Mendelian Mendelian GeneticsGeneticsGenetics Lecture Genetics Lecture

IIIIII

Biology Standards Biology Standards CoveredCovered

2c ~ students know how random 2c ~ students know how random chromosome segregation explains the chromosome segregation explains the probability that a particular allele will be probability that a particular allele will be in a gametein a gamete

2e ~ students know why approximately 2e ~ students know why approximately half of an individual’s DNA sequence half of an individual’s DNA sequence comes from each parentcomes from each parent

2g ~ students know how to predict 2g ~ students know how to predict possible combinations of alleles in a zygote possible combinations of alleles in a zygote from the genetic makeup of the parentsfrom the genetic makeup of the parents

Biology Standards Biology Standards CoveredCovered

3a ~ Students know how to predict the 3a ~ Students know how to predict the probable outcome of probable outcome of phenotypesphenotypes in a in a genetic cross from the genetic cross from the genotypesgenotypes of the of the parents and mode of inheritance parents and mode of inheritance ((autosomalautosomal or or x-linkedx-linked, , dominantdominant or or recessiverecessive))

3b ~ Students know the genetic basis for 3b ~ Students know the genetic basis for Mendel’s laws of segregation and Mendel’s laws of segregation and independent assortmentindependent assortment

Gregor MendelGregor Mendel

Thought of as the “founder” of Thought of as the “founder” of modern genetic researchmodern genetic research

He worked on an Austrian He worked on an Austrian monastery in the mid 1800’s monastery in the mid 1800’s

His work with the “garden His work with the “garden peas” explained many peas” explained many unanswered questions about unanswered questions about geneticsgenetics

Important Important VocabularyVocabulary

HeredityHeredity – the passing on of – the passing on of characteristics from parents to characteristics from parents to offspringoffspring

TraitsTraits – – characteristics that are characteristics that are inherited inherited (for example your eye (for example your eye color)color)

GenesGenes – – chemical factors that chemical factors that determine traitsdetermine traits

Important Important VocabularyVocabulary

GametesGametes – – individual male individual male and female sex cellsand female sex cells

PollinationPollination – – the transfer of the transfer of pollenpollen (male gametes) to the (male gametes) to the pistilpistil (female part of a flower) (female part of a flower)

FertilizationFertilization – – the uniting of the uniting of male and female gametesmale and female gametes

Chromosome NumberChromosome Number Each organism on Each organism on

this planet has a this planet has a specific number of specific number of chromosomeschromosomes Humans (for Humans (for

example) have example) have 23 23 pairspairs of of chromosomes with chromosomes with a a total of 46total of 46

Apes have 24 pairs Apes have 24 pairs for a for a total of 48total of 48

We share 99% of We share 99% of our genome with our genome with chimps and bonoboschimps and bonobos

Garden Pea plantsGarden Pea plants

The garden pea plants were The garden pea plants were “self pollinating” when “self pollinating” when Mendel first observed themMendel first observed them

This means that the offspring This means that the offspring of each pea plant was an of each pea plant was an exact copy of the parent plantexact copy of the parent plant

Garden Pea plantsGarden Pea plants

Mendel wanted to change the Mendel wanted to change the “exact copy” offspring “exact copy” offspring

He did so by “He did so by “cross – cross – pollinatingpollinating””

He basically pollinated one pea He basically pollinated one pea plant with a completely plant with a completely separate oneseparate one

Genes and DominanceGenes and Dominance

Mendel found that when you Mendel found that when you make certain “crosses” some make certain “crosses” some of the traits were hidden or of the traits were hidden or maskedmasked

When he made further crosses When he made further crosses some of these traits came some of these traits came backback

Genes and DominanceGenes and Dominance

This led Mendel to believe that This led Mendel to believe that there were there were Dominant traitsDominant traits and and less dominant or less dominant or recessive traitsrecessive traits

The The dominant traitdominant trait is expressed is expressed by a capital letter by a capital letter (for example (for example D)D)

The The recessive traitrecessive trait is expressed by is expressed by a lower case letter a lower case letter (for example (for example d)d)

Mendel’s ExperimentMendel’s Experiment The original The original parentparent pea plants pea plants

were called the were called the P generationP generation

Tall Short

P Generation

Mendel’s ExperimentMendel’s Experiment The first set of offspring in the The first set of offspring in the

experiment is called the experiment is called the FF1 1 – Generation– Generation

P Generation

Tall Short Tall Tall

F1 Generation

Mendel’s ExperimentMendel’s Experiment The second set of offspring from the FThe second set of offspring from the F11 – –

GenerationGeneration is called the is called the FF22 - Generation - Generation

P Generation F1 Generation

Tall Short Tall Tall Tall Tall Tall Short

F2 Generation

AllelesAlleles Alleles are different forms of a geneAlleles are different forms of a gene

By “form” we mean the amount or By “form” we mean the amount or variety of traitsvariety of traits If there are three different possible colors If there are three different possible colors for flowers on a pea plant, then there are for flowers on a pea plant, then there are three possible three possible AllelesAlleles

For example:For example: a a TALL alleleTALL allele may be written as may be written as TT a a short alleleshort allele may be written as may be written as

tt

Genotype vs. PhenotypeGenotype vs. Phenotype

The Genotype is the “genetic makeup”The Genotype is the “genetic makeup” HomozygousHomozygous TALL TALL plants would be:plants would be:

T TT T Heterozygous Heterozygous TALLTALL plants:plants: T tT t HomozygousHomozygous smallsmall:: t tt t

** The recessive trait is “short” and in The recessive trait is “short” and in order for the plant to be short order for the plant to be short bothboth alleles must be lowercase (recessive)alleles must be lowercase (recessive)

Genotype vs. PhenotypeGenotype vs. Phenotype The The PhenotypePhenotype is the “physical is the “physical

expression” of the gene expression” of the gene Dominance RuleDominance Rule::

If a dominant allele is present in the If a dominant allele is present in the Genotype, the Phenotype will show the Genotype, the Phenotype will show the dominant traitdominant trait TT is a dominant allele for TALLis a dominant allele for TALLT TT T will yield a TALL plant will yield a TALL plant T tT t will will alsoalso yield a TALL plant yield a TALL plant

* The * The ONLYONLY way a short plant will way a short plant will occur is if occur is if BOTHBOTH alleles are alleles are recessive!!recessive!! (tt) (tt)

F1 Generation Cross

Tt x Tt Cross This diagram This diagram

shows a cross shows a cross between the between the FF11 – – generationgeneration

Both plants being Both plants being crossed are crossed are HeterozygousHeterozygous

That is; they have That is; they have both both allelesalleles in in their their genotypegenotype

What would be the What would be the outcome?outcome?

Probability in GenotypesProbability in Genotypes

In a Heterozygous In a Heterozygous cross cross 1 out of 4 will be 1 out of 4 will be

homozygous homozygous dominantdominant (25%) (25%)

2 out of 4 (1/2) will 2 out of 4 (1/2) will be be heterozygous heterozygous dominantdominant (50%) (50%)

1 out of 4 will be 1 out of 4 will be homozygous homozygous recessiverecessive (25%) (25%)

Mendel’s PrinciplesMendel’s Principles Biological Biological characteristicscharacteristics are are inherited inherited

asas genes genes from parents to offspringfrom parents to offspring

Some forms Some forms (alleles)(alleles) of genes are of genes are dominantdominant and others are and others are recessiverecessive

In most sexually reproducing organisms, In most sexually reproducing organisms, each adult has two copies of a geneeach adult has two copies of a gene – – one from each parentone from each parent

The alleles for different genes usually The alleles for different genes usually segregate independently from each segregate independently from each otherother

Exceptions to Mendel’s Exceptions to Mendel’s PrinciplesPrinciples

Some alleles are Some alleles are neither dominant neither dominant nor recessivenor recessive

Many traits are Many traits are controlled by multiple controlled by multiple alleles or genes alleles or genes

Incomplete DominanceIncomplete Dominance – – when there isn’t a when there isn’t a dominant allele at alldominant allele at all This appears as a This appears as a thirdthird

allele (red, white, & allele (red, white, & pink)pink)

CodominanceCodominance – – a case in which a case in which bothboth alleles contribute to the phenotype of the alleles contribute to the phenotype of the organismorganism If a red and white flower were crossed, If a red and white flower were crossed,

codominancecodominance would result in a red flower with would result in a red flower with white stripes or white with red stripes white stripes or white with red stripes

Both alleles are expressed at the same time in the Both alleles are expressed at the same time in the offspringoffspring

Multiple AllelesMultiple Alleles – if a gene has more – if a gene has more than two allelesthan two alleles This means that more than two possible alleles This means that more than two possible alleles

exist in a populationexist in a population

Exceptions to Mendel’s Exceptions to Mendel’s PrinciplesPrinciples

Multiple AllelesMultiple Alleles

Polygenic traitsPolygenic traits – – when a trait is when a trait is controlled by two or more genes that controlled by two or more genes that can be found on similar or completely can be found on similar or completely different chromosomesdifferent chromosomes Human skin color is controlled by more Human skin color is controlled by more

than four individual genesthan four individual genes Human eye color is also considered Human eye color is also considered

polygenicpolygenic

Exceptions to Mendel’s Exceptions to Mendel’s PrinciplesPrinciples