CAIE IGCSE Biology 17.4.18 Genetic Diagrams to Predict Questions

Practise using genetic diagrams to predict monohybrid cross results involving codominance or sex linkage and calculate phenotypic ratios.

Exam points

write parental genotypes and gametes using the relevant allele notation for codominance or sex linkage
use a genetic diagram to predict possible offspring genotypes and phenotypes
calculate the resulting phenotypic ratio

[Maximum number: 5]

The eye is adapted for focusing on near and distant objects.
Fig. 3.1 shows the parts of the eye involved in focusing. The eye is focused on a distant object.

Fig. 3.1

Colour blindness can be caused by a mutation in a gene. The gene is located on the X chromosome.

Fig. 3.3 is a pedigree diagram of a family which has several people who are colour-blind.

Fig. 3.3

Use the information in Fig. 3.3 to complete the genetic diagram to show the probability of person 3 and person 4 having another child with colour blindness.

Table for Question CAIE IGCSE Biology 17.4.18 Genetic Diagrams to Predict Questions question 1 — CAIE IGCSE Biology

offspring
genotypes
offspring
phenotypes
probability of a child having colour blindness

1 correct genotype for person 3, i.e. $(X)^{A}(X)^{a}$ ;

2 correct genotype for person 4, i.e. $\mathrm{X}^{\mathrm{A}} \mathrm{Y}$ ;

3 correct gametes from the parental genotypes ;

4 correct offspring genotypes from their gametes / parental genotypes (in any order) ;

5 correct offspring correct probability ;

MP3 ecf from MP1 and MP2 MP4 ecf from MP3

MP5 ecf from their offspring genotype

expected answer:

genotype (person 3) $\mathrm{X}^{\mathrm{A}} \mathrm{X}^{\mathrm{a}} \quad \times\left(\right.$ person 4) $\mathrm{X}^{\mathrm{A}} \mathrm{Y}$ gametes $\quad \mathrm{X}^{\mathrm{A}}, \mathrm{X}^{\mathrm{a}}+\quad \mathrm{X}^{\mathrm{A}}, \mathrm{Y}$ offspring genotypes $X^{A} X^{A}, X^{A} X^{a}, X^{A} Y, X^{a} Y$

offspring phenotypes female with normal colour vision female with normal colour vision male with normal colour vision male with colour blindness)

probability $\quad 0.25$ / 25\% / 1 in 4 / $\frac{1}{4}$

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CAIE IGCSE Biology 17.4.18 Genetic Diagrams to Predict Questions question 1