BIOL190 - Exam 1

Phenotype

The observable physical or behavioral traits of an organism.

genotype

The specific set of genes or alleles inherited from parents.

gene

basic section of DNA that codes for a specific physical trait or body function

exons + introns

two copies of genes

alleles

a specific variant or version of that gene

homozygous

having two identical alleles (versions) of a gene

heterozygous

having two different alleles of a gene

single-gene inheritance (5)

• traits determined by a single gene

• rare, but they exist

• often call mendelian traits

• human disorders: ex. cystic fibrosis

• rules of single gene inheritance by MENDEL

Mendel (3)

• father of genetics

• didn’t know what genes were, how they influenced phenotypes, or how they were inherited at the cellular level

• used controlled crosses, pure-breeding strains, dichotomous traits, quantification of results, many trials

what term is used to refer to the progeny generation?

first filial generation— F1

pure-breeding

strains that consistently produce the same phenotype

what kind of cross reveals the segregation of alleles for ONE trait?

monohybrid crosses

Mendel’s 3 postulates of inheritance

• genes exist in pairs — 2 alleles per gene (3 in rare cases)

• some alleles are dominant and some are recessive

• alleles segregate independently during gamete formation

Mendel’s law of segregation

two alleles for each trait will separate from one another during gamete formation and each allele will have an equal probability (1/2) of inclusion in a gamete

random union of gametes at fertilization will unite one gamete from each parent to produce progeny in ratios that are determined by chance

**seen in meiosis 1

3 types of dominance

• dominance: phenotype of the heterozygote is the SAME as the phenotype of one of the homozygotes

• incomplete dominance: phenotype of the heterozygote is INTERMEDIATE between the phenotypes of the two homozygotes

  • neither allele is dominant; aka partial dominance, semi dominance

    • ex. red + white → pink

• codominance: phenotype of the heterozygote includes the phenotypes of both homozygotes

  • can see both alleles in phenotype, does not have to be 50/50

    • ex. blood type

lethal alleles (4)

• alleles that cause an organism to die; can be dominant or recessive

• time of death depends on when gene is expressed

• some alleles cause a dominant phenotype when present in the heterozygous state, but a recessive lethal phenotype when present in the homozygous state

• most lethal alleles are recessive

dominant lethal alleles

RARE.

To be maintained, the carrier must reproduce before they die.

-symptoms of lethality can show up over age, so can reproduce without knowing lethal present

ex. huntington’s

pedigree analysis

• often used in humans. can’d do experimental crosses in humans!

• basically family tree

what kind of cross does this show?

monohybrid crossing

what kind of dominance is this an example of?

incomplete

what kind of dominance is this an example of?

codominance

B

true-breeding: if you self-cross, progeny should remain the same as parent

• usually homozygous

D

what kind of pedigree analysis is this?

autosomal recessive trait

• tend to skip 1 generation, since recessive not expressed in heterozygous (carriers)

what kind of pedigree analysis is this?

autosomal dominant trait

• tend to show in every generation

mendel’s second law

• dihybrid (and trihybrid) crosses reveals the idea of independent assortment

• law of independent assortment: during gamete formation, segregating pairs of genes assort independently of each other

  • traits are inherited independently

  • all possible combos of gametes should be formed in equal frequency

product rule

predicts the frequency with which two independent events will occur simultaneously

• used instead of punnett square, for dihybrid crosses, trihybrid crosses

B

Epistasis

• when the effect of one gene masks or modifies the effect of another gene

• HAVE to have at least two genes to exhibit epistasis

**does NOT immerge from 2 alleles interacting → dominance

epistasis → 2 genes

**epistasis distorts expected phenotypic ratios; fewer phenotypes than expected

BbEe x BbEe → 9:3:3:1 in dihybrid cross BUT

BbEe x BbEe → 9:3:4 in epistasis

A

what does chi-square test for?

whether observed numbers are significantly different than expected numbers

**expected phenotypic ratios are based on large sample sizes — as sample sizes increase, deviation due to chance decreases (closer to expected numbers)

a) 3:1 ratio

b) x² = 2.55 0.20>p>0.05 → fail to reject hypothesis

what test is used to tell if variation is due to allelic differences or due to separate genes?

complementation test/analysis

complementation test

used to determine if two mutations causing a similar phenotype are alleles (or if they are at different loci)

**only works for recessive mutations

complementation = the production of a wild-type phenotype when two different recessive mutations are united

*mutations do NOT co-occur

what cross does this represent?

complementation

A

what allows x and y chromosomes to pair?

pseudoautosomal regions work for synapsis

x + y have NO similarity EXCEPT for the ends which are called the pseudoautosomal regions — allows the chromosomes to pair

**sex chromosomes were originally autosomes

sex-linked human traits (3)

• mainly x-linked because there are more genes on the x

  • Y is 1/3 size of X; Y has very few genes compared to X

• Y chromosome mutations almost always result in INFERTILITY; NOT passed on

• males are HEMIzygous for X-linked genes

**x-linked genes and traits can still do other processes: epistasis, etc.

predict the phenotype of the offsprings from these parents

what kind of pedigree analysis does this represent?

sex-linked transmission: X-linkage, recessive

**mostly males

more males than females have phenotype because of HEMIZYGOSITY

• females need BOTH recessive alleles, males only need one.

what kind of pedigree analysis is this?

sex-linked transmission: X-linkage, dominant

• equal #s of females and males affected

A

dosage compensation

balancing the dosage of X chromosome gene expression in females and males

• humans achieve dosage compensation via X-inactivation (make only one active X chromosome) in XX individuals

  • random inactivation; other organisms inactivate the paternal or maternal X chromosome BUT humans do randomly

• not always the way in other species

  • amplify X chromosome in males XY

X-inactivation

Females, XX — one X chromosome inactivated by one barr bodies

Males, XY — no X chromosome inactivated

→ females and males both have one active X chromosome

How many barr bodies are present in males?

none

how many barr bodies are present in XXX? XXY?

XXX — 2 barr bodies

XXY — 1 barr body; 1 inactivated X chromosome

** body works to only have 1 active X chromosome

when are females phenotypics mosaics?

if they are heterozygous at X-linked genes

**each cell can have different x-chromosome activated/inactivated

X: cell expresses only one independently BUT some groups of cells could have one gene expressed and others have other gene expressed.

ex. calico cats, patches on body, colorblindedness

what are the mechanisms of inactivation?

• epigenetics — modifications in gene function or phenotype that are NOT due to changes in DNA

• mechanism of inactivation involves modifying the chromosome so that genes cannot be transcribed

• occurs very early in pregnancy (within the 1st week)

• a “memory” is created so that the inactivation is carried over to all daughter cells

• NOT ALL genes are silent— ~15% escape complete silencing, but with reduced transcription (genes near PAR1)

  • usually genes closer to the pseudoautosomal region

  • XXX: despite 2 barr bodies, ~15% of genes likely to still express all 3

mitosis

a diploid (2n) cell produces two daughter diploid (2n) cells

• 2 copies of all chromosomes

chromosome classification (3)

• telocentric (not in humans) — common in fruit flies

**descriptors of chromosomes with respect to location of centromeres

B

steps of mitosis

**metaphase: chromosomes need to line up right

• each of the sisters align to the pole; spindles pull to cell

  • humans: 46 chromosomes lined up → 92 chromatids, each daughter gets 46

• order does NOT matter. could line up as a b B A or anything else

meiosis — what is it for?

• reduces the amount of genetic material by ½
  - if not, would have too much DNA (double the parent)

• necessary for sexual reproduction (diploid 2n → haploid n)

• Crossing over creates mosaics between maternal and paternal genetic material

• shuffling of homologous chromosomes creates even more diversity

  • alleles mix up randomly; mendel’s 2nd law independent assortment

what’s the big difference between meiosis 1 and mitosis?

meiosis I is reductional: diploid to haploid (2n → n)

• no haploid with mitosis

meiosis I pulls homologs or sisters to opposite poles?

homologs (based on similarity of sequence)

what is the outcome of this event?

prophase 1 - undergoes recombination which gets rid of bad alleles

• crossing over occurs at the chiasmata (formed between non sister chromatids)

what is the outcome of this event?

• homologous chromosomes randomly assemble at the metaphase plate

  • (mendel’s 2nd law)

what is the outcome of this event?

two haploid cells

• each cell contains one complete set of chromosomes

label

!! know difference between homologous chromosomes and sister chromatids

---

homologous chromosomes ~99% similar

proteins allow recombination to happen

describe how crossing over occurs in meiosis 1

crossover — double strand break

recombinants formed — DSB repaired but causes crossover/recombination

**could have DSB + repair across sister chromatids BUT not observed since same alleles

meiosis II

• during meiosis II, sister chromatids go to opposite poles

  • sister chromatids separate

• this is like mitosis

• result: 4 cells, each haploid

why is there synthesis before meiosis if the chromosomes are just going to break apart?

constraint based in evolution. S phase with mitosis/meiosis added on later by evolution.

which of mendel’s law does this represent?

1st law — law of segregation

• end up with 50/50

• separates homologs

• one gene → 2 alleles; one set of homolog with 2 alleles

which of mendel’s law does this represent?

2nd law — law of independent assortment

• end up with different haploid gametes

why is dosage compensation necessary in humans?

to obtain optimal gene expression levels for x-linked genes

when examining a pedigree, what’s a pattern you can use to differentiate an X-linked from an autosomal trait?

whether more males than females have trait due to males’ hemizygosity

what parts of meiosis correspond with mendel’s law of segregation and law of independent assortment?

law of segregation — homologous chromosomes separate during meiosis 1 for gametes (anaphase 1)

law of independent assortment — random shuffling of homologous chromosomes (metaphase 1)

in what case would mendel’s law of independent assortment not apply?

when genes are on the same chromosome — linked genes