10.2 Inheritance

Question 1

What is the law of independent assortment? (2)

Answer 1

Pairs of alleles are inherited independently of one mother if their gene loci are on separate chromosomes
- these are unlinked genes

Question 2

Why does the law of independent assortment occur?

Answer 2

Due to the random orientation of homologous pairs during MI of meiosis

Question 3

What does the independent segregation of unlinked genes result in? (3)

Answer 3

• results in a greater number of potential gamete combination
• greater variety of possible phenotypes
• more complex inheritance patterns (mono hybrid vs hybrid crosses)

Question 4

What does the a dihybrid cross determine?

Answer 4

• the genotype and phenotype combinations of offspring for 2 particular genes that are unlinked

Question 5

What is a linkage group?

Answer 5

Group of genes whose loci are on the same chromosome and don’t independently sort

Question 6

What are 3 characteristics of linked groups of genes?

Answer 6

• liked genes tend to be inherited together and don’t follow normal Mendelian inheritance for dihybid cross
• instead of phenotypic ratio will be more closely aligned to a monohybrid cross as the 2 genes are inherited are a single unit
• linked genes may be separated via recombination (crossing over during synapsids in meiosis I)

Question 7

What did Thomas Human Morgan’s experiment involving fruit flies demonstrate?

Answer 7

Demonstrated that linked genes were not independently assorted

Question 8

How did Morgan discover that there was a clear sex bias in phenotypic distribution? and

Answer 8

when cross-breeding red-eyed wild types with white-eyed mutants
- all female offspring of red-eyed, but all male offspring of a white-eyed females were also white

Question 9

what was the conclusion that Morgan made from the experiment of the fruit flies of red-eyed wild types and the white-eyed mutants?

Answer 9

Morgan described this distribution as ‘sex-limited’ inheritance and inferred it was caused by the gene for eye colour bein located on a sex chromosome

Question 10

what proposals did Morgan make based on the data which showed that the number of different traits in fruit flies did not conform to Mendelian ratio? (2)

Answer 10

• alleles for these traits were located on a shared chromosome (gene linkage) and hence did not independently sort
• linked alleles could be uncoupled via recombination (crossing over) to create alternative phenotypic combinations but these new phenotypes would occur at a lower frequency

Question 11

what conclusion did Morgan make when he observed that the amount of crossing over between genes differed depending on the combination traits? what did Morgan do with this information? (2)

Answer 11

• crossing over frequency may be a product of the distance between 2 genes on a chromosome - genes with a higher crossover frequency are further apart, whereas genes with a lower crossover frequency are closer together
• morgan used this to develop first gene linkage maps that showed relative positions of genes on a chromosome

Question 12

what are recombinants of linked gene?

Answer 12

combinations of genes not found in the parents

Question 13

what do recombinants occur as a result of? and what does this cause? (3)

Answer 13

crossing over of genetic material during prophase I of meiosis
if linked genes become separated by a chismata there will be an exchange of alleles between the non-sister chromatids
create new allele combinations that are different to the parent

Question 14

why would the frequency of recombinant phenotypes within a population be lower than the non-recombinant phenotypes?

Answer 14

because crossing over is a random process and chismata do not form at the same location with every meiotic division

Question 15

what is the relative frequency of recombinant phenotypes dependent on? (3)

Answer 15

• dependent on the distance
• recombinant frequency between 2 linked genes will be greater when the genes are further on the chromosomes
• because there are more possible locations where a chiasma could form between the genes

Question 16

how can recombinant phenotypes be identified?

Answer 16

test cross

Question 17

why would offspring with unlinked genes have an equal possibility of inheriting any potential phenotypic combination?

Answer 17

due to random segregation of alleles via independent assortment

Question 18

what will offspring with linked genes only express? (2)

Answer 18

the phenotypic combinations present in either parent
unless crossing over occurs

Question 19

what will occur less frequently than linked parental phenotypes?

Answer 19

unlinked recombinant phenotypes

Question 20

what are chi-squared tests?

Answer 20

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

Question 21

what does is suggest if observed frequencies do not conform to those expected for dihybrid crosses? (2)

Answer 21

• genes are linked and do no independently assort
• the inheritance of the traits are not random, but potentially affected by natural selection

Question 22

What is the process of doing a chi-squared test? (5)

Answer 22

• identify hypotheses H0 and H1
• construct table of frequencies (observed vs expected)
• apply chi-squared formula
• determine the degree of freedom
  identify the p value

Question 23

what can variation in phenotypes for a particular characteristic be?

Answer 23

discrete or continous

Question 24

what are monogenic traits? (3)

Answer 24

• characteristics controlled by a single gene loci
• tend to exhibit discrete variation
• with individuals expressing one type of a number of distinct phenotypes

Question 25

what are polygenic traits? (3)

Answer 25

• characteristic controlled by more than 2 gene loci
• tend to exhibit continuous variation
• with individuals phenotype existing somewhere along a continuous spectrum of potential phenotypes

Question 26

what does increasing the number of loci for a particular trait do in polygenic inheritance? (2)

Answer 26

• increases the number of possible phenotypes
• results in a phenotypic distribution that follows a normal distribution curve

Question 27

describe the overall inheritance of maize

Answer 27

the colour of maize is controlled by 3 gene loci (grain colour range from white to dark red)
- each gene has 2 alleles which for red pigment or white pigment
- the most frequent combinations have an equal number of the 2 allele types
- overall pattern of inheritance of maize show continuous variation

Question 28

what are phenotypic characteristics not solely determined by genotype?

Answer 28

environmental factors
- increases the variation seen for a particular trait

Question 29

what is an example of a polygenic trait that is influenced by environmental factors? (3)

Answer 29

human height
- controlled by multiple genes (bell-shaped spectrum of potential phenotypes)
- environmental factors such as diet and health can further influence an individual human’s height

Question 30

what is another polygenic trait influenced by environmental factors? (2)

Answer 30

skin colour
- controlled by multiple melanin-producing genes, also affected by factors such as sun exposure