CHAPTER 9Patterns of Inheritance

Overview:Mendel’s LawsVariations of Mendel’s LawsChromosomesSex linked genes

• 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

Purebreds and Mutts — A Difference of Heredity

• The parents of these puppies were a mixture of different breeds – Their behavior

and appearance is more varied as a result of their diverse genetic inheritance

• 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

MENDEL’S LAWSThe science of genetics has ancient roots

• Modern genetics began with Gregor Mendel’s quantitative experiments with pea plants

Experimental genetics began in an abbey garden

– Was the first person to analyze patterns of inheritance

– Deduced the fundamental principles of genetics

• Mendel studied garden peas

– These plant are easily manipulated

– These plants can self-fertilize

• 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

• He also created true-breeding varieties of plants

• Mendel then crossed two different true-breeding varieties, creating hybrids

• 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

• From his experimental data, Mendel deduced that an organism has two genes (alleles) for each inherited characteristic– One characteristic

comes from each parent

Mendel’s principle of segregation describes the inheritance of a single characteristic

• A monohybrid cross is a cross between parent plants that differ in only one characteristic

• Mendel’s principle of segregation

– Pairs of alleles segregate (separate) during gamete formation; the fusion of gametes at fertilization creates allele pairs again

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).

• A sperm or egg carries only one allele of each pair– The pairs of alleles

separate when gametes form

– This process describes Mendel’s law of segregation

– Alleles can be dominant or recessive

• An explanation of Mendel’s results, including a Punnett square

• Alternative forms of a gene (alleles) reside at the same locus on homologous chromosomes

Homologous chromosomes bear the two alleles for each characteristic

• Homologous chromosomes

Genetic Alleles and Homologous Chromosomes

– Have genes at specific loci– Have alleles of a gene at the same locus

• Homozygous

– When an organism has identical alleles for a gene

• Heterozygous– When an organism has different alleles for a gene

• 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

The principle of independent assortment is revealed by tracking two characteristics at

once

• Two hypotheses for gene assortment in a dihybrid cross

Mendel’s Principle of Independent Assortment

– Dependent assortment– Independent assortment

• Mendel’s principle of independent assortment

– Each pair of alleles segregates independently of the other pairs during gamete formation

• A testcross is a mating between

Using a Testcross to Determine an Unknown Genotype

– An individual of unknown genotype and

– A homozygous recessive individual

• 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

Mendel’s principles reflect the rules of probability

• The inheritance of many human traits follows Mendel’s principles and the rules of probability

Connection: Genetic traits in humans can be tracked through family pedigrees

• Most such disorders are caused by autosomal recessive alleles– Examples:

cystic fibrosis, sickle-cell disease

Connection: Many inherited disorders in humans are controlled by a single gene

• A few are caused by dominant alleles

– Examples: achondroplasia, Huntington’s disease

• Karyotyping and biochemical tests of fetal cells and molecules can help people make reproductive decisions– Fetal cells can be obtained through amniocentesis

Connection: Fetal testing can spot many inherited disorders early in pregnancy

• Phenotype

– An organism’s physical traits

• Genotype– An organism’s genetic makeup

• 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

VARIATIONS ON MENDEL’S PRINCIPLES

The relationship of genotype to phenotype is rarely simple

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

BEYOND MENDEL

• In incomplete dominance F1 hybrids have an appearance in between the phenotypes of the two parents

Incomplete Dominance in Plants and People

• In a population, multiple alleles often exist for a characteristic– The three alleles for ABO blood type in humans is an

example

Many genes have more than two alleles in the population

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

• Genetic testing can be of value to those at risk of developing a genetic disorder or of passing it on to offspring

Connection: Genetic testing can detect disease-causing alleles

• This situation creates a continuum of phenotypes– Example: skin color

A single characteristic may be influenced by many genes

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

Polygenic Inheritance

• Genes are located on chromosomes– Their behavior during meiosis accounts for

inheritance patterns

THE CHROMOSOMAL BASIS OF INHERITANCE

Chromosome behavior accounts for Mendel’s principles

• Certain genes are linked– They tend to be inherited together because they reside

close together on the same chromosome

Genes on the same chromosome tend to be inherited together

• This inheritance pattern was later explained by linked genes, which are

– Genes located on the same chromosome

– Genes that are typically inherited together

• This produces gametes with recombinant chromosomes

• The fruit fly Drosophila melanogaster was used in the first experiments to demonstrate the effects of crossing over

Crossing over produces new combinations of alleles

• 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

Geneticists use crossover data to map genes

• 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 CHROMOSOMES AND SEX-LINKED GENES

Chromosomes determine sex in many species

• 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

Sex-linked genes exhibit a unique pattern of inheritance

– Their inheritance pattern reflects the fact that males have one X chromosome and females have two

– These figures illustrate inheritance patterns for white eye color (r) in the fruit fly, an X-linked recessive trait

• 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

Connection: Sex-linked disorders affect mostly males