Y

(a) Both parents give a complete instruction manual to their offspring

The 23 pages of each instruction manual are roughly equivalent to the 23 chromosomes in each egg and sperm.

(b) Mutations are misspellings in the instructions. Maternal__ _ Paternal

Paternal

(b) Mutations are misspellings in the instructions. Maternal__ _ Paternal

Paternal

Normal allele: Mutant allele:

mutation with same meaning mutation with different meaning

Normal allele: Mutant allele:

mutation with same meaning mutation with different meaning

Paternal

Paternal

nzrve

mutation with no meaning

(nonsense mutation)

Figure 4.3 The formation of different alleles. (a) Each parent provides a complete set of instructions to each offspring. (b) The instructions are first copied, and different alleles for a gene may form as a result of copying errors. Some of these misspellings do not change the meaning of the word, but some may result in different meanings or have no meaning at all.

genes) by one-half is called meiosis, which will be described in detail in Chapter 6. However, our instruction manual analogy should help make it clear why the egg and sperm that produced you were different from the egg and sperm that produced your sibling.

The precursor cell to a human sperm or egg contain two copies of each page of the instruction manual. When a precursor cell develops into a sperm or an egg, it places one copy of every page into each of two daughter cells, and each page is placed in a daughter cell independently of all the other pages. In other words, the instruction manual you received from your mother is made up of a combination of pages from the manuals she received from each of her parents. In our analogy, each page is equivalent to a copy of a chromosome. Therefore, each egg or sperm contains a unique subset of chromosomes—and thus a unique subset of the alleles carried by the parent.

The physical process of separating the sets of chromosomes during meiosis results in what is known as independent assortment. As a result of independent assortment, an allele for an eye color gene ends up in a sperm or egg independently from an allele for the blood-group gene. Since the process of

Precursor cells to sperm and eggs have copies of each chromosome, that is, two full sets of instruction manual pages.

Sperm and egg cells have only one full set — a random combination of maternal and paternal instruction manual pages.

In humans, over eight million combinations are possible.

Possible combination #2

Possible combination #3

Precursor cells to sperm and eggs have copies of each chromosome, that is, two full sets of instruction manual pages.

Sperm and egg cells have only one full set — a random combination of maternal and paternal instruction manual pages.

In humans, over eight million combinations are possible.

Possible combination #1

Possible combination #2

Possible combination #3

Possible combination #1

Page 9 Blood group gene from dad

Page 15 Eye color genes from mom

Page 9 Blood group gene from mom

Page 15 Eye color genes from mom

Page 9 Blood group gene from mom

Page 15 Eye color genes from dad

Page 9 Blood group gene from dad

Page 15 Eye color genes from mom

Page 9 Blood group gene from mom

Page 15 Eye color genes from mom

Page 9 Blood group gene from mom

Page 15 Eye color genes from dad

Figure 4.4 Each egg and sperm is unique. The cells that are the source of eggs and sperm carry two of each chromosome—that is, two full copies of the instruction manual. When an egg or sperm is produced, it ends up with only one copy of each page. Since each egg and most sperm are produced independently, the set of pages in each is practically unique.

independently placing chromosomes into daughter cells is repeated every time an egg is produced, the set of alleles each child receives from a mother is different for all of her children. The egg that produced you might have carried the eye color allele from your mom's mom and the blood group from her dad, while the egg that produced your sister might have contained both the allele for eye color and the allele for blood group from your maternal grandmother (Figure 4.4). Due to independent assortment, about 50% of your alleles are identical to the alleles carried by your full sibling—that is, for each gene you have a 50% chance of being like your sister or brother.

In addition to independent assortment, there is another event during meio-sis that leads to diversity in egg and sperm. This process is called crossing over, and occurs when chromosome pairs "swap" information. In our instruction manual analogy, crossing over is equivalent to tearing a pair of pages in half and reassembling them so that the top part of the page is from one instruction manual and the bottom part is from the other instruction manual. The process of crossing over is discussed in more detail in Chapter 6. For now, it is sufficient to understand that the processes of independent assortment and crossing over create almost limitless variation in eggs or sperm from a single parent.

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