1. Chromosomes & Human Genetics
Discover Biology, 5th Ed., Anu Singh-Cundy & Michael Cain © 2012
W.W. Norton & Co., New York
Chapter 13, pp

2. Chromosomes in Inheritance
August Weismann, 1882, proposed chromosome # was halved in gametes
Meiosis discovered in 1887
August Weismann proposed hereditary material’s on chromosome
Idea that genes were on chromosomes  “chromosome theory of inheritance” established in early 20th Century
Homologous chromosomes pair up during meiosis (1 from each parent)
Genes are located at the same spot on homologous chromosomes = “loci”
Alleles are versions of the same gene
Same alleles = homozygous
Different alleles = heterozygous

4. Autosomes vs. Sex Chromosomes
22 pairs (1 from each parent)
Labeled # 1 - # 22 with the longest as # 1 and the shortest as #22
Each homologous chromosome is identical in length, shape, & genetic loci
Sexual & non-sexual characteristics
Sex chromosomes: XX = female; XY = male
X chromosome
Has many genes
Y chromosome
Has very few genes
Contains SRY gene (Sex-Determining Region of Y) specific for males

6. Individual Genetic Differences
Crossing-over
During prophase I of meiosis
Reciprocal exchange of segments of nonsister chromatids as tetrads
Typically near the ends of chromatids
Independent assortment of chromosomes
Maternal & paternal homologues randomly orient at metaphase plate
During anaphase II of meiosis, homologues separate to different sides
Fertilization
Diploid cells (23 chromosome pairs) = 223 combinations = 8,388,608 ways/ gamete

8. Genetic Linkage & Crossing-over
Thomas Hunt Morgan, 1909, experimented with fruit flies
Some genes were almost always inherited together
Seemed to defy Mendel’s law of independent assortment
Linked genes are located on the same chromosome
Linked genes located very near one another
Chromosome map
Shows linear order of genes on a chromosome [Alfred Sturtevant]
1 map unit = frequency of crossing-over of 1 %
Crossing-over reduces genetic linkage
Pea plants’ flower color & seed color are separated on Chromosome 1.
Because of distance between their loci, independent assortment was seen.

11. Human Genetic Disorders
Pedigrees to chart family trees for a particular trait
Somatic cell mutations are NOT passed down to offspring.
Only mutations found in germ cell are hereditary.
Dominant genes = display trait even if heterozygote
Carriers = do not display trait but carry gene as heterozygotes
Recessive alleles = display trait only if homozygous
Autosomal recessive genetic disorders are common
Cystic fibrosis (Chromosome 7)
Sickle-cell anemia (Chromosome 11)
Tay-Sachs disease (Chromosome 15)
Huntington’s disease (Chromosome 4)
Amyotrophic lateral sclerosis, ALS (Chromosome 21)

13. Sex-linked Inheritance Single-gene Mutation
~ 1200 of humans’ 25,000 genes are located on sex chromosomes
~ 1100 sex-linked genes are on X chromosome  X-linked
Males (XY) are more likely to display these traits.
Red-green color blindness
Pattern baldness
Duchenne muscular dystrophy Recessive allelic disorders
Hemophilia
Congenital generalized hypertrichosis (CGH)  Dominant X-linked disorder
~ 50 sex-linked genes are on Y chromosome  Y-linked
No well-documented disease-causing genes
~ 15 genes are shared on both X & Y chromosomes; NOT sex-linked

15. Inherited Chromosomal Abnormalities
Deletions
A piece breaks off and is lost from the chromosome
Inversions
Broken off piece reattaches to correct site but in reverse order
Translocations
Broken off piece of chromosome attaches to nonhomologous chromosome
Duplication
Homologous chromosomes do not split during anaphase
1 gamete gets 2 copies of same chromosome, while the other gets none.
Chromosome lengthens because it has 2 copies of a fragment joined together
Errors in crossing-over  1 chromosome has deletion while other has duplication

19. Mutations in Chromosomes Affect Gender
Translocation of SRY gene to X chromosome
XX individual develops as a male
Deletion in SRY gene on Y chromosome
XY individual develops as a female
AIS (androgen insensitivity syndrome) individuals are XY males
Unable to respond to male hormones such as testosterone
Appear female
“Feel” female, but ovaries fail to develop normally

20. AIS