17. Inherited Change

Question 1

Define ‘genotype’

Answer 1

The genetic constitution of an organism.

Question 2

Define ‘phenotype’

Answer 2

The observable or biochemical characteristics of an organism. It’s the result of the interaction between the expression of the genotype and the environment.

Question 3

Define ‘gene’

Answer 3

A length of DNA that normally codes for a particular polypeptide.

Question 4

Define ‘locus’

Answer 4

The position of a gene on a particular DNA molecule.

Question 5

Define ‘allele’

Answer 5

One of the different forms of a gene.

Question 6

Define ‘homologous chromosomes’

Answer 6

A pair of chromosomes, one maternal and one paternal, that have the same gene loci and therefore determine the same features. They are not necessarily identical, however, as individual alleles of the same gene may vary. They’re capable of pairing during meiosis.

Question 7

What does ‘homozygous’ mean?

Answer 7

If the allele on each of the chromosomes is the same.

Question 8

What does ‘heterozygous’ mean?

Answer 8

The 2 alleles on each chromosomes are different.

Question 9

What is a ‘dominant allele’?

Answer 9

An allele that is always expressed in the phenotype of an organism.

Question 10

What is a ‘recessive allele’?

Answer 10

The effect of the allele is only apparent in the phenotype of a diploid organism only in the presence pf another identical allele (has to be homozygous recesive)

Question 11

What does ‘homozygous dominant’ mean?

Answer 11

A homozygous organism with 2 dominant alleles.

Question 12

What does ‘homozygous recessive’ mean?

Answer 12

A homozygous organism with 2 recessive alleles.

Question 13

Define ‘co-dominance’

Answer 13

Both alleles for one gene in a heterozygous organism contribute to the phenotype.

Question 14

Define ‘multiple alleles’

Answer 14

A gene with more than 2 possible alleles, e.g. ABO blood group.

Question 15

Define ‘monohybrid inheritance’

Answer 15

The inheritance of a single gene.

Question 16

Define ‘pure breeding’

Answer 16

Homozygous individuals with the same genotype are constantly bred with each other, so their offspring have the same genotype/characteristics as parents.

Question 17

What is the F1 (first filial) generation?

Answer 17

The first generation of offspring.

Question 18

What is the F2 (second filial) generation?

Answer 18

The second generation of offspring

Question 19

Why do actual results of genetic crosses rarely the same as predicted results?

Answer 19

Discrepancies due to statistical error. It’s pure chance which gametes fuse with which.

Question 20

How do we increase the likelihood that the experimental value will mirror the theoretical value?

Answer 20

Use a larger sample size.

Question 21

A cross was carried out between a pea plant with green pods and a pea with yellow pods. The seeds from this cross were germinated and all of the 63 plants grown, all produced green pods.

a) State the probable genotype of the parent plant with green pods.
b) Explain why we cannot be absolutely certain of the parents phenotype.

Answer 21

a) GG- homozygous dominant
b) We can’t be sure because if the unknown genotype were heterozygous the gametes produced would contain both dominant G and recessive g. It’s a matter of chance which of these gametes fuses with those from our recessive parent- all these gametes have a recessive allele g. It’s just possible that in every case the gametes with the dominant allele will fuse so all offspring show dominant character. Provided the sample if offspring is large enough, we can be reasonably sure that the unknown genotype is homozygous dominant.

Question 22

In a cross between different peas plants with green pods and yellow pods, 96 plants were produced. 89 of these had green pods and 7 had yellow pods.

a) State the probable genotype of the parent with green pods.
b) Evaluate how certain we can be of the genotype of the parent plant with green pods.
c) In the cross, state the chance of any of the offspring having yellow pods.
d) Calculate the percentage of offspring with yellow pods that were actually produced in the cross described.

Answer 22

a) Gg heterozygous
b) Certain because 7 offspring display recessive character. These plants are homozygous recessive and must have obtained a recessive allele from each parent. Our unknown parental genotype must therefore have a recessive allele from each parent and be heterozygous. It’s theoretically possible that plants with yellow pods were due to a nutation but this is unlikely. The unexpectedly low number of plants with yellow pods is the result of random fusion of gametes.
c) 50%
d) 7.29%

Question 23

What is ‘dihybrid inheritance’?

Answer 23

When 2 characters, determined by 2 different genes, located on different chromosomes are inherited.

Question 24

What were the 2 characteristics of pea plants that Mendel investigated the inheritance of?

Answer 24

• Seed shape

- Seed colour

Question 25

List the gametes:

RRGG

Answer 25

RG

Question 26

List the gametes:

rrgg

Answer 26

rg

Question 27

List the gametes:

RrGg

Answer 27

RG, Rg, rG, rg

Question 28

Why are 4 types of gamete produced in the dihybrid cross of heterozygous individuals ?

Answer 28

E.g. The gene for seed colour and the gene for seed shape are on separate chromosomes. As the chromosomes arrange themselves at random on the equator during meiosis, any one of the 2 alleles for seed colour can combine with any one of the 2 alleles for seed shape. Fertilisation is random, so that any of the four types of gamete can combine with any of the four types from another plant.

Question 29

What’s the theoretical ratio for the dihybrid cross of 2 heterozygous individuals?

Answer 29

9:3:3:!

Question 30

Define Mendel’s ‘law of independent assortment’

Answer 30

Each member of a pair of alleles may combine randomly with either of another pair.

Question 31

In fruit flies, a pure breeding variety with red eyes and vestigial wings was crossed with a pure breeding variety with pink eyes and normal wings. All the F1 flies had red eyes and normal wings When these F1 flies bred with one another, the F2 generation produced the following types and numbers:
Red eyes, vestigial wings- 125
Red eyes, normal wings- 376
Pink eyes, vestigial wings- 41
Pink eyes, normal wings- 117

a) which characteristics dominant and which recessive?
b) Suggest suitable symbols to represent alleles involved

Answer 31

a) Red eyes and normal wings are dominant because these characteristics are expressed in the F1 generation while pink eyes and vestigial wings are not expressed so must be recessive. Red eyes and normal wings appear 3x more often in the F2 generation than pink eyes and vestigial wings.

b) R red eyes, r pink eyes.
N normal wings, n vestigial wings.

Question 32

In shorthorn cattle one allele codes for an enzyme that catalyses the formation of red pigment in hairs. The other allele codes for an altered enzyme that lacks the catalytic activity so doesn’t produce pigment and hair is white. The alleles are co-dominant. Describe the 3 coat colours of flower found

Answer 32

• Homozygous for first allele, both alleles code for the enzyme/ hence pigment produced. Cattle have a red coat.
• Homozygous for second allele, both alleles code for no enzyme and no pigment produced. Cattles have a white coat.
• Heterozygous, both coloured hairs produced so coat light red/roan

Question 33

Why can’t we use upper and lower case letters, only subscripts, to show alleles?

Answer 33

Would imply one allele is dominant to the other. Written as C^R C^R instead of RR

Question 34

What are the three alleles associated with gene I (immunoglobulin gene) for blood group

Answer 34

allele I^A, which produces antigen A
allele I^B, which produces antigen B
allele I^O, doesn’t produce either antigen

Question 35

Even though there are 3 alleles for the immunoglobulin gene, only 2 can be present in the genotype. Why is this?

Answer 35

There are only 2 homologous chromosomes and therefore only 2 gene loci.

Question 36

What are the possible genotypes for blood group A?

Answer 36

I^A I^A or I^A I^O

Question 37

What are the possible genotypes for blood group B?

Answer 37

I^B I^B or I^B I^O

Question 38

What are the possible genotypes for blood group AB?

Answer 38

I^A I^B, A and B are co-dominant so the individual has both antigens A and B.

Question 39

What are the possible genotypes for blood group O?

Answer 39

I^O I^O

Question 40

A man claims not to be the father of a child. The man is blood group O while the mother of the child is blood group A and the child’s blood group is AB. State whether the man could be the father.

Answer 40

No.
The man has the homozygous genotype I^O I^O, as O allele is recessive. Therefore the child would have inherited the O allele if the man was his father, so would be either blood group A or O.
The child however is blood type AB so must have the genotype I^A I^B. The child would’ve inherited the A allele from his mother, who is type A with genotype I^A I^A or I^A I^O. So the B allele must have come from his real father, who must be type B or AB.

Question 41

Siamese cats have dark hair on face, feet and tail of their coat. This is due to presence of pigment, which is controlled by the enzyme tyrosinase. Explain why Siamese kittens are born with a completely light coloured coat and only develop their characteristic markings some days later.

Answer 41

Kittens develop inside their mother and so are kept warm. As the kitten’s coat is light coloured, tyrosinase must be denatured. After birth, a kitten is exposed to cooler environmental temperatures and its extremities will be coolest as they’re furthest away from the main body where heat is generated and has largest surface area to volume ratio. Cooler temperature activates tyrosine. Tyrosinase therefore catalyses the production of dark pigment in these areas.

Question 42

Which sex gametes do females produce?

Answer 42

2 single X gametes

Question 43

Which sex gametes do males produce?

Answer 43

An X and a Y gamete

Question 44

Define sex linkage

Answer 44

Any gene carried on either the male or female chromosme.

Question 45

Why do sex-linked diseases occur more prevalently in males?

Answer 45

The X chromosome is much longer than Y. therefore, for most of the length of X, there’s no equivalent homologous portion of the Y chromosome. Those characteristics that are controlled by recessive alleles on this non-homologous portion of the X chromosome will appear more frequently in the male. This is because there’s no homologous portion on the Y chromosome that might have the dominant allele, in the presence of which the allele doesn’t express itself.

Question 46

Give an example of an X-linked disorder

Answer 46

Haemophilia- caused by defective gene on X. Condition almost entirely affects males.

Question 47

How is a male represented on a pedigree chart?

Answer 47

Square

Question 48

How is a female represented on a pedigree chart?

Answer 48

Circle

Question 49

How is a phenotypic disorder represented on a pedigree chart?

Answer 49

Shaded

Question 50

Define ‘autosome’

Answer 50

The remaining 22 chromosomes, excluding sex chromsomes.

Question 51

Define ‘autosomal linkage’

Answer 51

Where 2 or more genes are carried on the same autosome.

Question 52

Why do linked genes remain together during meoisis?

Answer 52

Assuming no crossing over, all linked genes remain together during meiosis and so pass into gametes and offspring together. They don’t segregate, in accordance with Mendel’s law of independent assortment.

Question 53

A and B are linked, as are a and b. Give the possible gametes produced, and contrast them to non-linked chromosomes.

Answer 53

Linked: AB, ab

Not linked: AB, Ab, aB, ab

Question 54

Give the phenotypic ratio if autosomal linkage occurs within 2 heterozygous organisms

Answer 54

3:1

Only 2 different gametes for each individual, rather than 4.

Question 55

Distinguish between sex-linkage and autosomal linkage

Answer 55

In sex-linkage the linked genes are on the same sex chromosome (usually X) whereas in autosomal linkage they’re on any chromosome other than sex chromosomes.

Question 56

Define epistasis

Answer 56

Arises when the allele of one gene affects or masks the expression of another in the phenotype.
E.g. gene 1- controls distribution of coloured hairs
gene 2- controls melanin pigment
Even if gene 1 is present, if 2 isn’t present the organism will be albino as there’s no melanin to create colour coating.

Question 57

Using the example of epistasis in mice, consider a cross between mouse 1 that is heterozygous for gene A (colour distribution) and homozygous recessive for gene B (melanin production) with mouse 2 that is heterozygous for both genes.

a) State the colour of mouse 1 and 2.
b) List the genotypes of their offspring
c) State the ratio of different phenotypes produced by this cross

Answer 57

a) mouse 1- albino, mouse 2- agouti colour
b) AABb, AAbb, AaBb, Aabb, aaBb, aabb
c) 4 albino, 3 agouti, 1 blacl

Question 58

Some varieties of corn have purple seeds due to the pigment anthocyanin in their coats. IN the absence of the pigment the seeds are white, The production of anthocyanin is controlled by both genes A and B. Suggest how epistasis can explain the production of anthocyanin in some plants but not others.

Answer 58

The production of anthocyanin uses a biochemical pathway that requires 2 functional enzymes each coded for by dominant allele of both genes A and B. If either gene is represented by 2 recessive alleles the enzyme it codes for is non-functional and the pathway can’t be completed. This is an example of epistasis as it affects the other gene in that, even if functional and produces its enzyme, its effects cannot be expressed because no pigment can be manufactured without the function of the other gene.

Question 59

Give the formula for chi squared

Answer 59

chi squared= sum of (observed numbers/expected numbers)^2/

expected numbers

Question 60

What does chi squared test measure?

Answer 60

A means of testing whether deviation between the observed and expected numbers is significant.
Used to test the null hypothesis: to examine the results of scientific investigations based on the assumption that there will be no statically significant difference between our sets of observations, any difference being to chance alone.

Question 61

What is the critical value of the chi squared test?

Answer 61

p=0.05: the attribution to chance, 5% due to chance. If the probability that the deviation is due to chance equal or greater than 0.005, the deviation is said to be not significant and the null hypothesis is accepted. If probability of deviation due to chance is less than 0.05, the deviation is significant, null hypothesis rejected.