Patterns of inheritance Flashcards
define genotype
genetic makeup of an organism
define phenotype
visible characteristics of an organism
define recessive
the allele of the two which doesn’t express itself (if other allele is dominant)
define dominant
the allele that’s expressed
chromosome vs chromatid vs chromatin
- chromosome consists of a single, double-stranded DNA molecule. ( single ‘unit’ before replication but doubles after replication before cell division, so X is now called chromosome)
- chromatids are two molecules of double-stranded DNA joined together in the centre by a centromere
- chromatin is the complex of DNA and histone protein
what are homologous chromosomes
one of a pair of chromosomes with the same gene sequence, loci, chromosomal length, and centromere location
what are sister chromatids
duplicated copies of a single chromosome that are attached to each other and are identical
centrosome vs centriole vs centromere
- Centrosome refers to a small region near the nucleus in the cell cytoplasm, containing the centrioles.
- centriole - two barrel-shaped organelles that make up the centrosome
- Centromere is the central region where the two chromatids are held together.
define heterozygous
- when the alleles on both (homologous) chromosomes are different
- each allele is a letter = a gene = length of DNA that codes for a polypeptide
- before replication one chromosome is composed of one DNA molecule
define homozygous
- when the alleles on both (homologous) chromosomes are the same
- each allele is a letter = a gene = length of DNA that codes for a polypeptide
- before replication one chromosome is composed of one DNA molecule
define codominance
when both alleles for a gene in a heterozygous organism equally contribute to the phenotype.
what are the different types of mutagenic agents?
- chemical
- physical
- biological
examples of physical agents
- x-rays
- gamma rays
- UV light
examples of chemical agents
- mustard gas
- aromatic amines
- benzopyrene ( found in tobacco smoke)
examples of biological agents
- some viruses
- food contaminants
- transposons (remnants of viral nuclei that have become incorporated in our genomes)
How are mutations that occur during gamete formation persistent?
they can be transmitted through many generations without change
How are mutations that occur during gamete formation random?
they are not directed by a need on the part of the organism
Through which processes can chromosomes mutate?
- deletion
- inversion
- translocation
- duplication
- non-disjunction
what’s deletion
part of chromosome is lost
what’s inversion
when a section of chromosome breaks off, turns 180 degrees and re-joins but can’t be expressed properly as some genes may be too far away from their regulatory nucleotide sequences
what’s translocation
piece of chromosome breaks off and attached to another chromosome
what’s duplication
a piece of chromosome is duplicated
what’s non-disjunction
chromatids fail to separate leaving one gamete with an extra chromosome (trisomy which causes down syndrome)
define aneuploidy
- where there is one extra chromosome or one missing chromosome (may cause trisomy)
- the chromosome number is not an exact multiple of the haploid number for that organism
define polyploidy
when a diploid gamete is fertilised by haploid gamete resulting in a triploid zygote (has 3 sets of chromosomes)
what’s independent assortment
When the pair of chromosomes splits up (in anaphase), each daughter cell will receive one chromosome. The allocation of this is completely random
during meiosis, genetic variation may result from…
- allele shuffling during crossing over in prophase 1 (at this stage chromosomes arrange themselves into homologous pairs)
- independent assortment of chromosomes during metaphase 1
- independent assortment of chromatids during metaphase 2
how is the number of gametes with different chromosome combinations calculated
2^n
n= haploid chromosome of that species
examples of variation caused by solely by the environment
- speaking regional dialect
- scar
- losing a digit or limb
examples of variation caused by the environment interacting with genes
- chlorotic (suffering from chlorosis) plants have the genotype for making chlorophyll but environmental factors ( dim light, soil with insufficient Mg) are preventing the expression of the genes.
define monogenic
determined by a single gene
what does P1 represent
parental generation
what does F1 represent
(first filial) generation
in a punnet square where do the female’s gametes go
vertical column
in a punnet square where do the male’s gametes go
horizontal row
define dihybrid
involving two gene loci
what are dihybrid crosses
investigations that examine the simultaneous inheritance of two characteristics ( yyrr = green + wrinkle seeded)
from Mendel’s results of his dihybrid crosses, what did he deduce?
- the alleles of the two genes are inherited independently of each other, so each gamete has one allele for each gene locus ( as gametes have 23 chromosomes)
- during fertilisation, any one of an allele pair can combine with any one of another allele pair
- The law of segregation predicts that each gamete has an equal probability of receiving any allele.
when two independent events occur simultaneously…
product of individual possibilities (multiplied together) = combined possibility of occurrence
What’s meant by a gene with multiple alleles
characteristic for which there are three or more alleles in the population’s gene pool
What are some example of multiple alleles
- the inheritance of human ABO blood groups ( also an e.g. of codominance)
- the inheritance of coat colour in rabbits
what are the 4 blood groups
A,B,AB,O
what do the genes for blood group encode for
isoagglutinogen (an antigen), I, on the surface of erythrocytes
what are the alleles for blood group present in the human gene pool?
- I^A ( codominant)
- I^B (codominant)
- I^O (recessive)
what happens if both I^A and I^B are both present in the genotype?
- they will both contribute to the phenotype.
- any individual will only have two of the three alleles within their genotype
what are the alleles for coat colour present in the rabbit gene pool?
- Agouti, C, (dominant to all other alleles)
- Chinchilla, C^ch, (dominant to himalayan C^h)
- Albino, c, (recessive to all other alleles)
What are some example of codominance?
- MN blood groups
- ABO blood groups
- Sickle cell anaemia
How is codominance shown in sickle cell anaemia
- sickle cell caused by mutation to gene that codes for the B-globin chain of Hb
- mutant allele is given the symbol Hb^S and the normal one is Hb^N
- In heterozygous people, at least half of their Hb will be normal and the other will be abnormal
define autosomal linkage
- gene loci present on the same autosome (non-sex chromosome) that are often inherited together
- offspring are likely to have same genotype as parent as linked alleles are inherited as a unit
What’s meant by linked gene loci
- When genes are close together on the same chromosome
- often inherited as one unit
why are linked genes not free to undergo independent assortment
the chromosome, not the gene, is the unit of transmission during sexual reproduction
when will linked genes always be inherited as one unit?
If they’re not affected by crossing over of NON-SISTER chromatids during prophase 1
crossing over of sister chromatids produced by crossing over in prophase 1 produces ……………… gametes
recombinant
what increases with the increase of distance between two gene loci on a chromosome?
the chance of recombinant gametes forming
what are sex chromosomes?
one of the 23 pairs of chromosomes that determines sex in humans
what are the 22 non-sex pairs of chromosomes called?
autosomes
what’s meant by autosomes being homologous?
they are the same length and contain the same genes at the same loci
what are the sex chromosomes in males
- XY (Y shorter than X so some genes in X will be missing in Y so males only have one gene for a specific gene locus)
- males are haploid or hemizygous for X-linked genes. They can’t be heterozygous or homozygous for X-linked genes
what are the sex chromosomes in females
XX
what’s meant by sex-linked
- genes present on (one of) the sex chromosomes
- If a gene is only found on the X chromosome it is X-linked.
What are sex - linked characteristics in humans?
- haemophilia A
- colour-blindness
why are the X and Y chromosomes not fully homologous?
- some genes on the X chromosome have no partner alleles on the Y chromosome
will a female (XX) have a genetic disease if she has one abnormal allele on one of her X chromosomes?
- unlikely as she probably has a functioning allele of the same gene on her other X chromosome
what happens if a male (XY) inherits his X chromosome with an abnormal allele for a particular gene from his mother ?
he will have a genetic disease as he won’t have a functioning allele for that gene
Effect of Haemophilia A
blood is unable to clot fast enough
Explain how Haemophilia A is a sex-linked characteristic
- a gene on non-homologous region of X chromosome codes for blood-clotting protein factor 8
- mutated allele codes for non functioning version of protein
- female with mutation can be a carrier as her other X chromosome has the unmutated allele for factor 8 so enough is produces to clot blood when needed
- if female passes the ‘faulty’ X chromosome to her son, he will suffer from the genetic disease as he has no functioning allele
Explain how colour blindness ( inability to distinguish between red and green) is a sex-linked characteristic
- inheritance pattern is the same as for Haemophilia A, that of a recessive sex-linked disorder
- functioning gene codes for proteins involved in colour vision
Do females have twice the number of X-linked genes being expressed?
No. In every female nucleus, one of the X chromosomes is inactivated randomly during early embryonic development
define epistasis
interaction of non-linked gene loci where one masks the expression of the other, affecting phenotypic characteristics
epistatic genes can work ………….. or …………..
- antagonistically (against each other)
- in a complementary fashion
what effect does epistasis have on genetic variation and why?
- reduces genetic variation
- gene loci not linked, they assort independently during gamete formation
- epistasis reduces the number of phenotypes produced in the F2 generation dihybrid crosses
what’s recessive epistasis (antagonistic)?
- homozygous recessive allele at first locus prevents expression of another allele at second locus
- alleles at first locus are epistatic to those in second, which are hypostatic to those in first
what’s dominant epistasis (antagonistic)?
- presence of dominant allele (heterozygous or homozygous) at first locus prevents the expression of another allele at second locus
- if alleles at second locus are recessive then they won’t be expressed anyway so it doesn’t matter if dominant epistasis is taking place (iicc, cc won’t be expressed)
How does complementary epistasis work?
- genes work together to code for two enzymes that work in succession, catalysing sequential steps of a metabolic pathway
- or gene may code for a transcription factor
example of complementary epistasis (coat colour in mice)
- gene locus C/c determines that the coat will have colour
- genotype CC or Cc produces fur colour
- cc produces no colour as no pigment develops and not distributed, mice will be albino REGARDLESS OF DOMINANT A
- A/a determines what the colour is by determining the distribution of pigment
- A produces agouti colour (grey)
- a produces ( when homozygous) produces black fur
- in presence of C, black pigment can be made from colourless substance
- in presence of A, this black pigment is deposited in hair, combined with a yellow band on each hair that produces agouti colouration
- colourless precursor molecule —–(allele C) –> black pigment —(allele A)—> agouti pattern
what’s the typical ratio expected in a dihybrid cross between two heterozygotes ( YyRr x YyRr)
9:3:3:1
ratio for recessive epistasis
9:3:4
ratio of dominant epistasis
13:3 or 12:3:1
ratio for complementary epistasis
9:7
what’s meant by pure-breeding?
- A purebred refers to offspring resulting from a true breeding. True breeding is a way to produce offspring that would carry the same phenotype as the parents
- pedigree offspring
- only have alleles for a particular characteristic (so homozygous):
yellow and round seeds = YYRR
green and wrinkled seeds = yyrr
example of complementary epistasis (flower colour in sweet peas)
- two strains of pure-breeding white-flowered sweet peas crossed
- all F1 progeny produced purple flowers
- F2 phenotypes contained white-flowered plants and purple-flowered plants in the ratio 9:7
- gene locus A/a and B/b yield this result
- colourless precursor molecule –(allele A)–> colourless intermediate product –(allele B) –> purple pigment
- both alleles code for enzymes that catalyse production of the next product
- At least one dominant allele for both gene locus present for flowers to be purple (e.g. AaBb = purple) as only alleles A and B code for the enzymes needed
define progeny
means “offspring” or “children
what’s chi-squared
statistical test designed to find out if the difference observed and expected data is significant or due to chance
when can chi-squared be used
- when data is in categories and isn’t continuous
- there’s a strong biological theory to use to predict expected values
- sample size is large
- data is only raw counts ( percentages or ratios can’t be used)
- there are no zero scores in the raw data count
what’s a null hypothesis
- statistical tests can’t be used to directly test a hypothesis, instead they test a null hypothesis
- null = there is no statistically significant difference between the observed and expected data. Any data difference is due to chance
formula for chi-squared (X^2)
X^2 = the sum of ([each observed number (O)- each expected number (E)]^2 / each expected number (E))
- differences may be +ve or -ve so they’re squared
procedure to calculate chi squared
- calculate X^2
- determine degrees of freedom (= no of categories -1)
- determine the value of p from distribution table.
- decide whether the difference is significant at p=0.05 level of probability
when do you accept null hypothesis
if calculated value of X^2 is smaller than the critical value
- ‘the calculated value of chi-squared is smaller than the critical value of chi-squared at p=0.05, so the difference is not significant and we can accept the null hypothesis’
when do you reject null hypothesis
if calculated value of X^2 is bigger than the critical value
- ‘the calculated value of chi-squared is larger than the critical value of chi-squared at p=0.05, so the difference is significant and we can reject the null hypothesis’
what’s continuous variation
- differences between individuals of a species where the differences are quantitative (measurable)
- intermediates between phenotypes
what’s discontinuous variation
- differences between individuals of a species where the differences are qualitative (categoric)
- no intermediates between phenotypes
examples of discontinuous variation
- gender
- ABO blood groups
- ear lobe (attached or free hanging)
characteristics that exhibit discontinuous variation are determined by…
- the alleles of a single gene locus
- monogenic
examples of continuous variation
- height
- skin colour
- leaf length
- hair colour
characteristics that exhibit continuous variation are determined by…
- many genes
- polygenic
interaction of genes and environment in continuos variation
- environment has greater effect on the expression of polygenic characteristics than monogenic ones
- example - each person has a genetic potential for height but without proper nutrition potential won’t be reached
describe natural selection
- mutations and migration introduce new alleles
- some individuals will be better adapted due to their genotype and phenotype due to selcetion pressures
- these individuals are more likely to survive and reproduce, passing on their advantageous alleles
- over time allele freq in a population will change
what are the 3 types of selection (sdd)
- stabilising
- directional
- disruptive
define directional selection
- natural selection that produces a gradual change in allele frequencies over several generations
- could arise due to selection pressure like change in environment
define stabilising selection
- natural selection that keeps allele frequencies relatively constant over generations
- occurs when environment remains unchanged
- favours intermediate phenotypes
example of stabilising selection
- babies birth mass close to 3.5kg are more likely to survive
- Very-low and very-high birth weights are selected against leading to the maintenance of the intermediate birth weights
- their offspring inherit alleles from them
define disruptive selection
- favours phenotypes at both extremes (could be that extreme phenotypes help with camouflaging to environment)
- intermediates selected against
define genetic bottleneck
- sharp reduction in population size due to environmental catastrophes
- as population grows, its less genetically diverse than before
define genetic drift
- when chance (instead of environmental selection pressures) affects which individuals in a population survive, breed and pass on their alleles
- random change in allele frequency
define the founder effect
- when only a small number of individuals from a large parent population start a new population
- gene pool not as diverse as parent population
- special case of genetic drift
sign that genetic drift is occuring
When there is a gradual change in allele frequencies in a small population due to chance and not natural selection
define population
members of a species, living in the same place at the same time, that can interbreed
what is population genetics
study of allele frequencies within a population’s gene pool, over time
factors that affect allele frequencies (therefore genetic diversity within a gene pool)
- population size
- mutation rate
- migration
- natural selection
- changes to environment
- isolation (founder effect)
- non-random mating
- genetic drift
- gene flow
what’s the Hardy-Weinberg principle
- states that if certain conditions are met then the allele frequencies of a gene within a population will not change from one generation to the next
- can be used to determine the frequencies of those carrying a recessive allele for a genetic disorder with a recessive inheritance pattern
what does the Hardy-Weinberg principle assume
- population is large enough to make sampling errors negligible
- random mating
- there’s no selective advantage for any genotype so no selection
- there’s no mutation, migration or genetic drift
equation for the Hardy-Weinberg principle
p + q = 1
p^2 + 2pq + q^2 = 1
- dominant allele is represented by p
- recessive allele is represented by q
- p^2 = homozygous dominant
- q^2 = homozygous recessive
- pq = heterozygous (symptomless carriers)
define speciation
the process by which new species are formed
define allopatric speciation
formation of two different species from one original species due to geographical isolation
define sympatric speciation
formation of two different species from one original species due to reproductive isolation with no geographical barrier
define sub-species
when two different populations of same species are still able to reproduce during the evolutionary process
how does speciation occur
- species split into two different populations
- mutations in one population not transmitted by interbreeding to other population
- different selection pressures at each different location and population will accumulate different allele frequencies
what are the two main types of isolating mechanisms
- reproductive
- geographical
what are examples of geographical barriers
lakes, river, mountains, oceans
give an example reproductive isolation
- mutation may lead to organisms being active at night rather than the day
- these organisms are unlikely to mate with those who are active during the day
how can genetic changes (change to chromosome number) lead to reproductive isolation
- prevents gamete fusion
- makes zygotes less viable
- leads to infertile hybrid offspring with odd no of chromosomes so chromosome pairing during meiosis can’t happen
mating between reproductively isolated populations may also be prevented by mutations leading to changes in
- courtship behaviour - time for mating or courtship rituals that precede mating
- animal genitalia or plant flower structure
define artificial selection
the process by which humans choose organisms with desirable traits and selectively breed them together to enhance the expression of these desirable traits over time and over many generations
during artificial selection, what is the agent of selection
humans
what’s inbreeding depression
- selective breeding reduces genetic diversity in the gene pool of selected breed
- related individuals crossed causing inbreeding depression
- chances of inheriting two copies of a recessive harmful allele are increased
What’s hybrid vigour
- when breeders outcross individuals to two different varieties to obtain those that are heterozygous at many gene loci
process of artificial selection via selective breeding
- The population shows phenotypic variation
- breeder selects an individual with the desired phenotype
- Another individual with the desired phenotype is selected. The two selected individuals should not be closely related to each other
- The two selected individuals are bred together
- The offspring produced reach maturity and are then tested for the desirable trait. Those that display the desired phenotype to the greatest degree are selected for further breeding
- The process continues for many generations: the best individuals from the offspring are chosen for breeding until all offspring display the desirable trait
what characteristics are animals bred for
- Cows, goats and sheep that produce a higher of milk or meat
- Chickens that lay large eggs
- Domestic dogs that have a gentle nature
- Sheep with good quality wool
- Horses with fine features and a very fast pace
what characteristics are plants bred for
- Disease resistance in food crops
- Increased crop yield
- Hardiness to weather conditions (eg. drought tolerance)
- Better tasting fruits
- Large or unusual flowers
What’s the importance of maintaining a resource of genetic material for use in selective breeding
- resource of genetic material close to the original wild type maintained
- ensures that the gene pool for a particular species doesn’t become too small
As inbreeding limits the size of the gene pool, there is an increased chance of:
- Organisms inheriting harmful genetic defects
- Organisms being vulnerable to new diseases (as there is less chance of resistant alleles being present in the reduced gene pool)
what are other ethical considerations of artificial selection
- domesticated animals are less able to defend themselves, making them easy prey
- livestock animals have more lean meat than fat so may die during winter if not housed
- coats of dogs (desirable to humans) make them less able to camouflage
- through inbreeding, dogs are susceptible to disease
- traits desired by humans may be selected against in the wild
where are samples of wild ancestral organisms kept to maintain a resource of genetic material
gene banks: seed banks, sperm banks, frozen embryo, botanic gardens. zoos
