Flashcards in Exam 3 Deck (65):
What is heterogamy?
- gametes differ (sperm/egg)
- XX and XY (or ZZ and ZY)
Can genes necessary for survival be on the Y chromosome?
Genes on Y chromosome are accessory genes
- such as to make sperm etc
If the Y chromosome contains accessory genes only, then what type of chromosome is the X chromosome?
X is a normal chromosome
- everyone needs it to be alive
- carries genes necessary for survival
What is isogamy?
no true male/female
- gametes from each mating time are identical
What is the difference between homogametic sex and heterogametic sex?
Homogametic sex: 2 like chromosomes (XX)
Heterogametic sex: 2 unlike chromosomes (XY)
What kind of sex determination system do bees have?
Females: Diploid (fertilized eggs)
Males: Haploid (unfertilized eggs)
- produce sperm through mitosis
No sex chromosomes
What is genic balance?
Sex determined by ratio of copies of X's to copies of autosomes
Typical female: 2 copies of X's & 2 sets of autosomes
- ratio = 1:1 (1.0)
Typical male: 1 copy of X & 2 copies of autosomes
- ratio = 1:2 (0.5)
According to genic balance, if a female accidentally had a XXY genetic makeup what would be the result?
The female would be normal and fertile because the ratio is still 2 copies of X & 2 copies of autosomes
- ratio still 1.0 so normal female
According to genic balance, if the ratio of X's to autosomes is different that 1.0 or 0.5 what occurs?
sterile,reduced viability or intersex
Which animal's gender is determined by genic balance?
drosophilia (fruit flies
In simplest terms, how is sex determined in humans?
Presence of Y = male
Absence of Y = female
What is the significant of the Y chromosome for male development?
One Gene on the Y chromosome needed for development of undifferentiated gonadal tissue into testes
Testes produces testosterone early and causes cascade in development of genitalia
What is the male gene on the Y chromosome?
SRY gene is the sex determining region of the Y chromosome
Master regulatory gene for sex determination
produces protein that is transcription factror
- this transcription factor binds DNA and turns transcription of male genes on
- testes development->testosterone->male characteristics
No SRY = female cascade
How can a person with the SRY gene develop as a female?
If you have 2 copies of the DAX-1 gene then it deletes the SRY gene
- default is female cascade
Regular is 1 copy of DAX-1
- normal male development`
To develop internal male genitalia what hormone is required? To develop external male genitalia what hormone is required?
Testosterone will develop internal male genitalia
Dihydrotestosterone will develop external male genitalia
What is the basic overview of human sexual determination?
Begin undifferentiated embryo
SRY gene triggers development of testes
- absence of SRY gene = development of ovaries (default)
Testes release Testosterone
- development of internal male genitalia
- dihydrotestosterone needed to develop external genitalia
Ovaries release estrogen
- development of internal genitalia+ external genitalia
What are the three genes that we covered that affect phenotypic sex? (changes in sexual development)
These are all mendellian (single gene)
1. Androgen insensitivity
3. Congenital adrenal hyperplasia
What is androgen insensitivity syndrome?
Have an XY chromosome
- genetically males
- phenotypically normal females
No menstruation (typically discovered at puberty)
- No cervix or uterus but undeveloped testes
Cause: gene that codes for androgen receptor protein not functioning = no testosterone gets into cells = no differentiation into male (like not even there)
Do ppl w/ androgen insensitivity syndrome (AIS) have the SRY gene?
Yes. They have testes that produce testosterone but no functional receptors on cells so it doesn't get in the cells
What kind of allele is AIS? What are the possible genotypes?
Sex linked Recessive
X^A X^A = Normal Female
X^A X^a = Female Carrier
X^A Y = Male
X^a Y = AIS Female
What is guevodoces?
Born w/ ambiguous genitalia
- at puberty definite masculinity occurs
- become 100% functional men
- autosomal recessive alleles affect testosterone metabolism
- gene that produces enzyme that converts testosterone to dihydrotestosterone (DHT) not functioning
- no conversion of testosterone to DHT so no development of external male genitalia
- huge surge in testosterone at puberty overrides need for DHT and develops external male genitalia
What enzyme converts testosterone to dihydrotestosterone?
What is congenital adrenal hyperplasia?
Masculinization of females
Males w/ it aren't feminized but have other complications
Defect in enzyme that converts precursor molecules in to cortisol
- Autosomal recessive (homozygous dominant or heterozygous = able to make cortisol)
- precursor molecules are converted to testosterone instead of cortisol
Do sex chromosomes follow diploid model?
Diploidy is having 2 copies of homologous chromosomes
- X & Y are not homologous (don't carry same genes)
What is hemizygous?
Males are hemizygous for whatever trait you're looking at
- 1 copy of X chromosome (w/ trait) 1 copy of Y
- the sex linked trait will be on X chromosome
- so male can be aY or AY for trait
Females can be homozygous or heterozygous
- AA, Aa, aa
- just like autosomal traits
What is sex linkage?
trait on the X chromosome
Male will have the trait or not
- know male genotype from his phenotype
Females can be carriers so you don't know genotype from phenotype
What is the difference between reciprocal crosses and test crosses?
Test crosses are used to determine genotype of parent
- offspring x homozygous recessive
Reciprocal crosses are used to determine if a trait is sex linked
- you'll get different results from crosses if sex linked than if autosomal
How can you determine if a trait is sex linked?
if you get males of one phenotype and females of another then the likely explanation is that the trait is sex linked
rr x RY can show that this trait is sex linked
r Rr rY Red female and White male
RR x rY doesn't necessarily show trait is sex linked (do both) b/c its just like autosomal trait (cross Homozygous Dominant x recessive)
R Rr RY Red female and Red male
For drosophilia eyes, what tare the results of this cross?
Rr x RY
R RR RY
r Rr rY
For sex linked traits keep sexes separate
- so 2:1:1 ratio
- 2 red females, 1 red male, 1 white male
- 3 phenotypic classes so 2 degrees of freedom
For drosophilia eyes, what tare the results of this cross?
Rr x rY
R Rr RY
r rr rY
- 1 red female, 1 white female, 1 red male, 1 white male
- 3 degrees of freedom
What kinds of traits show up in males more than females?
Sex linked recessive traits
What are the possible genotypes for hemophilia? How is it normally passed on?
- HH: normal, non carrier
- Hh: normal, carrier
- hh: hemophiliac
- HY: normal
- hY: hemophiliac
Normally passed from carrier mother to son
How does a female get a sex linked recessive trait?
The father MUST have the trait and the mother must either have it or be a carrier
That's why a sex linked recessive trait usually shows up in males more often
If father has the trait then the daughter will at least be a carrier (has to get his X chromosome)
Does the father have to have the trait in order for a son to receive a sex linked recessive trait like hemophilia?
H HH HY
h Hh hY
Can a sex linked trait be passed from father to son?
No, father is passing on his Y chromosome to his son
- not his X chromosome
If you're looking at a pedigree, how can you rule out sex linkage?
If you see a girl with the trait but her dad doesn't have it then the trait must be autosomal
For hemophilia you cross a carrier mom and normal father.
-What is the chance the offspring has hemophilia?
-What is the chance you have a girl with hemophilia?
- What is the chance you have a boy with hemophilia?
H HH HY
h Hh hY
1/4 chance offspring has hemophilia
0% chance girl w/ hemophilia
1/2 chance boy w/ hemophilia
Do traits skip generations?
They appear to skip generations when passed to carrier daughters
- can reappear next generation from carrier mom
What are X linked dominants and Y linked traits?
X linked dominants tend to be more prevalent in females
- opposite of sex linked recessive traits which show up more in males
Y linked traits only occur in males
- passed from father to each son
What is Dosage Compensation and X Inactivation?
Natures way of being fair
- females can be homozygous or heterozygous
- men can only be hemizygous
Females don't have twice the number of active X-linked genes as males
- mechanism so that one of the X chromosomes inactivates (condenses) and becomes a barr body
- if someone has more that 1 X chromosome then the extra X condenses into barr body
How many barr bodies does a female have? How many does a male have?
Female has 1 barr body
Male has 0 barr bodies
Is the barr body completely inactive? How do you know?
It is partially active
know this because in ppl who have an extra X chromosome that turns into barr body there is health issues
function best w/ just one copy of genes on X chromosome
What are some dosage compensation and X inactivation anomalies?
XO - Turner syndrome
- female w/ no Barr body
XXX - female
- 2 barr bodies
XXY - Klinefelter syndrome
- male w/ 1 barr body
What is the Lyon hypothesis? What impact does this have on homo and heterozygotes?
Which X inactivates to become a Barr body is random
- occurs early in embryonic development
- all descendents of that cell (mitotic copies) have same X chromosome inactivated
Homozygotes have no impact
- AA -> AO or OA
- aa -> aO or Oa
- same no matter which inactivates
- Aa -> AO or Oa
- end up w/ patches of cells w/ 1 X inactivated and others patches w/ the other X inactivated
What is mosaicism?
patches of cells w/ different phenotypes
heterozygote female has the different alleles inactivate during embryonic development and then replicate
What is genic interaction?
interactions between more than 1 gene to determine phenotype
How does dominance influence allelic actions?
The dominant allele will USUALLY code for production of a functional protein
- eg. P => purple pigment (mendels pea plant example)
The recessive allele does not code for a functional protein
- eg. p => no pigment
hemophilia - dominant allele gives you clotting factor and the recessive allele gives you nothing
If you have two alleles and its not a simple case of one working (dominant) and the other not (recessive), what occurs?
Incomplete dominance: phenotype is visible
Co-Dominance: detect phenotype any way other than visiually
Both alleles code for different but FUNCTIONAL protein
- heterozygote shows phenotype different from either homozygote
If carnations are incomplete dominance, R codes for red pigment and W codes for white pigment, what would RW be? Could you continuously produce this color offspring through crosses?
RW = Red+White = Pink flower
Can't continuously produce pink flowers b/c
R RR RW
W RW WW
1/2 are pink, 1/4 are red and 1/4 are white 1:2:1 ratio
What is a way to tell the difference between incomplete and co-dominance other than visual/non-visual?
If heterozygote shows a BLEND then alleles are incompletely dominant
If heterozygote shows BOTH then alleles are co-dominant
Bothe alleles are functioning!
What impact does having multiple alleles (greater than 2) have?
The individual person can only have 2 alleles at most for a gene at a particular locus (at most heterozygous)
However, multiple alleles is a population level phenomenon
- there may be 3 or more alleles in the population
If you have 3 alleles in the population, A B C, how many genotypes can you have?
- AA BB CC AB AC BC
For ABO Blood grouping, is this co-dominance or incomplete dominance?
These 3 alleles have Co-dominance and a dominant recessive relationship
- A is co-dominant to B but both are dominant to O
- O is non-functional (doesn't code for surface antigens)
- since O is not making anything it's recessive
Somebody has an AO genotype (ABO Blood grouping) what will their blood type be? What would be the results of the following cross AB x AO?
AO genotype = Type A blood
A AA AO
B AB BO
- 2 Type A, 1 type AB, 1 type B
C^+ is agouti, C^ch is chinchilla (dark grey), C^h is himalayan (white w/ black tips), C^a is albino
C^+ is dominant to all others
C^ch is incompletely dominant to C^h and C^a
C^h is dominant to C^a
C^+ and anything else => ?
C^ch C^ch => ?
C^ch C^a => ?
C^ch C^h => ?
C^h C^h or C^h C^a => ?
C^a C^a => ?
C^+ and anything else => agouti
C^ch C^ch => Chinchilla
C^ch C^a => Light Grey Blend
C^ch C^h => Light Grey blend w/ dark tips
C^h C^h or C^h C^a => Himalayan
C^a C^a => Albino
Many Genotypes (24) but only 6 Phenotypes
What are lethal alleles?
Alleles that have an unrelated secondary effect causing death
- tend to be RECESSIVE and don't code for a protein necessary to live
- dominant to one allele but lethal recessively
When Cuenot was breeding his yellow haired mice, why didn't he ever produce true breeding (homozygous) yellow mice?
Every time he would breed yellow mice he would see a 2:1 ratio (1/3 would be agouti)
He realized when he backcrossed the yellow's w/ agouti that the yellow's were heterozygotes
- so he should have seen a 3:1 ratio when crossing 2 yelllow mice
The yellow allele is recessive lethal (but dominant to agouti)
- 1/4 of the mice were dying as embryos b/c lethal allele
Yellow (A^Y A^+) X Yellow (A^Y A^+)
A^Y A^y A^y A^y A^+
A^+ A^y A^+ A^+A^+
The A^y A^y were dying off embryonically and the surviving were either heterozygote yellow or agouti
What is achondroplasia?
Form of short limbed dwarfism
- mutation in gene that codes for growth factor receptor protein = can't take up protein needed to grow long bones
Dominant but recessive lethal allele
- Homozygous = lethal
- Heterozygous = healthy but dwarf phenotypically
What are the results of 2 individuals w/ achondroplasia?
Dwarf (a^A a^+) X Dwarf (a^A a^+)
a^A a^A a^A a^A a^+
a^+ a^+ a^A a^+ a^+
1/4 die as embryo (a^A a^A)
2:1 Ratio Dwarf to normal
What are genic interactions w/ multiple alles?
Two or more genes acting on one trait
- gives you novel phenotypes
- eg AB = Blue, Ab = pink, ab = black, aB = Red
- epistasis: one gene can mask another gene
Crossed a Red eyed X White eyed fruit flies (drosophilia).
P: Red Eyed X White Eyed
F1: Red Eyed
- Cross Red Eyed X Red Eyed
F2: 9 Red Eyed, 3 Brown Eyed, 3 Scarlet Eyed, 1 White Eye
What is going on?
You would have expected to get a 3:1 ratio of Red Eyed to White Eyed
What we got is a Dihybrid ratio (9:3:3:1) indicating that there is a genic interaction of 2 or more genes acting on single trait to give you novel phenotypes (scarlet and brown colors came out of nowhere)
How are the new eye colors created in drosophilia?
Genic interaction of multiple genes acting on single trait (eye color) yielding novel phenotypes
S allele controls production of brown pigment
B allele control production of Scarlet pigment
- 2 independent pathways to make an enzyme that converts a compound into the colored pigment
CPD Y => Enzyme S => Brown
CPDX => Enzyme B => Scarlet
Brown Pigment + Scarlet Pigment => Red Pigment
- can have allele that doesn't create any enzyme = white
With incomplete or co-dominance what is the maximum number of phenotypes you can have with a monohybrid?
Monohybrid max is 3 phenotypes (AA, Ab, bb)
- 1 locus
Dihybrid max is 4 phenotypes (A_B_, A_bb, aaB_, aabb)
- 2 loci
What is epistasis?
One gene masks a second gene
Each gene codes for an enzyme critical in a step in a linear metabolic pathway
Any gene that acts earlier in a pathway will be epistatic
- it will mask the genes that come later in the pathway
Get weird ratios