Exam 2 Bio 303

Codominance

- *full expression of both alleles of heterozygotes* - genotype follows mendel's rules of inheritance - two differenet alleles are *fully* expressed (vs as a blend) - red + white = red +white

a test cross for 2 genes far apart on the same chrosome tends to ____ the true physical distance. why?

- *underestimate* - because of multiple (even #) of cross over events

Be able to define the Principle of Segregation

- 1st law

examples of sex limited traits

- DMD: is a X-linked disorder that is basically sex limited - it affects 1/3500 males but only 1/50million females - affect males die before reaching reproductive age or cannot phhysicallty ejaculate so affected females are extremely rare

white eye color of Drosophila is example of what

- X-linked recessive traits

What is the chromosomal basis of sex determination in humans and other animals?

- XX female - XY male

genetic interactions

- alleles of a gene exhibit independent assortment, but do not act independently in their phenotypic expression - ex: color of bell peppers

when does chromosomal sex determination occur?

- at fertiization

example of sex influenced trait

- baldness! - pattern baldness is seen more often in males than females bc for males it is autosomal dom but for women is autsomal rec - related to a genetic variant in X-linked male hormone recpetor aka you get it from ur mom

what is the first step in sex determination?

- chromosomal sex - diploid zygote is either XX or XY BUT not necessarily male of female for how long? (next slide)

sex LIMITED traits

- definition: are inherited by bothh sexes, BUT normally expressed ONLY in one sex, produce a phenotype in ONLY one sex

what is 3rd stage of determination?

- development of sexual phenotype

inheritance patterns of linked genes

- do NOT show mendel's 9:3:3:1 ratio in the F2s expected with independent assortment

"approx additive"

- doesnt have to be perfectly add up - longer the disntance between genes = accuracry DECREASES - that is because of multiple crossovers (multiples of 2) make it impossible to tell that a crossover actually happened - - even number of crossovers negates them

example of multifactorial trait

- genes + nutrition = height

2nd step in sex determination?

- gonadal sex determination - emphasis on formation of male and female gonads - if XY, expression of Y-linked ___ gene signals the indifferent gonad to develop as a testis - in XX lack of y linked expression causes x-linked genes to drive ovary formation

Be able to define the Principle of Independent Assortment

- hhis second law

gene mapping notes

- hypothesized crossover event occured more or less at random and up and down chrosoome - two genes father apart on the *same* chromosome are more likely to undergo a crossover - genes close togehter will often be linked and less likely to undergo cross over - propose recom freq would give estimates of the *relative* distances between genes on a chromosome - because geneitc distances are measured w RF are approx additive, we can construct a genetic map from recomb freq

sex chrosome inheritance is defined by

- if its x link rec, x link dom, or y linked

mosaicism*

- is a product of dosage compensation in a *heterozygous female* for x-linked genes -"the property or state of being composed of cells of two genetically different types." - google - think of black and orange cells drawing - deals with the X-chromosome

phenotypic sex determination in females

- lack of that hormone "prevents" development of external genitalia - " we are all inherently female"

lyon hypothesis*

- lyon deduced that in *heterozygous females* both alleles were active, but not in the same cells

hemizygosity

- males are hemizyogous for X chromosome because only have one x (XY) -" A chromosome in a diploid organism is hemizygous when only one copy is present" - google - "Hemizygosity is also observed when one copy of a gene is deleted, or, in the heterogametic sex, when a gene is located on a sex chromosome" - google

relationship between recombination and location of genes

- morgan and his student developed the idea that *physical distances* between genes on a chromosome are related to the rates of recombinatoin -you use recomb frew to find where genes are located in relation to eachother -it is all relative

Distinguish between the following types of traits (and know an example of each): discontinusous vs. continuous, polygenic vs. multifactoral

- pea plants are discontinous - humans are continuous

why are X and Y imporntant

- play major role in determining sex - carry genes that initiate and support development of male or femaleness in embryos -*carry many genes that can be indentified by their unique pattern of inheritance*

multifactorial traits

- polygenic + *known* amount of environmental influence = phenotype - polygenic AND show significant interactions with environmental cues - each gene is inherited in mendelian fashion - interactions of genes and environment yield variable phenotypes that don't show mendelian ratios

concept of dosage compensation*

- remember that females have multiple X- chrom's and males only have 1 - however, females do not produce teice as much X-linked gene product as males - males anf females produce thhe same amount of x-linked gene product - this equality is because of dosage compensation - the mechhanism causes either: 1. one x chrom hhas been inactivated or 2. both x chromsome only working at 50% - thhis mechanism is random - when the cell reproduces, that same x chrom will be inactivated after reproduces. - the inactivated x chromsome appears as *barr bodies* in the cell

Mary Lyon*

- studied the inheritance of coat color in cats and mice - created a hypothesis about doage compensation - female mice heterzygous for x-linked coat color genes had unique coat colors - coat colors were not the same as parents and alsno NOT a blend of parent colors - instrad, fur had rancom patches of the 2 parental colors - hemizyogius males always showed uniform coat color

phenotypic sex determination in males

- testosterone is converted to a hormone, which helps the formation of genetalia

notes on recomb stuff

- these chromsomes segregate during meiosis - each gamete receives one of the 2 homologs - independent assortment: in meiosis, each pair of homologous chromosomes assort independently of other homologous pairs

polygenic traits

- traits controlled by 2 or more gene pairs - as the number of loci (many genes, 2 loci at a time) controlling a trait increases, then number of phenotype classes also increases

complete linkage

- use F1 heterozygote to perform testcross for linkage by crossing it with *homozygous recessive* - gametes produced are only AB and ab - because no crossing over, these are the ONLY gametes produced by the heterozygote (only parental genotypes)

what is it called when ur testing for linkage and what are you crossing

-"back cross" - hheterozygote for both characteristics x homosygous recessive

expressivity

-"if yes, then how much?" - definition: the *degree* to which a trait is expressed; related to penetrance

penetrance

-"yes or now" -definition: the percentage of individuals having a particular genotype expressing the expected phenotype - ex: if 38/42 people with the allele for said phenotype, then there is 90% penetrance

what is this hormone called?

-DHT aka: di hydroxy testosterone

tortise shell cat

-Dominant O allele (X+) produces orange/yellow color - recessive o allele (Xo) produces black color - heterzygous females (X+/Xo) have parthces of orange fur and patches of black fur - (white fur us caused by an autosome gene)

example of X-linked dominant disorder

-Hypophosphatemia - affected indicudials have low phosphate levels in blood = skeletal deformities like bowlegs bone disease degradation of spine

cis configuration

-aka coupling - arragnement occurs when wildtype alleles are on one chromosome and mutant alleles are on another chromosome

trans configuration

-aka repulsion -occurs when a wild type allele and a mutant allele of a diff gene are found on the same chrosmome

units for genetic maps

-centimorgans (cM) or map unites (m.u.)

how are they opposites?

-crossing over mixes genes up and confirms with mendels ratios - linkage keeps genes together and skews mendels ratios

cross over and recombination frequency relationship

-crossing over results in recombination, recomb freq is the calculated amount of times recomb happens -recomb -> breaks up combos of alleles of genes close together on the same chromosome

Pseudohermaphroditism

-disorders result in individuals with both male AND female structures, but at different times in their lives (remmeber weird picture in slides of baby) - one autsomal form prevents conversion of testosterone into DHT so XY individuals appear to be female - scrotum resembles the labia - penis resembles an enlarged clit at birth

continuous characteristic

-ex: human height - a distribution of phenotypic characters from one extreme to another in an overlapping fashion - "continuing variation" - often controlled by 2 or more gene pairs

age

-ex: huntingtons disease is lethal due to a dominant allele, it is often "early onset" or whatevr, - ex: greying of hair

crossing over defined

-exchahnge of genetic material between non sister chromatids during prophase 1 of meiosis 1 -genes occasionally switchh from one *homologous* chromosome to another

how can sex phenotypes change at puberty?

-female -> puberty -> male -development of functional testes and male sex organs occur as a result of: *increases testosterone levels associated with puberty*

what does , formation of male/female structures depend on?

-gene action AND -interactions: 1. within the embryo 2. with other embryos in the uterus 3. the maternal environment

linked genes

-genes located close together on the same chromosome - these are 2 loci for seperate genes tgat lie close to one another (so allele A and allele B are linked) -belong to the same linkage group -travel together during meiosis, eventually ending up in the same gamete -mendel examined traits that allll segregated independently -many characteristis do not adhere to mendels 2nd law

albinism

-homozygous recessive individuals (aa) cannot make melanin - heterzgotes have enough to make skin, hair and eye colo

more examples of complex traits

-hypertension -obesity -cardiovascular disease

example of incomplete dominance

-red + white flowers = pink flowers of snapdragons -heterozygous withh alleles that are producing both red and white pigments - controlled by *two dominant genes* - p gen: R1R1 x R2R2 - F1 gen: R1R2 - F2 gen: (1/4) R1R1 aka red (1/2) R1R2 aka *pink* (1/4) R2R2 aka white

incomplete dominance

-the expression of phenotype that is an *INTERMEDIATE* to those of the parent - genotype strictly mendelian - multiple dominant alleles that can give an intermediate phenotype

recombination

-thru this, gametes contain new combos of alleles - definition: sorting of alleles into new combos - means the combo of alleles in an org's gametes may differ from the combo of gametes ffrom its parents - recomb gametes have differnt genotypes than their parents - can now restate mendels principles in terms of chromosomes - segregation: 2m orgs posses 2 allese for a trai each at the same locus on *homologous chromosomes*

Understand the influence genes on the sex chromosomes have on sex-linked characteristics

-y-linked SRY gene signals gonad to form into testis - if female, has no y chromo so sry gene not there and not expressed = "prevents" testis formation

sex INFLUENCED traits

0. sex-influenced traits are expressed in BOTH sexes, BUT differently in males and females 1. controlled by autosomal genes that are usually dominant in one sex BUT recessive in the other 2. illustrate the effect of *hormonal* differences on gene expression; aka different hormones will stimulate different genes

__1__ of loci results in __2__

1 independent assortment 2 *recombination*

Review how mutations can uncouple normal sex determination

1. chromosomal events may exchange of X and Y chromosomes 2. mutations may affect ability of a cell to respond to y-linked gene products 3. autosomal genes may control events on X and/or Y chromosome (*epistasis*) 4 androgen insensitivity 5. Pseudohermaphroditism

Be able to cite more than one way in which the sex of an organism is determined

1. chromosomal sex 2. gonadal sex 3. phenotypic sex

points to keep in mind when constructing genetic maps from RF

1. for large frequencies (50%+) we cannot distinguish btwn genes of diff chrom or genes far apart on same chrom 2. a testcross for 2 genes far apart on the same chrom tends to *underestimate* the true physical distance between them - this is becase it does not reveal double crossovers that may occur between loci

the unique patterns include ideas of:

1. males can only give X to daughter 2. males can only give Y to son 3. heterzygote females have a 50% chance of passing x-linked traits to *male* offspring (think of punnet square)

how it works (lyon hypothesis)*

1. one x chromosome is active in the somatic cells of females 1a. the other is inactivated and tightyl coiled into a barr body (think of crumpled paper analogy) 2. inactivated x can be maternal or paternal (random) 3. inactivation takes place in: *early development and occurs randomly* 4. inactivation is permanent (except in germ cells) 4a. all decendants of a particular cell will have the same x chrom inactivated 5. random inactivation of x crhom in females equlizes the activity for male and female

how can phenotypes arise during embryonic development that may be different than the *chromosomal sex*

1. phenotype *opposite* of chrom sex 2. phenotype an *intermediate* 3. phenotype can have characteristics and genitalia of *both sexes*

3 types of sex-related phenoytpic traits

1. sex influenced traits 2. sex limited traits 3. imprinting

mosaicism examples*

1. tortoiseshell cat (coat color) 2. blaschko's lines 3. anhidrotic ectodermal dysplasia (sweat glands)

1 in how many births result in a gonad/phenotypic sex that is different from the indiv's chromsomal sex?

1/2,000

what percentage penetrance do mendel's pea plants have?

100%

large frequences , above ___%, we cannot distinguid between genes on diff chromosome and genes located far apart on same chrosome

50%

what is the max recomb frequency and why? for LINKED genes (idk for bothh)

50% bc only 50% on the chromatids of a homologous pair are close enough to switch over

chromosomal sex determine: how long till may it take before be male of female

7-8 weeks

homologous chromosomes

Chromosomes that have the same sequence of genes and the same structure but can have diffferent allele on each chromosome (A and a)

whhat is an example of sex limited trait

DMD (muscle ripping disease)

does the presence of a gene guarantee its expression?

NO

Are X and Y homologous?

NO! - x and y have *different* patterns of inheritance because: they carry *diffferent genes*

example of recombination

P1: AA BB x aabb gametes: AB ab F1: AaBb x AaBb gametes: AB ab same as P1 = nonrecomb Ab aB diff than P1= recombinant gametes

what gene , that is Y-linked, signals gonad to become testis?

SRY gene

what chromosomes are sex chromosomes?

X and Y

red/green colorblindess is an example of what

X- linked reccesvie traits - red/green affects ~8% of males in US

chromosomal sex

XX is female, XY is male

androgen insensitivty

a mutaion in the X-linked gene for the androgen receptor - causes XY males to become phenotypic females

example of follow mendel's laws in humans

albinism

phenotypic sex

appearances

crossing over an dlinked genes

are opposites!! in effects

blue colorblindness is

autsomal dominant

what is an example off sex influenced trait

baldness

how can certain x-linked recessive disorders be expressed in heterzygous females

becuase of dosage compensation, inactivate the dom x

example of codominance

blood type type A: IAIA or IAIO type B: IBIB or IBIO type AB: IAIB type O: IOIO

example of genes with more than 2 alleles

bloog type -> ABO

what are variations in phenotype expression casued by?

can be caused by: 1. age 2. genetic interations 3. interactions with the environmet

height is what kind of trait?

complex trait -> multifactorial

dosage compensation

definition: mechhanism that regulates the expression of sex-linked gene products

imprinting

definition: phenemenon in whcih gene expression depends of whether it is paternally or maternally inherited

complex trait

determined by 1. several gene pairs, 2. nongenetic factors, and 3. environmental interactions

principle of segregation

diploid (2n) organisms have two alleles at each locus that seperate in meosis - one allele goes to each gamete

independent assortment

during the process of separation, both alleles at a locus act independently of alleles at other loci

polygenic trait example

eye color (8 genes controlling it)

sex phenotypes cannot change at puberty true or false

false, they can!

human genetics starts with the study of:

family histroy

who is mosaics for x-linked genes male or female?

female

genetic map vs physical map

genetic map: - all relative - no actual distances between genes - just know relationship between genes to be able to order the genes on a chromosome physical map: - have actual data of how far away from eachtoher, -have actual dna sequence

incomplete penetrance

genotype doesnt always produce the expected phenotype

what would be examples of penetrance

having or not having: 1. polydactyly 2. camptodactyly

what would be examples of expressivity

if does have polydactyly, then if left right or both hands affected?

Distinguish between dominance, codominance and incomplete dominance

incomplete dom. = blend, an intermediate codom. both are shown, not a mix

mendels 2nd law

independent assortment

closer genes are together on same chromsomes = ___ likely for crossing over to occur

less likely (think chalk throwing at baord analogy)

how many y-linked traits have been discovered to date?

less than 50

what if some genes are on the same chromosome?

linked genes

crossing over relatinship with linked genes

linked genes close to each other are usually inherited together BUT crossing over can prevent thhis - genes can occasionally (randomly) switch from one homologous crhom to another - results in RECOMBination; breaks up genes close together on the same chromosome

when does crossing over occur?

meiosis 1 -> prophase 1

human traits are mostly this kind

multifactorial traits

do linked genes sort independently?

no

are mendels principles wrong?

no just didnt account for inheritance of all the genetic characteristics

is there mosaicism in XY males?

no not for this class

can males be homo or heterozygous for x chromosome?

no, they are only hemizygous bc have on copy

complete linkage yeilds what type of gametes?

nonrecombinant - the progeny all display the original combos of traits in parents and are called nonrecombinant progeny

epistasis

one gene can mask the affect of another gene at a *different* locus - involved different *genes* not alleles of the same gene as in gene interaction. - note: not the same as "gene interaction"

gonadal sex

ovaries vs testes

what is essential to human genetics

pedigree construction

polydactyly and campodactlyly are releavant for what?

penetrance and expressvity

What was the prevailing pre-Mendelian theory of inheritance?

pre = preformationism during = blending inheritance

example off incompletely linked genes

purple and pink plants w oval and round shape P: puple oval & pink round F1: all purple and oval f2: pr o, pr r, pk o, pk r but ration higher ffor parental characteristics

Lethal yellow gene in mice

recessive in its effect on development (lethality) , but dominant for coat color

Note that genotype does not always determine phenotype (penetrance, expressivity)

see below

dosage compensation, Barr bodies, Lyon hypothesis

see below

What are the differences between chromosomal sex, gonadal sex, and phenotypic sex?

see below for answer

what provides a *genetic* pathway for embryo development that guides it towards a phenotypic sex?

sex chromosomes

discontinuous trait examle

tall and short pea plants - there are 2 distinct phenotypes

what can be done to refine such a genetic map?

testcross for an additional gene:*3 point testcross*

when creating a genetic map which alleles do you start with?

the ones farthest apart (ex: a and b) and go from there keep in mind that if c is this far away from A andlso see how far away it is from B to know on which side of a C goes.

color blindness and dosage compensation

the same C chrom is not always inactivated in the retinal cells ( carries can have trouble identifying diff shades of green and red)

what happens (in polygenic traits) as the # of classes increases?

there is LESS phenotypic difference between individual classes

T or F? phenotypic expression of a trait is different in males and females sometimes.

true

universal acceptor

type AB

universal donor

type O and get most cookies

recomb frequency hhas been used for what and by who first?

used by thomas hunt morgan to devise first genetic maps of chromosomes

What are the implications of a trait that is X-linked or Y-linked on heredity?

x can pass to son or duaghter, think about x rec and x dom, y can only be passed from father to son, females dont have y