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Genetics – Patterns of Inheritance
It is almost a part of daily conversation to hear people comment about how a child looks or acts like his or her parents. We see all around us that offspring resemble their parents more than theyresemble other random individuals in the population. These patterns of resemblance are a consequence of passing characteristics from parents to offspring through their genes. This passing of characteristics from parent to offspring is called heredity. Genetics is the science of studying how hereditary works and the overall patterns of inheritance that exists in biology.
Mendel and his Peas
In the early 1800s, before Mendel’s work, scientists proposed the blending hypothesis to explain how offspring inherit traits from both parents. According to the blending hypothesis, if a red flower and a yellow flower were crossed, their hereditary material would blend and their offspring would be orange. The work of many scientists, including Gregor Mendel, led to this hypothesis being discarded because it could not explain how traits that disappear in one generation can reappear in later ones.
Gregor Mendel was an Austrian monk in the mid-19th century. In the 1840s, he studied math, physics, zoology, and botany before working in the monastery garden. Mendel made observations about the patterns of inheritance of pea plants and did a series of experiments that revealed the basic rules underlying genetics. Mendel identified a trait as a characteristic or feature of an organism, such a flower color. By collecting careful data about how the traits of pea plants were passed between generations, Mendel formed the following hypotheses that are the foundations of genetics today.
- Rather than passing on the trait itself, each parent puts a single set of instructions for building a trait into every egg and sperm it makes.(We now know these instructions are genes.)
- Offspring receive two copies of instructions for any trait, one from each parent. These instructions can contain the same version of the instructions (the same alleles) or different versions of the instructions (different alleles).
- The trait observed in the individual depends on the two copies of the gene it inherits from its parents.
In his experiments, Mendel used garden pea plants. He could control each cross by carrying male pollen (sperm) to fertilize the female carpel (egg). A cross is the mating between a male and a female of a species. Mendel started his experiments by repeatedly breeding together similar plants until he had distinct populations he called true-breeding. True-breeding for a trait means that the offspring produced by parents always produce offspring identical to the parent. For example, peas with white flowers in a true-breeding population always produced offspring with white flowers. Mendel established true-breeding strains for seven different traits that allowed him to piece together the basic rules of all genetics.
Single Trait Crosses – A Most Simple Example
One of the most amazing parts of Mendel’s work is that not only was he incredibly detailed and thoughtful, but he also got very lucky. When Mendel crossed true-breeding plants for a single trait, such as purple-flowered plants and white-flowered plants, he noticed that offspring from these crosses were always purple. The offspring of two different true-breeding parents are called hybrids. For this reason, Mendel called the purple-flower trait dominant and the white-flower trait recessive. In genetics, a dominant trait hides the effect of a recessive trait when an individual contains the instructions for both versions of the trait. This means that a dominant allele codes for the protein whose function can be seen in the individual while the recessive allele codes for a protein whose function cannot be seen in the individual. In some cases, a recessive allele codes for a nonfunctional protein. Today, we can use additional vocabulary to describe how the phenotype (outward appearance of an organism) corresponds to itsgenotype(combination of alleles).We describe an organism that has two of the same alleles for a trait ashomozygous. A homozygous individual gives the phenotype that corresponds to the allele that it has two copies of. We describe an individual that has two different alleles for a trait as heterozygous. A heterozygous individual gives the phenotype of the dominant trait. In Mendel’s experiments, the original true-breeding parents were homozygous, while the purple-flowering offspring were heterozygous.
When Mendel crossed the heterozygous (hybrid) purple-flowering offspring from the cross between the homozygous (true-breeding) purple-flowering parents and homozygous (true-breeding) white-flowering parents, he found that approximately one quarter of the offspring now had white flowers. The original recessive trait had reappeared in the population meaning the instructions for building white flowers, present in the grandparents, were still present in the purple-flowering parents. These observations were the basis that allowed Mendel to develop what are now known as the Laws of Mendelian Inheritance.
It should be noted that a dominant and recessive pattern of inheritance is only one way that alleles interact. Many traits follow patterns that are more complicated than a single-trait dominant and recessive pattern.
Mendel’s First Law: The Law of Segregation
Mendel ‘s experiment allowed him to observe that not only does an organism contains two alleles for each gene, but that the organism only contributes one of its two copies of the gene to its own offspring. The other parent will contribute the other allele. Mendel was able to make this prediction without any knowledge of the process of meiosis, which allows organisms to form gametes with a single set of genetic information.
Mendel’s Second Law: The Law of Independent Assortment
Mendel studied a total of seven different traits. When he combined his experiments and tested two traits at once, he was able to expand his conclusions about the patterns of inheritance. Mendel knew that round pea shape was dominant to wrinkled pea shape, and yellow color was dominant to green. When Mendel crossed plants that were true breeding for both round peas and yellow color to plants that had wrinkled peas and green color, he again observed a reproducible pattern. He found that that a particular allele for a gene can be paired with either allele for another gene. Again, Mendel reached this conclusion without any knowledge of meiosis. When homologous chromosomes are separated during meiosis, each distinct homologous pair separates independently, and therefore randomly, compared to every other homologous pair. Consequently for Mendel, the gene for round or wrinkled peas segregated independently of the gene for yellow or green pea color. As a result, it is possible for the gametes produced by an individual to have any of the possible allele combinations based on the genotype of the individual.
Punnett Squares are a Tool to Predict the Outcome of Genetic Crosses
Today, we can predict the outcome of a genetic cross. First, symbols are assigned to represent the different alleles of a gene. Generally an upper-case letter is used to represent the dominant allele while a lower-case letter is used to represent the recessive allele. This means that for Mendel’s flower color, the allele for purple flowers would be represented as P while the allele for white flowers would be represented as p. Therefore the genotype of the homozygous parents can be represented as PP for the purple-flowering plants and pp for the white-flowering plants. The genotype of the heterozygous offspring would be represented as Pp. If it is unknown which of the two possible genotypes the individual has, we can write P_, where _ is a placeholder for the unknown second allele.
The possible outcomes of a cross between two individuals can be predicted using a tool called a Punnett square. For a single-trait cross, a Punnett square is a 2 x 2 box. Along the top of the square, the two alleles that one parent contains are listed individually. Along the left side of the box the two alleles that other parent produces are listed individually. The alleles are separated this way because, although the individual carries two alleles, only one of the alleles is contained in the sperm of egg produced. In thefour boxes of the Punnett square, the genotypes of all the possible offspring resulting from the cross are entered. The genotype of each box is then the allele given at the head of the column (one parent’s contribution) and the allele at the left row (the other parent’s contribution). This represents the two gametes that come together at fertilization and produce the offspring with the genotype predicted in the box. A Punnett square allows us to predict how many offspring of a given genotype will be produced by a cross. Similarly, a Punnett square allows us to predict how many offspring of a given phenotype will be produced by a cross.
Genetics is Complicated Stuff. Mendel Got Lucky.
In each of the traits he was examining, Mendel saw a clear pattern of dominant and recessive inheritance in which inheriting a single copy of the dominant allele was enough to produce the dominant phenotype, while inheriting two copies of the recessive allele was required to produce the recessive phenotype. In reality, patterns of inheritance are significantly more complicated. Nevertheless, Mendel’s work was foundational and is still the basis for our understanding of genetics.
Single gene traits can follow more patterns than simple dominant and recessive behavior. For example, some traits follow a pattern of intermediate inheritance, when the heterozygous phenotype is in between the phenotype of the two homozygous phenotypes. Other traits have more than two possible alleles, and many possible alleles exist in a population. Human blood type is an example of this, with three alleles present in the population. Human blood type also follows a pattern of codominance, meaning that a heterozygous individual shows the phenotype of both dominant alleles. Some traits, such as skin color or height, are influences by more than one gene. These are called polygenic traits, meaning many-genes. This increases the potential combinations of alleles, and thus the range of phenotypes, for a trait. In addition, some genes can affect more than one gene.