Mendel and the Gene Idea

Two possible explanations existed for how traits are passed from one generation to the next. Blending hypothesis Genes from parents blend to make a new set of characteristics

Two possible explanations existed for how traits are passed from one generation to the next. Particulate hypothesis Genes from parents are discrete units with fixed values for the trait

Mendel used the scientific approach to determine how traits are inherited and to identify two laws of inheritance.

Not PEAS again! Can’t you study something else for a while?! What did Mendel find? Mendel determined that inheritance from the parental generation to the filial generation is particulate. Mendel used peas to test his ideas. Not PEAS again! Can’t you study something else for a while?!

Mendel used 7 traits in his initial testing.

Peas usually self-pollinate, but Mendel controlled all of the reproduction. By crossing (mating) two true breeding varieties of an organism, scientists can study patterns of inheritance. In this example, Mendel crossed pea plants that varied in flower color. When pollen from a white flower fertilizes eggs of a purple flower, the first-generation hybrids all have purple flowers. The result is the same for the reciprocal cross, the transfer of pollen from purple flowers to white flowers.

Naming the Generations P refers to the parent (true- breeding) organisms F1 refers to the first generation of offspring from the P generation F2 refers to the generation of offspring produced by breeding two F1 organisms

Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation

Mendel’s Laws are based on the evidence from his experiments. Mendel reasoned that In the F1 plants, only the purple flower factor was affecting flower color in these hybrids Purple flower color was dominant, and white flower color was recessive

Mendel related four concepts in creating his model that explained the 3:1 ratio he found in the F2 generations.

First, alternative values of genes (alleles) account for variations in inherited characters Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers

Second, for each character an organism inherits two alleles, one from each parent

Third, if the two alleles at a locus differ then one, the dominant allele, determines the organism’s appearance The other allele, the recessive allele, has no noticeable effect on the organism’s appearance

Fourth the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes (Meiosis Happens!)

Mendel’s Laws Law of Dominance Law of Segregation If present, a dominant allele will be represented. Law of Segregation Alleles segregate during gamete formation. Law of Independent Assortment Chromosomes line up randomly and independently in metaphase I This law came later!

A phenotype is a description of the appearance of a gene while a genotype is the genetic code for a trait.

Terminology Homozygous- having identical alleles for a trait Aka: pure or true breeding Heterozygous- having different alleles for a trait Aka: hybrid

Using a test cross will allow you to figure out if an organism that displays the dominant trait is heterozygous or homozygous. By mating the unknown organism with an individual that has the recessive trait you can determine the genotype of the original organism.

If even one of the offspring displays the recessive trait, the original organism can be determined to be heterozygous.

A dihybrid cross allows us to determine that genes are inherited independently.

The laws of probability govern Mendelian inheritance.

The multiplication and addition rules can be applied to monohybrid crosses The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities P(A and B) = P(A)∙P(B)

P(A and B) = P(A)∙P(B) Probability of getting a heart = 1 in 4 Probability of getting a king = 1 in 13 Probability of getting a king of hearts = 1 in 52

Probability in a monohybrid cross can be determined using this rule HH Hh hh

The multiplication and addition rules can be applied to monohybrid crosses The rule of addition states that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities P(A or B)= P(A) + P(B)

P(A or B)= P(A) + P(B) Probability of drawing a heart or a spade from a deck of cards. Probability of drawing a heart = 1 in 4 Probability of drawing a spade = ¼

Solving Complex Genetics Problems with the Rules of Probability We can apply the rules of probability To predict the outcome of crosses involving multiple characters

Combining the rules works as well. What is the probability of drawing a king of spades or a king of hearts? P(K♠ or K♥) = P(K)∙P(♠) + P(K)∙P(♥)

What is the probability of drawing a king of hearts AND a king of spades. To make the problem easier, lets say we are using separate decks! P(K♥ and K♠)=

Challenge Problem: What is the probability of getting a royal flush from a single deck of cards?

You can look at the outcomes of multiple genes at the same time using a larger Punnett Square A dihybrid or other multicharacter cross Is equivalent to two or more independent monohybrid crosses occurring simultaneously In calculating the chances for various genotypes from such crosses Each character first is considered separately and then the individual probabilities are multiplied together

The Math: x=2n Possible number of different gametes (x) n is the number of heterozygous traits in the parent Possible number of different genotypes (x) n is the number of dominant traits in the individual

How many different gametes can be made from the genotype? AaBbCCDdEEffGg There are 4 heterozygous traits, so there are 24 or 16 possible gametes

How many possible genotypes are possible for the phenotype? Purple flowered, round, green peas with yellow inflated pods and tall axial flowers. There are 5 dominant traits so n=5 25=32 different possible genotypes for the phenotype

Frequency of Dominant Alleles Dominant alleles Are not necessarily more common in populations than recessive alleles

Inheritance patterns are often more complex than predicted by simple Mendelian genetics. The relationship between genotype and phenotype is rarely simple.

The inheritance of characters by a single gene may deviate from simple Mendelian patterns Multiple types of dominance Linkage Multiple alleles Pleitropy Sex linkage Epistasis Polygenism

The Spectrum of Dominance Complete dominance Occurs when the phenotypes of the heterozygote and dominant homozygote are identical In codominance Two dominant alleles affect the phenotype in separate, distinguishable ways In incomplete dominance The phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties

Complete dominance Occurs when the phenotypes of the heterozygote and dominant homozygote are identical

Codominance occurs When two dominant alleles affect the phenotype in separate, distinguishable ways

Incomplete dominance occurs When the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties

Mutation

P Generation F1 Generation F2 Generation Red CRCR Gametes CR CW x White CWCW Pink CRCW Sperm Cw Eggs CR CR CR CW CW CW

Multiple Alleles Most genes exist in populations in more than two allelic forms

The ABO blood group in humans Is determined by multiple alleles

Pleiotropy In pleiotropy A gene has multiple phenotypic effects

Some traits may be determined by two or more genes

In epistasis A gene at one locus alters the phenotypic expression of a gene at a second locus

Polygenic Inheritance Many human characters Vary in the population along a continuum and are called quantitative characters

Nature and Nurture: The Environmental Impact on Phenotype Another departure from simple Mendelian genetics arises when the phenotype for a character depends on environment as well as on genotype

The norm of reaction is the phenotypic range of a particular genotype that is influenced by the environment

Multifactorial characters Are those that are influenced by both genetic and environmental factors The ph of the soil, in conjunction with the genetics of the organism, determine the final phenotype.

An organism’s phenotype Includes its physical appearance, internal anatomy, physiology, and behavior Reflects its overall genotype and unique environmental history

Many human traits follow Mendelian patterns of inheritance Many human traits follow Mendelian patterns of inheritance. Humans are not convenient subjects for genetic research, however, the study of human genetics continues to advance

Pedigree Analysis A pedigree Is a family tree that describes the interrelationships of parents and children across generations

Inheritance patterns of particular traits Can be traced and described using pedigrees Can also be used to make predictions about future offspring Ww ww WW or First generation (grandparents) Second generation (parents plus aunts and uncles) Third generation (two sisters) Ff ff FF or Ff FF Widow’s peak No Widow’s peak Attached earlobe Free earlobe (a) Dominant trait (widow’s peak) (b) Recessive trait (attached earlobe) Figure 14.14 A, B

Many genetic disorders are inherited in a recessive manner Recessively inherited disorders Show up only in individuals homozygous for the allele Carriers Are heterozygous individuals who carry the recessive allele but are phenotypically normal

Cystic Fibrosis Symptoms of cystic fibrosis include Mucus buildup in the some internal organs Abnormal absorption of nutrients in the small intestine

Sickle-Cell Disease Sickle-cell disease Symptoms include Affects one out of 400 African-Americans Is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells Symptoms include Physical weakness, pain, organ damage, and even paralysis

Mating of Close Relatives Matings between relatives Can increase the probability of the appearance of a genetic disease Are called consanguineous matings

Some human disorders are due to dominant alleles One example is achondroplasia A form of dwarfism that is lethal when homozygous for the dominant allele

Huntington’s disease Is a degenerative disease of the nervous system Has no obvious phenotypic effects until about 35 to 40 years of age

Multifactorial Disorders Many human diseases Have both genetic and environment components Examples include Heart disease and cancer

Genetic Testing and Counseling Genetic counselors Can provide information to prospective parents concerned about a family history for a specific disease

Counseling Based on Mendelian Genetics and Probability Rules Using family histories Genetic counselors help couples determine the odds that their children will have genetic disorders

Tests for Identifying Carriers For a growing number of diseases Tests are available that identify carriers and help define the odds more accurately

Fetal Testing In amniocentesis In chorionic villus sampling (CVS) The liquid that bathes the fetus is removed and tested In chorionic villus sampling (CVS) A sample of the placenta is removed and tested

Fetal testing Figure 14.17 A, B

Newborn Screening Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the United States