understanding sex determination in humans:
Improved karyotype methods
Study of aneuploidy of sex chromosomes
Aneuploidy is the wrong number of a particular chromosome.
Aneuploidy results from non-disjunction

14. Abnormalities in chromosome number result from non-disjunction
Homologues fail to separate during Meiosis I.

15. Abnormalities in chromosome number result from non-disjunction-2
Sister chromatids fail to separate during Meiosis II.

16. Evidence for XX/XY If XXY is still male and XO is female
47, XXY Klinefelter syndrome
male in appearance, but some feminization; sterile.
slow to learn, but not retarded.
XXXY etc. similar, but more severe symptoms
45, XO Turner syndrome
Monosomy, the only one occurring in humans
female, sterile, short webbed neck, broad chest, short.
majority aren’t born
If XXY is still male and XO is female
Y must be determinant of maleness

17. About the Y Y chromosome has been shrinking.
Now missing many of genes that X has.
Two regions: PAR and MSY
PAR= pseudoautosomal region
Regions near p telomere and q telomere are homologous to X chromosome. Crossing over can occur there during meiosis. Because of this, genes in this location do not behave as sex-linked traits, thus said to be pseudoautosomal because they behave like genes on autosomes rather than sex chromosomes.

18. Structure of Y
universe-review.ca/R11-14-Ychromosome.htm
Human Genome project has revealed much about the Y chromosome.

19. Male specific region Y (MSY)
X-transposed region
99% identical to X chromosome region, but only 2 genes; the rest are not expressed.
X-degenerative region
Contains DNA related to X chromosome regions
Several functional genes and pseudogenes
Contains SRY that codes for testis-determining factor, necessary for maleness during development.
Ampliconic region
Highly similar or repeated genes, some related to male development and fertility.

20. Evidence for SRY SRY contains gene for testis determining factor
Crossing over in meiosis
Males with two X chromosomes; SRY found on one
Females with X and Y; SRY is missing from Y
Transgenic mice
Remove SRY from Y chromosome
Mice are XY but are female
Reciprocal experiment also done

21. Article about Y chromosome
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