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1/30/2012 Genetics

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Sex Chromosomes

-Discovery of X-linked (sex-linked) inheritance shoed that not all traits follow simple Mendelian rules

-In 1901 C.E. McClung first to suggest sexes were due to the inheritance of particular chromosomes

-Sex chromosomes

-Autosomal chromosomes (everything but sex chromosomes)


Various Systmes of Sex Determinination

-X-Y System (mammals)

-Males XY (Heterogametic)

-Female XX (Homogametic)

-XO System (many insects)

-Male XO (only 1 sex chromo

-Female XX (Pair of sex chromosomes

ZW System (Birds & some fish)

-Male ZZ Homogametic

-Female ZW hete


Inheritance pattern of an X-linked trait

Red-eyed female (R) wild type; red eyed (true breeding line)


White-eye male (mutant)


F2 generation 50% chance males will be white-eyed

0% chance females will be white-eyed


F1 Female

Homozygous (XRXR) or Heterozygous (XRXr)?


Test cross (Cross an unknown with a known)

White eyed male x F1 Red-eyed female


Reciprocal Cross

red-eyed male x white-eyed female






White-eyed male x red-eyed female (homozygous)



Male transmits X chromosome only to female offspring; female contributes an X chromosome to both male & female offspring.


Sex Linkage Terms

-Hemizygous: term used to describe the single copy of an X-linked gene in males (XRY)

-Holandric genes: the few sex-linked gene located on the Y chromosome (XYA)

-Pseudoautosomal inheritance: because there is a short region of homology between the X and Y chromosomes some sec-linked genes have the same inheritance pattern as autosomes (males can transmit such genes to both male and female offspring)


Extension of Mendelian Inheritance Chapter 4

Types of Mendelian Inheritance


-X linked

-Lethal alleles

Incomplete dominance



Incomplete penetrance

sex-influenced inheritance

sex-limited inheritance



-Wild Type Allele: most prevalent in population

-Mutant Allele: rare, usually decreased or eliminated protein synthesis or functional activity; ofter inherited in a recessive fashion

If herterozygotes of mutant alleles can survive, what does that say about the production of the wile type protein among homozygotes?

-That one good allele can produce 50% of what a normal allele would, is producing 2 times as much of that protein that is needed to produce that phenotype

-One good allele that you have is having to be expressed twice as much


Lethal Phenotype

-Essential Gene: codes for a protein without which the organism cannot survive (~1/3 of all genes)

-Lethal Allele: an allele that has the potential to cause the death of an organism

-usually inherited in a recessive manner

-Why? If you could just have one, it wouldn’t be in the population very long, there would be no such thing as a carrier.


-Conditional lethal alleles will kill organism only if or when certain environmental conditions arise (ex. Temperature-sensitive lethal alleles)


-Semilethal alleles: within a population this allele will kill some individuals, but not all.

-Not well understood