CHAPTER 11MENDEL & HEREDITYSC STANDARD B 4:

The student will understand the molecular basis of heredity.

ESSENTIAL QUESTIONHow does segregation of alleles contribute to genetic variation?

ORIGINS OF HEREDITARY SCIENCE Mendel” Breeding Experiments

Genetics: is the science of heredity &the mechanism by which traits are passed from parents to offspring

Mendelborn in Austrian Empire (today Czech Republic) in 1822

Studied physics & mathematics @ University of Vienna

Joined monastery in 1843 where he was put in charge of the gardens

MENDEL’S EXPERIMENTSMendel spent 2 years preparing his control plants to insure they were true breedersdescribes organisms that are homozygous for a specific trait so always produce offspring that have the same phenotype for that trait

MENDEL’S EXPERIMENTS

MENDEL’S EXPERIMENTS crossed true breeding, purple blossomed pea plants with true breeding, white blossomed pea plants and all the offspring had purple flowers

Then let the offspring self-pollinate and some of the plants in that generation had purple flowers & some had white

MENDEL’S EXPERIMENTSmale parts were removed from 1st flower

pollen taken from male parts of 2nd flower

pollen from 2nd brushed onto female parts of 1st flower

MENDEL’S EXPERIMENTSVocabulary:- character: a recognizable inherited feature or characteristic of an individual- trait: one of two or more possible forms of a character

~ phenotype: physical characteristics

~ genotype: genetic makeup , what alleles an organism has

MENDEL’S EXPERIMENTSVocabulary:- hybrid: the offspring of a cross between parents that have contrasting traits- generation: the entire group of offspring produced by a given group of parents

MENDEL’S EXPERIMENTS 3 reasons why the garden pea plant was good

choice:1. Several characters appear in

contrasting forms2. These flowers can self-pollinate

because each flower has both male & female parts

3. Plant is easy to grow1. Matures quickly2. Needs little care3. Produces many offspring

MENDEL’S EXPERIMENTSMonohybrid Cross

MENDEL’S EXPERIMENTS Monohybrid Cross: 3 Steps

1. Produced a true-breeding parent generation (P generation)

2. Produced 1st filial generation ( F 1 generation)

3. Produced 2nd filial generation ( F 2 generation)

MENDEL’S EXPERIMENTSTrue breeding purpleTrue breeding white

MENDEL’S EXPERIMENTSStep 2: cross pollinated parents

F 1 generation all purpleSelf-pollinated

F 2 generation 3 : 1 purple to white

MENDEL’S EXPERIMENTSMendel repeated these experiments with 7 different traits in pea plants:For each of the 7 characters he found a similar 3 : 1 ratio of contrasting traits in the F 2 generation

MENDEL’S EXPERIMENTS

MENDEL’S EXPERIMENTSRatios in Mendel’s Results

F 1 generation expressed the same trait for any of the 7 characteristics he studied

When F 1 plants allowed to self-pollinate he always saw a 3 : 1 ratio of contrasting traits

MENDEL’S THEORYExplains simple patterns of inheritance2 of several versions of a gene combine & result in 1 of several possible traits

Allele: one of two or more alternative forms of a gene each leading to a unique trait

MENDEL’S THEORY Dominant: describes an allele that

is fully expressed whenever the allele is present

Recessive: describes an allele that is expressed only when there is no dominant allele present

MENDEL’S THEORY Law of Segregation of Alleles:

When an organism produces gametes, each pair of alleles on homologous chromosomes separate in Meiosis I and each gamete has an equal chance of receiving either one of the alleles

MENDEL’S THEORY: LAW OF SEGREGATION OF ALLELES

MENDEL’S THEORY GENOTYPE: a specific combination

of alleles in an individual….. the “genes” an individual hasexample: AA, Aa, or aa

PHENOTYPE : the detectable trait or traits that result from the genotype of an individual….. the “physical appearance” an individual hasexample: normal, normal,

albino

MENDEL’S THEORYGENOTYPE DETERMINES PHENOTYPE !

MENDEL’S THEORYThe genotype of each of the peas is

____________.

MENDEL’S THEORYThe phenotype of each of the

following is _____.

MENDEL’S THEORY Homozygous: describes an individual that carries two identical alleles of a geneExample: PP or pp

Heterozygous: describes an individual that carries two different alleles of a geneExample: Pp

MENDEL’S THEORYMendel’s 2nd Experiments

Dihybrid crosses: involves test crossing two characters

Law of Independent Assortment: during gamete formation, the alleles on non-homologous chromosomes segregate independently

MENDEL’S THEORY

PROBLEM SOLVING: PRODUCING TRUE-BREEDING SEEDSTextbook page 271

Work in table groupsDefine the problemOrganize informationCreate solutionPresent to class

MENDEL’S THEORY

When genes are close together on same chromosome they will rarely separate independently so are said to be “linked”.

MODELING MENDEL’S LAWSPunnett Square: a graphic used to predict the results of a genetic cross

MODELING MENDEL’S LAWSA Punnett Square shows all the

genotypes that could possibly result from any given cross match.

MODELING MENDEL’S LAWSMonohybrid Homozygous CrossDraw a Punnett Square crossing homozygous Y (for yellow seed color) with homozygous y (for green seed color)

What is the ratio of yellow to green seeds ?

Monohybrid Heterozygous CrossDraw a Punnett Square crossing 2 plants that are heteroygous for Y

What is the ratio of yellow to green seeds?

MODELING MENDEL’S LAWSTest Cross: used to test an

individual whose phenotype for a given characteristic is dominant but its genotype is unknownIndividual is crossed with a

known homozygous recessive If unknown is homozygous dominant all offspring will show dominant phenotype

If unknown heterozygous for the trait then ½ the offspring will show dominant phenotype & ½ will show recessive trait

MODELING MENDEL’S LAWSUsing Probability

Probability: the likelihood that a specific event will occur; expressed in mathematicsProbabilities are used to predict the likelihood that specific alleles will be passed down to offspring

QUICK LAB: PROBABILITIES: PAGE 268 Notebook: page 15 Everyone completes this:

Follow procedureAnswer analysis questions 1 - 2

MODELING MENDEL’S LAWSPedigree: a diagram that shows the occurrence of a genetic trait in several generations of a family

Genetic Disorder: an inherited disease that is caused by a mutation in a gene or by a chromosome defect

PEDIGREES

PEDIGREES

PEDIGREES

MODELING MENDEL’S LAWS Pedigrees can help answer 3

aspects of inheritance:1. Sex linkage2. Dominance3. Heterozygocity

MODELING MENDEL’S LAWS1. Sex-Linked Gene

1. Gene located on either the X or Y chromosomes

2. Females have 2 X chromosomes so rarely show the recessive phenotype; males have just 1 X chromosome so will show the trait for a single recessive allele for genes on the X chromosome

3. If find a trait that is more common in males than females it is likely sex-linked

GENES ON SEX CHROMOSOMES

Sex-Linked

MODELING MENDEL’S LAWS2. Dominant or Recessive? If a child shows a trait and neither parent shows the trait it is likely a recessive trait

MODELING MENDEL’S LAWS3. Heterozygous or

Homozygous? Recessive trait in a

child shows parents had to be heterozygous for the trait

BEYOND MENDELIAN HEREDITYPolygenic Character: a character influenced by more than 1 geneincludes many characters in humansEye colorSkin colorHeight

BEYOND MENDELIAN HEREDITYIncomplete Dominance: the phenotype for a heterozygous individual is intermediate between the homozygous dominant phenotype and the homozygous recessive phenotype

BEYOND MENDELIAN HEREDITYGenes that are said to have 3 or

more possible alleles are said to have multiple allelesExample: human’s ABO blood

types

BEYOND MENDELIAN HEREDITYCodominance: a condition in which both alleles for a gene are fully expressed in the phenotype

BEYOND MENDELIAN HEREDITYGenes Affected by the

EnvironmentNutrients available or

temperature can affect the expression of the genotype

ExamplesSome animals have fur that changes color with the seasons

BEYOND MENDELIAN HEREDITYDuring meiosis, genes that are close together on the same chromosome are less likely to be separated than genes that are far apart

Genes that are close together and the traits they determine are said to be linked (not just sex-linked)

BEYOND MENDELIAN HEREDITYLinked Genes