Mendelian Inheritance Unit 7
Family resemblance: your mother and father each contribute to your genetic makeup. • allele: alternative version of a gene
Family resemblance: your mother and father each contribute to your genetic makeup. How we inherit our genes Each human offspring inherits one maternal set of 23 chromosomes and one paternal set.
Family resemblance: your mother and father each contribute to your genetic makeup. Unlucky catch A baby inheriting two copies of the defective FMO3 gene will develop fish odor syndrome.
Homozygous vs. Heterozygous • Homozygous refers to having identical alleles for a single trait. • The gene for seed shape in pea plants exists in two forms, one form or allele for round seed shape (R) and the other for wrinkled seed shape (r). A homozygous plant would contain the following alleles for seed shape: (RR) or (rr).
Dominant vs, Recessive • one form appears dominant over the other (it masks the presence of the other allele) • the dominant allele does not alter the recessive one in any way. • Both alleles can be passed on to the next generation unchanged.
Homozygous vs. Heterozygous • Heterozygous refers to having two different alleles for a single trait. • A heterozygous plant would contain the following alleles for seed shape: (Rr). • Remember that organisms have two alleles for each trait. When the alleles of a pair are heterozygous, one is dominant and the other is recessive. • Round: (RR) or (Rr), Wrinkled: (rr).
Family resemblance: your mother and father each contribute to your genetic makeup. • In this chapter we explore how heredity works • Inheritance follows some simple rules that allow us to • make sense of the patterns of family resemblance we see • predict accurately how many offspring are likely to inherit particular traits • predict the likelihood that an offspring will inherit a trait • Why is the behavior of some traits easy to predict while many other traits have less straightforward patterns of inheritance?
Family resemblance: your mother and father each contribute to your genetic makeup. TAKE-HOME MESSAGE • Offspring resemble their parents because they inherit genes—instruction sets for biochemical, physical, and behavioral traits, some of which are responsible for diseases—from their parents. Challenge Question • How many alleles of each gene do you have? Where did you get them?
Some traits are controlled by a single gene. • heredity: passing of characteristics from parents to offspring through their genes.
Some traits are controlled by a single gene. HEREDITY I AM the family face; Flesh perishes, I live on, Projecting trait and trace Through time to times anon, And leaping from place to place Over oblivion. The ears-heired feature that can In curve and voice and eye Despise the human span Of durance—that is I; The eternal think in man, That heeds no call to die. —Thomas Hardy, Moments of Vision and Miscellaneous Verses, 1917
Some traits are controlled by a single gene. • Some history: • In ancient Greece, the poet Homer extolled the tremendous benefits to society that came from the skillful breeding of horses. • Once breeders recognized the existence of heredity, they began selecting individual plants or animals with the desired traits to breed with each other, in the hope that their offspring would also have the sweeter corn, loyal dogs, docile livestock, beautiful flowers, and more.
Some traits are controlled by a single gene. • As scientists sought to understand more about the nature of inheritance of traits, hereditary processes were explained in ever increasing detail beginning at the populational level and going toward the molecular level. • Keeping this fact in mind will help in understanding the timeline which follows. • History of Genetics Timeline
Some traits are controlled by a single gene. • single-gene trait: A trait that is determined by instructions on only one gene
Some traits are controlled by a single gene. Chins, ears, and hairlines: single-gene traits. Some traits are determined by the instructions a person carries on one gene
Some traits are controlled by a single gene. TAKE-HOME MESSAGE • More than 9,000 human traits are determined by the instructions a person carries on a single gene, and the traits exhibit straightforward patterns of inheritance. Challenge Question • Give two examples of single-gene traits, genetic traits determined by alleles of a single gene.
Mendel learned about heredity by conducting experiments. Problem: children resemble both their mothers and their fathers, not just their fathers
Mendel learned about heredity by conducting experiments. • Another idea: offspring reflect a simple blending of their two parents’ traits via the blood • Problems: • Doesn’t explain how brown-eyed parents could give birth to blue-eyed children. • Or why one tall and one short parent sometimes produced a tall child, rather than always producing children of intermediate height
Mendel learned about heredity by conducting experiments. • Charles Darwin proposed another equally wrong theory known as "pangenesis“: • hereditary "particles" in our bodies are affected by the things we do during our lifetime. • modified particles were thought to migrate via blood to the reproductive cells and subsequently could be inherited by the next generation
Mendel learned about heredity by conducting experiments. • Lamarck’s Inheritance of Acquired Characteristics: • hypothesis about a mechanism of heredity by which changes in physiology acquired over the life of an organism (such as the enlargement of a muscle through repeated use) may be transmitted to offspring
Mendel learned about heredity by conducting experiments. • Mendel didn’t do anything radically new, but simply applied methodical experimentation and scientific thinking
Mendel learned about heredity by conducting experiments. • He chose a good organism to study: the garden pea. • relatively easy to fertilize manually by “pollen dusting” • easy to collect dozens or even hundreds of offspring from a single cross • pea plants are fast enough breeders that Mendel could conduct experiments that lasted for multiple generations
Mendel learned about heredity by conducting experiments. • Mendel chose to focus on seven easily categorized traits • flower color is purple or white • seed color is yellow or green • flower position is axial or terminal • pod shape is inflated or constricted • stem length is long or short • pod color is yellow or green • seed shape is round or wrinkled
Mendel learned about heredity by conducting experiments. • Mendel began his studies by first repeatedly breeding together similar plants until he had many distinct populations, each of which was unvarying for a particular trait • true-breeding: offspring of crosses of individuals within the population always show the same trait; i.e. the offspring of pea plants that are true-breeding for round peas always have round peas
Mendel learned about heredity by conducting experiments. Q Animation: Mendelian inheritance – the big picture
Mendel learned about heredity by conducting experiments. TAKE-HOME MESSAGE • In the mid-1800s, Gregor Mendel conducted studies that helped us understand how traits are inherited. He applied methodical experimentation and rigorous hypothesis testing, focusing on easily observed and categorized traits in garden peas. Challenge Question • Why was the pea plant a good organism with which to study heredity?
Punnett Squares • easy way to calculate mathematical probability of inheriting a specific trait • Reginald Punnett - 20th century English geneticist • shows all potential combinations of genotypes that can occur in children, given the genotypes of their parents AND • odds of each of the offspring genotypes occurring
Segregation & assortment • Mendel came to three important conclusions from his experimental results: • that the inheritance of each trait is determined by "units" or "factors" that are passed on to descendents unchanged (these units are now called genes) • that an individual inherits one such unit from each parent for each trait • that a trait may not show up in an individual but can still be passed on to the next generation
Segregation & assortment • Mendel's observations from these experiments can be summarized in two principles: • the principle of segregation • the principle of independent assortment
Segregation & assortment • Principle (Law) of Segregation: • for any particular trait, the pair of alleles of each parent separate • only one allele passes from each parent on to an offspring • which allele in a parent's pair of alleles is inherited is a matter of chance • segregation of alleles occurs during the process of sex cell formation (meiosis)
Segregation & assortment • Principle of Independent Assortment: • different pairs of alleles are passed to offspring independently of each other • result - new combinations of genes present in neither parent are possible • today, we know this is because the genes for independently assorted traits are located on different chromosomes
Segregation & assortment • These two principles of inheritance, along with the understanding of unit inheritance and dominance, were the beginnings of our modern science of genetics. However, Mendel did not realize that there are exceptions to these rules. • By focusing on Mendel as the father of genetics, modern biology often forgets that his experimental results also disproved Lamarck's theory of the inheritance of acquired characteristics. • Mendel rarely gets credit for this because his work remained essentially unknown until long after Lamarck's ideas were widely rejected as being improbable.
Phenotype vs. Genotype • Observing an individual’s phenotype is not sufficient for determining its genotype. • It is not always possible to determine an individual’s genotype from its phenotype. • A recessive allele’s effects may be masked by a dominant allele. • Genetic analysis makes use of clever experiments and Punnett squares.
Probability & Genetics • Chance is important in genetics for two reasons: • consequence of segregation • equally likely that the haploid gamete will include one or the other of the two alleles that the individual carries • impossible to know which allele it will be • fertilization is a chance event • all sperm/eggs produced by an individual are different from one another • any one of those gametes may be the gamete involved in fertilization
Test Cross • Genes are too small to be seen, and so determining an individual’s genotype requires indirect methods. • A test-cross enables us to figure out which alleles an individual carries.
Test Cross • You would like to produce white alligators via a mating program. • The problem is that you cannot be certain of the genotype of your alligators. • They might be homozygous dominant, MM, or they might be heterozygous, Mm. • In either case their phenotype is normal coloration. • How can you figure out which of these two possibilities is the actual genotype?
Test cross • In this test-cross, a homozygous white female alligator is bred with a normally colored male of unknown genotype. • The color of their offspring will help identify whether the male is homozygous dominant or heterozygous. • Q Animation
Test Cross • You mate a pigmented male alligator to a female albino alligator. The clutch of baby alligators includes both pigmented and albino individuals. What is the genotype of the father? • MM • Mm • mm • 1 and 2 are equally possible.
Test Cross Take-Home Message • In a test-cross, an individual with a dominant phenotype and an unknown genotype is mated with a homozygous recessive individual. • The phenotypes of the offspring reveal the unknown genotype.
Pedigrees • In cross-pollinating plants that either produce yellow or green pea seeds exclusively, Mendel found that the first offspring generation (f1) always has yellow seeds. However, the following generation (f2) consistently has a 3:1 ratio of yellow to green.
Pedigrees • This 3:1 ratio occurs in later generations as well. Mendel realized that this was the key to understanding the basic mechanisms of inheritance.
What is the genotype of the paternal grandmother? Homozygous recessive Heterozygous Homozygous dominant Cannot be determined
What is the probability that you (“Me” in the diagram) will be a carrier for this disease? 1/4 1/3 1/2 2/3 3/4
Sometimes it’s hard to figure out the pattern… • A Trait’s Mode of Inheritance Is Not Always Completely Obvious • Is it complete dominance or… • the influence of the environment?