Genetics chp. 4 (Extensions and Modifications to inheritance)

Chromosome sex determination of dif. organisms

• Humans: Male heterogamety (XX:XY)

• Birds: Female heterogamety (ZW:ZZ)

• Moths: Only one sex chromosome (ZO:OO)

• Grasshoppers: Only one sex chromosome (OO:XO)

• Bees: Ploidy determines sex (diploid:haploid)

Genic sex determination

No sex chromosomes only sex determining genes

ex. humans with SRY gene for male

Environmental sex determination

Environmental factors determine sex

ex. temperature for turtles

Roles of X and Y chromosome

X has genes essential for both sexes

Y has male determining gene (even with multiple X chromosomes as long as there is a Y, a male is the result)

Androgen insensitivy syndrome

defective androgen receptors will have affects for both male and female

Male affects: Poor bread growth, development of breasts, small testes

female affects: Constriction of aorta, under developed breasts and ovaries, distinct facial features, web of skin

Abnormal sex chromosome number and syndromes

Klinefelter: XXY, could be sterile, testes are underdeveloped

Poly X female: More than two X chromosomes, has no affects

Turner syndrome: XO, Female with underdeveloped sex characteristics

(Think about how mutations to shugoshin, cohesin, and seperase could cause these)

Environmental sex determination with turtles

• Exposure to cold and wet: Male specific proteins are activated

• Exposure to warm and dry: Female specific proteins are activated

X-linked recessive pedigree characteristics

• trait is rare in pedigree

• skips a generation

• More common in males

• Affected fathers cannot pass to sons

X-linked dominant pedigree characteristics

• trait is common

• Affected father will pass to all daughters

• Males and females have equal chance of being affected (only need one X to have it)

Dosage compensation

amount of protein produced by X-linked genes are randomly inactivated producing barr bodies (Purpose is to ensure that both females and males are getting the same amount of protein from X-linked genes)

Dosage compensation mechanisms

• Mice: females randomly inactivate one X chromosome to mimic the single X chromosome of the male

• Fruit Flies: Increase transcription of single X chromosome in males to mimic 2 X chromosomes of female

• Elegans: females express both X chromosomes by half to mimic the single X in males

Incomplete dominance vs codominance

• incomplete dominance: genes form new phenotype because neither is dominant nor recessive

• codominance: both genes are expressed to some degree together

Penetrance

Percentage of individuals having a certain genotype that express the phenotype

Expressivity

Degree to which a phenotype is expressed

gene interactions (synthesis, epistasis, suppression)

synthesis - the sum of two gene products leads to something else

epistasis - One genes determines the masking or expression of another gene

suppression - a mutant allele hides or modifies the effects of another mutant allele