The key separates four classes of vertebrates. The same species can also show inherited variation, such as coat colour in mammals. (a) Using the key, name t...
Question 1 Report
The key separates four classes of vertebrates. The same species can also show inherited variation, such as coat colour in mammals.
(a) Using the key, name the class of an animal that has wet scales and lays its eggs in water. [1]
(b) State the term used for animals whose body temperature changes with their surroundings. [1]
(c) Explain one advantage to a mammal of keeping a constant body temperature. [2]
(d) State two features, other than fur and live young, that are characteristic of mammals. [2]
(e) In a mammal, coat colour is controlled by a dominant allele B (black) and a recessive allele b (brown). State what is meant by a dominant allele. [2]
(f) A heterozygous black individual (Bb) is crossed with a brown individual (bb). State the genotypes of the offspring and the expected ratio of black to brown. [3]
(g) Explain why members of the same mammal species can differ in coat colour. [2]
Answer Details
This question tests using a classification key, vertebrate features, and a monohybrid cross.
(a) Class with wet scales, eggs in water. [1] Following the key (scales, then wet), the animal is a Fish [1].
(b) Term for changing body temperature. [1] Ectothermic (accept cold-blooded / poikilothermic) [1].
(c) One advantage of a constant body temperature. [2] The body's enzymes can work at a constant optimum rate [1]; so the mammal stays active in cold conditions / can live in a wider range of habitats [1].
(d) Two mammal features other than fur and live young. [2] Any two of: mammary glands that produce milk; external ears (pinnae); a diaphragm; different types of teeth; sweat glands [2, 1 each].
(e) Meaning of a dominant allele. [2] An allele that is expressed/shown in the phenotype whenever it is present [1], even in the heterozygote, where it masks the recessive allele [1].
(f) Cross Bb x bb. [3] The black parent (Bb) makes gametes B and b; the brown parent (bb) makes only b gametes:
| gamete B | gamete b | |
|---|---|---|
| gamete b | Bb | bb |
| gamete b | Bb | bb |
Offspring genotypes are Bb (black) and bb (brown), in the ratio 1 black : 1 brown [3: 1 for each correct genotype, 1 for the ratio].
(g) Why members of the same species differ in coat colour. [2] They carry different alleles/genotypes for coat colour [1], inherited from their parents (accept mutation as a source of new alleles) [1]. Exam tip: dominant needs the "even in the heterozygote" idea to score the second mark.
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