BIO 202 EXAM 4

mendels work

• mendels work led to the concept of gene and the foundation in understanding of inhertianc

• his work was ignored until 1900

sutton and boveri

• they noticed chromosomes and genes are both present in pairs in diploid cells

• homologous chromosomes separate during meiosis

• fertilization restores the paired condition for both chromosomes and genes——-this led to the chromosomal theory of inheritance-

chromosomal theory of inheritance

The chromosomal theory of inheritance is that genes are located on the chromosomes, and behavior of chromosomes during meiosis and fertilization explains the interitance of traits

Thomas Hunt Morgan

First person to associate a specific gene with a specific chromosome

Morgan’s experimental model

like mendel, he was insighful in his choice of model, drosphilia melanogaster

drosphilia melanogaster

a fly that east fungus form rotten fruits hence the name fruit fly

Fruit fly

• they are prolific breeders- make a 1000 of flies at once

• cheap to grow

• generation time span is about 2 weeks- whole life from birth to death in 2 weeks

• mutations produced phenotypes that are visible by naked eyes

• have only 4 pair of chromosomes

by 1910- does anyone knows the chromosomes functions?

no

what does nettie stevens discobered?

morgan and his graduate student- Nettie stevens discovered that not they have only 4 pairs of chromosomes but also that female flies have 2 identical x chromosomes

but males has very different X and Y chromosomes - which X is never present in female

morgan’s first mutant

he worked so hard by crossing over flies but didn’t find anything until 2 years

after 2 years he found a single white-eyed male as mutation in flies

Wild type W+

red eyes are wild type- very common in all will natural populations W+

Mutant phenotype W

White is the mutant phenotype- it is the mutation in wild type allele

What is the evidence that fly eye color is linked to gender ? 1-

• red eye female(homo) cross over with white eye male(hetero)

• F1 gen- all offspring have red eye color because red eyes are dominant over white

Crosses between F1 generation. 2-

as expected it produced 3:1 phenotypic ration- red: white : red

Unexpectedly results

The white-eyed trait appeared only in males

All the females had red eyes.

But half the males has red and other half has white eyes (again and again)

Se* chromosomes segregate during meiosis meaning-

• each sex chromosomes segregates into one gamete after meiosis

• xx produces haploid gametes- eggs- all containing X chromosomes

• XX- female

• XY- male

• X + Y- male

• X+ X- female

to remember- male has hat- so arrow upwards

nomenclature for drosophilia genetic

Gene name comes from first mutant discovered

W+

Red

WT

dominant

W

White

mutant

recessive

Hemizygous

when Males X w+ Y have only a single x chromosome- and there is no copies of the red eye allele

explain why in the F2 generation the white-eyed trait appeared only in males?

Because Males are red eyed if they inherit red-eye allele which is w+ but white eye if they inherit a white eye allele which is w, since there is no any additonal wil-type allele present to mask the recessive allele.

location of genes

morgan discovered that genes have physical location on chromosomes

Hoe morgan discovered that genes has physical locations on chromosomes?

Panel a- homozygous red-eyed female will produce- with red eyes even the father has white eyes——mother has red- offspring will have red too

panel b- mother is homo for white eyes- all male offspring will have white eyes—even father has red eyes

THEREFORE, MALE MUST INHERIT THE X CHROMOSOME FROM HIS MOTHER AND Y FROM HIS FATHER

THEREFORE, MALE MUST INHERIT THE X CHROMOSOME FROM HIS MOTHER AND Y FROM HIS FATHER

this suggested correlation between sex-linked chromosomes and the segration of factors that determine the eye color

Morgans 3 findings

1.

Eye color is linked to the X chromosome

Morgan studied fruit flies and noticed that eye color depends on the X chromosome.

• Females have two X chromosomes (XX) → they can have two copies of the eye color gene.

• Males have one X and one Y (XY) → they only get one copy of that gene (on the X).

• The Y chromosome doesn’t carry the eye color gene, so it doesn’t affect this trait.

• Red eyes are dominant, meaning if a red version is present, the fly will have red eyes.

Simple idea: The gene for eye color is on the X chromosome, and males are more affected because they only have one X.

2. genes reside on chromosomes - chromosomal theory of inheritance

Before Morgan, people didn’t know exactly where genes were.
He showed that genes are physically found on chromosomes (the structures inside cells that carry DNA).

Simple idea: Genes aren’t floating around—they live on chromosomes

3. each gene resides on a particular chromosomes

Morgan discovered that each gene sits in a specific spot (called a locus) on a chromosome.

Simple idea: Think of a chromosome like a street, and each gene has its own exact address.

what else these findings says?

that genes located on the X chromosome exhibit unique patterns of inheritance becasue of gender and because they were linked together

what chromosomes does not carrie hundreds or thousand of chromosomes ?

chromosome Y

Linked genes

genes located on the same chromosome - they tend to inherit together because the chromosome is passed along as unit

they do not separate independently during meiosis

do linked genes assort independently?

Linked genes don’t assort independently because they’re physically attached on the same chromosome, so they usually get inherited together.

linked genes- (located closer on same chromosome)

• all offspring are predicted to have parental phenotype when genese are linked in the same chromosome

• We have: AaBb × aabb

  And the important detail:
  A and B are linked (on the same chromosome, very close together)

  So instead of thinking of A and B separately, think of them as traveling together as a pair:

  • One chromosome carries AB

  • The other carries ab

  So the AaBb parent is really:
  AB / ab

What gametes are made? If genes were NOT linked (independent assortment):

You’d get 4 types:

• AB, Ab, aB, ab (all equally likely)

But since they ARE linked:

Because A and B are very close:

• They stay together

• No mixing (or extremely rare)

👉 So only two gametes form:

• AB

• ab

Final outcome

Only 2 types of offspring:

• AaBb (looks like one parent)

• aabb (looks like the other parent)

Ratio:
1 : 1

Why no new combinations?

Because:

• A and B are physically stuck together

• They don’t separate during meiosis

So you don’t see recombinant types (like Ab or aB)

takeaway

• Linked genes = travel together

• No independent assortment

• Offspring mostly look like the parents

• In this cross → only parental phenotypes (1:1 ratio)

Unlinked genes

When genes are unlinked (on different chromosomes), they follow Mendel’s law of independent assortment, meaning they separate independently during meiosis.

In the cross AaBb × aabb:

• The AaBb parent makes 4 types of gametes: AB, Ab, aB, ab (all equally likely)

• The aabb parent makes only ab

This produces 4 offspring types in a 1:1:1:1 ratio:

• AaBb (parental)

• aabb (parental)

• Aabb (non-parental)

• aaBb (non-parental)

Result:——

• 50% parental phenotypes

• 50% non-parental phenotypes

takeaway: Unlinked genes mix freely, so you get equal parental and new combinations.

what other experiemnt morgan did afterwards?

to test how linkage affects inheritance, morgan did other test including test crosses with fruit floes to see the inheritance of 2 characters

morgan crossed flies in traits of body color and wing size

what was expected ?

• If the genes were unlinked (different chromosomes) → ratio should be 1:1:1:1

• If the genes were completely linked (same chromosome, no crossing over) → ratio should be 1:1:0:0 (only parental types)

observed?

• The two large groups (965, 944) = parental phenotypes

• The two smaller groups (206, 185) = recombinant (non-parental) phenotypes

what does these all means?

What does this mean?

👉 The genes are linked (on the same chromosome) because parental types are more common
👉 BUT they are not completely linked because some recombinants still appear

why do recombinants show up?

Why do recombinants show up?

Because of crossing over during meiosis, which occasionally swaps parts of chromosomes and creates new combinations

takeaway

vast majority looked like one parent and other just some percent on another parent.

Final takeaway

• Genes are on the same chromosome (linked)

• They are close, but not extremely close

• Crossing over causes some recombination, so you don’t get a perfect 1:1:0:0 ratio

In one line:
Mostly parental types + some recombinants = linked genes with crossing over.

whats goign on? crossovers and recombination. what problem he noticed?

Most offspring looked like the parents (because genes were linked)
But a small number looked different

So something must be mixing the genes occasionally

morgans idea?

Thomas Hunt Morgan proposed that:
Chromosomes can swap pieces with each other

what happens in crossing over?

During prophase I of meiosis:

• Homologous chromosomes pair up (one from mom, one from dad)

• Non-sister chromatids cross and exchange matching DNA segments

This is called crossing over

why this matters?

When segments are swapped:

• Linked genes can get separated

• New combinations of alleles are formed

This creates recombinant gametes (new trait combinations)

simple understanding

Simple analogy

Think of two matching zippers:

• They line up perfectly

• Then swap a section

• Now each zipper has a mix from the other

takeaway

Crossing over explains why:

• Linked genes are usually inherited together

• But sometimes get separated, producing new combinations

One-line summary:
Crossing over swaps DNA between chromosomes, creating recombinant offspring even when genes are linked.

How do we know when genes are linked?

• can be based on phenotypic ratio of offspring in test cross

complete linkage

all offspring are identical in phenotype to one or the other parent for those genes

unlinked genes

an equal number of parental vs non parent phenotype among offspring 50& recombination frquency - the physical basis for this number is the random orientation of homologous chromosomes at metaphase 1 of meiosis

Frequency of recombinant

reflects - the distance between genes on a chromosome. the greater the distance between two genes, the more likely crossing over can occur between then and vice versa

chromosomal basis of gender varies with organis,- hermaphrodites

• have both male and female reproductive organs

• produce both sperm and eggs

• ex- worms, snails, some fish

Most animals including humans have 2 sexes

• either male or female

• each produce only one type of gamete

• male- sperm

• female- egg

chromosomal basis of sex varies

• humans

• xx- female

• xy- male

• birds- some reptiles

• zz- male

• zw- female

• insects- sex depends on the number of chromosomes

Y chromosome

• Y is the sex-determining chromosome

• it carrie the key gene (SRY) triggers male development

environmental sex determination

• seen in reptiles like crocodiles and some turtles

• temperature during development decide sex

• ex- warmer vs cooler nests- diff sexes

what happens in humans?

• In female xx and males xy- special cells in the ovaries and testes go through meiosis to make gametes (eggs or sperm)

• meiosis cuts the chromosome number in half—- halpoid gametes (one copy of each chromosome)

• Mother- she has 2 x chromosome- during meiosis- eggs get one x—-so all eggs x

• father- he has x and y

• during meiosis- half sperm gets x and other half y

• so sperm 50 % x and 50 % Y

what determines the baby’s sex?

xx- female

xy- male

since sperm are 50/50- theres about a 50 % chance of each sex

Aristotle 320 BC

he believed sex was based on temperature, but modern biology shows its determined by chromosome x and y

SRY

SRY is the gene found only on Y chromosome

Acts like a master switch for a male development

How sry gene determines the sex?

• if sry is present

• It turns on other genes

• embroys gonads develop into testes

• leads to male development

• if its absent

• no signal to become male

• gonads develop into ovaries

• so female development is the default pathway

What does SRY gene actually do?

• it makes transcription factor- a protein that turns other gens on/off

• these genes controls - testes formation, male characteristics, sperm production

Mouse vs sry gene

scientist added the sry gene to an xx female mouse embryo

the mouse developed as a male

• this proves SRY alone can trigger male development, even without Y chromosome

X vs Y chromosomes

• the x and y chromosomes are very different

Y chromosome

• smaller

• has very few genes (gene-poor)

• genes are mostly related to male traits

• not essential for survival

X chromosome

• larger

• has many genes 1000+

• many are essential for life

sex linked genes

genes are located on the x or y chromosomes are called sex-linked genes

what happened during meiosis?

• even though x and y are very diff, they still pair up during meiosis, and act like homologous chromosomes

how ?

they match up only in small shared region called pseusoautosomal region

crossing over

Because x and y are mostly not similar in dna sequence- they rarely exchange dna - crossing over is uncommon - crossing over is when chromosmes swap pieces, creating new combinations of genes

Y chromosome inheritance

• only males have a Y chromosomes

• female do not

• so a father passes his Y chromosomes only to his sons

• daughter get his x, not y

why y doesn’t change much?

• the x and y chromosomes are mostly not similar- not homologous

• becasue of that, y does not undergo crossing over( except tiny regions)

so the y chromosome passed down almost unchanged father to son, only small changes happen due to muttaions

why y chromosme matters

The above properties make the y chromosome useful for

• studying evolution

• tracing family lineage

• paternity testing

What about female having 2 x chromosmes ? wouldn’t females have double gene activity?

No, because of X-inactivation process

• in female, one x chromosome in each cell is turned off

• so only one x is active , just like in males

• it prevents females from having double gene dosage .

X- activation - Barr body

• females have 2 x chromosmes -xx

• in each cell, one x is turned off

• the inactive x becomes a condensed structure called barr body

• prevent having double the gene activity compared to male xy

what happens in egg cells?

• the inactive x is reactivated in the ovaries

• so every egg ends up with one active x

why don’t females get more x- linked disorders ?

Because - which x is turned off is random in each cell- some cells use maternal X and some use paternal x

Mosaic effect

• once a cell inactives the x, all its daughter cells keeps the same cell off, which creates a mosaic affects in females

• 50 % of cells use one off cell and 50 use other

• so females are mix of 2 cell types

mosaic effect example - cats

• in tortoiseshell/ calico cats

• one carries orange fur

• the other carriers black fur

• diff patches of cells activate different x chromosomes

• Result- patchy orange + black fur

Why are these cats almost always female ?

• males xy only have one x- only one color

• females have two xx- can show both colors because of x- inactivation

sex-linked genes - on the x chromosome

• x chromosome carries many genes not related to sex

• these are called x-linked genes

• they follow special inheritance patterns because - female - xx, and males xy- only one x effects show up more easily

X-linked recessive traits

A- normal- dominal

a- mutant- recessive

Males (XY)- only one x - if they get a they are affected

females (XX_- need 2 copies of aa to be affected

Aa female- carriers- usually not affected

Inheritance patterns 1- affected father (XaY) with normal mother (XAXA)

• all daughters - XAXA - carriers

• All sons - XAY- normal

• Father passes X to daughters

• Y to sons

2. carrier female (XAXa) with normal male (XAY)

• daughters 50 percent carrier- XAXa

• 50 percent normal- XAXA

• sons

• * 50 percent affected (XaY)

• 50 normal (XAY)

3. Carrier female (XAXa) × affected male (XaY)

• 50% of all children affected

• Daughters:

  • Some carriers, some affected

• Sons:

  • Some affected, some normal

why females are less affected ?

to get female more-

• mother must be carrier

• father must be affected

so males are more often affected by x-linked recessive traits

females are often carriers

father pass x - linked traits to daughters , not sons

X- linked disorders

• there are many genes app 1000 on the x chromosome

muttaions on x chromosome genes

can cause-

• red/green color blindness

• hemophilia

• duchenne muscular dystrophy

• male pattern baldness is also x- influences

• THESE ARE USUALLY RECESSIVE / X- LINKED

why disorders more in males ?

• Males = XY (only one X → “hemizygous”)

• If they get one mutant allele → they show the disorder

• Females = XX

• Need two mutant copies to be affected

• If only one → carrier

So:

• Males are affected much more often

Why sex chromosome abnormalities are tolerated ?

Compared to other chromosomes, abnormalities (aneuploidy) are less severe because:

• Y chromosome has few essential genes

• Extra X chromosomes get inactivated (Barr bodies)

Only one X stays active, no matter how many X’s there are

inheritance idea

• Carrier female → passes mutation to:

  • 50% of sons (affected)

  • 50% of daughters (carriers)

Males only need one copy → higher chance of disease
Females need two copies → much rarer

Central dogma

DNA- RNA- Proteim

• dna holds the instructiond

• RNA is copy of those instructions

• proteins are what actually do the work in cell

Genotype vs Phenotype

• genotype- DNa sequence

• phenotype- your traits - what you see, appearance

Traits depends on proteins made form DNA

Gene expression

• transcription- DNA- RNA - rna synthesis

• translation- RNA - PROTEIN- protein synthesis

Triplet codons

• DNA/ RNA is read 3 letters at a time

• each group of 4= 3 codons

• Each codon =. one amino acid

codons do not overalp

each codon has one meaning

How experiments helps scientists ?

these scientist figured out how genes makes protein

1- Archibald garrod -

In born errors of metabolism

• studies genetic diseases

• found that some people lack certain enzymes

• caused by mutations in genes

Genes control metabolism- chemical reactions in the body

george Beadle and Edward Tatum

One gene- one enzyme

• showed each gene makes one enzyme

• later update to- one gene- one polypeptide

dna and rna - nucleic acid - and protein is polypeptide

Francis crick and sydney brenner

Properties of the genetic code

• he discovered

• code is read in triplets- 3 bases

• it is non-overlapping

• reading frame is critical

Nirenberg and matthaei; khorana

cracked the genetic code

• figure out which codon= which amino acid

• example- UUU- phenylalanine

What is gene ?

A gene is piece of DNA that gives instructions to make a protein .

How does genes control phenotype?

• genes - makes proteins especially enzymes

• proteins control chemical reactions in the body- metabolism

• These reactions determines your traits - phenotype

gene- protein- traits

Idea from garrod related to gene

• he studies genetic disease and said

• genes control enzyme production

• enzyme control chemical reactions