Theory question 104
Mendelian InheritanceUse the information below to answer questions 4(a)(i) and 4(a)(ii). HbA represents normal haemoglobin. HbS represents sickled haemoglobin. (a) A female heterozygote for sickle cell married a sickler. With the aid of a genetic diagram, determine the: (i) possible genotypes of their offspring; (ii) phenotypic ratio of the offspring. (b) Explain briefly the reason why a Rhesus negative woman married to a Rhesus positive man might lose her second pregnancy. (c) Name two examples of features in animals that support the theory of use and disuse of body parts as used by Lamarck. (d) List three structures in mammals that are vestigial.
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(a) Parents: HbAHbS (carrier) × HbSHbS (sickler).
Gametes: HbA, HbS and HbS, HbS.
(i) Offspring genotypes: HbAHbS, HbAHbS, HbSHbS, HbSHbS (AS and SS).
(ii) Phenotypic ratio 1 carrier (sickle-cell trait, normal) : 1 sickler (sickle-cell anaemia).
(b) If the first baby is Rh positive, some of its blood enters the mother's blood at birth; she makes anti-Rh (anti-D) antibodies. In the second pregnancy with an Rh-positive baby these antibodies cross the placenta and destroy (agglutinate) the baby's red blood cells (haemolytic disease of the newborn), so the baby may die.
(c) Any two: long neck of the giraffe (stretching to reach leaves); webbed feet of water birds (stretching toes while swimming); long legs of wading birds; strong arm muscles of a blacksmith; loss of eyes in cave animals and of legs in snakes (disuse).
(d) Any three: appendix (vermiform appendix); coccyx (tail bones); ear muscles (pinna muscles); wisdom teeth; body hair; nictitating membrane (plica semilunaris) in humans.
In a genetic diagram always show parents, gametes, fusion and offspring.