Trait Inheritance Biology for Majors Mendels Experiments Mendel

Trait Inheritance Biology for Majors

Mendel’s Experiments • Mendel found that crosses between parents that differed by one trait produced F 1 offspring that all expressed the traits of one parent. When the offspring in Mendel’s experiment were self-crossed, the F 2 offspring exhibited the parental traits in a 3: 1 ratio, confirming that the “hidden” trait had been transmitted faithfully from the original P 0 parent.

Mendelian Genetics Observable traits are referred to as dominant, and non-expressed traits are described as recessive. Reciprocal crosses generated identical F 1 and F 2 offspring ratios. By examining large sample sizes, Mendel showed that his crosses behaved reproducibly according to the laws of probability, and that the traits were inherited as independent events.

Rules of Probability Two rules in probability can be used to find the expected proportions of offspring of different traits from different crosses. To find the probability of two or more independent events occurring together, apply the product rule and multiply the probabilities of the individual events. The use of the word “and” suggests the appropriate application of the product rule. To find the probability of two or more events occurring in combination, apply the sum rule and add their individual probabilities together. The use of the word “or” suggests the appropriate application of the sum rule.

Genotype vs. Phenotype • • • The observable traits expressed by an organism are referred to as its phenotype. An organism’s underlying genetic makeup, consisting of both physically visible and non-expressed alleles, is called its genotype. Homozygous individuals have two copies of the same gene for a trait while heterozygous individuals have two different versions of that gene.

Punnett Square Analysis

The Test Cross • A test cross can determine whether an organism expressing a dominant trait is a homozygote or a heterozygote.

Inheritance of Traits When homozygous individuals that differ for a certain trait are crossed, all of the offspring will be heterozygotes for that trait. If the traits are inherited as dominant and recessive, the F 1 offspring will all exhibit the same phenotype as the parent with the dominant trait. If these heterozygous offspring are selfcrossed, the resulting F 2 offspring will be equally likely to inherit gametes carrying the dominant or recessive trait, giving rise to offspring of which one quarter are homozygous dominant, half are heterozygous, and one quarter are homozygous recessive. Because homozygous dominant and heterozygous individuals are phenotypically identical, the observed traits in the F 2 offspring will be a ratio of three dominant to one recessive.

Laws of Inheritance Mendel postulated that genes (characteristics) are inherited as pairs of alleles (traits) that behave in a dominant and recessive pattern. Each gamete is equally likely to receive either one of the two alleles present in a diploid individual. In addition, genes are assorted into gametes independently of one another. That is, alleles are generally not more likely to segregate into a gamete with a particular allele of another gene. A dihybrid cross demonstrates independent assortment when the genes in question are on different chromosomes or distant from each other on the same chromosome. For crosses involving more than two genes, use the forked line or probability methods to predict offspring genotypes and phenotypes rather than a Punnett square.

Law of Independent Assortment: Dihybrid Cross

Tryhybrid Cross: The Forked-Line Method The forked-line method can be used to analyze a trihybrid cross. Here, the probability for color in the F 2 generation occupies the top row. The probability for shape occupies the second row, and the probability for height occupies the third row. The probability for each possible combination of traits is calculated by multiplying the probability for each trait.

Complicating the Law of Independent Assortment Although chromosomes sort independently into gametes during meiosis, Mendel’s law of independent assortment refers to genes, not chromosomes, and a single chromosome may carry more than 1, 000 genes. When genes are located in close proximity on the same chromosome, their alleles tend to be inherited together. This results in offspring ratios that violate Mendel’s law of independent assortment. However, recombination serves to exchange genetic material on homologous chromosomes such that maternal and paternal alleles may be recombined on the same chromosome. This is why alleles on a given chromosome are not always inherited together.

Recombination • Recombination is a random event occurring anywhere on a chromosome. Therefore, genes that are far apart on the same chromosome are likely to still assort independently because of recombination events that occurred in the intervening chromosomal space

Non-Mendelian Genetics • Not all traits display “simple” Mendelian inheritance patterns. • Beyond genes being linked on the same chromosome, some traits simply obey different inheritance rules.

Incomplete Dominance • Incomplete dominance is the expression of two contrasting alleles such that the individual displays an intermediate phenotype as in the pink flowers.

Codominance • Codominance is when both alleles for the same characteristic are simultaneously expressed in the heterozygote. The roan coat color in horses is an example of codominance: individual hairs are either chestnut or they are white, leading to the red roan overall appearance.

Sex-Linked Traits When a gene being examined is present on the X chromosome, but not on the Y chromosome, it is said to be X-linked. Males are said to be hemizygous, because they have only one allele for any X-linked characteristic. Hemizygosity makes the descriptions of dominance and recessiveness irrelevant for XY males. In some groups of organisms with sex chromosomes, the gender with the non-homologous sex chromosomes is the female rather than the male. This is the case for all birds. In this case, sex-linked traits will be more likely to appear in the female, in which they are hemizygous.

Punnett Square for a Sex. Linked Trait

Multiple Alleles Although individual humans (and all diploid organisms) can only have two alleles for a given gene, multiple alleles may exist at the population level such that many combinations of two alleles are observed.

Multiple Alleles: Coat Color in Rabbits

Penetrance is the probability of a gene being expressed. Despite the presence of a dominant allele, a phenotype may not be present. Complete penetrance means the genes are expressed in everyone with those genes. Incomplete or reduced penetrance means the genetic trait is expressed in only part of the population. The penetrance may depend on age. Reduced penetrance probably results from a combination of genetic, environmental, and lifestyle factors.

Expressivity Variable expressivity refers to the range of signs and symptoms that can occur in different people with the same genetic condition. As with reduced penetrance, variable expressivity is probably caused by a combination of genetic, environmental, and lifestyle factors, most of which have not been identified.

Pedigree Analysis • Geneticists use pedigree analysis to study the inheritance pattern of human genetic diseases.

How to Take a Family History A basic family history should include three generations. To begin taking a family history, start by asking the patient about his/her health history and then ask about siblings and parents. Questions should include: 1. General information such as names and birthdates 2. Family’s origin or racial/ethnic background 3. Health status 4. Age at death and cause of death of each family member 5. Pregnancy outcomes of the patient and genetically-related relatives It may be easier to list all the members of the nuclear family first and then go back and ask about the health status of each one. After you have taken the family history of the patient’s closest relatives, go back one generation at a time and ask about aunts, uncles, grandparents, and first cousins.

How to Draw a Pedigree

Sample Pedigree

Autosomal Recessive • Tay-Sachs is an autosomal recessive disease. This means that children can only inherit the Tay. Sachs if both of their parents are carriers for the disease.

Epistasis is a form of non-Mendelian inheritance in which one gene is capable of interfering with expression of another. This is often found associated with gene pathways where the expression of one gene is directly dependent on the presence or absence of another gene product within the pathway.

Epistasis (continued) • In mice, the agouti coat color (A) is dominant to a solid coloration. A gene at a separate locus (C) is responsible for pigment production. The recessive c allele does not produce pigment, and a mouse with the homozygous recessive cc genotype is albino regardless of the allele present at the A locus. Thus, the C gene is epistatic to the A gene.

Polygenic Inheritance • • • Some traits such as height have continuous variation (above) and a complex inheritance pattern. These traits are controlled not just by one gene, but rather, by multiple (often many) genes that each make a small contribution to the overall outcome. This inheritance pattern is called polygenic inheritance. Height and many other traits don’t just depend on genetics: they also depend on environmental factors, such as a child’s overall health and the type of nutrition he or she receives while growing up.

Polygenic Inheritance and Continuous Variation It is possible to produce a spectrum of slightly different phenotypes (something approaching continuous variation) with just three genes whose alleles display incomplete dominance.

Effects of the Environment Characteristics that are influenced by environmental as well as genetic factors are called multifactorial. The range of phenotypic possibilities is called the norm of reaction. While genes and genetic causes play a large role in health and phenotypes, the environment also plays an important role. Understanding this can enable the treatment of some disorders, such as the case with PKU in which limiting the intake of phenylalanine can prevent toxic build up of this amino acid. Often the norm of reaction is set by genetic factors but ultimately determined by environmental exposures.

Pleiotropy Genes that affect multiple, seemingly unrelated aspects of an organism’s phenotype, are pleiotropic.

Pleiotropy: Marfan Syndrome People with Marfan syndrome are tall, thin, and have eye and heart problems. The symptoms may not seem related, but they are all caused by the mutation of a single gene. This gene encodes a protein that makes elastic fibrils that give strength and flexibility to the body’s connective tissues. The mutation reduces the amount of fibrils produced. The eye and the aorta normally contain many fibrils that help maintain structure; these two organs are strongly affected in Marfan syndrome. In addition, the fibrils serve as “storage shelves” for growth factors. When there are fewer of them in Marfan syndrome, the growth factors cannot be shelved and thus cause excess growth (leading to the characteristic tall, thin Marfan build).

Practice Question A rare genetic disorder affects only the men in a family. What might be true about how that gene is inherited?

Quick Review • • What was the impact of Gregor Mendel on the field of genetics? How do his two laws of genetics apply? What factors can complicate the phenotypic expression of genotype, including mutations? How is a pedigree done? How do geneticists predict whether a disease will be passed through a family in one of three modes? What role does the environment play on phenotypes?