THOMAS MORGAN

Morgan’s DISCOVERY of LINKED GENES Characteristics of linked genes 1.WHEN GENES ARE CLOSE TOGETHER ON A CHROMOSOME THEY TEND TO BE INHERITED TOGETHER 2.LINKED GENES tend to not SEPARATE from one another during Crossing Over Prophase I During Cross Over in Prophase, they tend to stay together instead of separating and switching

USED DROSOPHILA Genus name –common name Fruit flies WHY? They Mature in 2 weeks They Produce large numbers of offspring They only have 4 pair of chromosomes One pair are the Sex CHROMOSOMES How Morgan Discovered Linked Genes

The Experiment for Linked Genes Part I crossed PURE BRED fruit flies for two traits Homozygous Dominant GRAY BODIES & NORMAL WING SIZE (GGWW) with Homozygous Recessive flies that had BLACK BODIES & SMALL WINGS (ggww) GG WW X gg ww WHAT Genotype did he expect based on Mendelian genetics? GgWw WHICH IS WHAT HAPPENED Okay Big deal! What do you think he did next? Think about Mendel’s experiments

MORGAN’S Experiment Part II He then crossed an F-1 of the GgWw hybrid offspring w/a recessive ggww What would you expect? Fill out this punnett-  he did not get as expected… ¼ GRAY NORMAL ¼ GRAY SHORT ¼ BLACK NORMAL ¼ BLACK SHORT INSTEAD, he got… GgWw x ggww GWgwGwgW gw

Experiment’s Results for Part II 2300 total offspring Expected Actual Results Gg Ww alleles gg ww alleles

MORGAN’S RESULTS 41.5% GRAY body/Normal wings 41.5% BLACK body/small wings 8.5% GRAY body/Small wings 8.5% BLACK body/Normal wings MORGAN’s Conclusion The genes for wing size and body color were so commonly inherited as only two combinations either gray body/normal wing or black body/small wing that they had to be … on the same chromosome! This indicated that the genes for body color and wing size were… LINKED onto one chromosome.

Crossing Over Explains the other 8.5% combinations of either Black Body/Normal Wing or Gray body/Small Wing Chromosome combinations for gametes Homologous chromosomes

This means that even though genes can be linked, They can sometimes separate from one another during Crossing Over in Meiosis

GENE MAP If you know the frequency of how often genes cross over, you can use the percentage to estimate how far apart the genes are from on another on a chromosome This is called a Gene Map So if two genes have an 8% frequency of crossing then are they far apart or close on a chromosome? c AB

Try this! Map this chromosome’s genes using these crossing over frequencies AE cross over 5% BA cross over 10% BE cross over 20% DE cross over 50% DB cross over 25% CB cross over 5% CA cross over 20%

D C B A E How did you do?

Gene Mapping Tutorial

GENES ON SEX CHROMOSOMES e/ge/m3/s2/index.htm e/ge/m3/s2/index.htm A gene located on a A SEX CHROMOSOME is called a SEX-LINKED GENE MORGAN DISCOVERED the 1 ST SEX-LINKED gene in fruit flies He crossed a PUREBRED Dominant RED- EYED FEMALE W + W + with a PUREBRED recessive WHITE-EYED WW MALE Let’s take a look at the 1 st cross.

Morgan’s 1 st Sex Linked Cross All offspring had red eyes

MORGAN’S 2 nd CROSS HE crossed from the F-1 generation a heterozygous FEMALE, W + W RED-EYED with a RED-EYED W + MALE (note only 1 allele! Let’s do the cross HE got a 3:1 RATIO of Red eye to White eye, but only MALES had WHITE EYES! Why? Since no FEMALES had WHITE EYES Morgan hypothesized that EYE COLOR must be a SEX-LINKED gene IT must be on the X CHROMOSOME The Y chromosome does not carry a gene for EYE COLOR The RECESSIVE TRAIT White eyes, is inherited more often in males that receive the r allele on their one and only X CHROMOSOME

MORGAN’S EXPERIMENTAL CROSS White eyed male Red eyed female All males had red eyes All females had red eyes 3:1 ratio red eyes to white eyes Only males had white eyes

Chromosomal mutations

associated associated-331 DUPLICATION, INVERSION, DELETION, TRANSLOCATION

Mutations Point mutation-single nitrogen base is misplaced Missense mutation –single point mutation A substitution may be okay May not be fatal, since there is redundancy in the amino acid codons Nonsense mutation, a single codon codes for a stop codon Deletion/insertion of a point mutation can cause a frame shift Can code for the incorrect amino acid Could create an incorrect protein

Nondisjunction ndex.htm ndex.htm Failure to separate chromosomes evenly during meiosis Chromosomal mutations involving whole or complete pairs of chromosomes 3n-triploidy 4n-tetraploidy Having more then one set of chromosomes-polyploidy Fatal in humans, beneficial in plants

SRY gene Sex determining Region of the Y chromosome The gene can trigger biochemical, physiological & anatomical features because it regulates other genes If it is missing embryonic gonads develop into ovaries

Sex linked genes Males have only 1 X gene Hemizygous also XY heterogametic Females XX homogametic If a male inherits a recessive allele from his mom he will express the trait Chance of a female inheriting a double dose is much less It is rare for X & Y to cross over

Sex linked disorders Duchenne muscular dystrophy- absence of an X linked gene for a muscle protein- dystrophin. Weak muscles, loss of coordination Hemophilia- blood clotting disorder, absence of a clotting factor.

Barr body, X chromosomes in females In females during development one X chromosome condenses into a Barr body. Most of its genes are inactive Females therefore are a mosaic of paternal X and maternal X as to which genes are activated In an X linked mutation for sweat glands, half her glands produce sweat, the others do not

X chromosomes Methyl groups attach to the inactive X chromosome One of the two X has an active XIST gene It produces multiple copies of rna that covers the X chromosome. This initiates X inactivation

Tortoise haired cats Female cats have patches of orange and black fur due to cells expressing an orange allele and others express no orange allele