6.2 Patterns of Inheritance Flashcards
Define monogenic
determined by a single gene, two distinct alleles
If you crossed a homozygous recessive genotype with a homozygous dominant genotype, for example for eye colour (B= brown, b=blue) what is the resulting phenotypic ratio?
100% heterozygous
Therefore 100% brown eyes
If you crossed two heterozgygous genotypes for eye colour for example (B = brown, b= blue), what is the resulting phenotypic ratio?
3:1 brown eyes to blue eyes
Define homozygous and heterozygous
Homo: identical alleles at a particular gene locus on a pair of homologous chromosomes
Hetero: different alleles at a particular gene locus on a pair of homologous chromosomes
What does dihybrid unlinked mean?
- inheritance of two characteristics simultaneously, involving 2 gene loci.
- if unlinked, there are 2 genes on two diff homologous pairs of chromosomes
Cross two pea plants that are heterozygous for seed shape and colour - what is the phenotypic ratio?
(yellowis dominant, green is recessive)
(round is dominant, wrinkle is recessive)
9:3:3:1
yellow round seed: yellow wrinkle seed: green round seed: green wrinkle seed
What is phenotype vs genotype?
pheno: visible characteristic
geno: genetic makeup
What are mutagens? What are some examples?
chemicals that increase mutation rate
physical: X-rays, gamma rays, UV
chemical: mustard gas, dyes, free radicals, aromatic amines
biological: viruses, transpoons, food contaminants from fungi
Mutations that occur during gamete formation are also: (2)
persistent: transmitted through generations without change
random: not directed by a need for it
What are non-disjunction chromosome mutations? WHat is an example?
failure to segregate - one gamete (and therefore zygote) has an extra chromosome. E.G DOWN SYNDROME/ TRISOMY 21
Define aneuploidy
abnormal chromosome number in a cell
Define polyploidy
if diploid gamete is fertilised by haploid gamete = triploid. 3 diploid gametes = tetraploid
How does sexual reproduction increase genetic variation? (4)
- allele shuffling during crossing over of non sister chromatids in prophase I
- independent assortment of chromosomes in metaphase I
- independent assortment of chromatids in metaphase II
- random fusion of gametes that are already genetically unique increases genetic diversity
How do environmental factors play into phenotypes? (2)
- ONLY ENVIRO include speaking with a dialect, scar after injury
- ENVIRO AND GENES: chlorotic plants don’t make enough chlorophyll due to environmental factors such as dim light or insufficient Mg
What is the genotype for males and females?
Male: XY
Female: XX
What is sex linkage? What is significant about male inheritance of certain sex linked characteristics? What are some examples?
- where genes are located on sex chromosomes
- if a male inherits an X chromosome with an abnormal allele for a particular gene from his mother, he will suffer from a genetic disease.
- hameophilia A and colour blindness and Duchenne muscular dystrophy
Males are —- for X linked genes
hemizygous
Describe haemophilia A
- unable to clot blood fast enough = bleeding or internal haemorrhage
Draw a genetic diagram of a haemophilia A carrier female with a normal male. Calculate the probability the offspring will suffer from haemophilia
25%
Describe colour blindness. When will a male suffer?
- recessive sex linked
- can’t tell difference between red and green
- a male with one abnormal allele on his X WILL since no protective allele on Y chromosome
in every female cell ——-, one ——- is inactivated during early —— development
prevents —— no of ——– genes being expressed
nucleus, X chromosome, embyronic
twice, X-linked
Define codominant
where both alleles present in the genotype of a heterozygous individual contribute to their phenotype
What is an example of codominance in animals?
coat colour in short horn cattle
alleles C^R C^R = red
alleles C^W C^W = white
allleles C^R C^W = roan
Draw a genetic diagram of a roan female and roan male and calculate the phenotypic ratio
1 white: 2 roan: 1 red
What is an example of codominance in humans?
MN blood groups
gene coding for a particular protein on surface of erythrocytes
Draw a genetic diagram of two individuals with an MN blood group
1 MM:2 MN:1NN
What is an example of codominance in plants?(hint: camelias)
if red flowered camellias are crossed with white, the offspring will have red and white spotted flowers (P^R and P^W)
What does multiple alleles mean?
characteristic for which there are 3 or more alleles in the population’s gene pool
Describe multiple alleles in human blood groups. What genotypes make what phenotypes?
- gene encoding isoagglutinogen, I, on surface of erythrocytes
- I^A and I^B are codominant, I^O is recessive
A is either I^A I^A or I^A I^O
B is either I^B I^B or I^B I^O
AB is I^A I^B
O is I^O I^O
Define autosomal linkage
gene-loci present on same autosome that are often inherited together
How does autosomal linkage with crossing over compare with no crossing over?
NO CROSSING: linked genes ALWAYS inherited as one unit -> 2 phenotypes
CROSSING: creates recombinant gametes (the further apart the gene loci on a chromosome, the more likely)
-> 4 phenotypes
Cross a purple flowered, long pollen grained plant (homozygous dominant x 2) with a red flowered, short pollen grained plant (homozygous recessive x2).
P or p, L or l. Assume there is autosomal linkage.
Note down the F1 phenotypes and genotypes.
Then cross the F1 generation and note down the phenotype ratio
Gametes are PL or pl
All purple flowered, long grains (PpLl)
3:1 ratio of purple flowered, long grain : red flowered, short grain
Define epistasis
Interaction of non linked gene loci where one masks the expression of the other - reduces genetic variation
What 2 main types of epistasis are there?
Antagonistic and complementary fashion (genes working to code for 2 enzymes that work in succession catalysing sequential steps of a metabolic pathway
Define recessive epistasis, epistatic and hypostatic
Homozygous presence of a recessive allele at the first locus prevents the expression of another allele at the 2nd locus
Alleles at first locus EPISTATIC to those at 2nd, which are HYPOSTATIC to those at first locus.
If pink variety of Salvia (AAbb) are crossed with white variety (aaBB) what do you expect? aa is epistatic to B/b, B is purple and b is for pink.
All AaBb = all purple flowers
For recessive epistasis, what would the phenotypic ratio be for the F2 generation of Salvia flowers when all purple flowers from F1 AaBb are bred?
9 purple: 3 pink: 4 white
Define dominant epistasis
When a dominant allele at a locus completely masks the alleles at a second locus
Chicken feather colour example:
I allele for I/i epistatic gene prevents formation of colour, even if C allele on hypostatic C/c gene is present ( coding for colored feathers). cc is also white therefore as no pigment is made.
A) what would the phenotype be of F1 if you bred white leghorn (IICC) and white Wyandotte (iicc)
B) if you bred the F1 generation together, what would the phenotypic ratio be? What is an exception?
A) all IiCc (white)
B) 13 white:3 coloured
Sometimes ratio is 12:3:1
Mice coat colour example:
CC/Cc produces coloured fur
cc = albino- doesn’t matter what A/a
AA/Aa = agouti colour (codes for distribution of pigment)
aa is black
What is the phenotypic ratio when you breed 2 agouti mice (AaCc) together?
9 agouti: 3 black : 4 albino
Pea plant example:
A/a codes for colourless intermediate (dominant) B/b codes for purple pigment (dominant)
A)
What are the offsprings phenotypes if two white pea plants, AAbb x aaBB are bred?
B) What is the phenotypic ratio when the F1 generation are bred?
- need at least one dominant B and one dominant A to be purple
- F1: all purple (AaBb)
- F2: 9:7 ratio of purple: white
Chicken combs:
At least one dominant of each is needed for walnut, pp is rose, rr is pea, and pprr is single
If you bred true breeding pea comb chickens with true breeding rose comb chickens, what is the genotype of the offspring
What is the phenotypic ratio of the F2 generation?
F1: all PpRr = walnut
F2: 9 walnut: 3 pea: 3 rose : 1 single
Just like Mendez’s dihybrid ratio
What are the characteristics of discontinuous variation? (4)
-discrete phenotypes
- no or few intermediates
- determined by alleles of single gene usually = monogenic
- bar chart
What are some examples of discontinuous variation?
- plant stem height, gender, seed shape, blood group
What are the characteristics of continuous variation? (4)
- many intermediates
- quantitative traits very by small amounts between each group
- polygenic
- histogram
What are some examples of continuous variation?
- height, birth mass, hair colour, heart rate, leaf length, mice tail length
Environment has a greater effect on expression of which types of traits?
greater for polygenic than monogenic
What is stabilising selection?
environment remains unchanged usually
favours intermediate phenotypes
What is directional selection?
gradual shift in optimum/mode/mean value -> favours an extreme
artificial selection is an example
What is disruptive selection?
selection pressure towards extremes creates two modal values, and less intermediates
What is genetic drift? Which populations are more at risk?
- random changes in allele frequency in a population that occur even when the habitat doesn’t change
- small populations -> each individual forms a great proportion of a population
What is a genetic bottleneck?
Population size shrinks and increases again
Non selective, catastrophic event
Genetic diversity decreases (but might coincidentally be beneficial still)
Sometimes fertility affected -> species become endangered/extinct
What is the founder effect?
- new population formed by a very small number of individuals who originate from a large population
- likely to have a small gene pool if isolated
Define population
Group of individuals of the same species that can interbreed
Define gene pool
Total number of alleles present within the population
What factors affect allele frequencies and genetic diversity in a gene pool?
Population size
Mutation rate
Migration
Natural selection
Genetic drift
Gene flow
Non-random mating
What is the p and q in the Hardy Weinberg equation and what is the purpose?
Q = recessive allele, P = dominant allele. Determine frequencies of those carrying a recessive allele (heterozygous) for a genetic disorder with recessive inheritance
What are the assumptions for Hardy Weinberg?
Large population, random mating, no selection, no mutation/migration/ genetic drift
1 out of 2000 is a sufferer (bb;q^2) of cystic fibrosis. Use this to calculate number of people that are carriers
Q^2 = 0.0005, q= o0.02236
P = 1-q
P = 0.9776
2 x p x q = 0.043
0.043 x 2000 = 87
Define speciation
Splitting of a genetically similar population into two or more populations that undergo genetic differentiation (accumulation of diff allele frequencies)due to reproductive isolation, leading to evolution of two or more new species.
Describe geographical isolation
- geographical features act as gene flow barriers
- different selection pressures = independent changes to allele frequencies/ chromosome arrangement
- eventually leads to reproductive isolation
= ALLOPATRIC SPECIATION
What are the types of reproductive/sympatric isolation? Give examples
Prezygotic: prevent fertilization. E.g.
- temporal (changing foraging behavior, diurnal populations unlikely to mate with crepuscular or nocturnal populations)
- ecological: similar regions but diff habitats
- behavioral: respond to diff courtship patterns
- mechanical: genital differences or plant flower structure differences
Postzygotic: prevents formation of fertile offspring
- hybrid infertility, reduced viability of hybrid zygotes, hybrid breakdown.
What is a deletion chromosome mutation?
part of gene is lost
What is an inversion chromosome mutation? What is the consequence?
segment breaks and reattaches in diff orientation on same chromosome
some genes become too far from regulatory sequences
What is a translocation chromosome mutation?
interferes with gene regulation
pieces of chromosome break off and join a different one
What is a duplication chromosome mutation?
overexpression of genes can disrupt metabolism
For centuries, artificial selection has been used to improve the quality of crop plants used for human consumption. Explain, with reference to selective breeding, why it is important to maintain viable wild populations of crop plant species. (6)
genetic variation
genetic resource / gene bank
source of useful alleles
can be cross bred with crop varieties allows introduction of different traits
unknown future requirements
potentially useful in changing climate
prevention of inbreeding depression
promotion of hybrid vigour
prevent dwindling gene pool
source of replacement if cultivated
population is in danger
plausible example(s) of any of the above
It is thought that the modern cheetah population has low genetic diversity because the population, relatively recently, experienced a genetic bottleneck. Explain why this can lead to low genetic diversity (2)
(many) alleles lost (when population dropped) ora
(modern population) descended from few survivors / AW
Scientists are concerned about genetic drift in the remaining cheetah populations.
Explain why genetic drift is likely to be of particular concern in the population of 100
cheetahs in Iran.
idea that one individual or allele has proportionally higher effect
on small population
(more likely that) alleles will be lost from population
(population) more vulnerable / likely to become extinct due , to
environmental change / AW
