Genetics: The Study of Inheritance [BIOLOGY]

• Heredity is the transmission of traits from one generation to the next. The study of heredity, genetics, started when the monk Gregor Mendel deduced the basic principles of genetics while studying garden peas. In 1866, Mendel published his work arguing that “heritable factors” are passed from parent to their offspring. These factors which we now refer to as genes retain their uniqueness generation after generation.
• A heritable feature that varies among individuals is called a character. Each variation of a character is called a trait. For example, a flower’s color would be its character while the colors that appear on the flower are its trait. Because Mendel’s experiments are rigorous, he was able to formulate many hypotheses about inheritance. Mendel’s experiments with pea plants allowed him to formulate four hypotheses.
• 1. There are alternative versions of genes that account for variations in inherited characters. These alternative versions of a gene are called alleles.
  2. For each character, an organism inherits two alleles of a gene, one from each parent. An organism with two identical alleles of the same gene is homozygous while those with two different alleles are heterozygous.
• 3. If the two alleles of an inherited trait differ, one allele determines the organism’s appearance while the other has no noticeable effect on the appearance. The allele that determines the appearance is called the dominant allele while the other is referred to as the recessive allele. Geneticists, scientists who study heredity, represent dominant alleles as uppercase italic letters and recessive alleles as lowercase italic letters.
• 4. A sperm or an egg carries only one allele for each character because allele pairs separate during the production of gametes. The process where the pairs separate is referred to as the law of segregation
• Punnett square, a means to quantify the probability that an organism will inherit a specific trait. Because appearance does not always reveal the composition of its genes, geneticists define an organism’s observable traits as phenotype and its genetic makeup as genotypE
• In his study, Gregor Mendel was able to see a pattern of inheritance of seven pea plant characters. The underlying mechanism for this pattern is stated in Mendel’s law of segregation: “Pairs of alleles segregate during gamete formation only to reform as pairs during fertilization.”
• In addition to the law of segregation, Gregor Mendel was also able to establish the law of independent assortment by tracking two characters at once. That is, a pair of alleles will not encroach on other pairs of alleles during gamete formation. Put another way, how one character is inherited will have no effect on how other characters are inherited
• Two statistical principles that are also present in genetics are the rule of multiplication, which is used to describe independent events (like determining the chance that two coins will appear with the same side up); and the rule of addition, which can help determine if an event will occur in two or more ways. These two rules when applied to segregation and independent assortment can help solve some rather complex genetic problems
• Genetic Disorders

  Combinations of certain alleles are not always beneficial. Genetic disorders may affect an individual’s appearance because of these kinds of combinations. Disorders that show Mendel’s inheritance patterns can be classified as recessive disorders or dominant disorders.
• Genetic Disorders

  When recessive traits cause a disorder to appear in an individual, it is an example of a recessive disorder. Most people with recessive disorders are born to normal, heterozygous parents, called carriers - as they have the allele for the disorder but do not affect their phenotypes. Dominant alleles may also cause a number of human disorders. Dominant alleles that cause lethal diseases are much less common than recessive alleles that cause lethal diseases.
• Non-Mendelian Patterns

  In Mendel’s study on pea plants, the traits that manifested wherein the first generation of offspring appears exactly like one of the two parents, or when the phenotype is expressed regardless if the allele is present in one or two copies, is called complete dominance.
• Non-Mendelian Patterns

  But in some of the first-generation (geneticists designate generations as Fn, where n is the number of generations after the original parents; for example, first-generation individuals will be designated as F1 ) offspring, the phenotype falls between those of the two parents. This situation is called incomplete dominance. Imagine that a white and red flower is bred and one of its offspring has pink flowers
• Non-Mendelian Patterns

  But in most organisms, genes can be found in more than two versions known as multiple alleles. An example of this would be our own ABO blood type.
• Non-Mendelian Patterns

  In people with blood types A and B, the alleles code for the presence of either antigen A or B, the last allele neither codes for the two hence blood type as a character is based on three alleles. This is called codominance as these two alleles can be expressed in heterozygous individuals who will have blood type AB. Care should be done to distinguish codominance with incomplete dominance as the former refers to the expression of two alleles in separate, distinct ways; while the latter is the expression of one intermediate trait
• Non-Mendelian Patterns

  Genes are not specific to only one character. It can also affect multiple characters called pleiotropy. Because of this, many characters can be observed like a continuum (visualize the color palette used on house and décor magazines or those showing skin tones). Such features, like our skin color and height, vary among the population and are due to polygenic inheritance.
• Chromosomes and Inheritance

  By combining these two ideas, one of biology’s most important concepts was formulated: the chromosome theory of inheritance. This theory states that genes are located on specific loci/positions on chromosomes, and these chromosomes are the ones that undergo segregation
• Chromosomes and Inheritance

  It was observed that genes located close together on the same chromosome tend to be inherited together. These genes are called linked genes and thus, may not follow Mendel’s law of independent assortment. This was found to be because of crossing-over in meiosis which produces new combinations of alleles. Thus, the discovery of how crossing over helps diversify gametes confirmed the relationship between chromosomes and heredity
• Chromosomes and Inheritance

  Among the studies which supported this relationship was that of Hunt Morgan on the fruit fly, Drosophila melanogaster. He introduced to us the concepts of wild-type, traits which are most common in nature, and mutants, less common traits. By mating wild-type fruit flies with mutants, he was able to observe offspring with varying combinations of characters from the parents.
• Sex Chromosomes and Sex-Linked Genes

  The X-Y system is only one of several sex-determining systems. In grasshoppers and other insects, for example, they have an X-O system where O stands for the absence of a sex chromosome. Females are XX and males have one sex chromosome, hence XO. As in humans, the sperm determines the sex of the individual since it produces two classes of sperm: one that may bear an X or another that lacks it.
• Sex Chromosomes and Sex-Linked Genes

  In contrast, the Z-W system is delegated to organisms where sex is determined by the egg. In certain fishes, butterflies, and birds, for example, males have ZZ genotype while females have ZW
• Sex Chromosomes and Sex-Linked Genes

  Aside from determining sex, these chromosomes contain characters unrelated to determining the sexuality of an organism. These are called sex-linked genes as they are genes located on either sex chromosome. In humans, the vast majority of sex-linked genes are X-linked genes. In terms of sex-linked genetic disorders, most of them are X-linked recessive conditions that affect males more often. This is because a man only inherits one X chromosome and for women to exhibit these kinds of disorders, they need to have two alleles to exhibit the trait.
• Sex-linked genes

  Examples of these disorders are hemophilia, characterized by excessive bleeding due to an abnormality in blood clotting proteins, and Duchenne muscular dystrophy, characterized by progressive weakening of muscles and lack of coordination due to a mutation of a gene that is located on the X chromosome which codes for muscle proteins.