Chromosomes

1
Chromosomes Heredity

• Mendel did not know about the existence of
  chromosomes in cells.
• In 1902, Walter Sutton and Theodor Boveri
  recognized the relationship between Mendels
  observations and the behaviour of chromosomes.
• This formed the basis of the chromosome theory of
  inheritance.

2
10.1.4 Chromosome Theory of Inheritance

• Genes are located on chromosomes and chromosomes
  provide the basis for the segregation and
  independent assortment of genes.

3
4.3.7 Sex-linked Inheritance

• The inheritance of some traits depends on the sex
  of the parent carrying the trait. This is
  because the genes for these traits are located on
  the sex chromosomes. A gene that is located on
  the X chromosome is called X-linked and a gene
  located on the Y chromosome is called Y-linked.
  (most of the traits are X-linked due to the
  larger size)

4
Thomas Morgan and the Fruit fly

• Fruit fly Drosophila melanogaster
• Thomas Morgan
• Worked with fruit flies in an attempt
• to duplicate Mendels experiments
• Looked at eye colour
• All flies had red eyes until
• one showed up with white eyes
• Many flies had red eye and fewer with white eyes,
  therefore red (wild type) is dominant over white
  (hybrid).
• Crossed a variety of flies and noticed that the
  flies with white eyes were always male.

5
(No Transcript)
6

• Morgan hypothesized that some traits were linked
  and therefore passed on together.
• If the white-eyed trait was found in a particular
  sex, it must be linked to the actual sex
  chromosome the X chromosome.

7
Conclusions

• Since males only have one X chromosome, they have
  a greater risk of inheriting any one of the
  genetic disorders that are associated with the X
  chromosome.
• Females inherit two X-chromosomes. This means
  that although she may inherit one gene for a
  particular X-linked disorder, it is likely that
  her other X chromosome will be unaffected.
• Females that have one affected X-chromosome are
  called carriers 4.3.10.

8
Why are guys the way they are?
9
4.3.8 Hemophilia Sex-Linked Disorder

• Hemophilia a hereditary condition in which the
  blood of a person does not clot normally.
• Sex-linked and recessive

10
(No Transcript)
11
Sex-linked traits and Punnett Squares

• Assume that the trait is X-linked unless
  otherwise stated.
• The gametes containing the X and Y chromosomes
  are placed on the outside of the Punnett square
• Alleles are written on the X chromosome and no
  allele is written on the Y chromosome.
• Sex is always included in the phenotypic and
  genotypic ratio.

12
Chromosomes and Gene Expression

• Even though females have two X chromosomes, one
  is inactivated (randomly), can be different among
  cells
• The inactivated X chromosome is called a Barr
  body

Animations
13
10.3.1 Polygenic Inheritance

• When a trait is controlled by more than one gene
• Many genes working together produce many proteins
  that together create a range of variation ?
  continuous variation in a trait

14
10.3.2
15
Modifier Genes? Epistasis

• Genes that work together with other genes to
  control the expression of a trait.
• In some cases, a gene at one locus alters the
  phenotypic expression of a gene at a second
  locus.
• Eye colour
• Brown eye ? presence of melanin
• (dominant)
• Blue eye ? absence of melanin
• (recessive)
• Hazel, violet, green ? result from the protein
  products of other genes that impact the alleles
  for the dominant or recessive eye colour
  characteristic
• Depending on the different combinations of
  alleles inherited, a person can have a range of
  proteins produced that produce a range of colour
  spectrum.

16
(No Transcript)