10.2 Inheritance

Question 1

What does it mean if alleles are unlinked?

Answer 1

According to the law of independent assortment, pairs of alleles are inherited independently of one another if their gene loci are on separate chromosomes – these genes are said to be unlinked

Question 2

Why are some alleles unlinked?

Answer 2

This is due to the random orientation of homologous pairs during metaphase I of meiosis

Question 3

What does independent segregation lead to in terms of variation?

Answer 3

The independent segregation of unlinked genes results in a greater number of potential gamete combinations, as well as a greater variety of possible phenotypes

Question 4

What does independent segregation lead to in terms of inheritance patterns?

Answer 4

This also results in more complex inheritance patterns (e.g. monohybrid versus dihybrid crosses)

Question 5

What does a dihybrid cross determine?

Answer 5

A dihybrid cross determines the genotypic and phenotypic combinations of offspring for two particular genes that are unlinked

Question 6

How many gamete combinations can there be in a dihybrid cross?

Answer 6

Because there are two genes, each with two alleles, there can be up to four different gamete combinations

Question 7

What is a linked group?

Answer 7

A linkage group is a group of genes whose loci are on the same chromosome and hence don’t independently assort

Question 8

What do linked genes not exhibit?

Answer 8

Linked genes will tend to be inherited together and hence don’t follow normal Mendelian inheritance for a dihybrid cross

Question 9

What will be the phenotypic ratio for linked genes?

Answer 9

Instead the phenotypic ratio will be more closely aligned to a monohybrid cross as the two genes are inherited as a single unit

Question 10

When may linked genes become separated?

Answer 10

Linked genes may become separated via recombination (due to crossing over during synapsis in meiosis I)

Question 11

Who provided insight into gene linkage?

Answer 11

Thomas Hunt Morgan provided a key contribution to our current understanding of gene linkage by discovering non-Mendelian ratios in Drosophila melanogaster (fruit flies)

Question 12

What did Thomas Hunt Morgan show in terms of linked genes?

Answer 12

His breeding experiments involving fruit flies clearly demonstrated that linked genes were not independently assorted

Question 13

What did Thomas Hunt Morgan show in terms of sex linkage?

Answer 13

When cross-breeding red-eyed wild types with white-eyed mutants, he discovered a clear sex bias in phenotypic distribution

Question 14

What were the phenotypes of the fruit flies which suggested sex linkage?

Answer 14

All female offspring of a red-eyed male were red-eyed, whereas all male offspring of a white-eyed female were also white-eyed

Question 15

How did Morgan explain his findings related to sex linkage?

Answer 15

Morgan described this distribution as ‘sex-limited’ inheritance and inferred it was caused by the gene for eye colour being located on a sex chromosome (i.e. X-linked)

Question 16

What other features did morgan find that did not abide by mendelian ratios?

Answer 16

Morgan went on to identify a number of different traits in fruit flies that did not conform to Mendelian ratios

Certain phenotypic combinations occurred in much lower frequencies than was to be expected

Question 17

What two explanations did Morgan present for the offspring not exhibiting Mendelian ratios?

Answer 17

The alleles for these traits were located on a shared chromosome (gene linkage) and hence did not independently assort

Linked alleles could be uncoupled via recombination (crossing over) to create alternative phenotypic combinations, but these new phenotypes would occur at a much lower frequency

Question 18

What did Morgan also observe in terms of crossing over?

Answer 18

Morgan also observed that the amount of crossing over between linked genes differed depending on the combination of traits

Question 19

What could Morgan conclude about crossing over?

Answer 19

This led to the idea that crossover frequency may be a product of the distance between two genes on a chromosome – genes with a higher crossover frequency are further apart, whereas genes with a lower crossover frequency are closer together

Question 20

What did Morgan develop with his findings?

Answer 20

Morgan used this concept to develop the first gene linkage maps that showed the relative positions of genes on a chromosome

Question 21

What can be seen from Morgan’s gene linkage map?

Answer 21

From the above diagrams it can be seen that:

Long aristae and red eyes are more likely to be separated via recombination (high crossover frequency)
Long aristae and long legs are less likely to be separated via recombination (low crossover frequency)

This indicates that the aristae and leg genes are located closer together, whereas the eye gene is more distant

Question 22

What are recombinants of linked genes?

Answer 22

Recombinants of linked genes are those combinations of genes not found in the parents

Question 23

What creates recombinants?

Answer 23

Recombinants occur as a result of crossing over of genetic material during prophase I of meiosis

Question 24

What occurs during prophase I which leads to recombinants?

Answer 24

If linked genes become separated by a chiasma, there will be an exchange of alleles between the non-sister chromatids

!This creates new allele combinations that are different to those of the parent

Question 25

Will the frequency of non/recombinants be higher in a population?

Answer 25

The frequency of recombinant phenotypes within a population will typically be lower than that of non-recombinant phenotypes

Question 26

Why will non-recombinant phenotypes be more rare?

Answer 26

This is because crossing over is a random process and chiasmata do not form at the same locations with every meiotic division

Question 27

What will the relative frequency of recombinant phenotypes be dependent on?

Answer 27

The relative frequency of recombinant phenotypes will be dependent on the distance between linked genes

Question 28

When will recombination frequency be greater?

Answer 28

Recombination frequency between two linked genes will be greater when the genes are further apart on the chromosome

Question 29

Why is recombination frequency greater when genes are further apart?

Answer 29

This is because there are more possible locations where a chiasma could form between the genes

Question 30

How can recombinant phenotypes be identified?

Answer 30

Recombinant phenotypes can be identified by performing a test cross (crossing with a homozygous recessive for both traits)

Question 31

How is it possible to determine whether genes are liked or unlinked?

Answer 31

It is possible to infer whether two genes are linked or unlinked by looking at the frequency distribution of potential phenotypes

Question 32

How is it possible to tell that offspring have unlinked genes?

Answer 32

Offspring with unlinked genes have an equal possibility of inheriting any potential phenotypic combination

Question 33

Why do offspring with unlinked genes have an equal possibility?

Answer 33

This is due to the random segregation of alleles via independent assortment

Question 34

How is it possible to tell that offspring have linked genes? (there is one exception)

Answer 34

Offspring with linked genes will only express the phenotypic combinations present in either parent unless crossing over occurs

Question 35

Which phenotype appears less often in offspring with linked genes?

Answer 35

Consequently, the ‘unlinked’ recombinant phenotypes occur less frequently than the ‘linked’ parental phenotypes

Question 36

What are chi-squared tests used for?

Answer 36

Chi-squared tests are a statistical measure that are used to determine whether the difference between an observed and expected frequency distribution is statistically significant

Question 37

What does it suggest if observed frequencies do not conform to those expected for an unlinked dihybrid cross?

Answer 37

Genes are linked and hence not independently assorted
The inheritance of the traits are not random, but are potentially being affected by natural selection

Question 38

How may phenotypes vary? (2 ways?

Answer 38

Variation in phenotypes for a particular characteristic can be either discrete (discontinuous) or continuous

Question 39

What are monogenic traits?

Answer 39

Monogenic traits (characteristics controlled by a single gene loci) tend to exhibit discrete variation, with individuals expressing one of a number of distinct phenotypes

Question 40

What are polygenic traits?

Answer 40

Polygenic traits (characteristics controlled by more than two gene loci) tend to exhibit continuous variation, with an individual’s phenotype existing somewhere along a continuous spectrum of potential phenotypes

Question 41

For polygenic inheritance, what does increasing the number of loci do?

Answer 41

Increasing the number of loci responsible for a particular trait increases the number of possible phenotypes

Question 42

What curve does the phenotypic distribution show for polygenic traits?

Answer 42

This results in a phenotypic distribution that follows a Gaussian (bell-shaped) normal distribution curve

Question 43

What is an example of a polygenic trait?

Answer 43

An example of a polygenic trait is grain colour in maize (wheat), which is controlled by three gene loci

Question 44

How may grain colour vary and why?

Answer 44

Grain colour can range from white to dark red, depending on the amount of pigment that is expressed

Question 45

What does each gene have in maize?

Answer 45

Each gene has two alleles, which either code for red pigment or white pigment

Question 46

What do the most frequent combinations have in terms of alleles for maize?

Answer 46

The most frequent combinations have an equal number of the two allele types

Question 47

What is rare in maize?

Answer 47

Conversely, combinations of one extreme or the other are relatively rare

Question 48

What pattern does polygenic genes in maize show?

Answer 48

The overall pattern of inheritance shows continuous variation

Question 49

What can phenotypic variations also be determined by, apart from the genotype?

Answer 49

Phenotypic characteristics are not solely determined by genotype, but are also influenced by environmental factors

Question 50

What is the role of environmental factors in determining phenotype?

Answer 50

The added effect of environmental pressures functions to increase the variation seen for a particular trait

Question 51

What is an example of a polygenic trait affected by environmental factors?

Answer 51

One example of a polygenic trait that is influenced by environmental factors is human height

Question 52

What is human height controlled by?

Answer 52

Human height is controlled by multiple genes (polygenic), resulting in a bell-shaped spectrum of potential phenotypes

Question 53

Apart from the genotype, what environmental factors can affect human height?

Answer 53

Environmental factors such as diet and health (disease) can further influence an individual human’s height

Question 54

Apart from height, what other trait is polygenic and affected by environmental factors?

Answer 54

human skin colour

Question 55

What is human skin colour controlled by?

Answer 55

Skin colour is controlled by multiple melanin producing genes, but is also affected by factors such as sun exposure