Notes – Ch. 15.1-15.3
Key Vocab.
Wild type
Mutant
Recombination
Recombinant
Parental
Genomic imprinting
Recombination Frequency
crossing over
independent assortment
linked genes
linkage map
X inactivation
Barr body
nondisjunction
aneuploidy
deletion
insertion
duplication
translocation
trisomy
monosomy
Chromosomal Inheritance – Bridge between Ch. 13 and 14 – explains the
relationship between meiosis and Mendelian inheritance
Meiosis
Mendelian genetics
Chromosomal Basis of
Inheritance
Morgan’s studies:
White eyed male (mutant) X red eyed female (wild type)
F1 = all red eyed flies (male and female)
F2 = 3:1 phenotypic ratio – but only males demonstrated the white eyed
mutation
 shows connection between sex and other traits
 Shows that not ALL traits assort independently from one another
 Supports chromosomal theory of inheritance – sex inherited WITH
another trait
Morgan continued his research to observe the inheritance of two traits at the
same time – higher proportions of parental phenotypes than would be
expected with independent assortment – so they must be inherited together
 Supports chromosomal theory of inheritance – traits on the same
chromosome may be inherited together
 linked genes tend to be inherited together because they are
located near each other on the same chromosome
 they are not ALWAYS inherited together – RECOMBINATION can
still occur due to crossing over
Recombination:
Mixes parental genetic info – produces recombinants.
A recombinant refers to offspring whose phenotype differs from either
parent
 crossing over accounts for the recombination of linked genes
o the closer two genes are to one another on a chromosome,
the lower the recombination frequency & the more likely
they are to be inherited together
o the farther apart two genes are from one another on a
chromosome, the greater the recombination frequency and
the less likely they are to have linked inheritance
o Recombination frequency – the percent (%) of recombinant
offspring
How to find the recombination frequency:
Number of recombinants
--------------------------------Total number of offspring
X
100
Sturtevant expanded on the work of Morgan and used recombination
frequencies to create linkage maps of chromosomes.
 helped to determine the relative locations of genes – the location
of one gene relative to others - based on recombination
frequencies
Sex linked genes
Genes present on the sex chromosomes will be inherited in conjunction with
sex and display their own distinct patterns of inheritance.
 we are most familiar with the X-Y sex inheritance that occurs in
humans and other mammals
 other systems exist (XX-X, Z-W, haploid-diploid) – I wouldn’t
expect to see these on the AP exam
Inheritance of affected X chromosomes in mammals
For each of the following crosses, create a Punnett square showing
inheritance and the offspring that are affected, are carriers, or are totally
normal.
Normal Mom X affected dad (XAXA x XaY)
Carrier Mom X normal dad (XAXa x XAY)
Carrier Mom X affected dad (XAXa x XaY)
X Inactivation – in female mammals, X inactivation occurs early in embryonic
development. This random process ensure only one X chromosome is active in each
somatic cell – which can cause a mosaic phenotype (some cells express one X, other
cells express the other.) This process keeps the transcriptional products of genes on
the X chromosome equal to what would be produced by males who have only one X
chromosome.