6. In daisies, thick stems (T allele) is dominant over thin stems (t), and white

Question

6. In daisies, thick stems (T allele) is dominant over thin stems (t), and white flowers (W)

is dominant over yellow flowers (w). These genes show independent assortment. When

a cross is made between a plant that is homozygous for both thick-stem, white flowers

and a plant that has thin stems and yellow flowers, what is the probability that they have offspring with yellow flowers and thick stems? Show your work and explain your reasoning.    (3 pts.)

7. Marfan syndrome is a human disorder causing defects in the lens of the eye, excessively lengthy fingers and toes, and characteristic defects of the heart. It results from the presence of a dominant allele. If a man with Marfan syndrome (who has an unaffected mother) has a child with a woman who does not have this disorder but whose mother did, what is the probability that their child will have the disease? (2 pts.)

8. A man, Waldo, whose sister died in early childhood from a recessive lethal disease marries a woman, Ethel, with the same family history. Because this man survived beyond childhood, he does not have the disease, but he may be a carrier ( = heterozygous, as may also be the case with his wife). What is the probability that their first child will suffer from the disease? [Hint: first calculate the probability that Waldo is heterozygous; then determine the probability that both parents are carriers. Remember that he has survived to adulthood when calculating this probability]. (2 pts.)

Explanation / Answer

TTWW crossed with ttww

Probability that they have offspring with thick stems with yellow flowers (T_ww) = nil

No such offsprings will be obtained in F1 generation., because all the offsprings will be heterozygous, TtWw (will have thick stems and white flowers).

In the F2 generation, Tt X Tt = 3 (thick):1 (Thin)

Ww X Ww, will give 3 white: 1 yellow

Probability of havig thick stem, yellow flower will be 3/4 *1/4 = 3/16

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