CIE iGCSE Co-Ordinated Science B16.3 Monohybrid inheritance Exam Style Questions Paper 3
Question
(a) Albinism is a condition in humans where no pigment is made by the skin.
The allele for no albinism is dominant and represented by the letter A.
The allele for albinism is recessive and represented by the letter a.
(i) Table 4.1 shows some genotypes, phenotypes and descriptions of the genotypes for three people.
Complete Table 4.1.
(ii) Person 2 and person 3 have a child together.
Complete the Punnett square diagram in Fig. 4.1 to show the possible genotypes of the offspring.
(b) Chromosomes carry genetic information.
(i) Complete the definition of the term chromosome.
A chromosome is a thread-like structure of …………………………… ,
carrying genetic information in the form of …………………………… .
(ii) State where chromosomes are found in human cells.
(iii) State the sex chromosomes found in male gametes in humans.
……………………………… or ………………………………
(c) State the name of the male gametes in humans.
(d) State the name of the organ where male gametes are produced in humans.
▶️Answer/Explanation
Ans : 4(a)(i) 
4(a)(ii) 
4(b)(i) DNA ;
genes ;
4(b)(ii) nucleus ;
4(b)(iii) X and Y ;
4(c) sperm ;
4(d) testes ;
Question
(a) A student has two peas.
Pea A is green. Pea B is yellow. The colour of peas is controlled by a single gene.
- The allele for green-coloured peas is g.
- The allele for yellow-coloured peas is G.
- Circle two words that can be used to describe the genotype of pea A.
allele dominant heterozygous homozygous phenotype recessive - State the two possible genotypes of pea B.
…… and ……
▶️Answer/Explanation
Answer:
1. Homozygous and recessive
2. GG and Gg
Explanation:
– Pea A is green, which is the recessive trait, so its genotype must be homozygous recessive (gg).
– Pea B is yellow, which is the dominant trait. It can be either homozygous dominant (GG) or heterozygous (Gg).
(b) Two pea plants are crossed.
Fig. 7.1 is an incomplete genetic diagram showing a genetic cross of two pea plants.
- Complete the genetic diagram in Fig. 7.1 to show the expected genotypes of the offspring.
Parental gametes G g Parental gametes G …… …… g …… …… - State the percentage chance of the parent plants in Fig. 7.1 producing offspring that have the genotype gg.
…… %
▶️Answer/Explanation
Answer:
1.
| Parental gametes | G | g | |
|---|---|---|---|
| Parental gametes | G | GG | Gg |
| g | Gg | gg |
2. 25%
Explanation:
– The genetic cross is between two heterozygous parents (Gg x Gg).
– The Punnett square shows the possible genotypes of the offspring: GG, Gg, Gg, and gg.
– The genotype gg has a 1 in 4 (25%) chance of occurring.
(c) Chromosomes, DNA, and genes are involved in inheritance.
- List these structures in order of size from smallest to largest.
…… → …… → …… - State the phenotype of a person with the sex chromosomes XX.
……
▶️Answer/Explanation
Answer:
1. DNA → genes → chromosomes
2. Female
Explanation:
– DNA is the smallest unit, followed by genes (which are segments of DNA), and chromosomes (which are made up of DNA and proteins).
– The sex chromosomes XX determine the female phenotype in humans.