Topic 25-26: Variation & Probability

Question 1

Describe what is meant by chromosomes, genes, and alleles

Answer 1

• Chromosomes: structures made up of DNA coiled around proteins (DNA protein complexes), carrying genetic information from cell to cell.
• Genes: sections along DNA that are coded for proteins, which are found at same location (locus) on homologous chromosomes.
• Alleles: variants of 1 gene which produce proteins that are slightly different.

Question 2

Describe somatic cells and gametes

Answer 2

• Somatic cells (non-sex): 46 or 23 pairs of chromosomes which derived from mitosis producing genetically identical daughter cells.
• Gametes (sex cells): 23 chromosomes which derived from meiosis producing genetically different daughter cells from each other and from the germ cells.

Question 3

What are homologous chromosomes?

Answer 3

Homologous chromosomes are similar in length and positions of centromeres. The genes are the same, but not necessarily the same alleles.

Question 4

Distinguish genotype and phenotype

Answer 4

• Genotype: combination of alleles for a particular gene
  + Homozygous dominant: mask recessive
  ~ NOT about preferential expression
  ~ NOT necessarily more common
  ~ NOT necessarily the “normal”
  + Homozygous recessive: being masked
  ~ Can be expressed normally, just the protein cannot work well
  ~ Sometimes, they are the code for “normal” protein
  + Heterozygous
• Phenotype: observable and measurable traits as interplay between genetic trait and environmental factors
  + Co-dominance: 2 phenotypes expressed equally
  + Incomplete dominance: intermediate between dominant and recessive phenotypes

Question 5

Explain the potential consequences of mutations

Answer 5

• Mutations can lead to diseases, but they are not always the case
  + Can be harmless: no phenotypic changes and unaffected protein functions
  + Can be inherited from parents in gametes
  + Can be acquired in lifetime in somatic cells, the principal driver of many diseases

Question 6

Explain how chromosomes determine the sex of a human individual

Answer 6

• Sex of a human is determined by X and Y chromosomes
• In Y chromosome, there is a section called sex-determining region Y (SRY) gene
  + Initiates testes development –> XY: male

Question 7

Explain different modes of inheritance for sex-linked disorders

Answer 7

• X-linked dominant:
  + Affected female: homo- or heterozygous genotype
  + Affected male: hemizygous (1 abnormal allele is sufficient to cause disease)
• X-linked recessive:
  + Affected female: homozygous recessive
  + Male: hemizygous
  + Mostly in males
  + Females tend to be carriers
• Y-linked: only inherited from father to son

Question 8

Explain different modes of inheritance for autosomal disorders

Answer 8

• Genes on chromosomes 1-22:
  + Autosomal dominant
  + Autosomal recessive

Question 9

What does it mean a carrier?

Answer 9

A person who has a copy of mutated or disease-causing gene and can pass down to offspring but do not express the phenotypes.