Chromosome Variations & Sex Determination

Question 1

Give the definition of euploid.

Answer 1

organisms with multiples of the basic chromosome set.

Question 2

Give the definition of monoploid.

Answer 2

an individual of a typically diploid species that has only one set of chromosomes.

Question 3

Give the definition of aneuploid.

Answer 3

Individuals whose chromosome number differs by one or a small number of chromosomes (can have a chromosome number greater/smaller than that of the wild type).

Question 4

How do cells end up with too many/few chromosomes?

Answer 4

• non-disjunction during meiosis or mitosis
• daughter cell has 1 too many chromosomes and the other is lacking
• risk of non-disjunction increases as mother age gets older

Question 5

Why do extra chromosomes affect the phenotype so drastically?

Answer 5

• gene balance, evolved in diploid organisms and disrupting that background disrupts their function (explains why aneuploids are so much more abnormal than polyploids)
• expression of deleterious alleles on monosomic autosomes

Question 6

Give possible changes in chromosome structure.

Answer 6

duplication (extra pieces), play an important role in the evolution of the genome
-deletion (missing pieces), can be small eg one gene, or large enough to be visualised on a karyotype
-copy number variations
-inversions and translocations
Inversions are balanced rearrangements, but can lead to duplications or deletions
Translocation is a rearrangement involving a part of one chromosome that has broken off and reattached to a different chromosome

Question 7

Give the key features of the Y chromosome.

Answer 7

• mostly repeated sequence
• few genes
• SRY is key maleness-determining gene
• inheritance father to son

Question 8

Give the key features of the X chromosome

Answer 8

• many genes all unrelated to sex-determination or sex function
• males are hemizygous for X chromosome
• X-linked traits in humans eg red-green colour deficiency, haemophilia, Duchenne muscular dystrophy

Question 9

Why does aneuploidy in chromosome 21 have no effect?

Answer 9

In female mammals one X chromosome is epigenetically inactivated early in development (lyonisation).

Question 10

How can X linked recessive traits can be deduced from certain clues?

Answer 10

• more males than females express the trait
• trait often skips a generation
• if a female expresses the trait, all of her male offspring will express the trait