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Chromosomal T heory of Inheritance. Chapter 15. The Theory. Genes are located on chromosomes Chromosomes segregate and independently assort during meiosis. Evidence. Cytologists (study cells): 1879 – Mitosis worked out 1890 – Meiosis worked out Geneticists (study genes):

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the theory
The Theory
  • Genes are located on chromosomes
  • Chromosomes segregate and independently assort during meiosis
  • Cytologists (study cells):
    • 1879 – Mitosis worked out
    • 1890 – Meiosis worked out
  • Geneticists (study genes):
    • 1860 – Mendel proposed laws of segregation & independent assortment
    • 1900 – Mendel’s work rediscovered
sutton b overi
Sutton & Boveri
  • 1902 – identified parallels between Mendel’s factors and behavior of chromosomes
  • Work led to chromosomal theory of inheritance
linkage chromosome maps
Linkage & Chromosome maps
  • Linked genes are genes that are located on the same chromosome and DO NOT assort independently
  • We also know that chromosomes will also ‘cross-over’ during gamete formation
  • Based on the number of crossovers observed, we are able to determine the order of genes, and relative distance of genes from one another
genetic recombination
Genetic recombination
  • Production of offspring with different traits than parents (also called recombinants)
recombination of unlinked g enes
Recombination of Unlinked Genes
  • In a testcross of YyRr x yyrr (yellow round x green wrinkled)
  • 25% yellow round
  • 25% green wrinkled
  • 25% yellow wrinkled
  • 25% green round
  • Happens due to random orientation of chromosomes during metaphase I (independent assortment occurs)


Types: 50%


Types: 50%

recombination of linked genes
Recombination of Linked genes
  • The first thing we need to do is to calculate the percent of crossovers between genes
  • Example using Morgan’s flies:
    • b+ = grey vg+ = normal wings
    • b = black vg = vestigial wings
  • Grey, normal winged fly x Black vestigial winged fly
    • b+bvg+vg x bb vgvg
  • Cross produced 2300 flies
the numbers that we expected
The numbers that we expected…
  • If genes are NOT linked, we would expect 50% to be normal (express the dominant trait) and 50% to express the recessive trait because independent assortment occurs
  • If genes ARE linked, we would expect 50% of offspring to be like the mother and 50% to be like the father because they will not sort independently
recombination frequenc y
Recombination frequency
  • When crossing over occurs, the genes are said to be recombinantgenes
  • To find the recombinant rate, we will add the two phenotypes where it was obvious that crossing over occurred and divide it by the total
  • Calculated by: # of recombinants ÷ Total # offspring
  • 206 + 185 = 391  391/2300 x 100% = 17%
  • These genes cross over 17% of the time
the conclusion
The Conclusion?
  • Genes are linked, but not totally
  • Crossing over in meiosis can cause genes to “unlink”
genetic maps

17 Map Units



Genetic maps
  • We can translate recombination frequencies to “map units”
  • 1% = 1 map unit
  • The genes b and vg are 17 map units apart from one another
making a linkage map
Making a Linkage Map
  • Let’s say we have 3 genes:
    • b = body color
    • cn = eye color
    • vg = wing size
  • We observed the following recombination frequencies:
  • What order should these genes be in?

17 %

18.5 m.u.







9.5 %

9.5 m.u

9 %

9 m.u.

  • Due to the smaller amounts of crossovers between b-cn and cn-vg, and the large amounts between b-vg there are some discrepancies in the numbers. In cases like this, geneticists add the smaller numbers together when making a map.
let s try 4 traits
Let’s try 4 Traits
  • The recombination frequencies are as follows:
  • Map this chromosome with the given information (given that these genes are linked)

The easiest thing to do is map the two furthest ones:

  • M and P were 11% apart
  • O seems to be equal between M and P (5.5%)
  • N is close to O (2.5%) But is it closer to P or M?
  • N-P= 8%; N-M= 3%
  • N is closer to M due to the lower percentage





3 m.u.

2.5 m.u.

5.5 m.u.

11 m.u.

sex determination
Sex Determination
  • Determined by presence of Y chromosome in humans
    • XX: female
    • XY: male
  • Other systems in birds, insects:
    • XO: insects
    • ZW: birds
    • Haplo-diploid: bees
sry gene
SRY gene
  • Sex determination region
  • On Y chromosome
  • Triggers events that lead to testicular formation
sex linkage or x linkage
Sex-linkage (or, X-linkage)
  • Some traits are only inherited because they are on the X chromosome
  • Typically recessive traits
  • Males inherit these more often
  • Why?
sex linked disorders
Sex linked disorders
  • Colorblindness
  • Hemophilia
  • Duchenne Muscular Dystrophy
sample cross
Sample cross
  • Colorblindness
    • XC = normal vision
    • Xc = colorblind
  • XCXc x XCY
  • In any sex-linked cross:
  • Affected males Xc get from Mom
  • Affected females get one Xc from Mom and one from Dad
  • More males affected than females
x inactivation
X inactivation
  • Females are XX but only need one X
    • one X condenses and genes become silenced
  • Inactivation happens during embryonic development
  • Inactivation is random
  • Females are mosaics of 2 cell types
    • paternal X inactive
    • maternal X inactive
chromosomal alterations
Chromosomal alterations
  • Mutations of chromosomes
back to mendel for a minute
Back to Mendel for a minute…
  • Looking at Mendel’s peas, it didn’t matter if the traits came from the maternal or paternal parent
  • Each trait would have the same bearing on the offspring
  • Scientists, within the past decade or so, have found some genes that are inherited differently depending on which parent passed it along
genomic imprinting
Genomic Imprinting
  • Variations in phenotype depending on whether the allele is passed on from the male or female parent
  • This differs from sex linkage due to the fact that most imprinted genes are found on autosomes
  • Occurs during gamete formation
  • These genes are expressed differently in eggs or sperm – the trait is ‘silenced’ in one gender
  • The developing embryo will only express the trait from one parent in all of its body cells
  • Effect is determined by which chromosome’s gene is expressed
example igf2
Example – Igf2
  • Insulin-like growth factor 2 (Igf2) is needed in mice for normal growth and development
  • In crosses between wild type and homozygous recessive dwarf mice, individuals produced heterozygous offspring
  • Offspring differed in phenotype depending on whether the gene was maternal or paternal
one more genetic exception
One More Genetic Exception…
  • Not all genes are found on nuclear chromosomes
  • Organelles such as mitochondria and chloroplasts have circular DNA molecules that carry genes
  • These are almost always passed on from the mother because the egg houses these organelles, while the sperm only carries chromosomes