Red-green colour blindness is a quite common recessive trait in humans. Approximately 10 per cent of males have some form of colour blindness. However, it is unusual for females to have this condition.
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Opticians use Ishihara test plates to determine colour blindness. What numbers can you see in the following test plates? Colour blind people will not see the same numbers as people with normal vision.
Fig. 4.1.8
Ishihara test plates. Can you see the numbers?
Genetic crosses
Scientists use genetic crosses to study the inheritance of different traits. Figure 4.1.9 shows a simple genetic cross that describes the inheritance of sex (gender) in humans.
Fig. 4.1.9 A simple genetic cross Sperm Egg
z Males are represented as ‘XY’ as they have a single X chromosome and a single Y chromosome. z Females are represented as ‘XX’ as they have two X chromosomes.
Working in pairs, answer the following questions:
1. Research Irish census data online. Calculate the ratio of males to females for each census. Do you see a trend emerging? Can you explain this trend using the genetic cross shown in Fig. 4.1.9?
2. Interpret the simple genetic cross in Fig. 4.1.9 to answer this question: who determines the sex of a human child – the father or mother?
