Chapter 25 Heredity

Genetic Information Cells contain chromosomes A person’s genetic information is carried in genes, which are segments along strands of DNA. In turn, DNA (and its accompanying genes) is packaged into chromosomes. Chromosomes consist of coiled DNA Segments of DNA are called genes

Genetic Information All human cells (except for germ cells) contain 23 pairs of—a total of 46—chromosomes. Chromosomes consist of long strands of tightly coiled DNA. Segments of DNA called genes contain the traits that each person inherits. Genes vary in size, ranging from a few hundred DNA bases to more than 2 million.

Karyotype Autosomes Sex chromosomes

Karyotype Of the 23 pairs of chromosomes contained within the cell’s nucleus, 22 are matched with a similar-looking (homologous) chromosome. This chart (called a karyotype) shows all the chromosomes, arranged in order by size and structure. Each pair of chromosomes consists of a chromosome inherited from the mother and a chromosome inherited from the father. These are called autosomes.

Karyotype The last pair of chromosomes is the sex chromosomes. In females, both chromosomes are relatively large and are designated by the letter X. In males, one sex chromosome is an X chromosome and one is a smaller chromosome designated by the letter Y. Gametes (eggs and sperm) are the only cells that contain a single set of 23 chromosomes. Female gametes contain only an X chromosome; male gametes may contain either an X or a Y chromosome.

Gender Determination When a sperm with an X chromosome fertilizes an ovum, the offspring is female (two X chromosomes). When a sperm with a Y chromosome fertilizes an ovum, the offspring is male (one X and one Y chromosome).

Question The genetic information in male offspring is: inherited from both the father and the mother. inherited from only the father. inherited from only the mother. unique from that of either parent.

Genes Each chromosome contains anywhere from a few hundred to several thousand genes. The location of a specific gene on a chromosome is called its locus. The locus of each gene does not vary from one person to another. (This allows the genes supplied by the egg to align with the similar genes supplied by the sperm.) Locus

Alleles

Alleles Even though homologous chromosomes carry the same gene at the same locus, they may carry an alternative form of that gene (called an allele). Alleles produce variations of a trait (such as brown versus blue eyes or curly versus straight hair). An individual may have two alleles that are the same or two alleles that are different. If a person has two alleles that are the same, the person is said to be homozygous for that trait. If the alleles are different, the person is said to be heterozygous. In heterozygous individuals, the trait that becomes detectable (called gene expression) depends on whether the allele is dominant or recessive.

Dominant and Recessive Alleles

Dominant and Recessive Alleles A dominant allele overshadows the effect of a recessive allele. Offspring express the trait of a dominant allele if both, or only one, chromosome in a pair carries it. For a recessive allele to be expressed, both chromosomes must carry identical alleles. Some alleles are equally dominant (codominant). In this instance, both alleles are expressed. An example of codominance is the AB blood type.

Question What is the location of a gene called? Karyotype Allele Genome Locus

Sex-Linked Inheritance Carrier of trait Trait will be expressed

Sex-Linked Inheritance Traits carried on the sex chromosomes are called sex-linked traits. Almost all of these traits, which are recessive, are carried on the X. An example of a common sex-linked condition is red-green color deficit (color blindness). If a woman inherits the allele for this condition, the allele on her other X chromosome would overpower the recessive allele. Consequently, she would be a carrier of the trait but would not exhibit any symptoms. Because a man has only one X chromosome, he does not have a dominant matching allele to overpower the X-linked recessive trait. As a result, he would be color-blind.

Genetic Disorders Single-gene disorders Chromosomal abnormalities

Genetic Disorders Genetic disorders may result from a mutation in a single gene or from the addition or subtraction of an entire chromosome or set of chromosomes. Single gene disorders cause a permanent change in genetic material (known as a mutation). Mutations may occur spontaneously, or they can result from exposure to radiation, certain chemicals, or viruses. A few of the diseases that result from mutations include sickle cell disease, severe combined immunodeficiency syndrome (SCID), phenylketonuria (PKU), Huntington’s disease, and cystic fibrosis. In some diseases (like Huntington’s disease), the defective gene is dominant; in other diseases (like cystic fibrosis), it is recessive.

Autosomal Dominant Inheritance

Autosomal Dominant Inheritance In a single-gene disorder when the defective allele is dominant, it overrides the normally functioning allele and the disorder results. This diagram illustrates how disease occurs. Because each child receives one copy of the gene from the mother and one from the father, he or she has a 50% chance of inheriting the defective gene and developing the disorder.

Autosomal Recessive Inheritance

Autosomal Recessive Inheritance In autosomal recessive disorders, the offspring must inherit two copies of the defective allele before the disorder manifests itself. Children who inherit a single copy of the allele become carriers of the disorder. (This means that they can pass the disorder on to their children, but they personally won’t develop the disorder.) A child has a 25% chance of inheriting the defective allele from both parents and, as a result, developing the disorder.

Question Which statement about sex-linked inheritance is true? Almost all these traits are dominant. Women inheriting such a trait would express the disorder. Almost all these traits are carried on the X chromosome. Sickle cell disease is a sex-linked disorder.

Chromosome Abnormalities Result when large segments of a chromosome, or an entire chromosome, are missing, duplicated, or altered Most common disorder (when homologous chromosomes fail to separate during meiosis) is nondisjunction

Disjunction During meiosis, homologous chromosomes separate to produce two daughter cells with 23 chromosomes each. The separation process is called disjunction. Fertilization supplies another 23 chromosomes, resulting in a complete set of 46 chromosomes.

Nondisjunction In nondisjunction, a pair of chromosomes fails to separate: Both chromosomes go to the same daughter cell, whereas the other daughter cell does not receive that chromosome.

Nondisjunction When fertilization adds the matching chromosome, one daughter cell has three of that particular chromosome (called trisomy), whereas the other daughter cell has one chromosome with no mate (called monosomy). Most survivable trisomy is Down syndrome (trisomy 21).

Nondisjunction of Sex Chromosomes The effects from nondisjunction of a sex chromosome typically are not as severe as nondisjunction of an autosomal chromosome.

Nondisjunction of Sex Chromosomes If a boy inherits an extra X chromosome (XXY), Klinefelter syndrome develops. This syndrome, which usually becomes apparent at puberty when secondary sexual characteristics fail to develop, is characterized by undeveloped testes, sparse body hair, unusually long arms and legs, enlarged breasts, low intelligence, and sterility. If a girl is born with only one X chromosome, Turner syndrome develops. This syndrome also becomes apparent at puberty when secondary sexual characteristics fail to develop. Other features include a webbed neck and short stature.

Multifactorial Inheritance Diseases are influenced by both genetics and environmental factors. Environmental factors influence disease progression or whether a disease even develops.

Question Most chromosomal abnormalities result when: homologous chromosomes separate repeatedly. homologous chromosomes fail to separate during meiosis. an egg is fertilized by more than one sperm. a mutation occurs.