Chapter 15: The Chromosomal Basis of Inheritance. Important Point:. If you are having trouble understanding lecture material: Try reading your text before attending lectures. And take the time to read it well!. Chromosomal Basis of Inheritance. Independent Assortment.
If you are having trouble
understanding lecture material:
Try reading your text
before attending lectures.
And take the time to read it well!
“Notice… that one-half of the offspring are expected to inherit a phenotype that matches one of he parental types.” p. 278, Campbell & Reece (2005)
Basis of recombinant types
Complete linkage give frequency of recombination of 0%
Frequency of recombination
Note that max = 50% (which occurs when loci are either on separate chromosomes or very far apart on the same chromosome)
Note that it is the recombinant types that there are fewer or
Linkage means that two loci are found on the same chromosome
Ordering loci on chromosomes
Best way to interpret this data is that b and vg are about twice as far apart as b is from cn or cn is from vg. Hence, cn must be in the middle
>50% frequencies of recombination are produced by adding together smaller frequencies of recombinationLinkage Maps
Know what a linkage map is
e.g., some forms of hemophilia
e.g., some forms of color blindness
e.g., Duschenne muscular distrophy
Male genotype 100% controlled by what is in the egg
Differences 100% dependent on what is in the egg
Males are more likely affected by recessive alleles, females by dominant alleles
Males express the genotype regardless of whether it is dominant or recessive
Dominant or Recessive Sex-Linked Inheritance?
One X chromosome is inactivated, to form what is known as a Barr body; this assures that both men and women have the same number of active X chromosome genes
X chromosomes are inactivated randomly and stay inactivated
Female mammals are mosaics
Polyploidies (e.g., in plants) are actually less serious than one too many (or one too few) autosomes (is a duplicate of all chromosomes)
An example of a monosomy
Turner Syndrome (XO), Incidence: 1 in 2500 female births
Klinefelter Syndrome (XXY), Incidence: 1:1000 male births
Also plastid mutation