Inheritance and Genetics: Biology 0610 (Cambridge O Level / IGCSE)

A pair of budgerigars is being bred for feather colour. In this species, feather colour is controlled by a single gene, which exists as two alternative forms: G, which produces green feathers, and g, which produces blue feathers. G is dominant to g.

Which statement correctly describes the relationship between this gene and its alleles?

A woman and a man, who already have two daughters, are expecting another child. A genetic counsellor is explaining to them how the sex of a baby is determined.

(a) State the sex chromosome combination found in the body cells of a human female, and in the body cells of a human male. [2]

(b) Using a genetic diagram, show the possible combinations of sex chromosomes that this next child could inherit, and use it to state the expected ratio of daughters to sons. [3]

(c) The couple's next two children are both sons. Explain why this does not mean that the expected ratio from part (b) was wrong. [2]

In a species of field cricket, wing length is controlled by a single gene with two alleles. The dominant allele, L, produces long wings; the recessive allele, l, produces short (vestigial) wings. Two long-winged crickets, both heterozygous for this gene, are bred together.

(a) State the genotype of each parent cricket, and explain what is meant by the term heterozygous. [2]

(b) Use a genetic diagram (Punnett square) to work out the possible genotypes of the offspring of this cross, and use it to give the expected phenotypic ratio of long-winged to short-winged offspring. [4]

(c) One of the offspring has short wings. State the genotype of this cricket, and explain how two long-winged parents can produce a short-winged offspring. [2]

In a species of cotton plant, fibre colour is controlled by a single gene. Brown fibre is produced by the dominant allele, F; white fibre is produced by the recessive allele, f. A plant breeder has a brown-fibred cotton plant of unknown genotype and wants to find out whether it is homozygous or heterozygous for this gene.

(a) State the two possible genotypes of the brown-fibred plant. [1]

(b) Describe how the breeder could use a test cross to determine the genotype of this plant, including the result that would be expected for each possible genotype. [3]

(c) The breeder crosses the unknown brown-fibred plant with a white-fibred plant. Of the offspring produced, 48 have brown fibre and 51 have white fibre. Use a genetic diagram to explain what this result shows about the genotype of the original brown-fibred plant. [3]

Haemophilia is a blood-clotting disorder caused by a recessive allele, h, carried on the X chromosome. The dominant allele, H, allows normal blood clotting. A man who has haemophilia (genotype XhY) has children with a woman who has normal blood clotting but is a carrier of the h allele (genotype XHXh).

Which statement correctly gives the chance that a child of this couple is an unaffected carrier daughter, and correctly identifies which parent contributes the h allele she carries?

A species of aquarium fish has 24 chromosomes in each body (somatic) cell, arranged as 12 homologous pairs. A skin cell from an injured fin divides by mitosis to replace damaged tissue as the fin heals.

Which statement correctly describes the daughter cells produced by this mitotic division?

The aquarium fish species described above has 24 chromosomes in each body cell (12 homologous pairs). In the reproductive organs of an adult fish, cells divide by meiosis to produce gametes (sperm or egg cells).

(a) State the number of chromosomes present in one gamete of this fish species, and explain why this number is different from the number in a body cell. [2]

(b) State three ways in which meiosis differs from mitosis, referring to the number of divisions involved, the number of daughter cells produced, and whether the daughter cells are genetically identical to the parent cell. [3]

(c) Explain why it is important that meiosis halves the chromosome number, in terms of what happens when two gametes fuse at fertilisation. [2]

Albinism is an inherited condition in which the skin and hair lack pigment. A family's pedigree is described below.

Generation I: Individual 1 (unaffected male) and Individual 2 (unaffected female) are married. They have three children, who make up Generation II: Individual 3 (unaffected female), Individual 4 (male, has albinism), and Individual 5 (unaffected male).

Individual 3 marries an unrelated, unaffected male, Individual 6 (also Generation II). Individual 3 and Individual 6 have one child, Individual 7 (Generation III), who has albinism.

(a) Using evidence from Individuals 1, 2 and 4, state whether the allele for albinism is dominant or recessive, and explain your reasoning. [2]

(b) Using N to represent the dominant (normal pigmentation) allele and n to represent the recessive (albinism) allele, state the genotypes of Individual 1 and Individual 2. [2]

(c) Individual 3 and Individual 6 are both unaffected, yet their child, Individual 7, has albinism. State the genotypes of Individual 3, Individual 6 and Individual 7, and explain how two unaffected parents can have a child with albinism. [4]

Human ABO blood group is controlled by a gene with three alleles: IA and IB are codominant with each other, and both are dominant to the recessive allele i. A woman with blood group AB (genotype IAIB) has children with a man with blood group O (genotype ii).

Which statement correctly gives the possible blood groups of their children?

Human ABO blood group is controlled by a gene with three alleles: IA and IB are codominant with each other, and both are dominant to the recessive allele i. A father has blood group A, but a blood test shows he also carries the recessive i allele. A mother has blood group B, and a blood test shows she also carries the recessive i allele.

(a) State the father's genotype, and explain what is meant by the term codominance, using IA and IB as examples. [2]

(b) Use a genetic diagram to work out the possible genotypes and blood groups of the couple's children, and state the expected ratio of the four possible blood groups. [4]

(c) One of their children has blood group O. Explain how this is possible, given that neither parent has blood group O. [2]