The Science of Heredity
When your smart phone doesn’t work as you expect it to, you try to figure out the problem. Sometimes the solution is as simple as turning on a power button. However, sometimes the problem is more complex. The problem may be internal, making it impossible to solve without further investigation into the phone’s computer code. Heredity is similar to a smart phone in that it has visible features and invisible programming. Heredity is the transmission of traits from parents to their young, or offspring. Transmission is the process of spreading or passing along from one to another. The traits you observe in the offspring represent its phenotype. The offspring’s genotype, however, is its set of genes, which reside on the organism’s chromosomes.
Two organisms may share the same phenotype, that is, they may look or act in similar ways. However, they can share the same genotype only if they share the same genes.
Geneticists are scientists who study heredity and genetic codes. To
Heredity and the Allele
A gene can have different forms, or alleles, for a specific trait. You inherit one allele for a specific trait from each parent. There are usually two kinds of alleles, with one being stronger than the other. These alleles are called dominant and recessive.
More than one pair of alleles controls many of the traits you see in yourself and in other organisms. However, a single pair of alleles does determine some traits, such as the length and color of a cat’s hair.
There are special terms used to describe the pairs of alleles that appear on chromosomes. To understand those terms, it is helpful to know the meanings of several word parts.
The prefix homo means “the same.” The prefix “hetero” means “different.” The base “zyg” means “union,” as in zygote, which is the fertilized egg that forms after the union of an egg and sperm cell.
When two alleles are identical, that is, when they are both dominant or they are both recessive, they form a homozygous pair. When two alleles are different, meaning one is dominant and the other is recessive, they form a heterozygous pair.
When a dominant allele exists within an organism’s genetic code, it is always expressed. A recessive trait is expressed only when both alleles are recessive. The allele for short hair in a cat is dominant. Conversely, the allele for long hair is recessive. A cat with homozygous dominant alleles will have short hair. A cat with heterozygous dominant alleles will also have short hair. Only a cat with homozygous recessive alleles has long hair.
The Punnet Square
A Punnet square is a tool you can use to determine possible combinations of alleles among offspring. Recall Gregor Mendel’s work with pea plants. He learned through experimentation that when purple-flowered plants were crossed with white-flowered plants, the offspring expressed the same phenotype. This first generation of offspring all had purple flowers. Then Mendel studied the results of self-pollination among the offspring. The results in the second generation included plants with both purple and white flowers in a ratio of 3:1.
Among pea plants, pairs of genes control the expression of several traits. Flower petal color is one trait. Others include the color of the pea pod and stem length. Another trait is the appearance of the pea pod, which may be inflated or pinched.
A single pair of genes does not determine all phenotypes. Multiple genes control some traits, called polygenic traits. Among humans, for example, eye color is a polygenic trait influenced by variations and interactions among several genes.
Some pairs of genes do control some human phenotypes, however. One pair determines the nature of human earwax. The dominant allele results in wet, sticky, yellowish-brown earwax. The recessive allele results in dry, crumbly, gray-to-tan earwax.
You can use Punnet squares to show the results of different parental crosses. In a mono-hybrid cross, each member of the parental generation, or P generation, is different for a particular trait. Suppose that in the case of earwax, for example, one parent is WW for wet earwax and the other is ww for dry earwax.
The offspring of the P generation are called the filial generation, or F generation. If you use a Punnet square to model the possible allele combinations, you see that all of the first filial generation, or Fl, are Ww, or heterozygous for wet, sticky, yellowish-brown earwax.
In a dihybrid cross, the parents in the P generation are different for two traits. So consider another human trait, freckles. Freckles, or dark spots on the skin, are caused by the uneven distribution of skin’s natural pigment, or melanin. Although the quantity and darkness of freckles is influenced by exposure to the sun, there is also a genetic factor at work. People with freckles have at least one dominant allele for a protein that controls melanin production.
When you build a Punnet square to model the possible combinations among the Fl generation, you begin by writing all of the possible allele pairs for each parent. In this example, you can use W and w to represent alleles for earwax and F and f to represent alleles for freckles.
When you list the possible allele combinations among the offspring, you see that there is a greater probability that some combinations will occur before others. The production of sex cells in each parent yields four possible combinations of alleles: WF, Wf, wF, and wf. When the parents are crossed, the allele combinations form the ratio 9:3:3:1.
The sexes of each species differ in their sex chromosomes. In humans, for example, females have two X chromosomes. Males have one X and one Y chromosome. The Y chromosome is smaller than the X chromosome and carries less genetic information.
Because females inherit an X chromosome from each parent and males inherit one X chromosome from the mother, genes that appear on an inherited X chromosome are called sex-linked or X-linked. Scientists have found hundreds of genes on the X chromosome. Almost all of the genes carried on this X have no corresponding genes on the Y. Consequently, a male expresses whatever allele he inherits on the X chromosome, even if it is a recessive allele.
Among X-linked traits are genes for red-green color blindness, male pattern balding, and hemophilia, a blood disorder. Red-green color blindness is a recessive trait. A female may carry the recessive trait on one of her X chromosomes, but because the trait is recessive, it does not express itself, and the woman has normal vision. However, as a carrier of the trait, the woman can pass the trait to her offspring.
Probability is described mathematically as the number of desired outcomes out of all possible outcomes. There are four possible outcomes in the offspring of a cross between a female carrier for red-green color blindness and a male with normal vision. The probability of the couple having a red-green colorblind son is 1 out of 4, which can also be expressed as t, or 25 percent.
The probability of having a female carrier is also 1 out of 4, or t, or 25 percent. Both of the remaining offspring, or 2 out of 4 (!, or 50 percent), will have normal vision.
Exceptions to Dominance
There are two exceptions to the rules of dominance among alleles. One is incomplete dominance and the other is co-dominance.
Incomplete Dominance In examples of incomplete dominance, one allele in a pair is unable to express its phenotype fully. The result is a combined phenotype in which the dominant allele isn’t fully dominant. Take, for example, the color of flowers called snapdragons. Use the Punnet squares below to examine the results of two crosses. The first is a mono-hybrid cross of homozygous red and homozygous white snapdragons. The second is a cross between two members of the F1 generation.