lecture 14: beyond mendel Flashcards
Do most characters follow Mendelian rules?
No, most have more complicated patterns of inheritance as Mendel’s principles of genetics are rather simplistic
What are examples of characters that follows a more complicated pattern than Mendelian rules?
Height, skin colour…
What is autosomal inheritance?
Inheritance of characters governed by genes on autosomes
What is sex-linked inheritance?
Inheritance of characters governed by genes on sex chromosomes (X or Y)
- Sex-linked characters are inherited differently in males and in females
Where was sex-linked inheritance first discovered?
in fruit flies
What is a wild-type trait?
The most common phenotype for each character
- uppercase W = dominant wild-type allele
What is a mutant trait?
Less common phenotype attributable to a mutation in a gene (recessive mutant allele)
- Lowercase w
- The gene is named after the mutant phenotype
What is the mutant eye colour of fruit flies?
White = ww
What is the wild-type eye colour of fruit flies?
Red = WW or Ww (dominant allele)
How was sex-linked inheritance discovered in fruit flies?
Reciprocal crosses produce DIFFERENT phenotype results in males and females —> so the sex/trait combo (mother with red eyes with father with white eyes, mother with white eyes with father with red eyes, etc) MATTERED in the inheritance pattern
How is sex defined in animals?
- with sex chromosomes
- X/Y chromosome system in mammals, but other sex determination systems exist too
- sex determination can also be controlled by the environment (ex: sex determined by nest temperature in alligators)
What are the difference between X and the Y sex chromosome and their roles?
- X = bigger and Y = smaller —> only two small homologous parts at top and bottom called pseudoautosomal regions (PARs) —> these parts allo these chromosomes to behave as homologues during meiosis —> PARs contain genes for development of males and females
- X chromosome genes = essential for development of females and males
- Y = only necessary for male development and male fertility
What are the two types of sex-linked inheritance?
- X-linked: character governed by a gene on the X chromosome
- Y-linked: character governed by a gene on Y chromosome
Is eye colour in fruit flies X or Y-linked?
X-linked —> only exist on X chromosomes
What were the 3 possible genotypes for eye colour in female fruit flies vs the 2 possible genotypes for male fruit flies?
FEMALE:
- X(W)X(W) = red
- X(W)X(w) = red
- X(w)X(w) = white
MALE:
- X(W)Y = red
- X(w)Y = white
—> so mutant eye colour (white) is more common in males (50% chance)
What are some examples of X-linked recessive disorders in humans?
- Red-green colour blindness
- Hemophilia A and B
Difference (1) between autosomal inheritance vs sex-linked inheritance and the 4 consequences of this difference?
Because males only have one X and most genes present on the X have no counterpart on the Y
- Males (XY) are therefore HEMIZYGOUS dominant or recessive —> cannot be homozygous/heterozygous
- Far more males have sex-linked recessive disorders (bc of 50/50 chance to have the disorder) —>females are more protected from X-linked recessive disorders
- Mothers (XX) can pass alleles to sons & daughters while fathers (XY) can ONLY pass on alleles to daughters
- Mothers can be carriers for X-linked recessive disorders (X(W)X(w)) —> males cannot bc they have the disorder OR they don’t (50% chance)
What is a polymorphic character and the two types?
Character that has more than 2 phenotypes in a population
- Qualitative characters: various trait CATEGORIES
- Quantitative characters: display CONTINUUM of traits (human height)
—> Eye colour = both qualitative and quantitative
Which 3 factors increase the number of phenotypes for a character in a population?
- Spectrum of dominance: not only complete dominance
- Multiple alleles: not only two alleles for 1 character
- Interactions between genes: one trait is usually governed by many different genes, not only two
—> Increase number of genotypes, and consequently phenotypes
What are the 3 categories on the spectrum of dominance?
- Complete dominance: only the dominant phenotype is expressed in HETEROzygotes
- Incomplete dominance: phenotype is a MIXTURE of both in heterozygotes (“blending” hypothesis)
- Codominance: both phenotypes are expressed in heterozygotes (ex: splash of white and purple colours on the petals)
How do incomplete dominance/codominance differ with Mendelian genetics?
Mendel: only 2 phenotypes as homozygous dom. and heterozygous give the SAME phenotype
On spectrum of dominance: 3 phenotypes —> heterozygous gives a NEW phenotype, different from the one given by homozygous dom. —> an INTERMEDIATE phenotype
How many phenotypes does codominance give?
3 phenotypes: 1 new phenotype = a blend between the 2 parental phenotypes
Explain how having multiples alleles for one gene increases the number of phenotypes in a population.
- Three or more alleles can exist for one single gene, not only two
- One person can only have two of all these alleles
—> So many possible genotypes giving rise to many different phenotypes
How does the concept of multiple alleles for one gene differ with Mendelian genetics?
Mendel = only two alleles giving rise to two phenotypes possible (1 dominant and 1 recessive)
How are ABO blood groups inherited?
- Multiple alleles: 3 alleles for the gene that governs our ABO blood group (I(A), I(B), and i)
- Codominance: I(A) and I(B) are codominant
- Complete dominance I(A) and I(B) and completely dominant over the i allele
I(A)I(A) and I(A)i = Type A
I(B)I(B) and I(B)i = Type B
I(A)I(B) = Type AB
ii = Type O
—> Combination of multiple alleles and codominance leads to 4 phenotypes
How does ABO blood groups differ and why is it important to identity one’s blood type?
I(A), I(B), and i reflect various types of cell membrane glycoproteins found on red blood cells
- Type A has a certain type of glycoproteins (antigens)
- Type B has another type of glycoproteins (antigens)
- Type O does not have any antigens
Matching blood types for blood transfusion: to prevent a reaction of in the immune system —> mismatched blood causes a transfusion reaction —> immune system judges the red blood cells as foreign (different glycoproteins) and the tissue will be attacked
Which blood type is the universal donor and the universal acceptor and why?
Donor = Type O, because type O does not have any antigens so everyone can accept O blood and not recognize it as foreign Acceptor = Type AB, because has both types of antigens and O does not have any
What do interactions between two genes for one character give in terms of phenotypes?
- 4 different phenotypes for 1 character: 2 parental and 1 appearing at each generation (F1 and F2)
- Qualitative
How does the concept of interactions between genes differ with Mendelian genetics?
- Mendel = only 1 gene (2 alleles) defining 1 character —> MONOGENIC characters
- With interactions between two genes = 4 alleles in total = 4 possible genotypes (as one persons only inherits 2 alleles of the 4) = 4 possible phenotypes
—> POLYGENIC characters
What is epistasis?
Special case of gene interaction where a gene at one locus MASKS the effect of a gene at another locus
—> Two genes involved: the one masking the other = the EPISTATIC gene, the MODIFIER/MASKING gene
- The epistatic gene can act in a dominant or recessive fashion (masks the other gene when the epistatic one is inherited as homozygous dominant OR as homozygous recessive)
How does epistasis work in coat coloration in mice?
- TWO genes: Gene B and C
- Gene B = black (B) or brown pigment (b) where black allele (B) is DOMINANT
- Gene C (at another locus) determines whether the pigment produced (black or brown) is DEPOSITED in the hair —> if no pigment is deposited, the coat is WHITE
- Dominant allele = C, Recessive = c
- CC and Cc genotypes ALLOW the pigment to be deposited while cc genotype does NOT
—> Gene C = EPISTATIC to the pigment colour gene B because it can MASK THE EFFECT of gene B, but in a RECESSIVE (cc) manner
—> So if cc in the genotype, it does not matter if there is dominant BB or Bb —> the coat will be white
How can polygenic inheritance produce quantitative characters?
- Many genes interact to affect the character of one character + Incomplete dominance between the alleles
- But there are ADDITIVE effects of alleles for each gene
—> Give quantitative characters with a CONTINUUM of phenotypes (ex: human height)
—> Distribution of phenotypes gives bell-shaped curve on a graph
—> Increase in the number of genes involved and combination of incomplete dominance will INCREASE THE NUMBER OF PHENOTYPES
Explain how the effects of alleles can be additive.
- Incomplete dominance —> gives phenotypes that are mix of the 2 parental phenotypes, where one of the phenotype is the DOMINANT one
- The more number of dominant alleles you have, the more your phenotype is close to the dominant phenotype
EX: Kernel Colour:
PARENTS: aabbcc (pure-line white) + AABBCC (pure-line red)
—> So 3 different genes with 2 alleles each contribute to 1 genotype/1 phenotype/to the colour of the kernel
F1: AaBbCc = medium red (equal mix of red and white) with 3 dominant red alleles and 3 recessive white ones
SELF-FERTILIZED F2: gives SEVEN possible phenotypes ranging from pure white to pure red —> in the middle = shades of mixtures of white+red
—> The more there are red (ABC) alleles in one genotype, the more the colour will be close to the parent’s pure red
Most characters are polymorphic. What does polymorphic mean?
Multiple phenotypes
