Drosophila Genetics: Chromosomes, Crosses, and Mendelian Laws

Drosophilia melanogaster

Scientific name for fruit fly

How many chromosomes do fruit flies have

8 chromosomes (4 pairs)

Three large pairs, one very small pair

Labeled I-IV in largest to smallest order

Lygaeus mode

Presence of different sex chromosomes

Male fruit flies

XY

Female fruit flies

XX

Sex chromosome in fruit flies

Pair I

Distinguishing female and male fruit flies

Females are larger than males

Females have a "knob" on the tip of the abdomen

Males have a solid abdomen while females have stripes

Parental flies will ALWAYS be

homozygous

Homozygous

Same allele on each of their pair of chromosomes

Wild type fly

AA or +/+

Mutant fly

aa or mut/mut

F1 Cross

Two heterozygotes together

Aa x Aa

Can help discover sex-linked traits and different dominance patterns

F2 generation

First generation where all new combinations of phenotypes can occur at the same time

Backcross

Crossing one of F1 progeny back to original parents to produce F2 generation

Can isolate a specific trait in a related population of organisms

Recessive backcross

Aa x aa; 1 Aa: 1 aa

Phenotype

the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment

Genotype

the part of the genetic makeup of a cell, and therefore of any individual, which determines one of its characteristics

Homolog

a gene inherited in two species from a common ancestor

Diploid (2N)

Carry two alleles at each locus on their chromosomes

One allele from maternal homolog and one allele from paternal homolog

Pure A cross

AA x AA; all AA

Pure a cross

aa x aa; all aa

Parental (P1) cross

Aa x aa; all Aa

F cross

Aa x Aa; 1 AA:2 Aa: 1 aa

Dominant backcross

Aa x AA; 1 AA:1 Aa

Test cross

Experimental cross when an organism that has the dominant phenotype with an unknown genotype is crossed with a homozygous recessive genotype (and phenotype)

Chi-square tests

Used to compare observed data against expected data

Estimates how much results we obtain deviate from what was predicted in hypothesis

Chi-square formula

x^2 = Σ (Observed - Expected)^2/ Expected

The larger the Chi-squared statistic...

the worse the goodness-of-fit

Rejecting the hypothesis

Test statistic exceeds the appropriate table value at p=0.05

Observed data deviated from our predicted outcome to a greater amount than expected by chance alone

Failing to reject the hypothesis

Observed test statistic is less than the table value

Degrees of freedom

n-1

What column do we use for the chi square table?

0.05 column

Fruit flies have _________ PAIRS of chromosomes.

4

The "standard" phenotype for a trait is referred to as ___________________.

wild type

For the Mendelian option, the Fly Colony program is designed so that the F1 progeny are all heterozygotes. This result simulates exactly what happens if we were crossing real flies and occurs because the male Parent is _________ dominant while the female Parent is selected to be _________ recessive.

homozygous/homozygous

Diploid organisms have ____________ possible genotypes produced at each locus.

3

Diploid organisms have a limited number of potential crosses for any single particular locus. These crosses are referred to as: __________________________.

Mendel's Big Six

Imagine you are conducting an experiment where you hypothesize that apples weigh more than bananas on average. After calculating the weight of 100 apples and 100 bananas you find the average weight of apples is 200 grams and the average weight of bananas is 100 grams. You conduct a statistical test and find this difference is significant according to the test.

True or False: You have proven that apples weigh more than bananas and this is now scientific fact.

False

What do you call the physical expression of a gene?

Phenotype

Dihybrid crosses

Mating experiment between two organisms that express two discreet traits

2 alleles on specific loci

Allele

One of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome

Loci

The position of a gene or mutation on a chromosome

Dihybrid heterozygous

Heterozygous at two different genetic loci

Heterozygous

Refers to a particular gene having two different alleles of homologous chromosomes

Law of Independent Assortment

The alleles of two (or more) different genes get sorted into gametes completely independently of one another

an allele occurrence in one gene does not affect the occurrence of an allele in another gene.

Mendel's Pea experiment

Pairs of traits in the parental generation sorted independently from one another from one generation to the next

9:3:3:1 phenotypic ratio

Law of Segregation

only one of the two alleles for any particular gene present in an organism is distributed to each gamete (sperm or egg) and that this allocation of the alleles is completely random

lets us predict what happens with a single trait associated with a single gene

Segregation deals with

phenotype

The product rule

states that the probability of two (or more) independent events occurring together can be calculated by multiplying each events' individual probability together.

Mendel's principle that states that only one of two alleles for a particular gene is distributed into each gamete is called The Law of _________________________.

Segregation

AaBb x aabb. Phenotypic ratio?

1:1:1:1

Aabb x AaBb. Phenotypic ratio?

3:1:3:1

AaBb x AaBb. Phenotypic ratio?

9:3:3:1

In fruit flies, the eye shape Lobe is dominant over wild type. Wild type body color is dominant over black body color. Using the Electronic Fly Colony you select the following parental cross: Wild, Wild Male x Lobe eye, Black body Female.

Select the 2 Parental genotypes that would correspond to these phenotypes.

aaBB male

AAbb female

____ Ratios can be produced from two or more Segregation Ratios.

Assortment

In a dihybrid cross, following normal Mendelian Laws of Heritability, if two heterozygotes (for both genes) are crossed once, what will be the expected phenotypic ratios?

9:3:3:1

An allele is:

One of several possible forms of a gene

Dihybrid backcross

mating to an individual that is homozygous recessive for TWO different traits (aabb)

Simple assortment ratio (1:1:1:1)

deviation from the simple assortment ratio can indicate that genes are linked together and give very clear indications of the chromosomal location

Failure to reject the predicted dihybrid ratio

Assortment was independent

Rejection of predicted dihybrid ratio

the two loci did not separate randomly during Meiosis. This would indicate the loci are indeed linked on the same chromosome.

Genetic linkage

the tendency for alleles close together on the same chromosome to be transmitted together as an intact unit though meiosis.

Can produce significantly more progeny with parental phenotypes and significantly fewer progeny with non-parental phenotypes than are expected by random chance.

Linked genes

Genes that are likely to be inherited together because they are physically close to one another on the same chromosome; the physical relationship between genes.

Crossing over

The exchange of genes between homologous chromosomes, resulting in a mixture of parental characteristics in offspring.

May occur if genes on same chromosome are located too far apart

results in recombination of alleles on chromosomes.

allows alleles on one DNA molecule to change positions from one homologous chromosome segment to another

Recombination

The rearrangement of genetic material, especially by crossing over in chromosomes or by the artificial joining of segments of DNA from different organisms.

creates substantial new genetic diversity favored by evolution and randomizes the arrangement of alleles of linked genes on chromosomes.

_____ is responsible for generating the genetic diversity in a species or population

Genetic recombination

Bateson, Saunders and Punnett's Pea Plants

Because the parental phenotypes reappeared more frequently than expected, the three researchers hypothesized that there was a coupling, or connection, between the parental alleles for flower color and pollen grain shape, and that this coupling resulted in the observed deviation from independent assortment.

Thomas Morgan

Bred fruit flies, and supported the the theory of chromosomal inheritance by finding that a specific gene is carried on a specific chromosome

If genes are linked...

parental gametes that contain parental combinations of the alleles will be produced significantly more often than predicted by chance

Demonstrated in F2 generation

In genetic linkage, the rate of recombination between linked genes is ___ and the parental phenotypes is higher than ___

low; 50%

When genes are linked,

Fewer recombinant phenotypes in F2 generation

Complete linkage

Equal presence of parental and recombinant gametes

Coupling (cis) configuration

two dominant alleles together and the two recessive alleles together

Repulsion (trans) configuration

Dominant and recessive allele together

If the loci are completely or partially linked, then prior to meiosis

alleles aB will be located on one chromosome, and alleles Ab will be on the other chromosome

Linkage is considered to be a violation of Mendel's Principle of _________.

Independent assortment

All loci located on Chromosome pair _______ of fruit flies would exhibit sex-linked mode of inheritance.

1

The Dihybrid Dominant Backcross is the primary cross that we will use throughout this laboratory to examine linkage.

False

The Fly Colony module used this week has a major difference from the previous weeks. Once you select a F1 offspring to use as an F2 parent, the program automatically selects the other parent to be a Dihybrid Double Recessive genotype. This choice assures that the expected Mendelian assortment ratio will always be 9:3:3:1.

False

When one parent has two mutant alleles and the other parent has both wild type alleles for the same trait, the configuration is referred to as ______________________.

Cis or coupled

You have performed a dihybrid recessive backcross, and find that you do NOT get a 1:1:1:1 ratio. This means ____________?

The genes are linked - they are on the same chromosome

When you cross two heterozygous parents (Aa), where A is a recessive lethal, what is the most likely phenotypic ratio of the progeny?

2:1

The ratio 3:1 is a "signature" phenotypic ratio for the Dominant/Recessive Mode of Inheritance.

True

What type of cross would one use to determine if a trait was sex-linked?

reciprocal cross

What type of dominance is demonstrated by the progeny expressing the phenotype of both parents?

Codominance

You cross 1000 fruit flies with the genotype Aa and obtain the following results:

AA: 208

Aa: 511

What mode of inheritance best fits these results?

Lethal alleles

Let's say you cross a red flower with a white flower. You observe that in your F1 generation, the flowers have both white petals and red petals present. What is the likely mode of inheritance for this trait?

Codominance

Which of these indicates dichotomous traits?

Red flowers vs. white flowers

When you have an unidentified genotype

cross it to a known homozygous recessive and you can easily determine the genotype of the unknown

Dominant/recessive inheritance pattern

one trait completely masks the another trait, so that the phenotype of a heterozygous organism is indistinguishable from that of an organism homozygous for the dominant allele

If you observe that one trait is completely masked in an F1 generation

MOI is dominant/recessive

Incomplete dominance

Neither allele is dominant over the other

F1 phenotypes are blending of two parental phenotypes

If you observe that the parental phenotypes appear as "blended" in the F1 generation

MOI is incomplete dominance

Co-dominance

detectable expression of both alleles in heterozygotes

If you observe that both parental phenotypes are expressed in each offspring of the F1 generation

MOI is co-dominance

Recessive lethal alleles

low frequencies in populations, although they may persist in some populations over a long period of time

"hidden" by dominant wild-type alleles in heterozygous genotypes, thus evading natural selection.

If you observe a 2:1 ratio in the F2 generation

MOI is lethal allele inheritance pattern

Sex-linkage

If the gene for a particular trait is only located on the X chromosome, then the inheritance pattern is going to result in a gender-specific phenotypic expression.

X-linked inheritance

The inheritance of genes on the X chromosome

Reciprocal cross

Crosses between different genetic stocks, strains or species where the sexes of the parents are reversed

Dichotomous key

Tool used to identify organisms

Series of paired statements

Process is repeated through entire key