Understanding genetics requires an understanding of the concepts of genotype and phenotype.
We often say that our genes give us our traits. That’s true, but only indirectly. Genes are sequences of DNA, and those genes serve as instructions for how to make corresponding sequences of RNA. Those sequences of RNA, in turn, serve as instructions for how to build proteins. Proteins are the final product of gene expression, and the particular proteins that are built in the cells of an organism are what give that organism its traits. The traits are directly determined by the proteins, but the traits are indirectly determined by the genes, because the genes indirectly instruct the cells how to build the proteins. For any given characteristic (like hair color, for example), there might be multiple possible traits (like black hair and brown hair, for example). The particular trait exhibited by an individual for a particular characteristic (like the black hair trait for the hair-color characteristic) is known as that individual’s phenotype. That phenotype depends on the proteins produced, which depends on the version of the corresponding gene (DNA) that individual possesses. We use the term "genotype" to refer to that individual’s particular DNA sequence for that particular gene. Therefore, genotype indirectly determines phenotype. Bolster your understanding of these terms, how they relate, and how they differ by reading the Phenotypes and Genotypes subsection.
A mutation is an accidental change in the DNA sequence of an organism. An organism’s DNA is arranged in chromosomes. Each chromosome is a long sequence of DNA nucleotides. A gene is a subsequence of DNA nucleotides within the longer sequence making up the chromosome. A mutation arises when the sequence gets changed either by an error during DNA replication or by some other accident (including exposure to certains chemicals or forms of radiation, for instance). The reason a gene is able to serve as a code (stored information) is because of its particular sequence of DNA nucleotides. Mutation changes that sequence. Therefore the code is changed, and when that altered code is used to make RNA, the RNA will also be altered. Similarly, when the code in that altered (mutated) version of RNA is used to build a protein, that protein might be different from the unmutated version. Since proteins directly determine phenotypes, a mutant protein (resulting from a mutant form of DNA for the corresponding gene) might result in an altered phenotype. Mutation is the original source of genetic variation, and it is the reason why there are different species and why there are differences between individuals of the same species. As you continue to study genetics, try to keep in mind that differences in genotypes and phenotypes are ultimately due to mutations that have accidentally occurred.
If the genotypes (for a particular characteristic or gene) are known for two parents, then it is possible to compute the probabilities of various genotypes or phenotypes in the offspring produced by those parents. This is because the genotype of an offspring results from randomly selecting one of the two alleles for the gene in one parent and randomly selecting one of the two alleles in the other parent. Those randomly selected alleles are carried by sperm and egg, and when the sperm and egg fuse (fertilization) to produce the zygote of the offspring, the genotype of the offspring will be a combination of an allele from the father and an allele from the mother. If we consider an example in which a male with genotype Aa mates with a female with genotype aa, then we can symbolize this mating (or cross) as Aa × aa.
Having the AA genotype requires receiving an A allele from the father and an A allele from the mother. The mother in this example has no A allele, so the probability is 0.
Having this genotype requires receiving an A allele from one parent (either one) and receiving an a allele from the other parent. The probability of receiving an A allele from the father is one-in-two (50%, or 0.5). The probability receiving an a allele from the mother is two-in-two (100%, or 1.0), because that’s all she have to give. The overall probability for Aa is the product of those two individual probabilities. In this case, it’s 0.5 × 1.0 = 0.5. In other words, there is a 0.5 (50%) probability that any given offspring of these parents will have the Aa genotype.
Using reasoning similar to that in the computation for Aa, there is also a 0.5 probability for the aa genotype, because there is a 0.5 probability of receiving an a allele from the father and a 1.0 probability of receiving an a allele from the mother. The overall probability is 0.5 × 1.0 = 0.5. In this example, aa is the only other possible genotype besides Aa.
Make sense of the fact that fertilization is much like randomly choosing a card from a two-card hand held by the father, then randomly choosing a card from a two-card hand held by the mother. The two cards drawn (representing sperm and egg) determine the two-card hand (representing the genotype of the zygote) held by the offspring. Keep this in mind as you review Mendel’s Laws of Inheritance.
Understanding genetics requires a thorough understanding of several important terms.
When organisms are studied genetically, scientists usually focus on a single gene to study. A gene is a stretch of DNA coding for a particular protein. As such, a gene controls a particular characteristic. Alternative version of a gene (with different nucleotide sequences) are called alleles. A diploid cell features two alleles for any particular gene, because a diploid cell features two of every kind of chromosome, one from each parent). An individual’s genotype is a sequence of symbols representing the particular alleles that an individual possesses for the gene under study. Since alleles code for proteins, and proteins give organisms their traits, alleles indirectly determine phenotype, which is the observable trait exhibited by an individual for the characteristic being studied. Since a diploid cell contains two alleles for a gene, there are two possibilities:
One allele is typically dominant over another allele, and that other allele is therefore recessive to the dominant allele. If this is the case, then the dominant phenotype will result if one or two dominant alleles occur in the cell. Notice that if there is just one dominant allele and the other allele is recessive, then that single dominant allele will dominate over the recessive allele, so that the dominant phenotype will be expressed and the recessive phenotype will not be expressed. The only way for the recessive phenotype to be expressed is for both alleles to be recessive. Such an individual is homozygous recessive. A heterozygous individual features one dominant allele and one recessive allele and expresses the dominant phenotype. A homozygous dominant individual (both alleles are dominant) also expresses the dominant phenotype. Codominance is a special case in which two different alleles are both simultaneously expressed, rather that one dominating over the other. A good example is Type AB blood (expressing both Type A and Type B phenotypes). Sex-linkage refers to a gene that is part of a sex chromosome. Because of this, the expression is different in males compared to females. In humans, for example, females have two X chromosomes, so they have two alleles for a sex-linked genes, whereas males have one X chromosome and one Y chromosome, and the Y chromosome doesn’t have all of the genes that an X chromosome has. Therefore a male has only one allele for a sex-linked gene. Before proceeding, be sure to review these important definitions as you study the laws of inheritance in this section.
A pedigree is a symbolic representation of the phenotypes and sexes of individuals and the ancestral relationships between those individuals. Keeping in mind the laws of inheritance, a pedigree can be studied to determine whether a given trait is dominant or recessive. Once the dominance/recessivity relationship is determined, a pedigree can also be used to determine the possible genotypes of individuals, based on the indicated phenotypes.
For example, in the pedigree shown above, the original parents (square and circle at the top) both exhibited the unaffected phenotype (because their symbols are not filled). They produced some offspring that were unaffected and some that were affected. The only way for this to be possible is for the affected phenotype to be the recessive trait. An offspring with the affected (recessive) phenotype must be homozygous recessive; otherwise that individual would have the unaffected (dominant) trait. Therefore, each original parent must be heterozygous, because each parent must pass on a recessive allele to produce a homozygous recessive offspring. We can make the same inference about the two parents listed rightmost in the generation enclosed by the red dashes, because they also produced recessive offspring. By studying pedigrees like this, you will gain a better understanding of the laws of inheritance.
A genetic cross is an incidence of sexual reproduction involving a particular male and a particular female of a particular species. A monohybrid cross is a cross in which only one gene (one characteristic) is being studied, and typically the two parents differ in their genotypes. For example, one parent might be homozygous dominant (AA) and the other might be homozygous recessive (aa). Knowing the genotypes of the parents allows us to draw a Punnett square to represent the possible fertilization events involving these two parents. Fertilization is the fusion of a male gamete and a female gamete to produce a zygote. A Punnett square lists (on adjacent sides of the square) all possible gametes that can be produced by each parent. By matching up each possible gamete from one parent with each possible gamete from the other parent, the boxes within a Punnett square list all possible zygotes that can be produced by those parents. Moreover, the ratio of different possible genotypes (the genotypic ratio) and of different corresponding phenotypes (the phenotypic ratio) can be computed for the generation created by the particular cross. After reviewing crosses in these videos, you should practice constructing Punnett squares to ensure you understand the concepts being represented.
Like a monohybrid cross, a dihybrid cross is a genetic cross. This means that it involves fertilization of a female gamete (from the female parent) by a male gamete (from the male parent) to produce zygotes that can develop into adult offspring. The fundamental difference between a monohybrid cross and a dihybrid cross is that a monohybrid cross tracks only gene (corresponding to one characteristic), whereas a dihybrid cross simultaneously tracks two different genes (corresponding to two different characteristics). Since two different genes are studied, the genotypes in a dihybrid cross are longer sequences of symbols (both for the alleles and for the zygotes), because they need to indicate alleles for two genes. Also, because there are more possible combinations of alleles, a dihybrid cross will features more boxes in its Punnett square compared to a monohybrid cross. These differences are illustrated in the generic examples shown below.
Monohybrid cross: Aa (heterozygous male) × Aa (heterozygous female)
Number of genes under study: 1 (Gene A)
Genotypes of possible male gametes: A, a
Genotypes of possible female gametes: A, a
Dihybrid cross: AaBb (doubly heterozygous male) × AaBb (doubly heterozygous female)
Number of genes under study: 2 (Gene A and Gene B)
Possible male gametes: AB, aB, Ab, ab
Possible female gametes: AB, aB, Ab, ab
Review the dihybrid example in this video to ensure you understand how dihybrid crosses differ from monohybrid crosses.
Natural selection is the only mechanism of evolution that is consistently adaptive, because natural selection favors alleles that confer a fitness advantage to the individuals with those alleles. One would expect that, over many generations, deleterious alleles (that is, alleles that reduce fitness, perhaps by causing genetic disorders) would be eliminated from a population, because natural selection selects against those deleterious alleles.
One mechanism for a deleterious allele being maintained by natural selection is known as heterozygote advantage. In the case of heterozygote advantage, heterozygous individuals have greater fitness than both types of homozygous individuals (homozygous dominant and homozygous recessive). For example, a mutant allele for a human hemoglobin gene codes for a mutant form of hemoglobin protein. This is deleterious, because it causes sickle cell disease. However, the mutant form of hemoglobin also confers higher resistance to malaria. A homozygous recessive individual will develop sickle-cell disease, which drastically reduces fitness. A homozygous dominant individual will not develop sickle-cell disease, but that person will not have enhanced malarial resistance. A heterozygous individual will not develop sickle-cell disease (because the one normal allele will prevent the disorder) and will also have increased resistance to malaria (because of the mutant allele). Therefore, heterozygous individuals will have the greatest chance of passing on their alleles to the next generation, and that includes the mutant allele. The sickle-cell allele will be maintained in the population by natural selection. To make sense of heterozygote advantage, review the laws of inheritance in this section, and keep in mind that each allele is a code for a particular protein, and therefore each allele corresponds to a particular trait.
Breakthroughs in genetics have created ethical questions that did not previously exist. This is because knowing a person’s genetic makeup allows us to forecast the genetic fate of a person or that person’s child. For example, recessive genetic disorders (like sickle cell disease or cystic fibrosis) occur only in people who are homozygous recessive. A heterozygous individual is considered only a carrier of the disorder. Since a carrier features one normal allele and one mutant allele, the mutant allele can be randomly passed on to a child. If both parents are carriers, then a child could randomly receive the mutant allele from both parents, and this would ensure that the child would develop the disorder. This leads to ethical dilemmas.
There are no easy or definitive answers to such questions. However, an understanding of the genetic principles underlying such ethical dilemmas is important in formulating arguments in either direction. As you consider difficult ethical questions like these, refer to the laws of inheritance that you learned in this section to gain a better understanding of the possible arguments.
This vocabulary list includes terms that might help you with the review items above and some terms you should be familiar with to be successful in completing the final exam for the course.
Try to think of the reason why each term is included.