6.2-Inheritance of Linked Genes SBI3U

Recall: Mendel’s Observations Mendel Demonstrated: Law of Segregation Law of Independent Assortment The alleles for a gene segregate independently during meiosis. This applies to alleles located on homologous chromosomes This theory was proven to be incorrect.

Theory of Linked Genes See pg. 251- Does not show the 9:3:3:1 ratio

Theory of Linked Genes Alleles on the same chromosome do not assort independently. Some genes that are on the same chromosome are inherited together and are called linked genes. An example of this additional complex pattern is found in sweet pea plants.

Linked genes: on the same chromosome and tend to be inherited together. Linkage Group: Genes on a chromosome that are always inherited together (unless crossing over occurs)

Crossing Over & Linked Genes All genes on any one chromosome are called a linkage group b/c they tend to be inherited together Crossing over can prevent linked genes from staying together. Ex. 2 genes are on the same chromosome The alleles for the linked genes switch Now they are unlinked and will migrate to different gametes  

UNIT 2 Chapter 6: Complex Patterns of Inheritance Section 6.2 Linked Genes Recombinant alleles: linked genes that have been separated during crossing over In most of the gametes formed, there is no crossing over—they maintain the linkage of the alleles. In a small minority of gametes, crossing over occurs and alleles of previously linked genes become unlinked.

Chromosome Mapping Used to determine location of genes on a chromosome Useful for organisms that reproduce rapidly (e.g. fruit flies) Frequency of crossing over is connected to how close the pairs of linked genes are Closer together = low frequency Further apart = high frequency

Sex-Linked Inheritance Thomas Hunt Morgan studied the fruit fly (Drosphila melanogaster) He crossed a pure red eyed female with a white eyed male and this is what he found…

Genotype Phenotype XR XR Female with red eyes (homozygous dominant) XR Xr Female with red eyes (heterozygous) Xr Xr Female with white eyes (homozygous recessive) XR Y Male with red eyes Xr Y Male with white eyes

Sex-Linked Inheritance cont’d XR XR In the F1 generation: 100% of offspring had red eyes. Xr XRXr XRXr Y XR Y XR Y

Sex-Linked Inheritance cont’d In the F2 generation: 100% of the females have red eyes. 50% of the males have white eyes 50% of the males have red eyes. XR Xr XR XRXR XRXr Y XR Y XrY

Experimental Conclusions Conclusion: The gene for eye colour is connected to gender and located on the X chromosome. Traits that are controlled by genes on either the X or Y chromosome are called sex-linked traits.

Sex Chromosomes The X chromosome contains 2000 genes The Y chromosome only contains 100 genes Sex chromosomes contain some genes that are unrelated to sex determination. Each sex-linked gene can contain dominant or recessive alleles,

Sex-Linked Traits in Humans

X-Linked Dominant Vs. X-Linked Recessive Affected males pass on only to daughters (100% chance) Females pass to both sons and daughters (100% chance) Very rare Ex. Fragile X Syndrome : Affect more males than females because males only need one allele to be affected, while females need 2 recessive alleles to be affected Ex. hemophilia, red- green colour vision deficiency X-Linked Dominant: X-Linked Recessive

X – linked recessive

X – linked dominant

X-Link Recessive: Colour Vision Deficiency (CVD) An X-linked recessive trait like CVD will affect more males than females in a family. These males will have difficuty distinguishing between shades of red and green

Test Your Understanding… Determine the probability that a woman who is a carrier for hemophilia and a man without hemophilia will have a child with hemophilia. What proportion of the female children with have hemophilia? What proportion of male children will have hemophilia? Is it possible for these parents to have a female with hemophilia?

Test Your Understanding… A human female with red-green color deficiency (CVD) marries a normal male. What proportion of their children will have CVD? Is it possible for the parents to have a female with CVD? Why or why not?

Homework: Read and make notes 6.2 Complete pg. 258 Q# 11-13, 16, 17