Lecture 9: Mendel, Genes, and Inheritance Flashcards
final focus
- rbc in sickle-cell disease
SICKLE CELL ANEMIA
- non flexible shape making it hard to move through capillaries
- mutated gene that codes for hemoglobin so it doesn’t form O2 properly, making it difficult to deliver to cells
important bc we need o2 as final e acceptor for atp
= EXAMPLE OF INCOMPLETE DOMINANCE TOO
- even w/ a heterozygote (where recessive is the mutated), you can still take on a milder form of the disease
What is blending theory of inheritance
- hereditary traits blend evenly in offspring through mixing of parents blood
what does blending theory of inheritance NOT explain
- extremes do not gradually disappear
(sometimes offspring has traits that’s EXACTLY like 1 parent, brown eyes for ex even if the other parent has blue eyes) - offspring sometimes have traits that differ from both parents (or its not a blend, where blue+brown eyes=brown eyes NOT A BLEND OF COLOURS)
= proven false
who is Gregor Mendel
founder of genetics
-first to use scientific method to study inheritance
Mendel’s hypotheses
- 4 total, and he used experimental results to support 2 principles
1) principle of segregation
2) principle of independent assortment
Why did Mendel choose the garden pea
- easy to grow (reproduce fast)
- clearly defined characteristics (colour)
- variation in chatacter-TRAITS
characters are passed onto offspring as
genes (discrete hereditary factors)
-determine the traits for characteristics you have
true breeding garden peas
true breeding varieties
-self fertilized plants, same trait in each generation (purple flower produces all purple flowers through self-pollination)
cross-pollination
- between different parent plants
(same and different traits in each generation); i.e. white + purple flower
P vs F1 vs F2
P- parents
- plants used in the initial cross
- each pea produced contains an embryo
F1-filal (offspring)
- first generation
F2
- second generation
cross pollination proves what
about blending theory
- that blending theory isn’t true because even with a purple flower and a white flower that produces a purple offspring, its not a blend of the colours
FLOWER COLOUR CROSS (purple and white)
P- purple crossed w/ white
F1- all F1 seeds formed purple
- purple flower offspring crossed
F2- purple flowers: 75%
- white flowers: 25% (reintroduction of trait seen in P generation not in F1, but F2)
= 3:1 RATIO
- NO BLENDING !!!!!
What was Mendels First hypothesis
1) genes for genetic characteristics occur in pairs
- one gene inherited from each parents
- alleles are different versions of a gene
- diploid= two copies of a gene
- non-applicable to bacteria
What was Mendel’s Second hypothesis
2) If two alleles of a gene are different, one allele is dominant over the other
- dominant allele is expressed
- recessive allele is masked
- recessive is only expressed when two copies are present (rr)
What was Mendel’s Third Hypothesis
3) 2 alleles of a gene segregate and enter gametes singly (when they need to form gametes)
- half of the gametes carry 1 allele, half carry the other (haploid)
- PRINCIPLE OF SEGREGATION
- two gametes will fuse=zygote nucleus, containing 2 gametes (diploid)
i.e. Pp plants produce two kinds of gametes….1) P, 2)p
What is a monohybrid cross
crossing individuals to look at 1 characteristic
- done through Punnett squares
- 3:1 ratio
- results support mendels 3 hypothesis, leading to PRINCIPLE OF SEGREGATION
Homozygous vs Heterozygous
homo: both alleles are the same
- PP (dominant)
- pp (recessive)
hetero: two different alleles
- Pp
Genotype vs Phenotype
geno:
- genetic constitution
- PP, Pp, pp (3 options therefore 3:1)
pheno:
- appearance
- purple vs white flowers (2 options)
Mendel’s Predictions (what could he predict)
- classes of offspring (traits)
- proportions of those offspring
- done through sum rule in probability
sum rule and product rule
sum: DIFF EVENTS, =OUTCOME
- probability of 2 different events producing 1 outcome
- individual probabilities added
(heterozygote)
product: INDEPENDENT EVENTS
- probability of 2 independent events occurring in succession
- individual probabilities multiplied
i.e. sex of child 1 has 0 affect on outcome of child 2 (homozygote)
Results if we validate mendels hypothesis with a test cross
- cross an unknown genotype with a homozygous recessive individual
- determines if the unknown is a heterozygote or homozygote
- can validate that F2 is more heterozygous, can tell us if a purple flower is Pp (1:1 ratio, 1 white, 1 purple..see below)
crossing Pp with pp
1) Pp-purple
2) pp-white
crossing PP with pp
- all purple
1:0 ratio
What was Mendel’s fourth hypothesis
4) alleles of genes that govern 2 different characters (during meiosis) segregate independently during gametes)=PRINCIPLE OF INDEPENDENT ASSORTMENT
- occurs to independent assortment during meiosis
Dihybrid Cross results
P generation: RRYY x rryy
- RR YY produces RY gametes
- rr yy produces ry gametes
F1:
- all offspring are Rr Yy (all dominant)
Crossing F1
F1: Rr Yy x Rr Yy
- produce 4 gametes:
1/4 R Y
1/4 R y
1/4 r Y
1/4 r y
F2: offspring have 4 phenotypes
- 9/16=both dominant
- 3/16= 1 dominant one recessive
- 3/16= 1 dominant one recessive (just opposite traits as above)
- 1/16= both recessive
9:3:3:1 ratio (using sum and product rules)
Independent assortment In a cross refers to
the combinations of gametes produced by each generation
F1 x Homozygous recessive dihybrid testcross
- testcross: crossing with homozygous recessive
- 1/4=both dominant
- 1/4= 1 dominant one recessive
- 1/4= 1 dominant one recessive (just opposite traits as above)
- 1/4= both recessive
- 1:1:1:1 RATIO
supports that f1 are heterozygous
Chromosome theory of inheritance
- Walter sutton: theorized that parallels between genes and chromosomes in meiosis and fertilization
- applies only to diploids (bc they separate and enter gametes)
1) chromosomes occur in pairs in diploid organisms
2) chromosomes of each pair are separated and delivered singly to gametes
many parental traits appear unchanged in offpsring in 1 generation and then reappear the next due to segregation of chromosomes
3) independent assortment of chromosomes
4) each chromosome of each pair id derived from each parents
- paired back in zygote 1 per parents
where are discrete hereditary factors found (said by Walter sutton)
- on chromosomes
we now call these genes, he was essentially saying that the behaviour of chromosomes during meiosis matched segregation and independent assortment patterns of mendels hereditary factors
Homologs Chromosomes have what that is occupied by a gene
LOCUS
- site occupied by a gene on a chromosome
particular DNA sequence that usually encodes a protein or RNA product responsible for phenotype
- alleles on different homologs chromosomes have the same loci
- can be = (homo) or diff (hetero)
What human traits follow mendelian principles
- albinism (autosomal recessive, follows principles segregation)
- webbed fingers (autosomal dominant, follows dominance and segregation)
- short limbed dwarfism (autosomal recessive/co-dominant, follows segregation and dominance)
What were some modifications to Mendel’s Hypothesis
1) Incomplete dominance- dominant alleles don’t fully mask recessive alleles
2) Codominance- effects of different alleles are equally detectable in heterozygotes
3) Multiple Alleles- 2+ alleles are present in a populations
**(note: one individual can only rlly carry 2 alleles of a gene, multiple alleles have nucleotide differences at locations in DNA sequences that create the decreeable alterations in structure)
4) Epistasis- genes interact, activity of 1 influencing the other
5) Polygenic Inheritance- character is controlled by common effects of several genes (QUANTITATIVE TRAITS=bell curve)
6) Pleiotropy- 2+ characters are affected by a single gene (i.e. sickle cell affects the heme, and the structure and function of RBC and can increase risk of disease and complications)
Incomplete Dominance
- dominant alleles IS NOT FULLY DOMINANT over recessive (not always strong enough)
- heterozygote phenotype : different from either homozygote phenotype
P: Red flower (CRCR) x White flower (CWCW)
F1: all offspring: CRCW
- all pink flowers (incomplete dom=mix, 100% heterozygous offspring)
From F1 generation:
CRCW x CRCW (both pink)
F2 generation:
- 1/4 red
- 1/4 white
- 1/2 pink
= 1:2:1 ratio (NOT BLENDING ANYMORE, because they come back)
Incomplete dominance in human traits
SICKLE CELL:
- homozygous recessive is a carrier, heterzygote has a milder trait
= Incomplete dominance
FAMILAL HYPERCHOLESTEROLEMIA:
- homozygous recessive has severe form of disease
- heterozygote has mild form of disease
= Incomplete dominance
TAY-SACHS DISEASE
- homozygous recessive has serious symptoms
- heterozygote has 0 symptoms but detectable biochemical effects
= Incomplete dominance
Co-Dominance
+ example w/ blood
different alleles of gene have equal effects in heterozygotes
- both alleles are expressed
I.E. blood types
Human M,MN,N blood types (glycoproteins on RBC)
1) LMLM= M glycoprotein present: blood type M
2) LNLN= N glycoprotein present: blood type N
3) LMLN= both glycoproteins present: blood type MN
- similar inheritance to incomplete dominance as it is to co-dominance can’t distinguish
1:2:1 RATIO
what cant the 1:2:1 ratio tell you
if its codom or incomplete Dom for f2
Multiple Alleles
More than 2 types alleles for a gene
- found among all individuals in a population
- diploid individuals only have 2 of the alleles
Phenotype depends on relationship between different pairs of alleles
- still follows mendels principles
- Small differences in DNA sequences result in multiple alleles *
= 1 change results in new allele, changes phenotype in F1/F2
Example for all 3 (incomplete, codom, and multiple alleles)
HUMAN ABO BLOOD GROUP
1) ANTIGENS
- glycoprotein on surface of RBC (triggers immune response)
* IA allele produces A antigen (dominant)
* IB allele produces B antigen (dominant)
* i allele produces neither A/B (recessive)= blood type O
(A antigen doesn’t make B and vice versa)
2) BLOOD TYPES (phenotypes) *based on above
-IAIA or IAi= type A
- IBIB or IBi= type B
- ii= type O
- IAIB= type AB
not rlly incomplete bc no allele is partly expressed
Human ABO Blood group
- purpose and phenotype
- immune system produces antibodies against antigens not found on its own RBC
because you don’t make 3 naturally, the body recognizes it as a foreign substance, produces an immune system if you got blood B instead of A for instance
UNIVERSAL ACCEPTOR: AB
UNIVERSAL DONOR: O
- I^A and I^B alleles are co- dominant to each other, and each are dominant to the i allele.
Co-dominance, multiple alleles, and dominant/recessive alleles
Epistasis
genes interact:
- allele of one locus inhibits or masks effects of allele at a different locus
- some expected phenotype don’t appear among offspring
Example: Labrador retrievers
MELANIN
- B allele= black
- b allele= brown
(dependent on …(below))
PIGMENT DEPOSITION (to determine if the colour acc appears in fur)
- E allele= normal amount of pigment (black/brown)
- e allele= blocked pigment (yellow fur)
PHENOTYPE
- BLACK FUR: BB EE, BB Ee, Bb EE, Bb Ee
- BROWN FUR: bb EE, bb Ee
- YELLOW FUR: BB ee, Bb ee, bb ee
What can be said about the epistasis genes in labradors
e- E gene is EPISTATIC to B gene
- 9:3:4 ratio (follows independent assortment because genotype ratio matches, not the phenotype since the genes affect each others expression)
Polygenic Inheritance
- several genes at different loci interact to control 1 character
- produces variation
Phenotypic Distribution: Bell-shaped curve
Often modified by environmental effects
= Quantitative Traits
Example
- height, weight, skin colour, hair colour
(based on nutrition, hormones, environment)
- perfect bell curve indicated continuous variation in population, larger sample the closer it is to perfect shape
Pleiotropy
- 1 gene affects more than 1 character
sickle cell disease (mutation to B globin gene which makes hemoglobin) - recessive allele affects hemoglobin function and structure
- leads to blood vessel, tissue, and organ damage
- different symptoms result
OPPOSITE OF POLYGENIC INHERITANCE
characters vs traits vs genes vs alleles
character- variety of heritable qualities
trait- variation in character
genes- discrete herditary factors (characteristics being passed on)
allele- different versions of a gene w/ different DNA sequences and different traits of a character
scientific theory and Mendel
- observations
- hypothesis
- testing hypothesis w experiments
- used statistical analysis *
each seed on the pea plant contained a
zygote
T/F dominant alleles do not directly inhibit recessive alleles
T: they don’t
What does Mendel’s hypothesis help explain
why individuals mat differ genetically but look the same
- i.e. Pp codes for dominant the same way PP does
Would the alleles of different characters be inherited independently or would they interact to alter their expected proportions in offspring
- from Mendel’s discoveries:
- they would be inherited independently of each other (product rule)
alleles of genes that govern characteristics segregate independently of each other=independent assortment
