BIO152H5 Lecture Notes - Lecture 7: Chromosome, Meiosis, Barr Body

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20 Oct 2015
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Lecture 07: Chromosomal Basis of Inheritance
Midterm Information
Week 1-4 (everything before this lecture)
50 multiple choice questions
Everything in class, NOT textbook
Mendelian Inheritance: chromosomes are present in diploid cells, homologous chromosomes separate
during meiosis, alleles segregate during meiosis, fertilization restores 2n condition
Chromosome theory of inheritance:
genes have specific locations on chromosomes
Morgan's experimental evidence
association between genes and chromosomes discovered by Dr. Thomas Morgan
studied fruit flies because they're fast breeders, many offspring, large sample size
Looking for variation in eye colour (red = wild type, white = mutant phenotype)
All flies have red eyes: red eye colour dominant to white
Female flies have red eyes, males have white: eye colour must be on x chromosome
A trait determined by genes on one of the sex chromosomes is sex linked.
X-chromosome is larger, so more traits are X-linked than Y-linked.
Autosomes: non sex-linked chromosomes
X and Y chromosomes act like homologs during meiosis.
Each egg contains one X chromosome, half the sperm are X and half are Y.
Sex determination is a 50-50 chance.
Example: color-blindness, X-linked and therefore more likely in males
hemophilia, X-linked and therefore more likely in males
one copy of allele needed for expression in males, two in females
daughters who receive one are heterozygous carriers
male with mutation pass it on to daughters, sons get Y
X Inactivation in Female Mammals
one X inactivated, females like males now have one copy of X-linked genes
called a 'Barr body', occurs randomly and independently in embryos
tortoiseshell cats: orange and black are on X chromosomes, depends on which is activated
Aneuploidy: abnormal number of chromosomes (trisomic, monosomic)
Polyploidy: more than 2n chromosomes, common in plants
Female - XXX (triple X), XO (Turner Syndrome)
Male - XXY (Klinefelter syndrome), OY (nonviable)
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