Biology 205 Chromosomal Basis of Inheritance: Sex linkage and Determination

Chapter 3

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1. State the chromosomal theory of inheritance and define basic concepts

• Locus
• Allele
• Homologous Chromosomes
• Autosomes vs Sex Chromosomes

2. Explain the X and Y chromosome system for sex determination in humans

• Define homogametic and heterogametic sex
• Humans - Y chromosome mechanism of sex determination - Y chromosome absent - gonads develop into ovaries
• Note that in birds and butterflies the female is the heterogametic sex

3. Sex linkage

• Define sex linkage and be able to do problems involving sex linkage
• Distinguish between homozygous and hemizygous
• X linked recessive traits
• Examples: Hemophilia, colorblindness (some forms)
• X linked dominant traits
• Example: Soft tooth enamel
• Y linked inheritance - Holandric SRY gene

4. Nondisjunction of sex chromosomes

Define non disjunction.

Learn basic syndromes in humans related to non disjunction.

• Turner syndrome
• Klinefelter syndrome
• XYY males

6.Sex Determination

Mammals: Sex determined mainly by the Y chromosome

Explain the concept of dosage compensation p 61-63

• Define the Barr body and explain its significance
• Note that X chromosome inactivation is called Lyonization and it happens at about 16 days.
• List the steps in X inactivation. p 64.
• Comment: Note that X controlling element(XIST) is expressed on the inactive chromosome. It produces an RNA which prevents the inactive chromosome from being expressed.
• Give the expected number of Barr bodies for different forms of nondisjunction. See table 3.2
• Why are mammalian females often to be mosaics?
• Explain the origin of the calico cat phenotype and anhidrotic ectodermal dysplasia in humans

Other Chromosomal Systems

• Drosophila - XX is female XY is male but XXY is also female -balance autosome and X chromosome number
• C. elegans - Similar to fruit fly but here the sexes are either hermaphroditic or male. p 65
• Plants more complex - some are monoecious others dioecious
• Hymenoptera - haplodiploidy - actually individuals that are homozygous or hemizygous at a sufficient number of sex determining loci become male and individuals that are heterozygous at a sufficient number of these loci become female. Diploid males sometimes occur.

 

Genic systems - mating strains + Vs - mating strains as in yeast, other fungi and many protists

Environmental sex determination

Some fish and reptiles. Turtles - eggs above 32C become female, eggs below 28C become male