Bio102: Introduction to Cell Biology and Genetics
Genes and Chromosomes: Sex Linkage and Karyotyping
Key Terms:
chromatin
histones
linked genes
sex-linked genes
karyotype
euploid
aneuploid
trisomy
monosomy
polyploid
autosomes
nondisjunction
Key Questions:
 How does the behaviour of chromosomes during meiosis explain Mendel's Law of
Segregation?
 How does the behaviour of chromosomes during meiosis explain Mendel's Law of
Independent Assortment?
 What kinds of genetic abnormalities can be detected by karyotyping? What kinds cannot?
 How is sex determined in mammals?
 How would Mendel's dihybrid cross results have been different if he'd happened to use two
genes that were located on the same chromosome?
 Why must the cells be stopped in M phase for a karyotype?
Lecture Outline:
Chromosomal Theory of Inheritance
Mendel:
Boveri & Sutton
alleles segregate in gamete formation
homologous chrmsomes separte in meiosis
different genes segregate independently
different chromosomes segregate independtly
offspring gets one allele per parent
offspring gets one chromosome per parent
therefore, genes are on chromosomes!
each gene has one consistent location.
Genes on the same chromosome are said to be “linked” so these don’t assort independently
Karyotype is the visible chromosomes of an individual
for humans, euploid cells have 46 chromosomes
if the wrong number of chromosomes is present, the cell is aneuploid
polyploid cells have extra sets of chromsomes
In humans gender is determined by the 23rd pair of chromosomes, the sex chromosomes
XX is female. XY is male
X has many genes on it. Y has very few, including the one for ‘maleness’
common human aneuploidies include
Down’s Syndrome (trisomy 21; three copies of chromosome 21)
Turner’s Syndrome (XO for the sex chromosomes)
Klinefelter’s Syndrome (XXY for the sex chromosomes)
most aneuploidies are not tolerated and are lethal
aneuploidies caused by nondisjunction at different points in meiosis
sex-linked genes are located on the X chromosome. No corresponding gene on Y
thus always move with that chromosome
can’t be inherited father-to-son
females are diploid (homo- or heterozygous) for these genes; males are hemizygous