Lecture 10 Flashcards
Genetics involves mitosis or meiosis? and why?
meiosis b/c it involves sexual reproduction
Finish the sentence: Genetics is a branch of biology that deals with the way biological characteristics are ________.
inherited
A human genome consists of what range of genes?
20 000 - 25 000
Humans are identical in (what %) of the sequences of their genes
99.9%
Where are genes located?
on the chromosomes
A gene contains info needed to produce what?
a specific protein
Finish the sentence:
Proteins are the (what) of the cell
the work-horses
They affect events within cells and ultimately what?
organisms
they can play a role in determining traits
like what?
eye color, height, hair texture, blood type, etc.
What is an allele?
varieties of a gene pair
What is a locus in genetics?
a specific place on a chromosome where a gene is located
What does it mean the genes don’t share the same locus?
= These genes are not allelic to
one another
(they are not at the same location on their corresponding chromosomes)
True or false: Alleles contain the same info
FALSE,
-> Alleles govern the same
character but do not
necessarily contain the same
information
What is a dominant allele?
an allele that masks the presence of another allele
What is a recessive allele?
an allele whose presence can be masked by a Dominant Allele
Which allele is always a uppercase letter?
Dominant Allele = uppercase (B)
Recessive Allele = lowercase (b)
Name all 3 possible combinations of alleles:
- homozygous dominant
- heterozygous
- homozygous recessive
In a situation where B = brown eyes and b = blue eyes
which combination is which which letters?
- homozygous dominant = BB
- heterozygous = Bb
- homozygous recessive = bb
What is the phenotype expressed if it’s homozygous dominant (BB)?
Dominant phenotype expressed
(+ 2 dominant alleles present)
What does it mean if Bb?
heterozygous
= different alleles + Dominant phenotype expressed
What does it mean if bb?
Homozygous recessive
= 2 recessive alleles +
Recessive phenotype expressed
Difference b/w a phenotype and a genotype?
Genotype = combination of alleles
Phenotype = appearance of the characteristic
So in a situation of flower color, if the genotype is PP (homozygous dominant) then what would be the phenotype?
P = purple
phenotype is purple (appearance of the characteristic)
True or false: Phenotype does
not always reveal genotype
TRUE
Recessively inherited disorders occur only in individuals __________________
in individuals homozygous (recessive) for the allele
– ex. ff, pp, bb, yy, etc.
What do we call a carrier of the recessive allele (heterozygote)?
Someone who displays the dominant phenotype but is heterozygous for a trait
Dominantly inherited disorders occur in individuals _____________ _____________
in individuals homozygous (dominant) or heterozygous for the dominant allele
– ex. FF or Ff; BB or Bb …
What would it mean for people with ff genotypes (homozygous recessive) in a dominantly inherited disorder?
they would be “normal” (no disorder)
(+basically the reverse of a recessive disorder)
Name the person who was credited with developing a formal explanation of how characteristics were passed from one generation to the next
Gregor Mendel (1822-1884)
he was an Austrian priest
= “father of modern genetics”
He mainly worked on what plant?
pea plants
Why did he choose characteristics like flower color?
b/¢ it’s easy to observe and quantify
Careful mathematical analysis of the results gave rise to WHAT?
to Mendel’s PRINCIPLES (Mendel’s Model)
His model can be divided into how many concepts?
4 concepts
Add info on this concept:
1) There are alternative forms of genes
called alleles
= units that determine heritable traits
Add info on this concept:
2) For each characteristic, an organism has two genes, one from each parent.
They could be the same or different alleles
Add info on this concept:
3) When the two genes of a pair are different alleles
one may be DOMINANT and the other may be RECESSIVE
Add info on this concept:
4) A sperm or egg carries only one allele for each inherited trait
because allele pairs separate (segregate) from each other during the production of gametes (Law of Segregation)
(+When sperm and egg unite at fertilization, each contributes its allele, restoring the paired condition in the offspring.)
To make just everything clear, what is the Law of Segregation?
two members of a pair of alleles separate during gamete formation
(remember 4 genetically different daughters)
So, when a sperm and egg unite and fertilize together, what happens?
each contributes its alleles; we then RESTORE the paired condition in the offspring
What was Mendel’s first experiment?
He took true-breeding plants (parents would produce offspring that would carry the SAME phenotype)
-> What would happen if a plant with purple flowers was artificially crossed with one with white flowers?
What are the 5 steps for his first experiment?
1) removed stamens from purple flower
2) transferred pollen from stamen of white flowers to carpel of purple flower
3) pollinated carpel matured into pod
4) planted seeds from pod
5) examined offspring
What was the result of this experiment?
ALL of the offspring had purple flowers!
What was the genotype?
Genotype =heterozygous
Pp (1 dominant + 1 recessive allele)
In the second experiment, the offspring crossed with each other, what was the result of this?
75% had Purple flowers
25% had white flowers
What does this mean for the genotype?
This means that P (purple genotype) is dominant over p (white genotype)
What are the 3 things Mendel concluded after this experiment?
1) The factor determining flower color occurred in 2 forms (white & purple)
2) if the 2 forms were present in the same plant from each parent,
the purple form was somehow Dominant over the white
3) The factors that controlled flower color somehow separated from each other when sex cells were formed
Despite separating, each factor retained its ability to affect flower color in the offspring, what important concept is this referencing?
the Law of Segregation
Did Mendel know about meiosis?
No, He did all this
mathematically
Remind the result of meiosis:
diploid (2N) organisms produce haploid (n) gametes
= Result is 4 sex cells each with 1 set of chromosomes
Why are we referencing the law of segregation?
B/C both alleles will always
end up in different gametes after meiosis
Difference b/w Monohybrid and Dihybrid crosses
Mono: focusing on ONE trait
Di: focusing on TWO traits
What matrix/concept can you relate to all of this? what does it show?
the Punnett Square (grid device); it shows the expected frequencies of genotypes
Think about it, what would be the result of Parent A: PP
and Parent B: pp
(genotype and phenotype)
100% Pp (genotype)= heterozygous
100% Purple (phenotype)
Think about it, what would be the result of Parent A: Pp
and Parent B: Pp
(genotype and phenotype)
Phenotype: 3 purple and 1 white (75% and 25%)
Genotype: PP homozygous purple
Pp heterozygous purple
pp homozygous white
25%:50%:25% (1:2:1)
Give me the probability of having a girl first
½
Give me the probability of having a girl second
½
Give me the probability of having a girl AND a girl
½ . ½ = ¼
Give me the probability of having a girl OR a boy
½ + ½ = 1
Something very important to know/understand about chance
Chance has no memory
What does it mean to say that Chance has no memory?
past events do not influence on the
probability of the occurrence of future events – events are independent
What did we learn so far from Mendel’s experiment?
He derived the Law of Segregation by performing breeding experiments in which he followed only a single character (such as
flower color)
= monohybrid cross
What is a dihybrid cross?
different alleles on 2 loci
Mendel identified the Law of WHAT by following 2 characters at the same time.
law of independent
assortment
For this, we need 2 pairs of alleles on homologous or nonhomologous chromosomes
NONhomologous chromosomes
TRUE OR FALSE:
Each pair of alleles is inherited codependently
FALSE: inherited independently
During gamete formation (meiosis), each pair of alleles __________________ (continue the sentence)
each pair of alleles segregates
independently of other pairs of
alleles
For the Law of Independent Assortment, what do you need? (the requirements)
organism with 2 pairs of chromosomes (2N = 4)
= 1 pair of LONG and 1 pair SHORT homologous chromosomes
What are the 2 ways the homologous pairs of chromosomes can align in Metaphase I?
1) Long Blue| Long Red
Short Blue |Short Red
2) Long Blue| Long Red
Short Red|Short Blue
Is the arrangement done randomly?
YES
Are both ways equal?
Yes, 2 equally probable arrangements of chromosomes at metaphase I
In this sequence: AaBBccDdEE
1) how many chromosomes are there?
2) identify which is homozygous? heterozygous…
5 chromosomes
Aa - Heterozygous
BB - Homozygous dominant
cc - Homozygous recessive
Dd Heterozygous
EE - Homozygous dominant
Because 3 genes are homozygous, only 2 genes are heterozygous and will make a difference in the genetic makeup; so what are the number of possible DIFFERENT gametes?
2^2 = 4
so what is the TOTAL number of possible gametes?
2^5 = 32
(and max # of actual gametes produced will always be 4)
What fraction of the offspring of parents each with the genotype Kk Ll Mm will be kk ll mm? (draw it out to make it clear)
Each character, K, L, and M, will assort independently. The chance of any specific homozygous
recessive appearing is 1/4. The chance of all three appearing
is therefore (1/4) (1/4) (1/4) or 1/64 (multiply together)
On a sheet of paper draw a grid for parents that are both RrYy
1) Shape of seeds
* R – smooth ; * r - wrinkled
2) Colour of seeds
* Y – yellow ; * y – green
x-axis: RY Ry rY ry
y-axis: RY Ry rY ry
On a sheet of paper draw a diagram where the parents are: BBSS and bbss
*Black color dominant over brown:
Black = BB or Bb, brown=bb
*Short hair is dominant over long:
Short =SS or Ss, long = ss
ALL F1 BbSs
What would happen/ expect ratio in a dihybrid (heterozygote) F2 cross if 2 alleles are on nonhomologous chromosomes
so both parents are BbSs
(draw a diagram)
9/16 = Black short-haired
3/16 = Black long-haired
3/16 = Brown short-haired
1/16 = Brown long-haired
George has phenylketonuria or PKU (a recessive trait) and is
myopic (a recessive trait). Marilyn is a carrier for both genes.
Identify ONLY your variables:
P = normal
pp = PKU
M = normal vision
mm = myopic
If George has phenylketonuria or PKU (a recessive trait), what would be his genotype?
If Marilyn is a carrier for both what would her genotype be?
George: ppmm
Marilyn: PpMm
What is the probability that George and Marilyn will have a son who is myopic but does not have PKU? *Before answering this, what are the possible genotypes the son would have?
Myopic + 🚫PKU
PPmm
Ppmm
Which one is impossible to have (PPmm OR Ppmm) considering the parents’ genotypes (ppmm, Ppmm)
George is ppmm, so the only possibility is to look for Ppmm
b/c George only has small p’s to give
Now what are the 2/3 main things we are looking for?
son AND myopia / no PKU
Let’s break down the math to get the probability of Ppmm:
1) what is the probability of having a son?
2) probability of Ppmm?
1) ½
2) 4/16 = ¼
So now, the probability of a son with Ppmm is equal to…
½ . ¼ = 1/8 = 0.125
Let’s do a summary of what we learned on Mendel’s laws, what are the 2 laws we discussed?
1) the Law of Segregation
2) the Law of Independent Assortment
Which one does monohybrid crosses and which one does dihybrid crosses?
Law of Segregation: MONO (dom/rec. alleles)
LIA: DI (2 separate genes)
For the law of Segregation, during gamete formation (meiosis), 2 alleles separate b/c…..
b/c homologous chromosomes move toward opposite ends of cell
For the law of Ind. Assortment, each chromosome is inherited __________
inherited independent of the other chromosomes
So in general what is the main difference b/w the law of segregation and ind. assortment?
The law of segregation describes how alleles of a gene are segregated into two gametes and reunite after fertilization. The law of independent assortment describes how alleles of different genes independently segregate from each other during the formation of gametes.
