MODULE 3 EXERCISES

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Chromosomes contain material that is transmitted from parent to   A recessive trait is not expressed when an individual has one domichild. Chromosomes are thus defined as the   nant and one recessive gene for that trait. The gene for brown eyes

is dominant over the gene for blue eyes. An individual with one gene

DNA is found on chromosomes. DNA is the basic

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for brown and one for blue eyes will have - eyes.

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Genes seem to be segments of DNA molecules. Genes are:

a. smaller than DNA molecules. b. carried on chromosomes.

c. determiners of traits. d. (none of these)

A human zygote, or fertilized ovum, normally contains 46 chromosomes. How many of these came from each parent?

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Identical twins have identical heredity. Therefore they must develop from

Suppose two individuals, each with one gene for blue eyes and one for brown eyes, want to have children. Using the following steps, calculate the likely eye color distribution in their children.

a. Fill in the possible ovum and sperm types in the chart below. Use B for the brown gene and b for the blue

gene.

Picture

Fraternal twins are no more alike than ordinary siblings. Therefore . they must develop from

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Put an X in the following chart wherever the characteristic applies.

b. Now fill in the squares with the combinations of sperm and ovum gene types.

c. Your chart shows that 1/4 of the combinations are BB (two dominants), '/z are Bb

and 1 /4 are bb

TWINS

CHARACTERISTICS   SIBLINGS IDENTICAL FRATERNAL

SAME SEX

d. The combination bb produces eyes.

DIFFERENT SEXES

SAME CHROMOSOMES

The combination Bb produces eyes.

SHARE SOME, BUT NOT ALL HEREDITY

The combination BB produces eyes.

e. Of four children, how many are likely to have brown eyes?

B

a single zygote (fertilized ovum) 2   carriers of hereditary material

5 brown   6 23

3 7

hereditary material    4   two separately fertilized ova (zygotes)

Identical twins    8   a, b, c

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