Ch.2 Mendelian Genetics Flashcards
What is Lamarckism?
a theory of evolution based on the principle that psychical changes in organisms during their lifetime (such as greater development of an organ or a part through increases use) could be transmitted to their offspring
T/F All traits are passed on to the next generation.
FALSE. Only genetic traits (genetically determined traits) are passed on.
T/F Traits which are acquired during lifetime are passed on.
FALSE. Physical changes like the giraffe are not passed on.
Blending hypothesis (1800s)
Genetic material from 2 parents blends together like blending paint colors.
What could the blending hypothesis not explain?
Cannot explain how traits can skip generations
Gregor Mendel (Father of genetics)
Studied inherited in pea plants
1st to use an experimental approach and the scientific method to study patterns of inheritance
Why did Mendel use garden peas? (5 things)
Easy to grow Many readily distinguishable varieties Short generation time Large number of offspring Mating could be controlled; plants could be allowed to self fertilize or could be cross fertilized
What were the 7 visible traits Mendel experimented with?
Shape of seed (round or wrinkled) Color of seed (yellow or green) Color of petals (purple or white) Shape of pods (inflated or pinched) Color of pods (green or yellow) Flowers (axial or terminal) Stem length (long or short)
Phenotype
physical appearance
True breeding
usually pea plants have a system enforcing self fertilization
Female and male gametes from the same flower unite and produce seeds: inbreeding
How much variation results from true breeding?
Results in little, if any, genetic variation from one generation to the next
Cross fertilization (cross-pollination)
preventing inbreeding, artificially crossing 2 individuals Transfer antlers (male) from 1 plant onto other plant that has had its anthers removed
Reason’s for Mendel’s success
Chose a good model system (pea plants)
Restricted his experiments to one or very few pairs of contrasting traits
He kept note books with accurate quantitative records
Analyzed the collected data
Derived his postulates based on the interpretation of his data
Monohybrid Cross
crossing pea plants which different in 1 character (plant height) with 2 distinct visible traits (tall and dwarf)
Observations of monohybrid cross
Parent generation- homo dominant and homo recessive
F1 - dwarf trait disappears, all F1 are tall
F1 generation self fertilized and the two tall plants could have a dwarf offspring in the F2 gen
What was the ration that traits appeared in in the monohybrid cross?
3:1
Mendel’s Postulates
1. Unit factors exist in pairs
Genetic characters are controlled by unit factors existing in pairs in individual organisms
Each diploid individual receives one factor from each parent
Bc the factors occur in pairs, 3 combos are possible: 2 factors for dwarf, 2 for tall, or one of each
Every individual possesses 1 of these 3 combos, which determines its stem height
Mendel’s Postulates
2. Dominance/ Recessiveness
When 2 unlike unit factors responsible for a single character are present in a single individual, one unit factor is dominant to the other, which is said to be recessive Latent traits (dwarf): recessive allele Trait which get expressed (tall): dominant allele The trait expressed is controlled by the dominant factor. The trait not expressed is controlled by the recessive factor.
Mendel’s Postulates
3. Segregation
During the formation of gametes, the paired unit factors separate randomly so that each gamete receives 1 or the other with equal likelihood.
If an individual has a pair of like factors (both tall) all gametes will have the same kind of unit factor.
If an individual has a pair of unlike unit factors (tall and dwarf), each gamete has a 50% chance of receiving either one.
Unit factors
Chromosomes (the units of inheritance)
Character
gene (a heritable feature, such as stem length) the instruction for a trait, located on a chromosome, at a specific site (gene locus)
Trait
a variant of a character (tall and dwarf) an allele, a version of a gene
Phenotype
physical expression of a trait (tall, blue eyes)
Genotype
genetic makeup of an individual; alleles written in pairs (DD, Dd, or dd)
Homozygous
both alleles are the same (DD, dd)
Homozygous dominant
DD
Homozygous recessive
dd
Heterozygous
the two alleles are different (Dd)
Dihybrid Cross
a cross involving 2 pairs of contrasting pairs (seed shape and seed color)
What is the phenotypic ratio of the F2 generation for a dihybrid cross?
9:3:3:1
Mendel's Postulates #4 Independent Assortment
During gamete formation, segregating pairs of unit factors assort independently of each other.
Each gamete receives one member of every pair of unit factors.
This means the 2 different characters are inherited independently.
Walter Flemming 1879
discovers chromosomes in the cell nuclei of salamander cells
Sutton and Boveri 1902
Chromosomal Theory of inheritance: genetic material in living organisms is contained in chromosomes
Movements and behavior of chromosomes during meiosis explains what??
Mendel’s principles of segregation and independent assortment
Diploid number (2n)
The specific number of chromosomes in each somatic cell nucleus of a diploid species.
During formation of gametes via meiosis the chromosome number is precisely halved to haploid (n).
When 2 gametes combine during fertilization, the diploid number is reestablished
Rule of Segregation and chromosomes
Homologous chromosomes segregate during anaphase 1 (pairs seperate)
Rule of Independent Assortment and chromosomes
chromosomes assort independently in gamete formation
Separation of sister chromatids in anaphase II
Mendel’s rules can be used to?
Predict the outcomes of crosses
Wild-type traits
Traits prevailing in nature. Not necessarily specified by dominant alleles.
(Dominant alleles are not necessarily more common. Dominant alleles are not necessarily “better” than recessive alleles.)
Punnett squares
Reginald Punnett
Shows the possible gametes and allele combinations among offspring
You can think of the outcome of a cross as a?
Probability
Tall: dwarf =3:1
Means there is a 3/4 chance (75% chance) or a probability of .75 that an offspring will be tall
T/F In a Punnett square each box has the same probability
TRUE. Monohybrid cross each box is 1/4. Dihybrid cross each box is 1/16
Alternative to dihybrid cross
you can make 2 Punnett squares and multiply the 2 probabilities
Forked line method
for an intercross (cross between 2 F1 hybrids)
Trihybrid cross; can be considered as 3 separate crosses
3:1
Combine the separate rations into overall ratios of phenotypes by multipications
What is the probability of an event?
The frequency of that event in the sample space (the possible combinations)
Sample space = the collection of all events
Independent events
one event provides no info on the other (or the outcome of one event has no influence on the outcome of another event)
EX: if you draw an ace from a deck you have no clue about the card’s suit
Joint occurrence
2 events happen at the same time.
EX: drawing the ace of hearts represents the joint occurrence of 2 events (ace and heart)
If 2 genes are located on 2 different chromosomes, how do they segregate?
They segregate independently of each other during meiosis
EX: Seed shape and seed color in peas: being round and green represents the joint occurrence of 2 events
The multiplicative rule
If the events A and B are independent, the probability that they will occur together, denoted P(A and B), is P(A) x P(B)
What is the probability of drawing the ace of hearts?
Probability of drawing an ace = 4/52
Probability of drawing heart =1/4
4/52 x 1/4 = 1/52
AaBbCcDd x AaBbCcDd
What is the probability of an offspring being aabbccDd?
aa = 1/4 bb= 1/4 cc= 1/4 Dd= 1/2 1/4 x 1/4 x 1/4 x 1/2 = 0.0078
Mutually exclusive
if 2 events do not overlap in the sample space
Additive Rule
P (C or D) = P(C)+ P(D)
Probability of drawing an ace OR a king
P(A) = 4/52
P (K) = 4/52
4/52 + 4/52 = 8/52
Multiplication and Addition Rule tricks
if it say AND then you multiply
if it says OR then you add
Binomial expansion
used when the outcome falls into 2 classes (affected/unaffected, boy/girl)
(p+q)^n with n = size of the group and p+q=1
Factorials
Can be used if you only want to know the outcome of a certain combination
x=one class of events
y=other class of events
[n!/x!y!]p^xq^x
When calculating probability of a certain scenario or outcome…
keep in mind that each birth is an independent event- the fact that the first child is affected, makes it not more or less likely that the second one will be too
Family pedigrees can be used to identify what?
Carriers - carries an allele (prob w/out knowing) not affected but can pass on allele
In a simple dominant-recessive inheritance of dominant A and recessive a, a recessive phenotype always results from?
a homozygous recessive genotype (aa)
In a simple dominant-recessive inheritance of dominant A and recessive a, a dominant phenotype can result from?
Either the homozygous dominant genotype (AA) or a heterozygous genotype (Aa)
Why does it matter if a person is homozygous dominant or heterozygous when dealing with family pedigrees?
Parent AA: 100% chance that a child inherits A
Parent Aa: 50% change that a child inherits A
Inheritance of a dominant trait
Does every individual who carries the dominant allele manifest the trait?
Yes
Inheritance of a dominant trait
Every affected individual (AA or Aa) is expected to have?
At least one affected parent (AA or Aa)
These means every generation has at least 1 person that shows the trait.
If two parents are affected but their child is not, what does these mean about the parents’ genotype?
The parents must be both heterozygous
Inheritance of a Recessive Trait
An affected individual (aa) has what kind of parents?
Unaffected (Aa x Aa)
Inheritance of a Recessive Trait
Can the trait “skip” a generation?
Yes
Inheritance of a Recessive Trait
What is the risk of affected children?
25%
Inheritance of a Recessive Trait
What percentage of unaffected children can be carriers?
2/3
Inheritance of a Recessive Trait
Carriers
persons with the recessive allele but unaffected phenotype. They can pass on the allele.
Difficulties genetic counselors face: Nonpolypoid Colorectal Cancer
Type of hereditary cancer appears at median age of 42
Missing info - can’t get all info
Individuals passed but cause is unknown (can’t know if it was from cancer or not)
Individuals could pass at younger age, before onset of disease
Not every individual has children
Things to consider
Births of children are treated as independent events (the outcome of one event has no influence on the other)
Genetic counseling is performed on various levels (individual, couple - pre and post conception counseling and testing)
We can predict the outcome of genetic crosses as probabilities or chances - nothing more.
Genetics is not as straightforward as Mendel proposed it
- If a plant has the genotype TtRRyy, what fraction of the gametes will have the genotype T-R-yy?
1/2
- The _______ method is a straight forward, simple method for figuring out crosses by combining the gametes systematically to diagram the genotypes of offspring.
Punnett square
- Mendel used true breeding plants to perform the first generation crosses. We now know these true breeding plants can be described with the genotype of?
heterozygous?
homozygous?
- When an allele is masked by another allele, the masked allele is described by the word?
Recessive
- Two pea plants both with round seeds are crossed. Round is dominant over wrinkled. You analyze the offspring from the 1st 2 generations of this cross. All the offspring in every case have round seeds. What do you know about the genotypes of these parents?
they are both homozygous dominant
- What is the definition of phenotype?
the observable characteristics of an organisms
- According to Mendel’s law of segregation
Allele pairs separate in gametic formation
9.What would you use to determine the probability of either drawing a heart or a club from a deck of cards?
Additive rule
- In analyzing a human pedigree, you find the trait to skip generations. Males and females are equally affected. An affected child will often both parents unaffected. The trait is most likely?
Recessive
- You play cards with your friends. If you want to calculate the probability of having all 4 aces on your hand, you would use?
Multiplication rule
- A sexually reproducing individual has 2 unlinked genes, one for head shape (H) and one for tail length (T). Its genotype is HhTt. Which combination is possible in a gamete from this organism?
HT
- In certain plants, the tall trait (T) is dominant to the short trait (t). If a heterozygous plant (Tt) is crossed with a homozygous tall plant (TT), what is the probability that the offspring will be short (tt)?
0
- In pedigrees with dominant inheritance patterns, it is typical that every affected individual has at least 1 affected parent.
TRUE
- Assuming no crossing over between the gene in question and the centromere, when do alleles segregate during meiosis?
Anaphase 1
- What is independent assortment?
During gamete formation, segregating pairs of unit factors assort independently of each other.
- A certain type of congenital deafness in humans is caused by a rare autosomal dominant gene. In a marriage between a deaf man and a deaf woman, could all the children have normal hearing?
Yes, assuming that the parents are heterozygous (because the gene is rare), it is possible that all the children have normal hearing.
- Which phase of meiosis is most directly related to the law of independent assortment?
metaphase 1
- Albinism, results from an autosomal recessive gene (a). Two parents with normal pigmentation have an albino girl. What is the probability that their next child will be an albino girl?
1/4 x 1/2 = 1/8
- Albinism, results from an autosomal recessive gene (a). Two parents with normal pigmentation have an albino girl. What is the probability that their next 3 children will be albino?
1/4 x 1/4 x 1/4 = 1/64
