Key Concepts for Week 8: Lectures 20-22
Monday, Nov. 16, Lecture 20: Mendel and the Gene
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Gene: unit of heritable information
Genetics: the study of heredity and heritable variation
Study of genetics started with Mendel and the pea plant experiment
o Crossed different plant varieties (purple and white plant) and observed a 3:1 ratio
 Dominant versus recessive allele
Law of segregation: two alleles for a trait segregate into different gametes
Law of independent assortment: each pair of alleles segregates independently of each other pair of alleles
during gamete formation
Test cross: a way to identify genotype of unknown parent by breeding it with a homozygous recessive
Wednesday, Nov. 18, Lecture 21: Human Genetic Disease
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Degrees of dominance:
o Complete – dominant or recessive
o Incomplete – neither allele is dominant (a mixed blend)
o Codominance – two alleles each affect phenotype in separate ways
 Ex: AB blood type
Pleiotropy: single gene has more than one phenotypic effect
Epistasis: gene that affects another gene at a different locus
Genetic disorders
o Recessively inherited
 Carriers: phenotypically normal but has a recessive allele that can be passed to offspring
o Dominantly inherited
Friday, Nov. 20, Lecture 22: Alterations of Chromosomes
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Sex linked genes
o Chromosomal basis of sex
o X or Y-linked genes
o X inactivation in females: an inactive X chromosome forms a Barr body (these genes are not
expressed)
Linked genes: genes located near each other on the same chromosome are often inherited together
o Sex-linked versus linked genes NOT THE SAME!
Genetic recombination: offspring produced that are different from parents
o Recombination of linked genes happens during meiosis I, called crossing over
Linkage maps show recombination frequencies of genes
Alterations in chromosome structure:
o Deletion
o Duplication
o Inversion
o Translocation