Genetics is the study of heredity, the passing of inherited traits from one generation to the next. The determiners of hereditary traits are located on chromosomes, consisting of DNA and proteins. It is the DNA that controls inheritance and directs cellular functions.
Each human body cell contains 46 chromosomes that exist as 23 unique pairs. Chromosome pairs 1 through 22 are called autosomes because they control most inherited traits except gender. Gender is determined by chromosome pair 23, the sex chromosomes. There are two types of sex chromosomes, a large X chromosome and a small Y chromosome. Males possess one X chromosome and one Y chromosome (XY). Females possess two X chromosomes (XX).
A person’s chromosomes, including the sex chromosomes, may be examined by making a karyotype. The chromosomes in a dividing cell are photographed during metaphase and the photograph is enlarged. Then the chromosomes are cut out, matched in pairs, and arranged by size and location of the centromere. Figure 18.14 is a karyotype of a normal male. Note the X and Y chromosomes and that the chromosomes are arranged in pairs.
Recall that gametes are formed by meiotic cell division, a process that places one member of each chromosome pair in each gamete. Each human gamete contains 23 chro- mosomes-22 autosomes and 1 sex chromosome. We will consider only the sex chromosomes here.
Because a female has two X chromosomes in her cells, all of her gametes contain an X chromosome. A male has both an X chromosome and a Y chromosome in his cells. Therefore, half of his gametes are X-bearing, and half are Y-bearing. If a secondary oocyte is fertilized by an X-bearing sperm, the child will be a girl. If a secondary oocyte is fertilized by a Y-bearing sperm, the child will be a boy. Obviously, the probability of any zygote becoming a girl (or a boy) is one-half or 50%. Figure 18.15 illustrates the determination of sex.
DNA consists of a double strand of nucleotides that are joined by complementary pairing of their nitrogenous bases: adenine (A), thymine (T), cytosine (C), and guanine (G). The sequence of these bases forms the genetic code, which contains the information for producing proteins that regulate cellular functions and determines the inheritance of genetic traits.
A gene is a unit of inheritance. It consists of a specific sequence of DNA that codes for a unique molecule of RNA. This RNA molecule will either be directly involved in the synthesis of a polypeptide or indirectly involved in regulating the production of a polypeptide. Genes occur in a linear sequence along a chromosome and a single chromosome may contain hundreds of genes.
Because chromosomes occur in pairs, genes also occur in pairs. An inherited trait is determined by at least one pair of genes. There may be two or more alternate forms of a gene controlling the expression of a particular trait. These alternate forms are called alleles (ah-lels), and each allele affects the expression of a trait differently. So, in the simplest case, a trait is determined by one pair of alleles present in a person’s cells. If the two alleles for a trait are identical, the person is homozygous for that trait; if they are different, the person is heterozygous for that trait.
Each person’s chromosomes contain a unique catalog of genes, the genotype for that person. The expression of those genes yields observable traits known as the phenotype. Though the phenotype is what is seen, the genotype is responsible for the inheritance and expression of those traits.
Dominant And Recessive Inheritance
Some alleles are dominant, and some are recessive. A dominant allele is always expressed, whereas a recessive allele is expressed only when both alleles are recessive.
Consider the example of skin pigmentation. Normal skin pigmentation is controlled by a dominant allele (A). The absence of pigment (albinism) is controlled by a recessive allele (a). Note that it requires only one dominant allele to express the dominant trait but that both recessive alleles must be present for the recessive trait to be expressed.
AA – Normal
Aa – Normal
Aa – Albino
Examples of Traits Determined By Dominant And Recessive Alleles
|Traits Determined by Dominant Alleles||Traits Determined by Recessive Alleles|
|Freckles||Absence of freckles|
|Dimples in cheeks||Absence of dimples|
|Dark hair||Light hair or red hair|
|Full lips||Thin lips|
|Free earlobes||Attached ear lobes|
|Feet with arches||Flat feet|
|Huntington disease||No Huntington disease|
|Panic attacks||No tendency to panic attacks|
|Extra fingers or toes||Normal number of digits|
|No cystic fibrosis||Cystic fibrosis|
|Type A, B. or AB blood||Type O blood|
|Type Rh+ blood||Type Rh— blood|
|Normal color vision||Red-green color blindness‘|
|No gout tendency||Gout’|
Incomplete dominance is a type of inheritance where the two alleles for a gene can create three different phenotypes. Each genotype-homozygous dominant, heterozygous, and homozygous recessive-has a different phenotype. An example is sickle-cell disease, a condition characterized by defective hemoglobin that cannot carry adequate oxygen. Erythrocytes with the defective hemoglobin assume a characteristic sickled or crescent shape. Sickle-cell disease occurs among people whose ancestors lived in central Africa. About 8.3% of black Americans possess the allele for sickle-cell disease.
A person who inherits both recessive alleles for sickle-cell disease (HSHS) produces abnormal hemoglobin, leading to the formation of sickled cells that cannot carry sufficient oxygen. Because of their shape, the sickled cells tend to plug capillaries. Symptoms include pain in joints and the abdomen and chronic kidney disease.
In the heterozygous state (HHS), some hemoglobin molecules are normal but others are abnormal. Fortunately, few RBCs become sickled when oxygen is at normal levels and clinical symptoms are absent at such times. However, more RBCs become sickled during times of decreased blood oxygen level, a characteristic that allows detection of carriers of the sickle-cell allele. The heterozygote state affords some protective advantage against the pathogen causing malaria. The homozygous dominant genotype produces the phenotype of all normal hemoglobin.
In some traits, both alleles are expressed and affect the phenotype. This type of inheritance is referred to as codominance. An example of codominance can be seen with the ABO blood group. There are three alleles involved: a dominant IA that causes the production of the A antigen; a dominant IB that causes the production of the B antigen; a recessive i that has no function. If both IA and IB are present, both alleles are expressed. Since the recessive i has no function, genotype i i produces neither A nor B antigens. This is called type O blood, which simply means there are no A or B antigens. The possible genotypes and phenotypes for the ABO blood group are
Many traits are controlled by polygenes, a number of different genes that may be located on the same or different chromosomes. Each gene contributes to the phenotype, though some genes may have more influence on the trait than others. To add to the complexity of polygenic inheritance, each gene involved may possess a number of different alleles. Environmental factors may also exert influence over the expression of a phenotype. For these reasons, it is difficult to predict the inheritance of polygenic traits. Examples of traits controlled by polygenes are height, skin pigmentation, and intelligence.
The ABO blood group is also governed by polygenes. The gene for the H antigen is found on chromosome 19. The H gene possesses two alleles: a dominant H that causes the production of H antigen and a recessive h that is nonfunctional. Individuals who are homozygous dominant (HH) or heterozygous (Hh) possess the H antigen. The IA and IB alleles, which are located on chromosome 9, produce enzymes that add to the H antigen and produce either A or B antigens. Many people mistakenly conclude that type O blood has no antigens because the i alleles have no function. However, most people with blood type O actually have H antigens. Individuals with genotype hh do not produce the H antigen and have what is called the Bombay phenotype. These individuals will be Type O even if their genotype contains the IA, IB, or both IA and IB alleles because, without the H antigen, A and B antigens cannot be formed.
A few traits are determined by genes on the X chromosome. These are X-linked, or sex-linked,
traits. Recessive X-linked traits affect males more frequently than females. Males only possess one X chromosome. If a recessive trait is carried by the X chromosome in a male, the trait will be seen. Females possess two X chromosomes. To see the recessive trait, a female must possess two recessive alleles. If the female possesses one dominant “normal” allele, the recessive trait will not be seen.
Red-green color blindness is a common X-linked recessive trait. A color-blind male inherits the allele for color blindness from his mother, who provides his X chromosome. The mother may either have normal color vision or be red-green color-blind (table 18.5). It is important to note that if the mother has normal color vision, she still possesses the allele for color blindness and is considered a carrier for the color-blindness trait.
Possible Genotypes And Phenotypes For Red-Green Color Blindness, An X-Linked Trait
|Females||Normal color vision|
|xcxc||Normal color vision carrier|
|Males||XCY||Normal color vision|
Parents often wonder about the chances of their child developing certain inherited traits. This can be predicted for some traits for which the inheritance pattern has been determined and if the genotypes of the parents are known. Such predictions indicate the probability, rather than absolute certainty, that a trait will be inherited.
Let’s consider freckles. Freckles are determined by a dominant allele (F), and a nonfreckled phenotype is determined by a recessive allele (f). The possible genotypes and phenotypes are
FF – Freckled
Ff – Freckled
Ff – Nonfreckled
Figure 18.16 shows how to determine the probability of the freckled or nonfreckled trait in the next generation if the genotypes of the parents are known. In this example, the parents are known to be heterozygous for freckles. What is the probability that their children will be freckled?
Because each parent is heterozygous, meiotic division during gamete formation causes half of the gametes of each parent to contain an allele for freckles (F), and half to carry an allele for normal pigmentation (f). The union of sperm and secondary oocyte occurs at random (by chance), so we must allow for all possible combinations of gametes. This is accomplished by using a Punnett square (a chart named after Reginald Punnett, a geneticist).
The alleles in ovum are placed along the horizontal axis, while the alleles in sperm are placed along the vertical axis. Next, the allele of each ovum is written in the squares below each ovum and the allele of each sperm is written in the squares to the right of each sperm. The Punnett square now shows all possible genotypes that may occur in the next generation.
From this information, the predicted genotype ratio may be determined. Then, knowing that the trait for freckles is dominant and that the presence of a single dominant allele (F) produces freckles, the predicted phenotype ratio may be determined. Note in figure 18.16 that it is possible for two heterozygous freckled parents to have a child with normal pigmentation. However, if one parent is homozygous dominant for freckles and the other is heterozygous for freckles, all children would be freckled.
The inheritance of any dominant/recessive trait may be determined in a similar manner.