Chapter 15

Chromosomal Basis of Inheritance

Mendel’s inheritance with independent assortment

Mendel had factors, we now know these factors were

specific genes at specific loci, (locations) on specific

chromosomes. This is the chromosome theory of

inheritance.

Morgan’s experiments with Fruit Flies

A.Morgan used fruit flies to trace a gene to a specific chromosome

1. Drosophila Melangaster – wildtype phenotype = most common and mutant phenotypes = caused by mutations of the wildtype gene.2. Sex-linked- using the gene for white eyes Morgan found that if a female had the trait and mated to a male with wildtype, only her sons would be white-eyed.

Sex Linkage

This meant the gene must be on the X and not the Y.

A daughter would receive the wildtype X from the father which would override the faulty X from mom.

Linked genes – genes close together on the same chromosome

• Morgan found that flies have linked genes for body color and wing size.

• Gray and normal wings were wildtype, black and vestigal wings were mutant types.

• Gray flies with vestigal wings and black flies with normal wings are much harder to breed than the ones in b.

Independent assortmentGenes which are not linked sort

independent of one another.When they come together in a zygote it is called recombination.

a. A frequency of 50% or more recombination means the genes are not linked

b. Linked genes do not sort independently, they travel together, making recombination percentages low.

Recombination information can be used to map the genes on a

chromosome.

• The measurements are called centimorgans

• If the recombination frequency between cn and b is 9%, cn and vg 9.5%, b and vg 17% the map would look like this;

• 17 centimorgans

b cn vg

• If the genes are far apart they behave as if they were on different chromosomes

• This slide shows recombination during meiosis.

Recombination calculations

Sex chromosomes

A. In humans XX is female and XY is male

1. The SRY gene has been shown to trigger the development into a male fetus at about 2 months old.

2. SRY probably regulates other genes

3. Some XX male and XY females exist with mutated SRY genes

Sex-linked

• Genes on the X chromosome will only be present in one copy in males, as they only have one X and the Y does not contain the same genes.

Hemophilia

Males are said to be hemizygous for these traits

a. Colorblindness is an example of a non- lethal sex-linked trait- females can acquire it from their father (colorblind) and mother (carrier).

b. Duchene muscular dystrophy – weakening of the muscles,1/3500 males- usually die by 20s

c. Hemophilia- absence of certain clotting factors.

X inactivation

• In females one X becomes inactivated and condenses into a Barr body, which one is inactive is random, and will be different in different cells.

Errors

A.Nondisjunction- when meiosis is not accurate and some cells are missing a chromosome (anueploidy) and one has too many chromosomes(polyploidy)

Turner’s

Metafemale

Down’s Syndrome-1/700 births trisomy 21

Klinefelter – XXY, XXXY

XYY

Other nondisjunctions

• Metafemale - XXX2. Turner – X0

Other errorsA. Deletions – Chromosome fragment is lost

1. Cri-du-chat- chromosome 5B.Duplication – chromosome fragment attached to homologueC.Inversion – reverse orientation of fragment on chromosomeD.Translocation – fragment on a nonhomologue

1. Implications of cancer - CML

Imprinting

• Genomic imprinting – One active copy either maternal or paternal.

• Fragile X – caused in part by maternal imprinting.

Extranuclear genes

A. Small circular DNA in mitochondria and chloroplasts

B. Maternal inheritance is the rule