Patterns of Patterns of InheritanceInheritance

Chapter 9Chapter 9BIOL 1010BIOL 1010

GENETICS: the scientific study of heredity

Genome: complete set of an organism’s gene

Gene: unit of heredity which codes for a protein

Gregor Mendel (1822-1884):

•Gained posthumous fame as the founder of genetics

•Was the first person to analyze patterns of inheritance: “heritable factors”

•Deduced the fundamental principles of genetics

Gregor Mendel: “Father of Modern Genetics”

• Rule of Multiplication: probability of a compound event is the product of the separate probabilities of the independent events

P(A and B) = P(A)*P(B)

• Rule of Addition: if events are mutually exclusive, then the probability of either/or is the sum of the separate probabilities of the independent events

P(A or B) = P(A) + P(B)

Rules of Probability

The probability of event A OR event B occurring is the chance of event A occurring

added to the chance of event B occuring

The probability of event A AND event B occurring is the chance of event A occurring multiplied by the chance of event B occuring

• Heredity: transmission of traits from one generation to the next

• Phenotype (Character): heritable feature (i.e. flower color) based on genotype (genetic makeup)

• Trait: variant of a character

• Wild-type: variant found most often in nature

• True-breeding: purebred, offspring are identical to the parent

• Hybrids: offspring of two different true-breeding parents

Genetic Nomenclature

Inherited Traits in Humans: Controlled by a Single Gene

• P generation: two different pure-breeding parental plants

• F1 hybrids: the first generation plants obtained from crossing two selected pure breeding plants.

• F1 hybrids do not produce seed that is the same as the parent plants

• F2 hybrids: second generation plants (result of self or cross fertilization of F1 hybrids

Genetic CrossesMonohybrid

Cross

• A monohybrid cross is a cross between purebred parents that differ in only one characteristic

• F1 generation: all show the trait of one parent (i.e. purple flowers)

• F2 generation: show the two traits of the parents in a 3:1 ratio (i.e. purple to white flowers)

Monohybrid CrossMonohybrid Cross

1. There are alternate versions of genes

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

• Homozygous: two identical alleles

• Heterozygous: two different alleles

3. If the two alleles differ (the individual is heterozygous)

• Dominant allele: determines the phenotype (character)

• Recessive allele: no noticeable effect on the phenotype (character)

4. Law of segregation: A sperm or egg carries only one allele for each character because the allele pair segregates during meiosis

Mendel’s Law of Segregation

• Show the results of random fertilization

• Each axis shows possible alleles from each parent

Punnett Squares

• Mating of an individual of dominant phenotype but unknown genotype to a homozygous recessive individual

Test Crosses: Determining Unknown Genotypes

Modern genetics

• Homologous chromosomes contain the same genes at the same loci but may contain different alleles (alternative forms of a gene)

• Gene locus (plural-loci): specific location of a gene on the chromosome

Modern Genetics

• A dihybrid cross is a cross between purebred parents that differ in two characteristics

• F1 generation: all show the dominant trait of the two characters

• F2 generation: show four combinations of traits in a 9:3:3:1 ratio

Dihybrid Cross

Each pair of alleles assorts independently of the other pairs of alleles during gamete formation (the inheritance of one character has no effect on the inheritance of another)

Mendel’s Law of Independent Assortment

Mendel’s Laws and MeiosisChromosome Theory of Inheritance: genes are located at specific loci on chromosomes and that the behavior of chromosomes during meiosis and fertilization accounts for inheritance patterns

• Male

• Female

• Affected Male

• Affected Female

• Mating

Mating• between

related

individuals

Family Pedigrees

A family pedigree showing inheritance of free versus attached earlobes

Some Autosomal Disorders in HumansRecessive Disorders

•Albinism (1/22,000)

•Cystic fibrosis (1/1,800)

•Phenylketonuria (1/10,000)

•Sickle cell disease (1/500)

•Tay Sachs disease (1/3,500)

Dominant Disorders

•Achondroplasia (1/25,000)

•Alzheimer’s disease (early onset)

•Huntington’s disease (1/25,000)

•Hypercholesterolemia (1/500)

• Inbreeding: mating between close blood relatives

• Increases the likelihood of homozygosity of a recessive allele in children of inbred parents

• Most genetic disorders are not evenly distributed across all ethnic groups

• Result of prolonged isolation of certain populations

Inbreeding Increases the Likelihood that a Recessive Trait will be Inherited

Dd DD

DdDd DDDD

Dd Dd

dd=deaf

Dominant Disorders• One allele is all that is required to cause the

phenotype

– i.e. Achondroplasia: dwarfism (homozygosity=lethal)

• Dominant lethal alleles are rare in populations

– i.e. Huntington’s disease: causes degeneration of the nervous system in middle age

About 200,000 new breast cancer cases are diagnosed each

year

Approximately 10% of these are heritable; they run in families

There are options if the results of genetic testing show a

positive result for a BRCA mutation

Genetic Testing

• Carrier screening

• Prenatal diagnostic testing:

– i.e. amniocentesis

• Newborn screening

• Genealogical DNA testing

• Predicting adult-onset disorders

• Estimating risks of disease development

• Confirmational diagnosis

• Forensic/identity testing

Variations on Mendel’s Laws• Incomplete dominance: F1 has an

appearance in between the two parental phenotypes

• Codominance: both alleles are fully expressed in heterozygous individuals

• Pleiotropy: single gene influences more than one character

• Polygenic Inhertitance: additive effects of two or more genes on a single phenotype

• Environmental Factors: non-genetic, non-hereditary factors that contribute to phenotype

An Example of Incomplete Dominance

Hypercholesterolemia

An Example of Codominance

ABO Blood Groups

An Example of PleiotropySickle-Cell Disease

An Example of Polygenic InheritanceSkin Color

Linked Genes• Thomas Hunt Morgan: In 1916, published a paper on

genes in Drosophila melanogaster (fruit fly)

– Found the recombinant frequency to be 17%, not 50%

Linked Genes• Genetic recombination (crossing

over): usually ensures that genes on the same chromosome still assort independently

• Linked genes: genes so close together on a chromosome that they do not assort independently but tend to travel together

• Linkage map: diagrams describing relative gene locations using recombination frequencies

– The shorter the distance between two genes, the less likely a crossover event will happen between them

Sex Chromosomes and Sex-Linked Genes• Sex chromosomes: X and Y

– XX Female

– XY Male

• Sex-linked gene: any gene located on a sex chromosome

• X-linked genes: the X chromosome contains many more genes (~2000 genes) than the Y chromosome (~78 genes)

• Sex-linked disorders: disorders associated with a defective gene found on a sex (usually X) chromosome

– Is characterized by a malfunction of light-sensitive cells in the eyes.

– Males are more likely to be color blind than females

Figure 9.30

Copyright © 2004 Pearson Education, Inc. publishing as Benjamin Cummings

Sex-Linked Disorders in Humans

Red-green color blindness: recessive X-linked trait

Figure 9.32

Sex-Linked Disorders in HumansHemophilia: “The Royal Disease” • Sex-linked recessive blood-clotting trait that may result in

excessive bleeding and death after relatively minor cuts and bruises