BIOL1115: Lab 8 - Genetics

what is genetics?

the study of molecular variation in organisms and the inheritance of that variation

what do geneticists attempt to answer?

questions as to why organisms resemble their parents and siblings while still showing their own unique combinations of features

what are the major breakthroughs in genetics?

- Gregor Mendel's rules of inheritance

- the realization that genes (the units of heredity) are carried on the chromosomes

- the discovery of the molecular nature of genes

what should study organisms have in genetics?

- be easy to grow or rear

- have a rapid generation time

- produce many offspring

- take up little space

what did Mendel use for his genetic experiments?

garden sweet peas, because they were easy to grow, had many distinct observable characteristics, and he could control matings by transferring pollen between specific flowers

why is the fruit fly the most widely used model organism for genetic studies today?

- it breeds at a high rate

- it has only four pairs of chromosomes

what was the fruit fly used for?

to confirm that genes are located on chromosomes, that genes one the same chromosome do not assort independently, and to create the first ever genetic map

what do fruit flies possess?

unusually large chromosomes called polytene chromosomes, which can be easily seen and studied under a compound microscope

what are polytene chromosomes?

large chromosomes formed by repeated DNA replication without cell division, resulting in many aligned copies of DNA strands

what are thale cress'?

widely used for studying plant genetics, evolution, and development

why are thale cress used for genetic studies?

- has a rapid generation time

- easy to grow

- has a small genome size which makes it useful for genetic mapping and sequencing

what is the Law of Segregation?

allele pairs separate during gamete formation so each gamete carries only one allele

what does the Law of Segregation allow?

allows us to predict the outcome of monohybrid crosses?

what is a monohybrid cross?

a cross between two individuals that are both heterozygous for the character that we are interested in following

what are some human characteristics that have been attributed by a single gene?

- mid-digital hair

- widow's peak

- tongue rolling

- freckles

- cleft chin

- curly hair (incomplete dominance)

- cheek dimples

what is incomplete dominance?

when one allele is not completely dominant over the other

how do you determine gametes from a genotype?

each gamete gets one copy of the allele per gene

what gametes can aaBB produce?

aB

what gametes can EEFf produce?

EF and Ef

what gametes can ccdd produce?

cd

what gametes can BbGg produce?

BG, Bg, bG, bg

when does the the number of gamete possibilities increase?

as heterozygosity increases

how can the number of possible types of gametes be expressed as?

2^2 where n = the number of heterozygous genes

why are human genetics more difficult to study than that of other organisms?

- because of our long generation time

- small number of offspring

- breeding experiments being unacceptable

how are human genetics studied?

by constructing pedigrees

what are pedigrees?

a family tree that tracks certain traits, frequently those that cause diseases

what can inherited trait may be caused by?

either a dominant or a recessive allele

where can traits be carried on?

a sex chromosome (sex-linked traits) or on any other chromosome (autosome)

what is the challenge for geneticist when looking at a pedigree?

to determine if the trait/disease is carried by a dominant or a recessive allele

what does the ABO blood group involve?

three alleles (I^A, I^B, and i)

what does the ABO blood group alleles produce?

four phenotypes: A, B, AB, and O

what do all humans have?

antibodies (defensive proteins) in their blood

what are antibodies?

it can recognize and bind to foreign substances

what are substances that elicit an antibody response?

antigens

what are common antigens?

pollen, dust particles, and cells containing foreign surface markers

what does allele I^A produce?

a particular glycoprotein on the surface of red blood cells called antigen A

what does allele I^B produce?

a glycoprotein called antigen B

what does allele i produce?

no glycoprotein

what are alleles I^A and I^B?

dominant, meaning that one copy of the allele sufficient to express its phenotypic effect. they are codominant to each other

what does codominant mean?

both are expressed when they are both present

what is allele i?

recessive, and thus the O phenotype is only expressed when two copies of it are present

what is the genotype of type A blood?

I^AI^A or I^Ai

what is the genotype of type B blood?

I^BI^B or I^Bi

what is the genotype of type AB blood?

IAIB

What is the genotype of type O blood?

ii

what happens when antibodies attack the antigens on red blood cells?

they cause the cells to clump, or agglutinate

what can clumping cause?

block small blood vessels and lead to fatal consequences

how is a blood type determined?

by mixing a few drops of blood with a few drops of a liquid called an antiserum

what is antiserum?

a blood serum containing antibodies against specific antigens

what happens if agglutination occurs within a particular antiserum?

it is considered a positive response (e.g.,, if blood reacts with anti-A antisera, it means the person has type A antigens)

what happens to a person receiving a blood transfusion?

the antibodies in the received blood do not matter (there's not enough of them). only the antigens on the received blood cells matter

what other factor apart from the ABO blood group affects blood transfusion?

the Rh (Rhesus) factor

what is the Rh factor?

it involves eight alleles, but functions as if there were only two, Rh+ (dominant), and Rh-

what does the Rh+ allele produce?

an antigen on the surface of red blood cells, whereas the Rh- alleles do not

what do people with Rh- blood develop?

large numbers of antibodies against Rh+ blood, but only after an initial exposure

what does a second exposure to Rh positive blood do?

an agglutination response (think hemolytic disease of the newborn)

what may mutations confer?

an advantage to the individual and that results in the increased likelihood that the new allele will be passed down to the next generation

where can mutations to the genetic code occur within?

genes, which results in different versions of the gene, or it can happen in parts of the genome that control the expression of those genes. this can result in an increased, decreased, or cessation expression of the gene

what fish species was being investigated in the video?

threespine stickleback

how did marine fish end up in the freshwater lakes?

after the last ice age, glaciers retreated and sea levels dropped, leaving some marine sticklebacks trapped in newly formed freshwater lakes

what was the significant phenotypic difference between the marine form and freshwater form?

freshwater sticklebacks lost their pelvic spine

what selection pressure may have contributed to the loss of pelvic spines in freshwater fish?

predation by dragonfly larvae in which they grab onto the spines of the fish

what structure is the fish pelvis/spine homologous to in vertebrates?

hind limbs

what gene controls the development of the pelvic spines?

PITX1 gene

what chromosome is the PITX1 gene found on?

chromosome 7

when the scientists compared the genetic code for this gene in freshwater stickleback to its marine ancestor what did they find?

the PITX1 coding region was the same

in what parts of the body is the gene expressed in the marine stickleback?

jaw, pituitary, pelvic region

where is the gene NOT expressed in the freshwater stickleback?

pelvic region

what did the scientists think was causing the difference in expression of the PITX1 gene?

a mutation in the "switch" that controls expression of PITX1 in the pelvis. the gene is still present, but the switch that turns it on in the pelvic region is broken or deleted

when did they test this hypothesis?

added reporter genes to DNA fragments to look for a glowing protein to determine which one is expressing it on the pelvis

how long did it take to find the correct fragment that contained the regulatory switch?

5 years

what was the genetic difference between the freshwater stickleback and the marine stickleback with respect to the developmental control region?

there was a complete deletion of the switch controlling the proteins expression on the pelvis

what fossil evidence in the lakebed in Nevada showed about the sticklebacks?

the fossils showed that the loss of pelvic spines happened rapidly and repeatedly, other sticklebacks also lost their spines, showing this evolutionary change has occurred many times independently throughout history whenever sticklebacks colonize freshwater