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10 GENETICS

Learning Objectives

  1. Model and better understand Mendelian genetics.
  2. Determine personal phenotypes and genotypes for some observable traits. 
  3. Model and understand that most traits are not Mendelian (even those tested here). 
  4. Determine the ratio of dominant to recessive traits in our class. 

Activity

  1. How would you know if a trait follows a Mendelian inheritance pattern, assuming you know and can track the genotypes and phenotypes of an organism as it produces offspring?
  2. What is a Punnett square, and what does it show? Using a Punnett square, predict the offspring of a cross between two heterozygous parents for gene A (Aa × Aa).
  3. Discuss the answers to questions 1–2 with your group.

Mendel’s Experiments and Heredity

Johann Gregor Mendel (1822–1884) was a lifelong learner, teacher, scientist and man of faith. Supported by his monastery. in what is now the Czech Republic. In 1865, Mendel presented the results of his experiments with nearly 30,000 pea plants to the local Natural History Society. He demonstrated that traits are transmitted faithfully from parents to offspring independently of other traits and in dominant and recessive patterns.

Mendel’s work was accomplished using the garden pea to study inheritance. This species naturally self-fertilizes, preventing pollination from other plants. The result is “true-breeding” pea plants. When fertilization occurs between two true-breeding parents that differ in only one characteristic, the process is called a monohybrid cross, These are plants that always produce offspring that look like the parent. By experimenting with true-breeding pea plants, Mendel avoided the appearance of unexpected traits in offspring that might occur if the plants were not true breeding. The garden pea also grows to maturity within one season, meaning that several generations could be evaluated over a relatively short time. Finally, large quantities of garden peas could be cultivated simultaneously, allowing Mendel to conclude that his results did not come about simply by chance.

This illustration shows a monohybrid cross. In the upper P generation, one parent has a dominant yellow phenotype and the genotype upper Y upper Y, and the other parent has the recessive green phenotype and the genotype lower y lower y. Each parent produces one kind of gamete, resulting in an upper F subscript 1 baseline generation with a dominant yellow phenotype and the genotype upper Y lower y. Self-pollination of the upper F subscript 1 baseline generation results in an upper F subscript 2 baseline generation with a 3 to 1 ratio of yellow to green peas. One out of three of the yellow pea plants has a dominant genotype of upper Y upper Y, and 2 out of 3 have the heterozygous phenotype upper Y lower y. The homozygous recessive plant has the green phenotype and the genotype lower y lower y.

Figure 14.1: Pea plant with dominant yellow phenotype crossed with recessive green phenotype. This cross produces all heterozygotes with a yellow phenotype. A Punnett square analysis can be used to predict the genotypes of a cross of two heterozygotes.

Working with garden pea plants, Mendel found that crosses between parents differing in one trait produced F1 offspring expressing traits of one parent. Observable traits are referred to as dominant, and non-expressed traits are described as recessive. When the offspring in Mendel’s experiment were self-crossed, the F2 offspring exhibited the dominant trait (75%) and the recessive trait (25%) in a 3:1 ratio, confirming the recessive trait had been transmitted faithfully from the original P0 parent. By examining sample sizes, Mendel showed that his crosses behaved reproducibly according to the laws of probability, and that traits were inherited as independent events.

Gene variants which arise by mutation and exist at the same relative chromosomal locations are called alleles. Two alleles for a given gene in a diploid organism are expressed and interact to produce physical characteristics. The observable traits of an organism are referred to as phenotype. The organism’s underlying genetic makeup is referred to as  genotype. Mendel’s experiments demonstrate the difference between phenotype and genotype. When true-breeding plants in which one parent had yellow pods and one had green pods were cross-fertilized, all F1 offspring had yellow pods. Offspring were phenotypically identical to the parent with yellow pods. Allele donated by the parent with green pods was not simply lost, because it reappeared in some F2 offspring. This demonstrates F1 plants must have different genotypes from the parent with yellow pods.

The plants Mendel used in his experiments were homozygous for traits he was studying. Diploid organisms that are homozygous at a given gene have two identical alleles for that gene on their homologous chromosomes. Mendel’s parental pea plants always bred true because both gametes produced carried the same trait.When plants with contrasting traits were cross-fertilized, all offspring were heterozygous, meaning their genotype had different alleles for the gene being examined.

In all seven pea-plant characteristics, one of the two contrasting alleles was dominant, and the other was recessive. We now know this is due to genes on homologous chromosome pairs. For a gene that is expressed in a dominant and recessive pattern, homozygous dominant and heterozygous organisms will appear identical. This means they have different genotypes but the same phenotype. The recessive allele will only be observed in homozygous recessive individuals, so some refer to recessive alleles as masked.

Alleles do not always behave in dominant and recessive patterns. Incomplete dominance describes situations in which the heterozygote exhibits a phenotype that is intermediate between the homozygous phenotypes. Codominance describes the simultaneous expression of both of the alleles in the heterozygote. Although diploid organisms can only have two alleles for any given gene, it is common for more than two alleles of a gene to exist in a population. In humans, as in many animals and some plants, females have two X chromosomes and males have one X and one Y chromosome. Genes that are present on the X but not the Y chromosome are said to be X-linked, such that males only inherit one allele for the gene, and females inherit two. Finally, some alleles can be lethal. Recessive lethal alleles are only lethal in homozygotes, but dominant lethal alleles are fatal in heterozygotes as well.

Many human diseases are genetically inherited. A healthy person in a family in which some members suffer from a recessive genetic disorder may want to know if he or she has the disease-causing gene and what risk exists of passing the disorder on to his or her offspring. Of course, doing a test cross in humans is unethical and impractical. Instead, geneticists use pedigree analysis to study the inheritance pattern of human genetic diseases. Autosomal dominant disorders are present in both male and female humans and appear in each generation of a pedigree. An example of this in humans is Huntington’s disease, in which the nervous system gradually wastes away. People who are heterozygous for the dominant Huntingtonallele (Hh) will inevitably develop the fatal disease. However, the onset of Huntington’s disease may not occur until age 40, at which point the afflicted persons may have already passed the allele to 50 percent of their offspring.

Recessive alleles can be “carried” and not expressed by individuals. Such heterozygous individuals are sometimes referred to as “carriers.” Further genetic studies in other plants and animals have shown that much more complexity exists, but that the fundamental principles of Mendelian genetics still hold true. Autosomal recessive disorders will show unaffected carriers and can skip generations in pedigrees.

This is a pedigree of a family that carries the recessive disorder alkaptonuria. In the second generation, an unaffected mother and an affected father have three children. One child has the disorder, so the genotype of the mother must be upper case A lower case a, and the genotype of the father is lower case a lower casea. One unaffected child goes on to have two children, one affected and one unaffected. Because her husband was not affected, she and her husband must both be heterozygous. The genotype of their unaffected child is unknown, and is designated upper A question mark. In the third generation, the other unaffected child had no offspring, and his genotype is therefore also unknown. The affected third-generation child goes on to have one child with the disorder. Her husband is unaffected and is labeled 3. The first generation father is affected and is labeled 1; The first generation mother is unaffected and is labeled 2 The Visual Connection question asks the genotype of the three numbered individuals.

Figure 14.2: Alkaptonuria is a recessive genetic disorder in which two amino acids, phenylalanine and tyrosine, are not properly metabolized. Affected individuals may have darkened skin and brown urine, and may suffer joint damage and other complications. In this pedigree, individuals with the disorder are indicated in blue and have the genotype aa. Unaffected individuals are indicated in yellow and have the genotype AA or Aa. Note that it is often possible to determine a person’s genotype from the genotype of their offspring. For example, if neither parent has the disorder but their child does, they must be heterozygous. Two individuals on the pedigree have an unaffected phenotype but unknown genotype. Because they do not have the disorder, they must have at least one normal allele, so their genotype gets the “A?” designation.

Because human males need to inherit only one recessive mutant X allele to be affected, X-linked disorders are disproportionately observed in males. Females must inherit recessive X-linked alleles from both of their parents in order to express the trait. When they inherit one recessive X-linked mutant allele and one dominant X-linked wild-type allele, they are carriers of the trait and are typically unaffected. Carrier females can manifest mild forms of the trait due to the inactivation of the dominant allele located on one of the X chromosomes. However, female carriers can contribute the trait to their male children, resulting in the male exhibiting the trait, or they can contribute the recessive allele to their female children, resulting in the children being carriers of the trait. Although some Y-linked recessive disorders exist, typically they are associated with infertility in males and are therefore not transmitted to subsequent generations. X-linked or sex linked disorders affect more males than females, as females can be unaffected carriers.

A diagram shows an unaffected male with a dominant allele and an unaffected carrier female with an x-linked recessive allele. Four figures of offspring are shown representing the various resulting genetic combinations: unaffected male offspring, unaffected female offspring, affected male offspring, and unaffected carrier female offspring.

Figure 14.3: The male offspring of a person who is a carrier of a recessive X-linked disorder will have a 50 percent chance of being affected. A female will not be affected, but she will have a 50 percent chance of being a carrier like the female parent.

Single Trait Inheritance (Source: Professor Amy Moore)

Based on the principles set forth by Mendel, we can predict what genotypes and phenotypes offspring will have based on the genotypes and phenotypes of their parents. One efficient way to do this involves using a Punnett square. A Punnett square is a grid where all the alleles of one parent are provided as the column headers while all of the alleles from the second parent are provided as the row headers (Figure 14.4). When the alleles from the two parents are combined in the grid, the internal squares predict the genotypes of their offspring. In addition, multiple Punnett squares can predict offspring genotypes across several generations. The first filial generation (F1) is the offspring that results from crossing the original, parental generation. The second filial generation (F2) is the offspring that results from crossing F1 individuals.

 

 

A Punnett squareFigure 14.4: A Punnett square is used as a visual representation of crossed traits and the results of the crosses. Capital R represents the dominant trait, and lowercase r represents the recessive trait. The first square is a cross between the two dominant traits R and R. RR is the result.

Genetic Inheritance 

Genes are the basis of heredity, a sequence of DNA that codes for a polypeptide.  Genes result in proteins that collectively make up your traits (hair color, eye color, height, what enzymes you produce, and so on). Alleles are alternative forms of a gene.  An example is ear lobe attachment, one allele codes for detached or free earlobes, the other allele codes for attached earlobes.  Most sexually reproducing species have two sets of chromosomes, one from their biological mother and one from biological father, meaning a person will have two alleles for a specific trait.  Many genes have two allele forms, dominant and recessive.  The dominant form means if a person has that allele, they will show that trait.  In the earlobe example, if a person has free earlobes, they have at least one dominant allele.  A recessive allele is masked by the dominant allele if a person has one of each, and only displays that trait if they have both recessive alleles.  This means a person needs two alleles, both coding for attached earlobes to display that trait. 
A person’s genotype can be inferred from their phenotype.  The phenotype is the observable expression of a trait, such as a person with attached earlobes.  Here we can infer the person has both alleles for attached earlobes, so their genotype is homozygous recessive (the same allele on both chromosomes).  Genotype refers to the alleles a person has for a particular trait and is written using a capital letter to represent the dominant allele, often the first letter of that dominant trait, and the lowercase version of that letter for represent the recessive allele.  Since free earlobes are dominant the dominant allele would be represented by F and the recessive (attached earlobes) would be f. 
You will determine your phenotype and infer possible genotypes for the traits listed below.  Remember, if you show a dominant trait, you have at least one dominant allele, but may be homozygous dominant or heterozygous for that trait.  Suppose, however, that one of your parents shows the recessive trait.  In that case, the parent would have passed on the allele for the recessive trait and you would be heterozygous for that trait.  If neither of your parents shows the recessive trait, you may not know if you are heterozygous or homozygous dominant for that trait.  
In your notebook or handout, record whether or not you possess each trait by putting a √ (check mark) for yes and/or an X for no.  For traits that you cannot observe directly, ask your lab group for help.   

Ear lobes (E): Free earlobes have at least one dominant allele.  People with attached earlobes are recessive. 

Eye color (B): Inheritance of eye color is controlled by multiple genes, but people having the homozygous recessive genotype have blue or green eyes.  People who have a dominant allele may have different shades of brown or hazel eyes. 

Widow’s Peak (W):  A hairline that forms a downward point in the middle of the forehead is caused by a dominant allele.  A smooth hairline is caused by a recessive genotype. 

Bent Little Finger (L):  A dominant allele results in the end joint of the little finger of each hand bending inward.  Straight little fingers are a result of the recessive genotype. To test: Place your hands on a flat surface, palms down, and relax.  Check to see if the first joints of your little fingers are bent or straight. 

Mid-Digital Hair (H):  Individuals who have hair on the middle joints of their fingers have at least one dominant allele.  Those with two recessive alleles do not have hair on the joint.

Hair color (A):  Individuals with red hair have the recessive genotype.  Those with any other color hair have at least one dominant allele. 

Curly hair (C):  Individuals having curly hair have at least one dominant allele.  People having straight hair have the recessive genotype. 

Freckles (F):  The recessive genotype means the individual lacks freckles completely.  An individual with freckles will have at least one dominant allele. 

Dimples (D):  An individual without dimples is homozygous recessive, while an individual with dimples has at least one dominant allele (two dimples are homozygous dominant, while one dimple is heterozygous). 

Cleft Chin (M):  An individual with a genotype of homozygous recessive will have a cleft chin, while a person with at least one dominant allele will not have a cleft chin. 

Hitchhiker’s thumb (J):  A person that can bend the last joint of the thumb to approximately a 45 degree angle has the recessive genotype while an individual that cannot do it has at least one dominant allele. 

Index Finger Length (I):  If the index finger is shorter than the ring finger (4th finger next to the pinky), you have a dominant allele. If not, you have a recessive allele. 

Left-over-right thumb crossing (Q):  When the hands are folded in a natural fashion, the left thumb crosses the right thumb in a dominant genotype.  If reversed, a recessive genotype is present. 

Palmaris Longus Muscle (P): Presence of the muscle indicated at least one dominant allele.  Clench your fist tightly and bend fist towards you.  If you can see three tendons in your wrist, you have a dominant allele, if you only see two, you are homozygous recessive.  

PTC Tasting (T): the ability to taste phenylthiocarbamide (PTC) is dominant and those who do not taste it are recessive. 

 

 

Assessments

  1. If a man does not have Hitchhiker’s thumb, what are the two possible genotypes?
  2. Is anyone dominant for every trait? Is anyone recessive for every trait?  If not, what does this show about dominance of traits in people? 
  3. Two people that look alike have thousands of common traits.  How often do you think that genetic twins (aside from identical twins) exist? Explain your answer.
  4. Pick three traits and write the ratio of dominant individuals to recessive for those traits. Then, for each trait, write the percent of students in class dominant for the trait. 
The Mendel’s Experiments and Heredity portion of this chapter is adapted from Biology 2E  from OpenStax Licensed: CC BY
The Single Trait Inheritance portion of this chapter is by Professor Amy Moore. Licensed: CC BY NC SA
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