Patterns of Inheritance (OCR A Level Biology): Flashcards

A gene found on a region of a sex chromosome (usually the longer X chromosome) that is not present on the other sex chromosome.

Because their inheritance depends on the sex of the individual, they are called sex-linked genes (e.g. the gene for haemophilia).

When two or more genes are located on the same autosome (a chromosome that is not a sex chromosome). These linked genes do not assort independently during meiosis and tend to stay together in their original parental combination.

The alleles are written as a superscript next to the sex chromosome.

• A heterozygous female: X^GX^g

• A male: X^GY

Because the chromosomes of eukaryotic cells occur in homologous pairs (two copies of each chromosome), so cells have two copies of every gene, allowing different allele combinations within an individual.

True

False — autosomally linked genes do not assort independently and tend to stay together in their original parental combination.

The observable characteristics of an organism.

The difference in phenotypes between organisms of the same species.

• Genetic factors

• Environmental factors

• A combination of genetic and environmental factors

The complete phenotype of an organism is determined by the expression of its genotype and the interaction of the environment on this: Phenotype = Genotype + Environment

The four ABO blood groups in human populations arise because individuals carry two of three possible alleles for the single ABO gene.

Diploid organisms inherit two alleles of each gene, which may be the same or different.

The different alleles at the F8 gene locus (which codes for the blood-clotting protein Factor VIII) dictate whether normal Factor VIII is produced, and therefore whether the individual has haemophilia.

• The phenotype may be affected by a single gene or by several genes

• The effect of a gene on the phenotype may be large or small

Clones of plants with exactly the same genetic information (DNA) will grow to different heights when grown in different environmental conditions.

• Length of sunlight hours (which may be seasonal)

• Supply of nutrients (food)

• Availability of water

• Temperature range

• Oxygen levels

Normal grey fly larvae fed a diet of silver salts develop the yellow colour regardless of their genotype.

This shows that flies which should be grey (according to their genes) can become yellow due to an environmental factor (their diet).

The plant becomes yellow even though genetically it should be green.

The development of long stems with small, curled leaves in plants grown in the dark, despite the plants being genetically able to develop normally.

Genetic variation directly affects the DNA of the gametes, so it can be passed to offspring.

Variation in phenotype caused solely by the environment does not alter the genetic component of the gametes, so it cannot be inherited.

The recessive allele that causes sickle cell anaemia has a high frequency in populations where malaria is prevalent, because heterozygous individuals are resistant to malaria.

The plant develops long stems with small, curled leaves (known as etiolation) and becomes yellow, even though genetically it should develop normally.

False — only alterations to the genetic component of the gametes can be inherited.

True

The small differences in DNA base sequences between individual organisms within a species.

They have very similar genotypes, but two individuals will have small differences between their DNA base sequences.

Considering the size of genomes, these differences are small between individuals of the same species.

• Crossing over of non-sister chromatids during prophase I of meiosis

• Independent assortment of homologous chromosomes during metaphase I of meiosis

• Random fusion of gametes during fertilisation

Each results in a new combination of alleles in a gamete or individual.

During sexual reproduction, genetic variation is transferred from one generation to the next, and this generates phenotypic variation within a species.

The process by which non-sister chromatids exchange alleles during prophase I of meiosis.

• During meiosis I, homologous chromosomes pair up and are in very close proximity

• The non-sister chromatids cross over and get entangled; these crossing points are called chiasmata

• The entanglement places stress on the DNA molecules

• A section of chromatid from one chromosome may break and rejoin with the chromatid from the other chromosome

• This swaps alleles, resulting in a new combination of alleles on the two chromosomes

The crossing points where non-sister chromatids become entangled during crossing over are called chiasmata.

The production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle during metaphase I of meiosis.

• In metaphase I, homologous pairs are pulled towards the equator of the spindle

• Each pair can be arranged with either chromosome on top, completely at random

• The orientation of one homologous pair is independent of the orientation of any other pair

• Homologous chromosomes are then separated to different poles, so the combination of alleles in each daughter cell depends on how the pairs lined up

The formula is 2ⁿ.

For humans this is 2²³, which is 8 324 608 different combinations.

During fertilisation, any male gamete can fuse with any female gamete to form a zygote.

This random fusion of gametes creates genetic variation between zygotes, as each will have a unique combination of alleles. There is an almost zero chance of individuals from successive sexual reproduction being genetically identical.

Mutations cause genetic variation by generating new alleles, which may be advantageous, disadvantageous or have no apparent effect on phenotype.

n is the number of chromosomes in a haploid cell (the haploid number). For humans n = 23.

Because mutations occur all the time, not only during the processes of sexual reproduction. New alleles can remain hidden (unexpressed) in a population for several generations before contributing to phenotypic variation.

False — crossing over occurs during prophase I of meiosis.

True

The study of how the alleles of a single gene are passed on from one generation to the next.

A table within a genetic diagram that shows the possible combinations of gametes to predict the genotypes and phenotypes of offspring from a cross.

When gametes form by meiosis, there is an equal chance of either allele from a homologous pair entering a gamete.

Since either gamete may fuse at fertilisation, the zygote has an equal chance of inheriting either allele.

The predicted genotypes are all based on chance.

There is no way to predict which gametes will fuse at fertilisation, so real results can differ from the predictions.

Predicted phenotype ratio = 3 dominant : 1 recessive

The predicted genotype ratio from a Bb × Bb cross is 1 BB : 2 Bb : 1 bb.

A gene that has more than two possible alleles within a population.

However, a diploid individual still only inherits two of the possible alleles.

Where both alleles in a heterozygous genotype are expressed in the phenotype, so neither is recessive to the other (e.g. blood group AB).

• I^A and I^B are codominant with each other

• Both I^A and I^B are dominant to I^O

• I^A produces antigen A, I^B produces antigen B, and I^O produces no antigens

Using a capital letter for the gene and superscript letters for the alleles (e.g. I^AI^B), rather than a capital and lower-case letter.

1 blood type AB : 1 blood type A : 1 blood type B : 1 blood type O

Genetic diagrams are often used to present inheritance information clearly and precisely so that predictions about offspring can be made.

• The possible gametes that each parent can produce

• The phenotype as well as the genotype of the offspring resulting from the cross

Predicted genotype ratio = 1 BB : 2 Bb : 1 bb

True

False — predictions are based on chance, so real results can differ from the predicted ratios.

A genetic cross that looks at how the alleles of one gene are transferred across generations.

A genetic cross that looks at how the alleles of two genes are transferred across generations, showing the inheritance of two different characteristics in an individual.

A monohybrid cross follows the alleles of one gene, whereas a dihybrid cross follows the alleles of two genes at the same time.

Because two genes are followed at once, there are many more possible allele combinations, producing several more genotypes and phenotypes in the offspring.

A dihybrid cross is used to show the inheritance of two different characteristics in an individual.

Write the two alleles for one gene, followed immediately by the two alleles for the other gene.\n\nDo not mix up the alleles from the different genes.\n\nFor example, for a gene with alleles Y/y and another with alleles G/g, a correct genotype would be written as YyGg.

The two genes are located on different, non-homologous chromosomes, so they are inherited independently of one another.

Four types of gamete, each carrying one allele from each gene:\n\n- BE\n- Be\n- bE\n- be

Two types of gamete:\n\n- bE\n- be

A Punnett square, which arranges all possible gametes from each parent along the rows and columns and combines them to show every possible offspring genotype.

3 black coat, brown eyes : 3 chestnut coat, brown eyes : 1 black coat, blue eyes : 1 chestnut coat, blue eyes

Both use the same approach: identify parental genotypes, work out the possible gametes, and combine them in a Punnett square.\n\nThe difference is that a dihybrid cross tracks two genes at once, so there are more gametes and offspring combinations to consider.

False — unlinked genes are on different, non-homologous chromosomes, so they are inherited independently.

True

When two or more genes are located on the same autosome (a non-sex chromosome), so they tend to stay together in their parental combination and are inherited together rather than assorting independently during meiosis.

That the genes assort independently of one another during meiosis.

Because they are located on the same chromosome, so they show linkage and stay together in the original parental combination as they pass into the gametes.

The linked alleles are kept together inside brackets to show they are inherited together.

For example, the genotype FFGG would be written as (FG)(FG) if the two genes are linked.

By an unexpected phenotypic ratio in the offspring, which differs from the ratio predicted for unlinked genes and shows the genes lie on the same chromosome.

A 1 : 1 : 1 : 1 ratio:

• 1 normal tail, green scales

• 1 normal tail, white scales

• 1 short tail, green scales

• 1 short tail, white scales

A 1 : 1 ratio — 1 normal tail, green scales : 1 short tail, white scales.

Because the genes are located on the same chromosome, so the parental allele combinations stay together in the gametes.

When a gene is present on only one of the sex chromosomes (usually the X chromosome) and not the other, so an individual's sex affects which alleles it can pass on and how the characteristic is inherited.

Males (XY) have only one X chromosome, so a single recessive allele on it is expressed.

Females (XX) have two X chromosomes, so a dominant allele on the second X can mask a recessive allele.

A female with one recessive disease allele that is masked by a dominant allele on her other X chromosome; she does not have the disease but has a 50% chance of passing the recessive allele to each offspring.

Females have three phenotypes: 'normal', 'carrier' and 'has the disease'.

Males have only two: 'normal' or 'has the disease'.

Using pedigree diagrams and Punnett squares, and by noting that the affected phenotypes are not spread evenly across the sexes, with one sex (usually males) disproportionately affected.

Offspring that have a different combination of characteristics to their parents are called recombinant offspring.

Crossing over during meiosis exchanges genetic material, which breaks the linkage between the genes and recombines the parental characteristics, producing recombinant offspring.

False — a male (XY) has only one X chromosome, so a recessive allele on it is always expressed; only females can be carriers.

True

Occurs when two genes on different chromosomes affect the same feature, where one gene affects the expression of the other gene.

A deviation from the expected phenotypic ratios suggests epistasis is taking place (in the same way that deviation can suggest genes are linked).

When epistasis occurs, the offspring ratios differ from the standard dihybrid ratio.

A ratio of 9:3:3:1.

In a standard dihybrid cross between two double heterozygous parents, we expect a 9:3:3:1 ratio in the offspring, but epistasis causes this ratio to differ.

Ratios that differ from the standard 9:3:3:1, such as:

• 9:3:4

• 12:3:1

• 9:7

• 13:3

• Allele R codes for a pigment that produces grey feathers

• Allele r does not produce a pigment, resulting in white feathers

• Allele F codes for the production of an enzyme that stops the production of grey pigment, even if allele R is present

• Allele f does not produce an enzyme

Allele F codes for an enzyme that stops the production of grey pigment, even when allele R (which codes for grey pigment) is present.

So genotypes such as RRFF, rrFF and RrFf all produce white feathers.

Genotypes RRff and Rrff produce grey feathers.

Grey feathers require at least one R allele and no F allele (i.e. genotype ff).

Each parent produces four gamete types:

• RF

• Rf

• rF

• rf

13 white : 3 grey

Any genotype containing F gives white feathers, so only RRff and Rrff (3 in 16) are grey.

You are unlikely to be asked to complete large Punnett squares for epistasis. Instead you should be able to:

• recognise common ratios that occur with epistasis

• use the information provided to predict phenotypes in examples of epistasis

False — the two genes are located on different chromosomes.

False — 9:3:3:1 is the standard dihybrid ratio; a deviation from it suggests epistasis.

A statistical test that determines whether or not there is a significant difference between the observed and expected results in an experiment.

Categorical data, i.e. data that falls into distinct groups or categories.

It suggests the presence of a factor not being accounted for (a factor other than chance is causing the difference).

For example, linkage between the genes being analysed.

Any differences observed can be said to be due to chance alone.

chi-squared value = ∑ (O − E)² ÷ E

Where O = observed value and E = expected value.

• Obtain the expected (E) and observed (O) results

• Calculate the difference between each set of results

• Square each difference (positive or negative sign is irrelevant)

• Divide each squared difference by the expected value

• Add the resulting values together to obtain the chi-squared value

Because the difference is squared, which always gives a positive value regardless of the sign of the original difference.

It must be compared to a critical value, read from a table of critical values.

A probability level of 0.05 (5 %).

Degrees of freedom = number of classes − 1

degrees of freedom = number of classes − 1

For example, if there are 2 phenotypes then 2 − 1 = 1 degree of freedom.

There is a significant difference between the observed and expected results.

• A factor other than chance is causing the difference

• The null hypothesis is rejected

There is no significant difference between the observed and expected results.

• Any differences are due to chance

• The null hypothesis is accepted

There could be linkage between the genes being analysed (i.e. the genes are located close together on the same chromosome and are not inherited independently).

• The probability level used

• The degrees of freedom

There is only a 5 % probability that any difference between observed (O) and expected (E) results has occurred by chance.

False — it is used for categorical data that falls into distinct groups.

True

The differences that exist between at least two things (a level, amount, quantity or feature). In natural selection, it refers to the differences between individuals of a species (also called intraspecific variation).

The differences between individuals of a species where the differences are quantitative (measurable), with a range of values between two extremes rather than discrete categories.

The differences between individuals of a species where the differences are qualitative (categoric), falling into discrete, distinguishable categories with no intermediates.

• Quantitative differences are measurable and fall along a range of values between two extremes (continuous variation)

• Qualitative differences fall into discrete, distinguishable categories with no intermediates (discontinuous variation)

• Continuous: height or mass of a human

• Discontinuous: ABO blood group in humans (a person can only have one of the four groups)

• Continuous: a range of values with no distinct categories

• Discontinuous: distinct, separate categories when the data is plotted

An interaction between genetics and the environment.

• Different alleles at a single locus have a small effect on the phenotype

• Different genes can have the same effect on the phenotype, adding together to give an additive effect

• A large number of genes with a combined effect on the phenotype are known as polygenes

A large number of genes that together have a combined (additive) effect on a single phenotypic characteristic, giving rise to continuous variation.

It occurs solely due to genetic factors; the environment has no direct effect.

• Different genes have different effects on the phenotype

• Different alleles at a single gene locus have a large effect on the phenotype

In continuous variation, a large number of genes with a combined additive effect on a phenotype are known as polygenes.

Discontinuous variation is controlled solely by genetic factors, with the environment having no direct effect.

The F8 gene codes for the blood-clotting protein Factor VIII. The different alleles at the F8 locus dictate whether normal Factor VIII is produced, and therefore whether an individual has haemophilia — a clear-cut category with a large effect from a single gene.

Phenotype = genotype + environment

Phenotype = genotype

False — that describes discontinuous variation; continuous variation shows a range of values between two extremes.

True

An environmental factor that affects the chance of survival (and reproduction) of an organism, influencing which alleles are passed on to the next generation.

Natural selection that keeps allele frequencies relatively constant over generations, favouring intermediate phenotypes and selecting against the extremes.

Natural selection that produces a gradual change in allele frequencies over several generations, favouring one extreme phenotype.

Very-low and very-high birth weights are selected against.

This leads to the maintenance of intermediate birth weights, keeping allele frequencies relatively constant over generations.

When there is:

• A change in the environment (and therefore a change in selection pressures), or

• A new advantageous allele appearing in the population.

• There is phenotypic variation within the population

• A selection pressure favours a particular phenotype, which is produced by particular alleles

• Individuals with the favoured phenotype are fitter, so are more likely to survive, reproduce and pass on the advantageous alleles

• Over several generations the frequency of the advantageous allele increases while the frequency of other alleles decreases.

• Warmer seas speed up fish metabolism, increasing their need for oxygen, yet warmer water holds less oxygen

• Larger fish have greater metabolic (oxygen) needs, so are affected more strongly by the increased temperature

• Smaller fish are therefore fitter and better adapted, so are more likely to reproduce and pass on their alleles

• Over generations the frequency of alleles for small body size increases.

Primarily because of the effect that temperature has on enzyme activity.

A change in allele frequencies in a population due to chance (rather than environmental selection pressures) determining which individuals survive, breed and pass on their alleles.

In small populations, chance can strongly affect which alleles are passed on, so some alleles may be lost or favoured purely by chance.

In large populations, chance variations in allele frequency usually even out across the whole population, so natural selection plays a more influential role.

When a previously large population suffers a dramatic fall in numbers (e.g. due to a major environmental event).

This reduces genetic diversity as alleles are lost, and surviving individuals end up breeding with close relatives (inbreeding).

The bottleneck (after the last Ice Age) left cheetahs with a serious lack of genetic variation.

Low genetic variation reduces the likelihood that the species can respond to and survive future environmental changes.

When only a small number of individuals from a large parent population start a new population.

Only some of the total alleles from the parent population are present (not all of the gene pool), and which alleles are present is due to chance. Allele frequencies may then change in a different direction from the parent population.

A gene pool is the complete range of DNA sequences (alleles) that exist in all the individuals of a population or species.

False — genetic drift has a greater effect in small populations, where chance more strongly affects which alleles are passed on.

True

The principle stating that, if certain conditions are met, the allele frequencies of a gene within a population will not change from one generation to the next.

They allow the calculation of allele and genotype frequencies within a population, and allow predictions to be made about how these frequencies will change in future generations.

• Organisms are diploid

• Organisms reproduce by sexual reproduction only

• There is no overlap between generations (parents do not mate with offspring)

• Mating is random

• The population is large

• There is no migration, mutation or selection

• Allele frequencies are equal in both sexes

The assumptions it relies on (no migration, mutation or selection, random mating, a large population) are very rarely all present in nature. The principle is therefore most useful for building models and making predictions.

• p = the frequency of the dominant allele

• q = the frequency of the recessive allele

A frequency is expressed as a proportion of the population, i.e. a number out of 1.

For example, if half of the population shows a particular genotype then its frequency is 0.5.

p + q = 1

p² + 2pq + q² = 1

where:

• p² = frequency of homozygous dominant genotype

• 2*pq = frequency of heterozygous* genotype

• q² = frequency of homozygous recessive genotype

An offspring can inherit the dominant allele from the father and the recessive from the mother (p × q), or the dominant from the mother and the recessive from the father (p × q).

There are two ways to produce a heterozygote, so the frequency is p × q + p × q = 2*pq*.

When applying the Hardy-Weinberg equations you should always start your calculation from the proportion of individuals showing the recessive phenotype.

The recessive phenotype is the only one whose genotype can be worked out immediately, because it is always homozygous recessive (q²).

The dominant phenotype is shown by both homozygous dominant and heterozygous individuals, so its genotype cannot be determined directly.

Step 1: The recessive phenotype is homozygous recessive, so q² = 0.10.

Step 2: Find q by taking the square root: q = √0.10 = 0.32.

Step 3: Use p + q = 1, so p = 1 − 0.32 = 0.68.

• The equations are used to estimate the allele and genotype frequencies in a population.

• The principle states that there is an equilibrium between allele frequencies, with no change between generations.

True

False — allele frequencies stay constant between generations when the conditions are met.

The proportion of all the alleles of a gene in a population that are of a particular form.

The formation of new species from pre-existing species over time, as a result of changes to gene pools from generation to generation.

When two reproductively isolated populations no longer interchange genes, so changes in their allele frequencies are not shared and they evolve independently.

When the genetic differences between them lead to an inability of members of the populations to interbreed and produce fertile offspring.

Because they no longer interchange genes when producing offspring, so changes that occur in the allele frequencies of each group are not shared between them.

• Allopatric speciation

• Sympatric speciation

Speciation that occurs as a result of geographical isolation, when populations of a species become separated by a geographical barrier. It is the most common type of speciation.

• Populations become separated by a geographical barrier (natural or man-made), so they are reproductively separated and no genetic exchange occurs

• Sufficient selection pressures act to change the gene pools and allele frequencies within both populations

• Over time the populations diverge, differing physiologically, behaviourally and morphologically, until they can no longer interbreed to form separate species

• Natural barriers, such as a body of water or a mountain range

• Man-made barriers, such as a motorway

Speciation that takes place with no geographical barrier: two populations living in the same area become genetically isolated because no gene flow occurs between them.

• Ecological separation: populations are separated because they live in different environments within the same area

• Behavioural separation: populations are separated because they have different behaviours (e.g. differences in feeding, communication or courtship behaviours)

Soil pH has a major effect on plant growth and flowering, so a population growing in soil of a slightly different pH may flower at a different time from another population, leading to reproductive separation and eventually genetic isolation.

The difference in feeding behaviour (some feed on the bottom, others in open water) originally causes the separation.

A difference in courtship displays, which arises from genetic isolation, is what then prevents the populations from breeding together.

Without mutations there are no new alleles for selection to act on. The genetic changes caused by mutations produce the differences in physiology, behaviour and morphology between populations that eventually lead to speciation.

Allopatric speciation is the most common type of speciation and occurs as a result of geographical isolation.

True — allopatric speciation results from geographical isolation, where a barrier separates the populations.

False — sympatric speciation occurs with no geographical barrier; the two populations live in the same area.

The process by which humans choose organisms with desirable traits and selectively breed them together to enhance the expression of those traits over many generations.

By their phenotype, not their genotype.

Knowledge of the specific alleles is not required — breeders simply select the individuals that visibly show the desired characteristics.

• The population shows phenotypic variation

• A breeder selects an individual with the desired phenotype

• A second individual with the desired phenotype is selected (the two should not be closely related)

• The two selected individuals are bred together

• Offspring reach maturity and are tested for the desirable trait; those showing it to the greatest degree are selected for further breeding

• The process is repeated over many generations until all offspring display the desirable trait

Selecting two parents with the desired characteristics, breeding them and stopping there is not selective breeding and will not produce a new breed.

The process must be repeated for many generations — selecting the best offspring each time — until all offspring reliably display the desirable trait.

Because the genetics is not always understood, breeders can accidentally enhance other traits that are genetically linked to the desirable trait.

These linked traits can sometimes negatively affect the organism's health.

Any of, for example:

• Cows, goats and sheep with higher milk or meat yield

• Chickens that lay large eggs

• Sheep with good quality wool

• Domestic dogs with a gentle nature

• Horses with fine features and a fast pace

Farmers select female cows with the highest milk yield and breed them with bulls related to high-yield females, repeated over many generations to increase milk yield.

Because selection focuses on only one characteristic, little thought is given to other traits: selectively bred cows are more prone to ailments such as mastitis, milk fever and lameness than randomly bred cows.

Any of, for example:

• Disease resistance in food crops

• Increased crop yield

• Hardiness to weather conditions (e.g. drought tolerance)

• Better tasting fruits

• Large or unusual flowers

A fungus-resistant allele from another species of wheat is introduced by selective breeding.

The resulting hybrid wheat plants are not susceptible to fungal infection (e.g. Fusarium head blight), so crop yield increases.

Introducing the allele into the crop population can take many generations.

It ensures the gene pool for the species does not become too small.

A small gene pool reduces variation and weakens the population, so keeping wild-type genetic material available preserves useful alleles for future breeding.

Heavily inbreeding maize (breeding plants with very similar genotypes) produces small, weaker plants with less vigour — this is inbreeding depression.

By outbreeding with wild-type varieties — breeding individuals that are not closely related.

This decreases the chance of harmful recessive alleles combining and being expressed, giving increased growth and survivability (hybrid vigour).

Selective breeding of closely related individuals reduces the gene pool, i.e. it reduces the number of alleles in a population.

Inbreeding reduces the gene pool, increasing the chance of organisms inheriting harmful genetic defects and being vulnerable to disease.

Some breeds suffer damaging inherited conditions, for example:

• Breathing problems in Bulldogs and Pekinese (shortened snouts)

• Hip dysplasia in very large dogs such as St Bernards and Great Danes

• Malignant blood vessel tumours in Golden Retrievers

It occurs because inbreeding increases the chance of harmful recessive alleles combining in an individual and being expressed in the phenotype, leading to decreased growth and survivability.

Taller, healthier maize plants with a greater yield, an effect known as hybrid vigour.

False — they are more prone to ailments such as mastitis, milk fever and lameness.

True