5.3 Chromosomes & Heredity Mendel did not know about the existence of chromosomes in cells. In 1902, Walter Sutton and Theodor Boveri recognized the relationship between Mendel’s observations and the behaviour of chromosomes. This formed the basis of the chromosome theory of inheritance.

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

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)

Thomas Morgan and the Fruit fly Fruit fly = Drosophila melanogaster Thomas Morgan: Worked with fruit flies in an attempt Worked with fruit flies in an attempt to duplicate Mendel’s experiments to duplicate Mendel’s experiments Looked at eye colour Looked at eye colour All flies had red eyes until 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). 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. Crossed a variety of flies and noticed that the flies with white eyes were always male.

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.

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.

Why are guys the way they are?

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

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.

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

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

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 Brown eye  presence of melanin(dominant) Blue eye  absence of melanin 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 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.