6.3Chromosome Structure Questions and Answers

6.3Chromosome structure – Questions and answers

Q1.

Bk Ch6 S6.3 Q1

Outline the contribution made by Walter Sutton and Theodor Boveri to the study of heredity.

A1.

Bk Ch6 S6.3 A1

Walter Sutton proposed the theory that inherited factors are carried on chromosomes after observing the movement of chromosomes during the process of meiosis in grasshoppers. Theodor Boveri demonstrated that a connection existed between chromosomes and the inheritance of characteristics.

Q2.

Bk Ch6 S6.3 Q2

Describe the relationship between chromosomes and genes. Use a diagram to illustrate your answer.

A2.

Bk Ch6 S6.3 A2

Sutton and Boveri realised that organisms had many more inherited characteristics than chromosomes. They were able to establish that chromosomes carry the units of inheritance. A chromosome is a strand of DNA on which inheritance factors or genes are located.

Q3.

Bk Ch6 S6.3 Q3

Define ‘nucleotide’, and outline its relationship to DNA. Use a diagram to explain your answer.

A3.

Bk Ch6 S6.3 A3

A nucleotide is a structural unit of DNA. Each nucleotide contains a sugar, a phosphate and one of four different base groups. The bases are adenine, guanine, thymine and cytosine and are symbolised by the letters A, G, T and C respectively.

Q4.

Bk Ch6 S6.3 Q4

The following diagram represents part of a single strand of a DNA molecule.

Draw the other strand of the DNA molecule, using the correct complementary base sequence.

A4.

Bk Ch6 S6.3 FA4

The other strand:

Q5.

Bk Ch6 S6.3 Q5

Explain how crossing over contributes to variation during the production of gametes. Use diagrams to illustrate your answer.

A5.

Bk Ch6 S6.3 A5

During crossing over in meiosis, chromosomal material is exchanged between homologous pairs. This increases the variation that occurs in gametes.

Q6.

Bk Ch6 S6.3 Q6

aDefine ‘sex linkage’.

bDescribe a specific example of the work by Thomas Hunt Morgan that led him to hypothesise about the existence of sex linkage.

A6.

Bk Ch6 S6.3 A6

aSex linkage refers to genes that are carried on the X chromosomes. Sex-linked characteristics occur more often in one sex than the other because they are inherited on the X chromosome.

bThomas Hunt Morgan bred fruit-flies (Drosophila) in the early 20th century in an attempt to replicate the work of Gregor Mendel. However, the characteristic of eye colour that he chose to investigate led him to the hypothesis that some characteristics are linked to the sex of individuals and are therefore sex-linked. When he mated a white-eyed male with a red-eyed female all of the F1 were red-eyed suggesting that red eye colour is dominant to white. However, when he crossed the red-eyed offspring the results he obtained were quite unexpected. Instead of red-eyed flies and white-eyed flies turning up in the 3 : 1 ratio he observed a spread of red-eyed males, red-eyed females and white-eyed males but no white-eyed females.

W: red eye colour (dominant); w: white eye colour (recessive).

Parents: XwYXWXW

F1: XWXw red-eyed females, XWY red-eyed males

F2:

XW / Xw
XW / XWXW / XWXw
Y / XWY / XwY

When he carried out a test cross of the original white-eyed male with an F1 red-eyed female, both red eye and white eye occurred in the male and female offspring.

XW / Xw
Xw / XW Xw / Xw Xw
Y / XWY / XwY

This led him to conclude that the gene for eye colour in Drosophila is carried on the X chromosome and since the Y chromosome is an unmatched pair for the X chromosome it would not carry the same genes. Since females have two X chromosomes this would account for the non-appearance of white-eyed females in the F1—they would have genotype XWXW or XWXw, both resulting in the red-eyed phenotype. Only when the test cross was performed did white-eyed females occur, resulting from the combination of X chromosomes carrying the white-eye allele from both the mother and the father. Morgan’s hypothesis reliably accounted for his observations.

Q7.

Bk Ch6 S6.3 Q7

aDefine ‘codominance’.

bThe MN human blood group is another example of codominance. If an MM homozygous male has a child with an NN homozygous female, what will be the blood group of their child? Use a diagram to illustrate your answer.

A7.

Bk Ch6 S6.3 A7

aCodominance refers to the expression of two alleles in the heterozygote, that is, neither allele is dominant or recessive to the other—both appear in the phenotype.

bThe child will have the genotype MN and have the phenotype of MN type blood, that is, the alleles M and N are codominant with both being expressed in the child.

N / N
M / MN / MN
M / MN / MN

6.3 Chromosomes QApage 1 of 4