BIO 202 Lecture Notes - Lecture 11: Centimorgan, Y Chromosome, Gonad
7 Aug 2018
School
Department
Course
Professor
Chapter 15: The Chromosomal Basis of Inheritance
• Mendelian genes have specific loci on chromosomes.
o Chromosomes undergo segregation and independent assortment.
o Morgan showed the first solid evidence associating a specific gene with a specific
chromosome.
o Convincing evidence that chromosomes held Mendel’s heritable factors.
• Linked genes- Genes on the same chromosome that tend to be inherited together.
• If genes are linked, there is an expected ratio of 1:1:0:0 of the two parent phenotypes and
hybrid phenotypes, respectively, when parents are homozygous.
o There is a little bit on deviation from this, due to genetic recombination.
• Unlinked genes are expected to have a 1:1:1:1 ratio of all phenotypes when a
heterozygous dihybrid is crossed with a homozygous recessive
• The percentage of recombinant offspring, the recombination frequency, depends on the
distance between genes on a chromosome.
o The farther apart two genes are, the higher the probability that a crossover will
occur between then and therefore the higher the recombination frequency.
o 1% of recombination frequency is expressed as 1 map unit of distance on the
chromosome, or 1 centimorgan.
• Two X’s is female, XY is male (homogametic and heterogametic).
o SRY is the primary sex determining region on the Y chromosome; it’s a
transcription factor for testis production (primary is type of gonad and gametes,
secondary is body type and puberty effects).
