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9.4
Abnormal Chromosome Inheritance

The normal number of chromosomes in human cells is 46, but occasionally humans are born with an abnormal number of chromosomes because improper meiosis occurred. Nondisjunction occurs during meiosis I when both members of a homologous pair go into the same daughter cell, or during meiosis II when the sister chromatids fail to separate and both daughter chromosomes go into the same gamete ( Fig 9.9 ). If an egg that ends up with 24 chromosomes instead of 23 is fertilized with a normal sperm, the result is a trisomy, so called because one type of chromosome is present in three copies. If an egg that has 22 chromosomes instead of 23 is fertilized by a normal sperm, the result is a monosomy, so called because one type of chromosome is present in a single copy.

Figure 9.9
Nondisjunction during meiosis.
Because of nondisjunction, gametes either lack a chromosome or have an extra chromosome. Nondisjunction can occur (a) during meiosis I if homologous chromosomes fail to separate, and (b) during meiosis II if the sister chromatids fail to separate completely.
Down Syndrome

Down syndrome is trisomy 21, in which an individual has three copies of chromosome 21. In most instances, the egg contained two copies of this chromosome instead of one. (In 23% of the cases studied, however, the sperm had the extra chromosome 21.)

Down syndrome is easily recognized by the following characteristics: short stature; an eyelid fold; stubby fingers; a wide gap between the first and second toes; large, fissured tongue; round head; palm crease; and unfortunately, mental disabilities, which can sometimes be severe ( Fig 9.10 ).

Figure 9.10
Down syndrome.
Down syndrome occurs when the egg or the sperm has an extra chromosome 21 due to nondisjunction in either meiosis I or meiosis II. Characteristics include a wide, rounded face and narrow, slanting eyelids. Mental disabilities are often present, but may vary greatly.

The chance of a woman having a Down syndrome child increases rapidly with age, starting at about age 40. The frequency of Down syndrome is 1 in 800 births for mothers under 40 years of age and 1 in 80 for mothers over 40 years of age. However, most Down syndrome babies are born to women younger than age 40, because this is the age group having the most babies.

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Abnormal Sex Chromosome Number

Nondisjunction during oogenesis or spermatogenesis can result in gametes that have too few or too many X or Y chromosomes. Figure 9.9 can be used to illustrate nondisjunction of the sex chromosomes during oogenesis if we assume that the chromosomes shown represent X chromosomes.

Just as an extra copy of chromosome 21 causes Down syndrome, having additional or missing X or Y chromosomes causes certain syndromes. Newborns with an abnormal sex chromosome number are more likely to survive than those with an abnormal autosome number, and the explanation is quite surprising. Normal females, like normal males, have only one functioning X chromosome. The other X chromosome (or additional X chromosomes) becomes an inactive mass called a Barr body (after the person who discovered it).

In humans, the presence of a Y chromosome, not the number of X chromosomes, almost always determines maleness. The SRY (sex-determining region Y) gene located on the short arm of the Y chromosome produces a hormone called testis-determining factor, which plays a critical role in the development of male genitals. No matter how many X chromosomes are involved, an individual with a Y chromosome is a male, assuming a functional SRY is on the Y chromosome. Individuals lacking a functional SRY on their Y chromosome have Swyer syndrome, also known as an “XY female.”

A person with Turner syndrome (45, XO) is a female. The number indicates the total number of chromosomes the individual has, and the O signifies the absence of a second sex chromosome. Turner syndrome females are short, with a broad chest and webbed neck. The ovaries, oviducts, and uterus are very small and underdeveloped. Turner females do not undergo puberty or menstruate, and their breasts do not develop ( Fig 9.11 a ). However, some have given birth following in vitro fertilization using donor eggs. These women usually have normal intelligence and can lead fairly normal lives if they receive hormone supplements.

Figure 9.11
Abnormal sex chromosome number.
a. A female with Turner syndrome (XO) has a short thick neck, short stature, and lack of breast development. b. A male with Klinefelter syndrome (XXY) has immature sex organs and some development of the breasts.

A person with Klinefelter syndrome (47, XXY) is a male. A Klinefelter male has two or more X chromosomes in addition to a Y chromosome. The extra X chromosomes become Barr bodies.

In Klinefelter males, the testes and prostate gland are underdeveloped. There is no facial hair, but some breast development may occur (Fig. 9.11b ). Affected individuals generally have large hands and feet and very long arms and legs. They are usually slow to learn but not mentally handicapped unless they inherit more than two X chromosomes.

As with Turner syndrome, it is best for parents to know as soon as possible that their child has Klinefelter syndrome because much can be done to help the child lead a normal life.

  1. Describe how a zygote could receive an abnormal chromosome number.

  2. Contrast monosomy with trisomy.

Answers: 1. If nondisjunction occurs during meiosis I or II, a gamete, and therefore a zygote, could have an abnormal chromosome number. 2. Monosomy results when an individual is missing one chromosome. Trisomy results when an individual has one extra chromosome.