CHAPTER 9 Patterns 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

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 genetics has ancient roots 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

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

One characteristic comes from each parent 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

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

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

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

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

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

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

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

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 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 Phenotype 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

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

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