CHAPTER 9 Patterns of Inheritance

CHAPTER 9Patterns of Inheritance Overview: Mendel’s Laws Variations of Mendel’s Laws Chromosomes Sex linked genes

Purebreds and Mutts — A Difference of Heredity • Genetics is the science of heredity • These black Labrador puppies are purebred—their parents and grandparents were black Labs with very similar genetic makeups • Purebreds often suffer from serious genetic defects

Their behavior and appearance is more varied as a result of their diverse genetic inheritance • The parents of these puppies were a mixture of different breeds

MENDEL’S LAWS The science of genetics has ancient roots • The science of heredity dates back to ancient attempts at selective breeding • Until the 20th century, however, many biologists erroneously believed that • characteristics acquired during lifetime could be passed on • characteristics of both parents blended irreversibly in their offspring

Experimental genetics began in an abbey garden • Modern genetics began with Gregor Mendel’s quantitative experiments with pea plants • Was the first person to analyze patterns of inheritance • Deduced the fundamental principles of genetics

These plant are easily manipulated • These plants can self-fertilize • Mendel studied garden peas

Mendel crossed pea plants that differed in certain characteristics and traced the traits from generation to generation • This illustration shows his technique for cross-fertilization

Mendel then crossed two different true-breeding varieties, creating hybrids • He also created true-breeding varieties of plants

Mendel studied seven pea characteristics • He hypothesized that there are alternative forms of genes (although he did not use that term), the units that determine heredity

Mendel’s principle of segregation describes the inheritance of a single characteristic • From his experimental data, Mendel deduced that an organism has two genes (alleles) for each inherited characteristic • One characteristic comes from each parent • A monohybrid cross is a cross between parent plants that differ in only one characteristic

Pairs of alleles segregate (separate) during gamete formation; the fusion of gametes at fertilization creates allele pairs again • Mendel’s principle of segregation Allele: Any one of the alternative forms of a given gene (e.g. the ABO gene has three major alleles: A, B and O alleles). Alternative forms of a gene (alleles).

The pairs of alleles separate when gametes form • This process describes Mendel’s law of segregation • Alleles can be dominant or recessive • A sperm or egg carries only one allele of each pair • An explanation of Mendel’s results, including a Punnett square

Homologous chromosomes bear the two alleles for each characteristic • Alternative forms of a gene (alleles) reside at the same locus on homologous chromosomes

Genetic Alleles and Homologous Chromosomes • Homologous chromosomes • Have genes at specific loci • Have alleles of a gene at the same locus

When an organism has identical alleles for a gene • Heterozygous • When an organism has different alleles for a gene • Homozygous

The principle of independent assortment is revealed by tracking two characteristics at once • By looking at two characteristics at once, Mendel found that the alleles of a pair segregate independently of other allele pairs during gamete formation • This is known as the principle of independent assortment

Mendel’s Principle of Independent Assortment • Two hypotheses for gene assortment in a dihybrid cross • Dependent assortment • Independent assortment

Each pair of alleles segregates independently of the other pairs during gamete formation • Mendel’s principle of independent assortment

Using a Testcross to Determine an Unknown Genotype • A testcross is a mating between • An individual of unknown genotype and • A homozygous recessive individual

Mendel’s principles reflect the rules of probability • Inheritance follows the rules of probability • The rule of multiplication and the rule of addition can be used to determine the probability of certain events occurring

Connection: Genetic traits in humans can be tracked through family pedigrees • The inheritance of many human traits follows Mendel’s principles and the rules of probability

Connection: Many inherited disorders in humans are controlled by a single gene • Most such disorders are caused by autosomal recessive alleles • Examples: cystic fibrosis, sickle-cell disease

Examples: achondroplasia, Huntington’s disease • A few are caused by dominant alleles

Connection: Fetal testing can spot many inherited disorders early in pregnancy • Karyotyping and biochemical tests of fetal cells and molecules can help people make reproductive decisions • Fetal cells can be obtained through amniocentesis

VARIATIONS ON MENDEL’S PRINCIPLES The relationship of genotype to phenotype is rarely simple • Phenotype • Mendel’s principles are valid for all sexually reproducing species • However, often the genotype does not dictate the phenotype in the simple way his principles describe • An organism’s physical traits • Genotype • An organism’s genetic makeup

BEYOND MENDEL • Some patterns of genetic inheritance are not explained by Mendel’s principles

Incomplete Dominance in Plants and People • In incomplete dominance F1 hybrids have an appearance in between the phenotypes of the two parents

Many genes have more than two alleles in the population • In a population, multiple alleles often exist for a characteristic • The three alleles for ABO blood type in humans is an example

A single gene may affect many phenotypic characteristics • A single gene may affect phenotype in many ways • This is called pleiotropy • The allele for sickle-cell disease is an example

Connection: Genetic testing can detect disease-causing alleles • Genetic testing can be of value to those at risk of developing a genetic disorder or of passing it on to offspring

A single characteristic may be influenced by many genes • This situation creates a continuum of phenotypes • Example: skin color

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

THE CHROMOSOMAL BASIS OF INHERITANCE Chromosome behavior accounts for Mendel’s principles • Genes are located on chromosomes • Their behavior during meiosis accounts for inheritance patterns

Genes on the same chromosome tend to be inherited together • Certain genes are linked • They tend to be inherited together because they reside close together on the same chromosome

This inheritance pattern was later explained by linked genes, which are • Genes located on the same chromosome • Genes that are typically inherited together

Crossing over produces new combinations of alleles • This produces gametes with recombinant chromosomes • The fruit fly Drosophila melanogaster was used in the first experiments to demonstrate the effects of crossing over

Geneticists use crossover data to map genes • Crossing over is more likely to occur between genes that are farther apart • Recombination frequencies can be used to map the relative positions of genes on chromosomes

SEX CHROMOSOMES AND SEX-LINKED GENES Chromosomes determine sex in many species • A human male has one X chromosome and one Y chromosome • A human female has two X chromosomes • Whether a sperm cell has an X or Y chromosome determines the sex of the offspring

Sex-linked genes exhibit a unique pattern of inheritance • All genes on the sex chromosomes are said to be sex-linked • In many organisms, the X chromosome carries many genes unrelated to sex • Fruit fly eye color is a sex-linked characteristic

These figures illustrate inheritance patterns for white eye color (r) in the fruit fly, an X-linked recessive trait • Their inheritance pattern reflects the fact that males have one X chromosome and females have two

Connection: Sex-linked disorders affect mostly males • Most sex-linked human disorders are due to recessive alleles • Examples: hemophilia, red-green color blindness • These are mostly seen in males • A male receives a single X-linked allele from his mother, and will have the disorder, while a female has to receive the allele from both parents to be affected

CHAPTER 9 Patterns of Inheritance