Mendelian Genetics- lecture #16 & 17 Flashcards
why did mendel use garden peas for his experiments?
many different versions avaliable
short generation times
produce large number of offspring
self fertilizing (each flower has male and female parts)
why didn’t Mendel breed with humans?
takes a long time to reproduce
infrequent to have more than 1 offspring
we have so many traits from ancestors…
what are characters?
varied heritable features
gene for something
what are examples of characters?
flower color, hair color, eye color
what are traits?
variant of a character
we get 2 traits inherited
what examples of traits?
purple flowers and white flowers
brown eyes vs blue eyes
brown hair vs blonde hair
how is a pea plant self-fertilizing? what parts makes it self fertilizing?
egg bearing carpel
pollen producing stamen
what allowed Mendel to control gametes used in fertilization?
removed the immature stamen
dusted the carpel with pollen taken from a different stamen
used true breeding individuals
what is a true breeding individual?
produce the parent variety only (all future generations will be of the same color as the parent only)
PP or pp
what is hybridization?
cross of two different true breeding plants
the two true breeding individuals form what generation?
parent generation
P gen
hybrids produced from the P gen become what generation?
F1 generation
not true breeding
what comprises the F2 generation?
two members from the F1 generation produce F2 generation
what is the law of segregation?
blending doesnt happen, flower is either purple or white, not light purple
if theres a cross between a white flower and a purple why might the next generation be mostly purple?
because its dominant (white is still in the genes of the next generation though, even if not expressed)
what were mendels 4 key observations?
- alternate versions of genes account for variation in inherited characteristics
- an organism will have two copies of each character
- if two alleles at a locus differ from one another, the dominant allele will determine the organism’s appearance
- law of segregation
what are alternate versions of a character called?
allele
DNA at the same locus of homologous pairs may exhibit what?
slight variation in nucleotide sequence
what does this variation in nucleotide sequence give rise to?
different allele of the same character
many alternate alleles are possible
- an organism will have two copies of each character, what does that mean?
one gene copy will originate from the father and one will originate from the mother
what does homozygous mean?
identical alleles of a gene
heterozygous
two copies different from one another, what is that called
why did the white pea plant ‘disappear’ in the F1 generation?
recessive allele
what is the process to determine which gametes are used in fertilization?
two alleles separate during meiosis
end up in separate gametes
if the two alleles are the same, all 4 gametes produced will have the same allele for the gene (all brown eyes)
if the two alleles are different then 2 gametes will have the dominant allele (brown eyes) and two will have the recessive allele (blue eyes)
what is homozygous dominant and recessive?
dominant: PP
recessive: pp
are heterozygotes true breeding?
no, Pp
what is a phenotype?
observable traits
the flower is purple
what is a genotype?
genetic makeup
the flower is purple but its genotype is 1:2:1 therefore there is white in its genes
in order to determine an unknown genotype what can we do?
test cross the unknown with a homozygous recessive organism
why do we test cross with pp?
we know the outcome of homozygous recessive
if there are white plants in the F1 generation then the unknown must be Pp
what do monohybrid crosses analyze?
one trait at a time
Gg x Gg
4 possibilities
what do dihybrid crosses analyze?
two traits simultaneously
YyRr x YyRr
16 possibilities
what is dependent assortment?
alleles of each trait are passed along together
what is independent assortment?
alleles for each trait are passed along as individual units
if the alleles are passed along together than the organisms produced in the F1 generation will form ___ and ___ gametes
YR and yr
4 possible gamete combinations
if the alleles are separated independently during gamete formation than the F1 generation will produce ___, ___, ___ and ___ gametes
YR, Yr, yR and yr
16 possible gamete combinations
mendel examined the offspring and found that the F2 generation had what phenotypic ratio?
9:3:3:1
9 yellow, round plants (YYRR, YYRr, YyRR, YyRr, YYRr, YyRR, YyRr, YyRr, YyRr)
3 yellow, wrinkled plants (YYrr, Yyrr & Yrr)
3 green, round plants (yyRR, yyRr & yyRr)
1 green, wrinkled plant (yyrr)
what is the law of independent assortment?
each pair of alleles will segregate independently of each other allele during gamete formation
what is the law of independent assortment?
each pair of alleles will segregate independently of each other allele during gamete formation
what is complete dominance?
dominant allele has the same phenotypic effect whether it is present in one or two copies
Mendel’s Law only explains ________ __________
complete dominance
Mendels laws fail to explain ______________ ____________
incomplete dominance
F1 hybrids mating a homozygous dominant and a homozygous recessive produce what?
heterozygotes
Bb
with an intermediate phenotype
what is the blending hypothesis?
mating heterozygotes would never again lead to the appearance of red or white
it results in 1 red: 2 pink: 1 white
why might pink be shown in the offspring of red and white parents?
blending hypothesis
most genes in the population have how many alleles?
many
how many alleles exist for the blood type gene?
I^A, I^B, i
what are the 4 different blood types from the possible allele combinations?
Type A blood: I^A I^A or IA i
Type B blood: I^B I^B or IB i
Type AB blood: I^A I^B
Type O blood: ii
how can I^A and I^B alleles both be expressed?
co-dominant
what can the A blood type recieve?
A and O blood
what can the B blood type recieve?
B and O blood
what can the AB blood type recieve?
A, B and O (the universal recipient)
what can the O blood type recieve?
only O (but is the universal donor
what is epistasis?
phenotypic expression of a gene at one locus, affects phenotypic expression of a gene at a second locus
what is an example of epistasis?
E deposits black or brown pigment
ee deposits no pigment regardless if the B was B or b
9:3:4
what is the phenomenon called pleiotropy?
many genes affect more than one phenotype
what is an example of pleiotropy?
sickle-cell disease in humans
what does sickle- cell disease do?
produces abnormal hemoglobin
causes red blood cells to change shape becoming sickle-cell in shape
the sickle-cells are quickly destroyed by the body, leads to what?
anemia and body weakness
because we’re losing o2 delivery
what does the deformed shape of the red blood cells do? what does this result in? what side effects?
creates blockages
results in fever and pain
how many people does sickle-cell disease kill worldwide?
100,000 per annum
sickle-cell disease only results in those that are what for the allele/trait?
homozygous
what happens if someone is heterozygotes and they have sickle-cells?
have less sickle-cells, nothing the body cant handle
normal and sickle-cell allele are both expressed
what is the spectrum of possibility for certain traits a consequence of?
polygenic inheritance
what is polygenic inheritance?
the added effect of two or more genes on a single character
what is polygenic inheritance the reverse of?
pleiotropy
what type of punnett square would we use for skin color?
dihybrid cross + 3rd trait
trihybrid punnett square
what are many genetic factors (including skin color) affected by?
the environment
what is an example of how our environment has an effect on our skin color?
sun exposure (tanning)
what is another example of a trait that results from a combination of heredity and the environment?
effect of experience on intelligence
nutrition on height
what is the special position of Mendelian genes on their chromosomes?
loci
chromosomes exhibit what type of assortment?
independent
chromosomes undergo what?
segregation (separation)
what happens to meiotic behavior of homologous chromosomes?
accounts for the segregation (separation) of alleles at the same locus in order to form gametes
what happens to meiotic behavior of non-homologous chromosomes?
account for independent assortment of alleles for 2 or more genes on different chromosomes
what did Thomas Morgan provide evidence to?
associate a particular gene with a specific chromosome
what did Thomas Morgan study?
Drosophila melanogaster (type of fruit fly)
what is a Drosophila melanogaster (fruit fly)? what does it eat?
fruit fly that is a fungi-eating insect
Why did Morgan decide to use fruit flies?
produces hundreds of offspring
reproduces fast (2 weeks)
3 autosomal, 1 sex pair (XX and XY)
how many autosomal cells do humans have? how many sex cells do humans have?
22 and one sex cell
what did Morgan initially discover?
male flies that had white eyes
usually flies had red eyes
most commonly occuring phenotype is the wild type, what does an alteration of the wild type produce?
mutants
if you mate a white eyed male with a red eyed female what was found in the F1 generation?
all have red eyes
if all the F1 generation had red eyes, what is the main takeaway?
allele behavior correlates with behavior of the chromosomal pair
A cross of the two F1 offspring produces the typical 3:1 ratio, which gender of the F2 offspring had white eyes?
male
all F2 females had red eyes
why were females unable to have white eyes? why were males only able to get white eyes?
eye color of the flies were linked to sex
can males carrying a recessive alle mask it by a dominant allele?
no, because they only have one X, therefore they can only be X^R or X^r, they cant be heterozygous (mask)
how many possibilities do females have to determine eye color?
X^R X^R and X^R X^r: red eyes
X^r X^r: white eyes
therefore, eye color is determined entirely by the chromosome provided by the mother
how many genes does the Y chromosome have? how many proteins do they encode?
78 genes, encode 25 different proteins
(78 is not many compared to X)
what are the proteins encoded by the genes on the Y chromosome necessary for?
normal testes development
creates differences in offspring
what is the SRY?
sex determining region (gene located on the Y chromosome)
What does the SRY trigger?
the development of the testes
determines the sex of fertilized egg
what happens when the SRY gene is absent?
ovaries develop XX
grasshoppers and some other insects have an X-O system, what does the O symbolize?
the lack of a second sex chromosome
females: XX
males:XO
what gamete determines the sex of the offspring in grasshoppers?
male gamete
(same as in humans)
what animals have sex determined by the female?
some fish
butterflies
birds
what is the genotype for birds sex chromosome? how is sex determined by the female?
male: ZZ
females: ZW
sex is determined according to whether the egg carries a Z or a W
what organisms do not have sex chromosomes?
bees and ants
how is the sex determined in organisms that dont have sex chromosomes?
depends on chromosome number
males develop from unfertilized eggs (haploid) (16 chromosomes)
females develop from fertilized eggs (diploid) (32 chromosomes)
any gene on a sex chromosome is called what?
sex-linked gene
what is an example of a trait that is carried on the sex chromosome (X chromosome)?
eye color
how many genotypes do females have?
3
how many genotypes do males have?
2
what is Rr an example of?
heterozygous
a carrier
why does the man (X^rY) pass r to his daughter and not his son?
man doesnt give the X chromosome to son so it would only go to daughter
mating between X^R X^R and X^rY will produce what offspring in females, and what offspring in males?
females: all heterozygous X^R X^r (Rr)
males will all have red eyes X^R Y (RY)
how might a white eyed female occur?
if the mother has white eyes
is a heterozygous carrier and the father has white eyes
why do sex-linked disorders mostly affect the male population?
because males are hemizygous
if a male receives an affected X chromosome for a particular trait than he will express that allele and be affected
what does hemizygous mean?
neither homozygous or heterozygous because the X and the Y chromosome are non-identical
why are females not normally affected by sex-linked disorders?
because women recieve XX
if one is affected, the other X might not be and she will not express the disorder because shes just a carrier Rr
both XX would need to be recessive in order to show the disorder rr
what are examples of sex-linked traits?
hemophilia
red-green colorblindness
Duchenne muscular distrophy
what is hemophilia?
blood clotting (genetic based)
what is duchenne muscular distrophy?
problem with muscle development and muscle strength
each human female contains XX chromosomes, are both these chromosomes active?
only one chromosome is active
in what form does the other X copy exist? what is this state called?
supercoiled almost totally inactive state called a Barr Body
why is one X chromosome inactive?
occurs to prevent different amounts of protein production in males and females
when does inactivatio occur?
randomly, very early in the stages of embryonic development
what does each embryonic cell consist of?
one X chromosome
becomes a Barr body at random
which X chromosome is activated? one one from the mother or father?
can be either
on a cat black fur is from father, yellow fur from mom
when black fur is active it is shown (from father) yellow fur X chromosome is condensed
when yellow fur is active it is shown (from mother) black fur X chromosome is condensed
in a heterozygous female for an X-linked gene what allele is turned on/off?
one allele turned on in some cells
other cells will have the other allele turned on
how do you tell if genes are inherited together?
genes that are located near to one another on the same chromosome are inherited together
when genes are located far apart from each other what does that mean?
they will not be inherited together
what does the cross of two heterozygotes result in for the F2 offspring? phenotypic ratio
Pp x Pp (P= purple, p= red)
3 purple
1 red
what does the cross of two heterozygotes result in for the F2 offspring? phenotypic ratio
Ll x Ll (L=long, l=short)
3 long
1 short
when the data from the two characters are combined what is the expected ratio?
9:3:3:1 (independent assortment)
what was the observed ratio of the purple, red, long and short in %?
75% of plants were purple and long
14% of plants were red and round
what does linked genes refer to?
each chromosome carries thousands of genes, those that are located near to one another tend to be inherited together
do linked genes follow Mendels law of independent assortment?
no
meiosis of unlinked genes will produce an equal amount of how many possible genotypes?
RR, Rr, rR, rr
what does crossing over give rise to? what does it lead to production of?
new allele combinations
leads to production of 4 different alleles
what happened when Thomas Morgan mated a wild-type fruit fly (gray body (G) and long wings (L)) with a (black fly (g) with undeveloped wings (l))
unlinked genes, a mating of Gg Ll and gg ll would produce equal amounts of all four genotypes
because the genes are linked what happened to the bulk of the offspring?
parental phenotypes
17% were recombinant phenotypes
what is recombinao?tion frequency equal t
percentage of recombinants produced
what were the observed phenotypes a consequence of?
crossing over which unlinked linked alleles
what did the new combinations involved in fertilization give rise to?
recombinant offspring
what was hypothesized by one of Morgan’s students?
that crossing over is equally likely at all points on a chromosome
how did the hypothesis that ‘the further apart two genes are located on a chromosome the greater the likelihood that crossing over will occur between them’ arise?
arose because the increased distance allowed a greater number of possibilities for crossing over to occur
allows more flexibility
recombinant data from fruit flies was then used to map out the location of various genes, what is this called?
linkage map
what is the distance between genes measured in?
map units
what is 1 map unit equal to in percent recombination frequency?
1 map unit = 1% recombination frequency
B+C= 5%
A+D= 4%
D+B= 3%
A+B= 7%
what is the order of these genes on the chromosome?
what is the recombination frequency of A+C?
A—4—-D—3—B—5—C
A——-7———-B
A————12————-C
order of the genes on the chromosome is A-D-B-C
the recombination frequency is 12
picture of this in downloads
is the primary hypothesis that recombination frequency should be higher between genes that are farthest apart from one another true?
yes, in the example the recombination frequency is 12 and is therefore the highest
is crossing over equally likely at all points on a chromosome?
no
