The Chromosomal Basis of Inheritance

1
Chapter 15

• The Chromosomal Basis of Inheritance

2
The Chromosomal Basis of Inheritance.

• Mendelian genes have specific loci on
  chromosomes, and it is the chromosomes that
  undergo segregation and independent assortment.

3
Thomas Hunt Morgans Experiments

• Morgan used fruit flies Drosophila melanogaster,
  in his genetic studies.
• Morgan discovered the sex-linkage of some genes.

4
Sex Linked Genes

• Genes located on sex chromosomes are sex-linked
  genes.
• Sex-linked genes can be located on the X or Y
  chromosome in humans.
• Morgan studied eye color in the flies and found
  that eye color for a mutant version, white eyes,
  is located on the X chromosome.

5
Linked Genes

• Each chromosome has many genes. Genes located on
  the same chromosome tend to be inherited
  together. These genes are said to be linked.
• Morgan found that the genes for body color and
  wing shape tend to be inherited together since
  they are on the same chromosome.

6
Genetic Recombination

• Genetic Recombination is the general term for the
  production of offspring with new combinations
  inherited from two parents.

7
Unlinked Genes Independent Assortment of
Chromosomes

• In Mendels experiments, seed color and seed
  shape were two characters studied.
• There are many different combinations of the
  traits for these characters.
• These two characters are located on different
  chromosomes and neither affects the inheritance
  of the other.

8
The Recombination of Linked Genes Crossing Over

• Linked genes do not assort independently because
  they are located on the same chromosome and tend
  to move together during meiosis and
  fertilization.
• Recombination can occur when crossing-over
  between homologous chromosomes occurs.

9
Mapping Genetic Loci

• A genetic map is an ordered list of the genetic
  loci along a particular chromosome.
• Sturtevant, one of Morgans students, discovered
  that the farther apart 2 genes are located on a
  chromosome, the higher the probability that a
  crossover will occur and thus provide more
  genetic variability.

10
Sex Chromosomes Mammals

• Mammals- XX is female XY is male.
• These are not homologous, but behave as such
  during meiosis.
• Y has a sex determining region (SRY). If SRY is
  present, then testes develop, if absent then
  ovaries develop.

11
Sex Chromosomes in Birds

• The sex chromosomes in birds are designated Z and
  W, and the male is the homomorphic sex (ZZ) and
  the female heteromorphic (ZW).
• In most avian species the Z chromosome is a large
  chromosome, usually the fourth or fifth largest,
  and it contains almost all the known sex-linked
  genes.
• The W chromosome is generally a much smaller
  microchromosome, containing a high proportion of
  repeat sequence DNA.

12
Reptile Sex Determination

• Many turtles, lizards, crocodilians, and a few
  snakes have a form of sex determination that
  depends on the incubation temperature of eggs.
• The female is responsible for the location in
  which she buries her eggs, she can control the
  sex ratio of her clutch.
• For example, some turtles have eggs that turn
  into males when they are incubated at low
  temperatures, and females when they are incubated
  at high temperatures.

13
Reptile Sex Determination 2

• Other species of turtles produce males at low
  temperatures, females at intermediate
  temperatures, and males again at the highest
  temperature.

14
Haplo-Diploid System

• The Haplo-diploid sex-determination system is a
  system that some Hymenopterans (bees, ants, and
  wasps), and coleopterans (bark beetles) use to
  determine the sex of their offspring, and
  preserve eusociality.
• In this system, there are no sex chromosomes.
  Instead, the sex is determined by the number of
  chromosomes. Haploid offspring (which are from
  unfertilized eggs) are males, while Diploid
  offspring (which are fertilized eggs) are females.

15
X0 System (Some Insects)

• Sex determination is by a chromosomal "balance"
  X0 system.
• In this system (grasshoppers, crickets, roaches,
  and some other insects) there is only one sex
  chromosome, referred to as X. Males only have one
  X chromosome (X0), while females have two (XX).
• The 0 in the males means zero or no chromosome.

16
Sex Linked Inheritance

• In sex-linked inheritance, alleles on sex
  chromosomes are inherited in predictable
  patterns.
• For example, in Drosophila the locus for eye
  color is located on the X chromosome. The allele
  for red eye color, which is normal in wild flies,
  is dominant to the mutant allele for white eyes.
• As females have two chromosomes X (with a locus
  for eye color), they might be homozygous or
  heterozygous for either allele.

17

• Males, who carry only one X chromosome, are
  always hemizygous. They carry only the one X
  chromosome inherited from their mother, and it
  determines their eye color.

18

• In the left hand example, homozygous red eyed
  females (RR) mate with hemizygous white eyed
  males (w-). In the offspring, all the daughters
  are red eyed heterozygotes (Rw) and all sons are
  red eyed hemizygotes (R-).
• In the right hand, homozygous white eyed females
  (ww) mate with hemizygous red eyed males (R-). In
  the offspring, all the daughters are red eyed
  heterozygotes (Rw) and all sons are white eyed
  hemizygotes (w-).

19
Human Sex-Linked Disorders

• X-linked disorders are those in which the
  defective gene lies on the X sex chromosome. We
  inherit the sex chromosomes along with the other
  44 (22 pairs) of non-sex chromosomes from our
  parents.
• For X linked disorders, in males as there is no
  second X chromosome carrying the normal copy to
  compensate.

20
Examples of X-Linked Disorders

• Typical examples of X-linked disorders include
  hemophilia and Duchenne Muscular Dystrophy.
• In such examples the sons of a non-carrier father
  and a carrier mother have a 50 chance of being
  affected. Daughters have a 50 chance of being
  carriers.

21
(No Transcript)
22
X Inactivation in Female Mammals

• X inactivation The phenomenon in a female by
  which one X chromosome (either the maternal or
  paternal) is randomly inactivated in an early
  embryonic cell, with fixed inactivation of that
  same X in all cells descended from that cell.

23
Barr Bodies

• The inactivated chromosome becomes a Barr body.
• Its compact appearance reflects its inactivity.
  So, the cells of females have only one
  functioning copy of each X-linked gene the same
  as males.

24

• After inactivation has occurred, all the
  descendants of that cell will have the same
  chromosome inactivated.
• Thus X-inactivation creates clones with differing
  effective gene content.
• An organism whose cells vary in effective gene
  content and hence in the expression of a trait,
  is called a genetic mosaic.

25
(No Transcript)
26
Errors in Chromosomal Inheritance

• Chromosomal abnormalities can occur during
  meiosis. The chromosomes can fail to separate
  during meiosis- nondisjunction.
• This can result in having the wrong numbers of
  chromosomes- aneuploidy.
• Having one too many, 3, is trisomy.
• Having one too few, 1, monosomy.

27
Other Abnormalities

• Abnormalities in chromosome structure follow a
  chromosome break and, during the repair process,
  the reunion of the wrong segments of the
  chromosome. If, following repair, there is a loss
  or gain of chromosomal material (an unbalanced
  rearrangement) there can be significant clinical
  consequences.

28
Deletion

• A chromosomes deletion is when a part of a
  chromosome(s) has been deleted. A deletion can
  occur on any chromosome, at any band, and can be
  any size (large or small). What a deletion causes
  depends on how big a piece is missing and what
  genes are missing in the section (i.e. where the
  deletion is).

29
Duplication

• A duplication is just that, a duplication of a
  section of a chromosome. A duplication is
  sometimes referred to as a 'partial trisomy'.
  Trisomy refers to three. Therefore if a
  duplication exists, that individual has three
  copies of that area instead of two. This means
  there are extra instructions (genes) present that
  can cause an increased risk for birth defects or
  developmental problems.

30
Inversion

• An inversion consists of two breaks in one
  chromosome. The area between the breaks is
  inverted (turned around), and then reinserted and
  the breaks then unite to the rest of the
  chromosome. If the inverted area includes the
  centromere it is called a pericentric inversion.
  If it does not, it is called a paracentric
  inversion.

31
Translocation

• Translocation is the term used to describe a
  rearrangement of chromosome material involving
  two or more chromosomes.
• The most common type of chromosome translocation
  is called a reciprocal translocation because
  material is swapped between two chromosomes.

32

• Another type is called a Robertsonian
  translocation, named after an American scientist,
  that only involves rearrangements between certain
  chromosomes numbers 13, 14, 15, 21 and 22.

33
Chromosomal Abnormalities Disorders

• Down syndrome or trisomy 21 is a genetic disorder
  caused by the presence of all or part of an extra
  21st chromosome.
• Individuals with Down syndrome can have a lower
  than average cognitive ability, often ranging
  from mild to moderate mental retardation.
• 1 per 800 to 1 per 1,000 births.

34

• Klinefelter's syndrome,or XXY syndrome is a
  condition caused by a chromosome nondisjunction
  in males affected individuals have a pair of X
  sex chromosomes instead of just one and are at
  additional risk for some medical conditions.
• Turner syndrome encompasses several chromosomal
  abnormalities, of which monosomy X is the most
  common.
• It occurs in 1 out of every 2500 female
  births1. Instead of the normal XX sex
  chromosomesfor a female, only one X is present
  and fully functional. This is called X0. In
  Turner syndrome, female sexual characteristics
  are present but generally underdeveloped.