Chromosomes & HeredityChromosomes & HeredityMendel did not know Mendel did not know about the existence of about the existence of chromosomes in cells.chromosomes in cells.

In 1902, Walter Sutton In 1902, Walter Sutton and Theodor Boveri and Theodor Boveri recognized the recognized the relationship between relationship between Mendel’s observations Mendel’s observations and the behaviour of and the behaviour of chromosomes.chromosomes.

This formed the basis of This formed the basis of the chromosome theory the chromosome theory of inheritance.of inheritance.

10.1.4: Chromosome Theory of 10.1.4: Chromosome Theory of InheritanceInheritance

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

4.3.7: Sex-linked Inheritance4.3.7: Sex-linked Inheritance

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

Thomas Morgan and the Fruit flyThomas Morgan and the Fruit flyFruit fly = Drosophila melanogasterFruit fly = Drosophila melanogaster

Thomas Morgan:Thomas Morgan: Worked with fruit flies in an attempt Worked with fruit flies in an attempt

to duplicate Mendel’s experimentsto duplicate Mendel’s experiments Looked at eye colourLooked at eye colour All flies had red eyes until All flies had red eyes until

one showed up with white eyesone showed up with white eyes Many flies had red eye and fewer with white eyes, Many flies had red eye and fewer with white eyes,

therefore red (wild type) is dominant over white therefore red (wild type) is dominant over white (hybrid).(hybrid).

Crossed a variety of flies and noticed that the flies Crossed a variety of flies and noticed that the flies with white eyes were always male.with white eyes were always male.

Morgan hypothesized Morgan hypothesized that some traits were that some traits were linked and therefore linked and therefore passed on together.passed on together.

If the white-eyed trait If the white-eyed trait was found in a was found in a particular sex, it must particular sex, it must be linked to the actual be linked to the actual sex chromosome – the sex chromosome – the X chromosome.X chromosome.

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

Why are guys the way they are?Why are guys the way they are?

4.3.8: 4.3.8: Hemophilia: Sex-Hemophilia: Sex-Linked DisorderLinked Disorder

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

recessiverecessive

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

Chromosomes and Gene Chromosomes and Gene ExpressionExpression

Even though females have Even though females have two X chromosomes, one is two X chromosomes, one is inactivated (randomly), can inactivated (randomly), can be different among cellsbe different among cellsThe inactivated X The inactivated X chromosome is called a chromosome is called a Barr Barr bodybody Animations

10.3.1: 10.3.1: Polygenic Polygenic

InheritanceInheritanceWhen a trait is When a trait is controlledcontrolled by more by more than one genethan one gene

Many genes working Many genes working together produce many together produce many proteins that together proteins that together create a range of create a range of variation variation continuous continuous variation in a traitvariation in a trait

10.3.2

Modifier GenesModifier Genes Epistasis EpistasisGenes that work together with other genes to Genes that work together with other genes to control the control the expressionexpression of a trait. of a trait.In some cases, a gene at one locus alters the phenotypic In some cases, a gene at one locus alters the phenotypic expression of a gene at a second locus.expression of a gene at a second locus.Eye colour:Eye colour: Brown eye Brown eye presence of melanin presence of melanin

(dominant)(dominant) Blue eye Blue eye absence of melanin absence of melanin

(recessive)(recessive) Hazel, violet, green Hazel, violet, green result from the protein products of result from the protein products of

other genes that impact the alleles for the dominant or other genes that impact the alleles for the dominant or recessive eye colour characteristicrecessive eye colour characteristic

Depending on the different combinations of alleles inherited, a Depending on the different combinations of alleles inherited, a person can have a range of proteins produced that produce a person can have a range of proteins produced that produce a range of colour spectrum.range of colour spectrum.