Question 1

Heredity is the transmission of genetic characteristics from parent to offspring. In the following discussion, we will examine a few basic principles of normal heredity,

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thus establishing a basis for understanding hereditary traits in later chapters. Hereditary defects are described in chapter 29 along with nonhereditary birth defects.

Question 2

As we have seen, the agents of heredity are the genes, and the genes are carried on the chromosomes. When we lay the 46 chromosomes out in order by size and other physical features, we get a chart called a karyotype12 (fig. 4.16).

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he chromosomes occur in 23 pairs; the two members of each pair are called homologous 13 chromosomes

Question 3

chromosomes

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One member of each pair is inherited from the individual's mother and one from the father.

Question 4

Except for the -----, two homologous chromosomes look alike and carry the same genes, although they may have different varieties of those genes.

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X and Y chromosomes

Question 5

Chromosomes X and Y are called the sex chromosomes because they determine the individual's sex; all the others are called

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autosomes

Question 6

A female normally has a homologous pair of X chromosomes

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whereas a male has one X and a much smaller Y.

Question 7

Any cell with 23 pairs of chromosomes is said to be diploid (2n). Sperm and egg cells, however, are haploid (n),

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meaning they contain only 23 unpaired chromosomes.

Question 8

Sperm and eggs, and cells on their way to becoming sperm and eggs, are called germ cells (sex cells).

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All other cells of the body are called somatic cells.

Question 9

In meiosis (see section 27.4a), the two homologous chromosomes of each pair become segregated into separate daughter cells leading to the haploid germ cells.

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At fertilization, one set of paternal (sperm) chromosomes unites with a set of maternal (egg) chromosomes, restoring the diploid number to the fertilized egg and the somatic cells that arise from it.

Question 10

The positions of specific genes on our 23 paired chromosomes have been well mapped since the massive international Human Genome Project of 1990 to 2003. A gene's position is called its locus.

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Two homologous chromosomes have the same gene at the same locus, but they can carry different forms of that gene, called alleles, which produce alternative forms of a particular trait.

Question 11

Typically, one allele is dominant and the other one recessive.

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If at least one chromosome carries the dominant allele, a person usually exhibits ("expresses") the corresponding trait.

Question 12

Even if a recessive allele is present on the other chromosome, its effect is masked by the dominant one. If a dominant allele codes for a particular protein, a recessive allele typically codes for a variant, nonfunctional version of that protein;

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recessive alleles are sometimes referred to as loss-of-function genes. Their effects are expressed only if present on both chromosomes of the pair. A specific example of this, sickle-cell disease, will be described shortly.

Question 13

Individuals with identical alleles of a gene on both homologous chromosomes are said to be homozygous for that gene.

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If the homologous chromosomes have different alleles for that gene, the individual is heterozygous.

Question 14

The paired alleles that an individual possesses for a particular trait constitute the genotype.

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An observable trait resulting from that genotype is called the phenotype.

Question 15

We say that an allele is expressed if it shows in the phenotype of an individual so it can be

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detected by anything from simple observation (such as eye color) to biochemical analysis (such as the presence of a certain enzyme).

Question 16

Dominant alleles are customarily represented by an italic capital letter such as A. The letter is arbitrary but typically stands for the trait under consideration. Recessive alleles are represented by the same letter in lowercase italic, such as a.

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A genotype is usually represented by two letters, such as A4 for a homozygous dominant pair of alleles, aa for homozygous recessive, and Aa for heterozygous.

Question 17

It's relatively rare for a trait to be determined by only one gene, but such cases are not insignificant.

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About 25 million Americans have diseases attributed to a single allele: cystic fibrosis, Huntington disease, muscular dystrophy, and sickle-cell disease, among others.

Question 18

Sickle-cell disease afflicts about 1.3% of Americans of African ancestry; another 8% are heterozygous carriers who don't have the disease, but carry the recessive allele and may pass it to their children.

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The carriers are said to have sickle-cell trait (SCT).

Question 19

Sickle-cell disease stems from a recessive allele for one of the proteins that constitute the hemoglobin molecule. In people with sickle-cell disease, hemoglobin turns to a gel at low oxygen levels, as when it passes through the veins.

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This causes the red blood cells (RBCs) to be distorted into elongated shapes with pointed ends (fig. 4.17a). They're also sticky and clump together, plugging small vessels and causing intense pain, disability. and stunted physical and mental development.

Question 20

Without medical care. a child with sickle-cell disease has little chance of living

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beyond 2 years

Question 21

To examine sickle, let us represent the dominant allele for normal hemoglobin H and the recessive allele for sickle-cell hemoblogin h. Homozygous dominant (HH) individuals don't have sickle-cell disease or trait; they can't pass it to their children.

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Heterozygous (Hh) individuals have the trait but not the disease; they have the potential to pass it to their children. Homozygous recessive (hh) individuals have sickle-cell disease.

Question 22

A diagram called a Punnett shows the pattern of inheritance of this gene and shows why sickle-cell disease can skip a generation. If a woman is heterozygous,

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50% of her eggs will carry allele H and 50% will carry allele h, as shown across the top of the Punnett square. Similarly, if a man is heterozygous, 50% of his sperm will carry each allele, as shown down the left side.

Question 23

If two heterozygous individuals have children,

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it's a matter of chance which sperm fertilize which eggs, but the four cells within the Punnett square illustrate the possible outcomes.

Question 24

If an H-bearing sperm fertilizes an H-bearing egg, the result is an HH child with normal hemoglobin (upper left cell of the chart).

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There is a one-in-four (25%) chance of any one of their children having this fortunate outcome.

Question 25

If an h-bearing sperm fertilizes an h-bearing egg, the result is an hh child with sickle-cell disease (lower right cell of the chart). There is also a one-in- four (25%) chance of any one of their children having this tragic outcome.

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But if an H-bearing sperm fertilizes an h-bearing egg, or an h-bearing sperm fertilizes an H-bearing egg, the result is a heterozygous (Hh) child with normal hemoglobin, but who has sickle-cell trait and could pass it to the next generation.

Question 26

But if an H-bearing sperm fertilizes an h-bearing egg, or an h-bearing sperm fertilizes an H-bearing egg, the result is a heterozygous (Hh) child with normal hemoglobin,

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but who has sickle-cell trait and could pass it to the next generation.
There is a two-in-four (50%) chance of this outcome.

Question 27

We can infer from this how a recessive trait can skip a generation. Either parent in this chart could have had a parent with sickle-cell disease; he or she would be disease-free (both being heterozygous);

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but these parents can have a child with sickle-cell. The disease "skipped" this generation of parents, lying hidden in a heterozygous state, but could appear in their parents as well as their children.

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4.4a