HERITABLE VARIATION AND PATTERNS OF INHERITANCE

            Heredity is the transmission of traits from one generation to the next.

            Genetics is the scientific study of heredity.

            Gregor Mendel

           worked in the 1860s,

           was the first person to analyze patterns of inheritance, and

           deduced the fundamental principles of genetics.

In an Abbey Garden

           Mendel studied garden peas because they

           were easy to grow,

           came in many readily distinguishable varieties,

           are easily manipulated, and

           can self-fertilize.

Monohybrid Crosses

           A monohybrid cross is a cross between purebred parent plants that differ in only one character.

 

           Mendel developed four hypotheses from the monohybrid cross, listed here using modern terminology (including gene instead of heritable factor).

1. The alternative versions of genes are called alleles.

2. For each inherited character, an organism inherits two alleles, one from each parent.

         An organism is homozygous for that gene if both alleles are identical.
         An organism is heterozygous for that gene if the alleles are different.

3. If two alleles of an inherited pair differ,

         then one determines the organisms appearance and is called the dominant allele and
         the other has no noticeable effect on the organisms appearance and is called the recessive allele.

4. Gametes carry only one allele for each inherited character.

         The two alleles for a character segregate (separate) from each other during the production of gametes.
         This statement is called the law of segregation.

           Do Mendels hypotheses account for the 3:1 ratio he observed in the F2 generation?

           A Punnett square highlights

           the four possible combinations of gametes and

           the four possible offspring in the F2 generation.

           Geneticists distinguish between an organisms physical appearance and its genetic makeup.

           An organisms physical appearance is its phenotype.

           An organisms genetic makeup is its genotype.

Genetic Alleles and Homologous Chromosomes

           A gene locus is a specific location of a gene along a chromosome.

           Homologous chromosomes have alleles (alternate versions) of a gene at the same locus.

Mendels Law of Independent Assortment

           A dihybrid cross is the mating of parental varieties differing in two characters.

           What would result from a dihybrid cross? Two hypotheses are possible:

1. dependent assortment or

2. independent assortment.

           Mendels dihybrid cross supported the hypothesis that each pair of alleles segregates independently of the other pairs during gamete formation.

           Thus, the inheritance of one character has no effect on the inheritance of another.

           This is called Mendels law of independent assortment.

           Independent assortment is also seen in two hereditary characters in Labrador retrievers.

Using a Testcross to Determine an Unknown Genotype

           A testcross is a mating between

           an individual of dominant phenotype (but unknown genotype) and

           a homozygous recessive individual.

The Rules of Probability

           Mendels strong background in mathematics helped him understand patterns of inheritance.

           The rule of multiplication states that the probability of a compound event is the product of the separate probabilities of the independent events.

Family Pedigrees

           Mendels principles apply to the inheritance of many human traits.

           Dominant traits are not necessarily

           normal or

           more common.

           Wild-type traits are

           those seen most often in nature and

           not necessarily specified by dominant alleles.

           A family pedigree

           shows the history of a trait in a family and

           allows geneticists to analyze human traits.

Human Disorders Controlled by a Single Gene

           Many human traits

           show simple inheritance patterns and

           are controlled by single genes on autosomes.

Recessive Disorders

           Most human genetic disorders are recessive.

           Individuals who have the recessive allele but appear normal are carriers of the disorder.

           Cystic fibrosis is

           the most common lethal genetic disease in the United States and

           caused by a recessive allele carried by about one in 31 Americans.

 

           Prolonged geographic isolation of certain populations can lead to inbreeding, the mating of close relatives.

           Inbreeding increases the chance of offspring that are homozygous for a harmful recessive trait.

Dominant Disorders

           Some human genetic disorders are dominant.

           Achondroplasia is a form of dwarfism.

             The homozygous dominant genotype causes death of the embryo.
             Thus, only heterozygotes have this disorder.

           Huntingtons disease, which leads to degeneration of the nervous system, does not usually begin until middle age.

Genetic Testing

           Today many tests can detect the presence of disease-causing alleles.

           Most genetic tests are performed during pregnancy.

           Amniocentesis collects cells from amniotic fluid.

           Chorionic villus sampling removes cells from placental tissue.

           Genetic counseling helps patients understand the results and implications of genetic testing.

 

VARIATIONS ON MENDELS LAWS

           Some patterns of genetic inheritance are not explained by Mendels laws.

Incomplete Dominance in Plants and People

           In incomplete dominance, F1 hybrids have an appearance between the phenotypes of the two parents.

           Hypercholesterolemia

           is a human trait that is an example of incomplete dominance and

           is characterized by dangerously high levels of cholesterol in the blood.

           heterozygotes have blood cholesterol levels about twice normal, and

           homozygotes have about five times the normal amount of blood cholesterol and may have heart attacks as early as age 2.

ABO Blood Groups: An Example of Multiple Alleles and Codominance

           The ABO blood groups in humans are an example of multiple alleles.

           The immune system produces blood proteins called antibodies that bind specifically to foreign carbohydrates.

           If a donors blood cells have a carbohydrate (A or B) that is foreign to the recipient, the blood cells may clump together, potentially killing the recipient.

           The clumping reaction is the basis of a blood-typing lab test.

           The human blood type alleles IA and IB are codominant, meaning that both alleles are expressed in heterozygous individuals who have type AB blood.

Pleiotropy and Sickle-Cell Disease

           Pleiotropy is when one gene influences several characters.

           Sickle-cell disease

           exhibits pleiotropy,

           results in abnormal hemoglobin proteins, and

           causes disk-shaped red blood cells to deform into a sickle shape with jagged edges.

Polygenic Inheritance

           Polygenic inheritance is the additive effects of two or more genes on a single phenotype.

The Role of Environment

           Many human characters result from a combination of

           heredity and

           environment.

           Only genetic influences are inherited.

 

THE CHROMOSOMAL BASIS OF INHERITANCE

           The chromosome theory of inheritance states that

           genes are located at specific positions (loci) on chromosomes and

           the behavior of chromosomes during meiosis and fertilization accounts for inheritance patterns.

           It is chromosomes that

           undergo segregation and independent assortment during meiosis and

           account for Mendels laws.

Linked Genes

           Linked genes

           are located close together on a chromosome and

           tend to be inherited together.

           Thomas Hunt Morgan

           used the fruit fly Drosophila melanogaster and

           determined that some genes were linked based on the inheritance patterns of their traits.

Linkage Maps

           Early studies of crossing over were performed using the fruit fly Drosophila melanogaster.

           Alfred H. Sturtevant, a student of Morgan,

           developed a method for mapping the relative gene locations,

           which resulted in the creation of linkage maps.

 

SEX CHROMOSOMES AND SEX-LINKED GENES

           Sex chromosomes influence the inheritance of certain traits. For example, humans that have a pair of sex chromosomes designated

           X and Y are male or

           X and X are female.

Sex Determination in Humans

           Nearly all mammals have a pair of sex chromosomes designated X and Y.

           Males have an X and Y.

           Females have XX.

Sex-Linked Genes

           Any gene located on a sex chromosome is called a sex-linked gene.

           Most sex-linked genes are found on the X chromosome.

           Red-green colorblindness is

             a common human sex-linked disorder and
             caused by a malfunction of light-sensitive cells in the eyes.

           Hemophilia

           is a sex-linked recessive blood-clotting trait that may result in excessive bleeding and death after relatively minor cuts and bruises and

           has plagued the royal families of Europe.