Chapter 12-Mendel's Experiments and Heredity

Gregor mendel

inheritance guy, explained how it worked

Demonstrated that traits are transmitted from parents to offspring independently of other traits and in dominant and recessive patterns

preformatiionism

Organisms develop from miniature versions of themselves.(homunculus)

Instead of assembly from parts, preformationists believed that = living things exist prior to their development.

changing ideas due to the invention of the microscope

homunculus

fully formed human is already in one of the gametes(eggs or sperm)

Historical misconceptions:blending inheritance

asserted that the original parental traits were lost or absorbed by the blending in the offspring, as offspring appear to be a “blend” of their parents’ traits

offspring inherit traits as an average of their parents' traits

inheritance of acquired characters-most traits are not passed on

What happens if there is too much blending?

Too much blending, everyone is going to start looking the same

Historical misconceptions: Pangenesis

Darwin’s attempt to explain genetic variety, but no evidence of gemmules found so disproven.

Suggest all cells, not just gametes, are involved.

Says that all cells in an organism release tiny particles called gemmules, which are then passed on to the next generation(the offspring).

gemmules

gametes were thought to be a mixture of gemmules

Gregor mendel timeline

1. grew up on a farm in Austrian empire(czech republic) gardening and beekeeping

2. 1840-studied philosophy and physics at University of Olomouc

3. 1843- at Saint Thomas Abeeyto, he trained for priesthood and taught science

4. 1850- failed high school teacher certification

5. 1851- University of Vienna for more formal training

6. 1853-returned to abbey, taught and experimented on plant breeding

7. 1865-presented his experiments to local Natural history society and published his work, experiments in plant hybridization.

8. 1867-became an about and gave up scientific research

Why peas(pisum sativum)?

• many simple dichotomous and true breeding traits-each character only has two forms

• Monoecious(m and f part on one plant), so easy to manipulate breeding

• each pea plant produced many seeds

• short generation times

• cheap and readily available

• bishop disapprove of his mice mating experiments

• allowed see that traits were not blended in the offspring or absorbed, but kept their distinctness and could be passed on

def character

specific observable form-like a category

ex: flower color, seed color, plant size

def traits

variations of a character(physical appearance of a heritable characteristic)-like options in the category

ex: white vs. purple

def phenotype

observable trait expressed by an organism(white and purple flowers, tall and short plants)

def genotype

genetic composition determining phenotype

underlying genetic makeup, consisting of both physically visible and non-expressed alleles

combinations of hereditary “factors” (genetic code) producing a phenotype

ex: Aa

def monohybrid cross

cross focuses on ONE character

resulting offspring are monohybrids

determined that each parent in the monohybrid cross contributed one of two paired genes to each offspring, and every possible combination of genes was equally likely

def dihybrid cross

cross focuses on TWO characters (proves the factors controlling the traits are inherited independent of one another-law of ind ass)

b/t two true breeding parents

asks: do characters affect each others inheritance factors

Mendel’s experiments (1856-1863)

• used two purebred (true breeding) individuals that have different traits and selectively bred them to produce only one specific trait

• he was the first to quantify results of these hybridization

• involved about 28,000 plants

• Notation-P gen, F1 gen(seeds grown from P gen), F2 gen(seeds from F1)

• each experiment performed a series of crosses focusing on one specific character(monohybrid crosses)

• replicated results in separate crosses involving diff characters

What was the result of mendel’s experiment?

• P generations only produced one phenotype in F1 gen

• F1 generation always produced one phenotype in greater numbers(dominant) than the other phenotype(recessive) in F2 generation

• Proportion of two phenotype ALWAYS close to 3:1 (in F2)

In short: P1 and p2 are purebred, crossed t1 and t2 respectively, then got trait 1 only 100% of time in F1(meaning t2 kinda “disappeared)”, then self fertilized f1 and in f2, saw t1 and t2 again in 3:1 ratio.

true breeding

ability of an organism to pass down the same phenotypic traits to its offspring(always produce offspring that look like the parent)

any crosses performed will result in offspring with the same phenotype

purebred

true breeding individuals-homozygous ones

Offspring resulting from a true breeding. True breeding is a way to produce offspring that would carry the same phenotype as the parents. Thus, a purebred would result when the parents are homozygous for certain traits

Selectively bred to produce only ONE specific trait

hybridization

the process of breeding two genetically distinct individuals to produce offspring that inherit traits from both parents

selectively bred

the process of choosing parents with certain traits to produce offspring with those traits

P generation

original parents in initial hybridization(the very top one)

first generation crosses

F1 generation

offspring from cross of P generation (the second/middle one)

F2 generation

offspring from cross of F1 generation

Law of segregation

allele pairs separate during cell division in gametes, gametes get only 1 allele for each gene, then come back together when fertilization occurs

Heritable factors occur in pairs, heredity factors separate during gamete formation, then heritable factors randomly recombine to form new individuals

During gamete production, each gamete receives one copy of each gene from a parent

paired unit factors (genes) must segregate equally into gametes such that offspring have an equal likelihood of inheriting either factor.

explains results of mendel’s monohybrid crosses

What did mendel hypothesize?

• Some sort of heredity factors are passed on to offspring

• two hereditary factors needed to produce traits(possessed two copies of the trait for the flower-color characteristic, and that each parent transmitted one of its two copies to its offspring)

• dominant trait(expressed unit factor)-trait masking the other trait

• recessive trait(latent unit factor)-trait that is masked (lacked all dom version of trait)

• factors randomly sep and recombine in offspring

test cross can verify genotype of F1 gen

alleles

specific versions of hereditary factors/different version of the same gene that occur at the same location on a chromosome and code for diff versions of same character

Gene variants that arise by mutation and exist at the same relative locations on homologous chromosomes

For example, there is a gene for fur color, and different alleles code for different variants of fur color, such as brown, red, black, white, or orange.

get two alleles total (one allele from each parent) for each gene

notation for dominant and recessive allele

dominant allele have a capital letter notation

recessive alleles have a lower case letter notation

Do alleles occur in pairs?

YES, one on each homologous chromosomes (one on each half x)

homozygote

pure breds, both alleles are the same(two identical alleles for a gene on their homo chromo), homozygous dominant(AA) and homozygous recessive (aa)

heterozygote

alleles are different(for gene), hybrids, heterozygous (Aa)

Gene

specific DNA sequence that CODES for a trait(like hair color)

The basic unit of heredity passed from parent to child.

these are mendel’s hereditary factors

locus

location on a chromosomes (plural-loci)

reciprocal cross

repeating previous cross but each parents has opposite trait

Mendel’s: a paired cross in which the respective traits of the male(short) and female(big) in one cross become the respective traits of the female(short) and male(big) in the other cross.

Ex: Cross a male with a trait of interest to a female without the trait; Cross a female with the trait of interest to a male without the trait

Goal: tests the role of sex on inheritance

test cross

a cross that test whether an organism that expressed a dominant trait is homozygous or heterozygous

figures out if the parent is PP or Pp by crossing it with a homo recessive.

figures out what is the genotype of the dominant one(if all heterozygous=org is homoz, if 1;1 ratio of heterozygous and recessive=org is hetero)

validates that pairs of alleles seg equally

wild type allele

most common allele in the population(most have this)

mutant allele

rare allele in population (more recent allele from a mutation)

a gene that has changed, or mutated, from its normal form

punnett squares

introduced by Reginald c. punnett in early 1900s

method of predicting all possible genotypes(outcomes) among the progenies(offspring) in a cross and expected freq.

based upon probabilities-predicts the likelihood of producing a specific genotype for each hybridization scenario

each square represents a probability (1/4 per sq for mono)

probability basics

• mathematical measures of likelihood

• probabilities describe chance/random events

• given as fraction or decimal equivalent

• events not due to chance are: probability of 0(event will never happen) or probability of 1(event will always happen)

• chance/random event=0<probability<1

• ex: chance of heads on a coin flip is one out of two possible side(1/2, 0.5, 50% prob)

how do you calculate probability?

1. ep: dividing the number of times the event occurs by the total number of opportunities for the event to occur.

2. tp: dividing the number of times that an event is expected to occur by the number of times that it could occur.

empirical probabilities vs theoretical probabilities

ep: comes from observations in the lab
tp: comes from knowing how the events are produced and assuming that the probabilities of ind. outcomes are equal

product rule: Multiplicative Law of Probability

• probability of two independent events occurring together can be calculated by multiplying the ind. probabilities of each event occurring alone.

• can be applied to independent transmission of characteristics as whatever characteristics someone got doesn’t impact the other.

• overall chance of two or more independent random events occurring together

• probabilities of all the events multiplied together

product rule example

chance of rolling a five on a die=1/6

chance of rolling a four on a die=1/6

overall chance of rolling a five AND(then too) rolling a four=1/6 × 1/6 =1/36

sum rule: additive law of probability

• the probability of the occurrence of one event OR the other event(two mutually exclusive event-can’t occur at same time) is the sum of their ind. probability.

• applied when considering two mutually exclusive outcomes that can come about by more than one pathway

• overall chance of multiple possible outcomes(one or another outcome occurring)

• probabilities of all the events added together

Sum rule Example

chance of rolling a five on a die=1/6

chance of rolling a four on a die=1/6

overall chance of rolling a five OR rolling a four first” 1/6 +1/6=1/3

Both laws for complex events:what is the probability of rolling a 5 on one dice and 4 on one dice?

chance of rolling 5 then 4(use product rule b/c then=and)

1/6×1/6=1/36

chance of rolling 4 then 5(use product rule b/c then=and)

1/6×1/6=1/36

if order of the numbers does not matter(use sum rule)

1/36 + 1/36= 2/36

punnett square probabilites

What do punnett squares parts represent?

• Top and sides: All possible combinations of the parental alleles are listed along the top (for one parent) and side (for the other parent) of a grid,

  • Representing their meiotic segregation into haploid gametes.

• Combinations of egg and sperm are made in the boxes in the table to show which alleles are combining.

• Each box represents the diploid genotype of a zygote, or fertilized egg, that could result from this mating.

• Because each possibility is equally likely, genotypic ratios can be determined from a Punnett square.

pedigree

Used to figure out who has a certain trait or genetic disease causing gene or what risk exists of passing the disorder to their offspring by working backwards in the family tree.

Trying to figure out if the condition is dominant or recessive.

Used to study inheritance pattern of human genetic diseases.

Occurs every generation=dominant

Skips generation=recessive

Mendel’s law of independent assortment

• genes do not influence each other with regard to the sorting of alleles into gametes, and every possible combination of alleles for every gene is equally likely to occur.

• hereditary factors for one character assort and segregate into gametes indep. of factors for other characters

• allows for many diff combo of factors from parents in the offspring

• explained by modern concept of ind ass during meiosis(where diff homo pair line up in random orientations)

demonstrated by results of mendel’s dihybrid crosses

dihybrid cross image

mixing of the traits-recombo

9:3:3:1 phenotypic ratio ALWAYS when cross heteros

can’t get ratio unless alleles separate indep from each other:aka, two traits aren’t linked together

trihybrid cross

used when there are more than two genes being considered

To prepare a forked-line diagram for a cross between F₁ heterozygotes resulting from a cross between AABBCC and aabbcc parents, first create rows equal to the number of genes being considered, and then segregate the alleles in each row on forked lines according to the probabilities for individual monohybrid crosses

then, multiply the values along each forked path to get the F2 offspring probs.

genetic uniqueness of gametes: walter sutton

1903-walter sutton presented evidence of homologous chromosomes(led to chromosomal theory of inheritance-the idea that chromosomes carry genes and are the basis of heredity)

Humans: homologous chromo and gamete number

23 pairs of homologous chromosomes

23 chromosomes in each human gamete

How many combo of chromo in each gamete w/o crossover? how about crossover?

8,324,608 possible combo of chromosomes in each gamete w/o crossover

• over 7 trillion combo of egg and sperm

with crossover, any combo in a gamete is unlikely to ever be produced again-all gametes gonna be a lil diff

What is mendelian inheritance?

simple dominance=complete dominance(mask rec)

1. Two alleles contribute to the expression of each character

2. one gene—> one character

3. Each allele expresses a distinct phenotype for the character

4. One allele will be dominant over the other

5. In the heterozygous genotype (having one of each allele), only the phenotype of the dominant allele is expressed (phenotype of the recessive allele is masked/not expressed), thus the term "complete dominance"

6. 2 possible alleles (dominant allele, recessive allele)→2 possible phenotypes (dominant phenotype, recessive phenotype)

7. 3 genotypes (homozygous dominant-AA, heterozygous-Aa, homozygous recessive-aa)

Are most traits due to more than one type of inheritance?

YES, most traits are due to more than one type of inheritance, but mendel doesn’t say that

what is non mendelian inheritance?

do not follow one/more conditions of mendelian inheritance-like blending of traits or more than one gene for a character

incomplete dominance

heterozygous org expresses an intermediate phenotype b/t dom and recessive, still one phenotype, but just a blend of dom and rec.

a genetic pattern where two alleles(dom and rec) blend to create a new phenotype(mix of the two)

ex: dom-red, het-pink, rec-white

multiple alleles

more than 2 alleles exist for a given gene(many combos of two alleles), more than 2 phenotypes expressed

multiple alleles exist for the gene that determines a character

ex:four alleles exist for the c gene. The wild-type version, C⁺C⁺, chinchilla phenotype- c^chc^ch, The Himalayan phenotype-c^hc^h, the albino- cc,

codominance

more than one dominant allele, express both phenotypes at the same time in hetero. USE SUPERSCRIPT. 2 distinct phenotype

both alleles for the same characteristic are simultaneously expressed in the heterozygote.

ex:Homozygotes (L^ML^M and L^NL^N) express either the M or the N allele, and heterozygotes (L^ML^N) express both alleles equally.

How does the complete dominance of wild type phenotype over all other mutants occur?

effect of “dosage” of a specific gene product, such that the wild-type allele supplies the correct amount of gene product whereas the mutant alleles cannot.

how does a mutant allele be dominant over the wild type?

the mutant allele somehow interferes with the genetic message so that even a heterozygote with one wild-type allele copy expresses the mutant phenotype.

One way in which the mutant allele can interfere is by enhancing the function of the wild-type gene product or changing its distribution in the body.

sex-linked inheritance

Occurs when gene is on the X or Y.

the passing of traits from parents to offspring through the sex chromosomes

Inheritance patterns not same in males and females. Female have 2 X alleles, male have 1 X allele-X-linked

Why do males have a higher chance of getting recessive?

Male have a higher chance of getting recessive bc only needs to be on the only X chromo to be affected=only needs to inherit one recessive X alelle to be affect

Woman need it on both=need it on both X

Carriers

females who are heterozygous for a disease that can pass it on, but they themselves don’t express it/are unaffected

For sex-linked inheritance, do reciprocal crosses prod same results?

reciprocal crosses DO NOT produce the same offspring ratios(due to Y being shorter and having less genes)

Autosomes

non-sex chromosomes,

Sex chromosomes

one pair of non-homologous chromosomes.

X-linked

When a gene being examined is present on the X chromosome, but not on the Y chromosome

hemizygous

have only one allele for any X-linked characteristic

lethal allele

a gene mutation that causes an organism to die.

Caused by mutations in genes that are essential for growth or development(as there is a nonfunctional allele).

AA and Aa die(lethal dom) aa-die (lethal rec.) die before birth

Don’t get expected ratio b/c some die bleh(changes expected ratio like if homo rec die in cross b/t 2 heteros, new ratio would be 2:1, not 1:2:1).

What happens to organisms with the lethal allele?

Because the gene is essential(if they get the nonfunctional ones), these individuals might fail to develop past fertilization, die in utero, or die later in life, depending on what life stage requires this gene.

recessive lethal

inheritance pattern in which an allele is only lethal in the homozygous form and in which the heterozygote may be normal or have some altered nonlethal phenotype

When a recessive lethal allele is inherited, only homozygous rec. organisms die.

only wild type homozygotes and heterozygotes would be observed in crosses as recessive died due to non func allele

dominant lethal

inheritance pattern is one in which an allele is lethal both in the homozygote and the heterozygote

Can only be transmitted if the lethality phenotype occurs after reproductive age.

When a dominant lethal allele is inherited, both homozygotes and heterozygotes die.

may not be expressed till adulthood(Huntington disease)

linked genes

genes that have very close loci/on same chromosome(meaning physically close to each other on same chromo), cross over together as a pair and occur together. NO indep assortment b/t 2 genes

genes get passed on together.

allele combinations are not inherited independently of each other(transmitted through meiosis together during recombo)

parental genotypes

been inherited intact from the parents of the individual producing gametes

As the distance between two genes increases, the probability of one or more crossovers between them increases, and the genes behave more like they are on separate chromosomes. (random info)

Why can two genes on the same chromosomes behave independently, meaning NOT linked?

b/c of recombo or crossing over during meiosis

What genes are ALWAYs sorted independently?

Genes that are located on separate non-homologous chromosomes will always sort independently.

Single observable characteristics are almost always…

under the influence of multiple genes(each with two or more alleles)

epistasis

not all genes leads to a particular trait, some genes are regulatory. some gene expression regulated by other genes; many genes, many characters, one gene, lots of characters

phenotypic expression due to interactions of one or more other genes

How can genes fxn?

• Several genes can contribute to aspects of a common phenotype without their gene products ever directly interacting. Each gene will add to the complexity and specificity of the something.

• Complementary or synergistic fashions, such that two or more genes need to be expressed simultaneously to affect a phenotype.

• Oppose each other, with one gene modifying the expression of another.

Epistasis from the textbook

One gene masks or interferes with the expression with another. One genes is silenced(hypostatic) while the other does the silencing(epistatic)

A circumstance where the expression of one gene is modified (e.g., masked, inhibited or suppressed) by the expression of one or more other genes

In which the expression of one gene is dependent on the fxn of a gene that follows it in the pathway

Diff epistasis

1. when a dominant allele masks expression at a separate gene.

2. Genes are silenced

3. can be reciprocal such that either gene, when present in the dominant (or recessive) form, expresses the same phenotype.

epistasis examples

ex: regulatory genes turning on/off various pigmentation genes for skin, hair and eye color

polygenic inheritance

many genes determine the phenotype, genes interact w/ each other.

trait is expressed by multiple genes, leads to continuous range of phenotypes

when multiple genes work together to influence a trait

Allele vs genotype

An allele is represented by a single letter, like "A", not "Aa" which represents a genotype containing two alleles (one "A" and one "a") at a specific gene locus;

pleiotropy

ONE gene produce many different phenotypes for different characters.

1 gene is involved in MANY characters. associated with disease, like sickle cell anemia

when a single gene influences multiple traits or characteristics(exhibits multiple phenotypic expression)

homeotic genes-homeobox genes

regulatory genes- fxn: developmental processes in animals, det. where body part is formed and at what time

Drug resistance

variation in the resistance to dif drugs in the parasite due to mutations or multiple mutations in different genes that leads to some parasites in that parasite population having a different response

change in gene expression not due to changes in gene sequences

not changing genetic code, but changing the expression of it and this can be passed on; genes turn on/off

epigenetics

changes in phenotypic expression NOT due to changes in genetic code(due to turning of genes on/off)

still poorly understood

many processes, such as DNA methylation and histone mod, can alter gene expression without altering actual genes sequences

ex: cell differentiation during development

the way your behaviors and environment can cause changes that affect the way your genes work. Epigenetics turns genes "on" and "off."

Lamarck

inheritance of acquired traits(soft inheritance)