FUNDAMENTALS OF GENETICS

Mendel’s Legacy

Genetics is everywhere these days – and it will continue as a dominant force in biology and society for decades to come. Wouldn’t it be nice if people understood it better?

GREGOR MENDEL

Biographymonk in Austria in mid-1800sstudied statisticsTeacherGardener

MENDEL’S GARDEN PEAS

heredity: transmission of traits from parents to offspringtraits: category in which one or more variations an be found (ex: flower color)Allele: value of a trait (purple flowers)Molecular Genetics: study of chromosomes and genes

MENDEL’S TRAITS

TRAIT TRAITVARIANTS

VARIANTS

MENDEL’S METHODS

mated known traits together methodicallycross-pollination: involves flowers of two separate plantsself-pollination: involves flowers of only one plantpeas usually self pollinate, but Mendel was able to force cross-pollination

Mendel’s Model Organism – the Garden Pea

A Method to the Madness – How Mendel Crossed Strains of Pea

Began with “pure” plantsCross-pollinated P generation to create F1 generation

Self-pollinated F1 generations to create F2 generation

Mendel was able to record the results of every cross

MENDEL’S EXPERIMENTS

MENDEL’S RESULTS AND CONCLUSIONS

Recessive and Dominant TraitsAll members of F1 generations had similar traits, but F2 generation showed variation

The Reality of “Round and Wrinkled” – Two Alternative Traits of the Seed Shape Character

It’s worth noting that each of these seeds is a new individual of a different generation from the plant that bears the peas.

MENDEL’S RESULTS AND CONCLUSIONS

Law of Dominancewhen present, a dominant allele will be expressed

DOMINANT AND

RECESSIVE

TRAITS

Phenotypes when dealing with different genotypes

MENDEL’S RESULTS AND CONCLUSIONS

Law of segregationa pair of factors is segregated, or separated, during the formation of gametes

homologous pairs split during anaphase I

Monohybrid Crosses and the Principle of Segregation

A cross between individuals differing in single character is a monohybrid cross.

The analysis of monohybrid crosses allowed Mendel to deduce the Principle of Segregation ....

Genes come in pairs that separate in the formation of sex cells (and these sex cells unite randomly at fertilization).

MENDEL’S RESULTS AND CONCLUSIONS

Law of independent assortmentfactors for different characteristics are distributed to gametes independently

the orientation of one homologous pair does not affect the orientation of other homologous pairs

Independent Assortment in Meiosis

Linkage

SECTION 9-2: GENETIC CROSSES

Each individual has exactly two alleles for each trait.

They may be the same or

different.

Each gamete has exactly one allele for each trait

GENOTYPE AND PHENOTYPE

Terms you need to know:Genotype

Phenotype

Homozygous

Heterozygous

PROBABILITY: LIKELIHOOD THAT A SPECIFIC EVENT

WILL OCCUR

outcomespossibleofnumber

outcomespositiveofnumberyProbabilit

B. PROBABILITY OF A RECESSIVE TRAIT IN F2

GENERATION: 1/4

Mendel’s Monohybrid Cross – P to F1

A Punnett square, something we’ll cover in a moment.

Homozygous x

homozygous PP

p

p

Pp Pp

Pp Pppp

PP

Staying the Course – Mendel Continued Crosses to the F2 (the grandchildren)

What was learned?

The green trait was not lost or altered, even though it disappeared in the F1.

One trait is dominant to the other in its expression.

The reappearance of the recessive trait in ¼ of the F2, suggests genes come in pairs that separate in the formation of sex cells.

PP Pp

Pp pp

Heterozygous x heterozygous

Pp x Pp

Pp

Pp

p

pP

P

Homozygous x

heterozygousTest crosses

PP x Ppor

pp x Pp

Pp

Pp pp

Pp pp

pp p

p

pP

Test Cross results for unknown

hybrid

Pp

Pp pp

Pp pp

pp p

p

pP

PP

p

p

Pp Pp

Pp Pppp

PP

Test Cross results for

unknown pure bred

Dihybrid: cross between individuals that involves comparing two traits

PREDICTING RESULTS OF DIHYBRID CROSSES

RRYY

rryyRY RY RY RY

ryRrYy RrYy RrYy RrYy

ryRrYy RrYy RrYy RrYy

ryRrYy RrYy RrYy RrYy

ryRrYy RrYy RrYy RrYy

Homozygous x homozygous to produce dihybrids

Pre

dict

the

gam

etes

Predict the gametes

Predict the F1 generation

MAKING THE GAMETES

RRYY Each gamete has exactly one allele for each trait

RY

MAKING GAMETES

RrYy

R R r r

FOIL(R+r)(Y+y)

All possibilities are equally likely

Y y Y y

independent assortment in action

Predict the F1 generation

RrYy

RrYyRY Ry rY ry

RYRRYY RRYy RrYY RrYy

RyRRYy RRyy RrYy Rryy

rYRrYY RrYy rrYY rrYy

ryRrYy Rryy rrYy rryy

Heterozygous x heterozygous to produce dihybrids

Predict the gametes

Pre

dict

the

gam

etes

Predict the offspring

With Complete Dominance, Different Genotypes Can Produce the Same Phenotype

Mendel Truly Stayed the Course, Creating an F3 (great-grandchildren) to Test the Hypothesis of Segregation

Are Different Characters Like Color and Shape Inherited Together or Inherited Independently?

Mendel performed dihybrid crosses to find out.Mendel’s conclusion: Different characters are inherited independently.

What Works for Peas Also Works for Humans

An albino woman

In the cross Aa x Aa, where A is a dominant allele for wild type (standard) pigmentation and a is a recessive allele for no pigmentation (albinism), ¾ of offspring will be wild type and ¼ will be albino.

RULES FOR WRITING GENOTYPES

1. ALWAYS KEEP ALLELES FOR EACH TRAIT TOGETHER2. ALWAYS PLACE DOMINANT ALLELES BEFORE RECESSIVE ALLELES

Some Alleles Are Related Through Incomplete Dominance

Dominance relationships may differ, but the Principle of Segregation remains the same.

Incomplete dominance is a partial expression of both alleles

Pleiotropy – When One Allele Influences Many Traits

EXAMPLES OF PLEIOTROPY Marfan Syndrome

can cause weakness of the aorta and extremely long thin bones

PKU

can cause mental retardation and reduced hair and skin pigmentation

Polygenic Inheritance – When a Single Trait is Influenced by Many Genes

Height is a polygenic trait

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SAsaSBsbSCsc

SAs a

SBs b

SCs c

Skin Color is polygenic

Fra

ctio

n o

f p

op

ula

tio

n

Skin color

1––64

15––64

6––64

20––64

SUMMARYPleiotropy

Gene

Trait

Trait

Trait

Trait

Trait

Trait

Polygenism

Trait

Gene

Gene

Gene

Gene

Gene

Gene

Multiple Alleles

Many genes are present in 3 or more versions (alleles) – this is known as multiple alleles.

The human ABO blood group is determined by three alleles (IA, IB, and i) of a single gene.

Codominance

The human ABO blood group illustrates another genetic phenomenon – codominance.

Codominance occurs when the phenotype associated with each allele is expressed in the heterozygote.

The AB phenotype (genotype IA IB) is an example of codominance

Chromosomes and Inheritance

Since genes are carried on chromosomes, knowledge of chromosome number and structure has far-reaching implications for basic genetics and human health.

A normal human male karyotype.

X-linked Inheritance

Behind the 8-ball? Colorblindness is an X-linked recessive trait.

X-linked Inheritance – When Men and Woman Play by Different Rules

X-linked Inheritance – When Men and Woman Play by Different Rules

What a color blind individual sees.

What a normal person sees.

Behind the 8-ball? Colorblindness is an X-linked recessive trait.

Many Genetic Diseases are Autosomal Recessive Traits

What’s an autosome?

Sickle cell anemia is a recessive autosomal disease common in areas where malaria is endemic.

What Works in Peas Works (genetically speaking) Works in People

¼ of offspring of two carriers of a recessive allele are expected to show the recessive trait; ½ of offspring are expected to be carriers.

Many Human Traits are Autosomal Dominant Traits

For disease traits, autosomal dominant inheritance is less common than autosomal recessive inheritance.

The Fundamental Question

What is the relationship between genes (genotype) and observable characteristics (phenotype)?

orNature versus nurture?

The answer?

Genotype + Environment Phenotype

Genes and Environment Determine Characters

Genetically identical hydrangeas growing in soils of different acidity (different environments).

The phenotype = genotype + environment principle applies equally to human traits.

CONCEPT CHECKThe figure diagrams one

of the genetic crosses that helped Mendel form his theories of inheritance. Which process distributes “P” alleles to approximately one half of the F1 gametes and the “p” allele to the other half?

Independent assortment.Mitosis. Meiosis. Chromosome crossover.

ANSWERThe figure diagrams

one of the genetic crosses that helped Mendel form his theories of inheritance. Which process distributes “P” alleles to approximately one half of the F1 gametes and the “p” allele to the other half?

3) Meiosis.

CONCEPT CHECK

Mendel’s principle of independent assortment holds true only for traits with genes that

are on homologous chromosome (linked).are on separate chromosomes (unlinked). have loci far apart on homologous chromosomes. are both “1” and “2”.

ANSWER

Mendel’s principle of independent assortment holds true only for traits with genes that

4) are both “1” and “2”.

CONCEPT CHECKThe traits studied by Mendel were mostly discrete

traits with two phenotypes. Many traits such as human height vary continuously. Which of the following describe the inheritance of continuously varying traits using Mendel’s rules of inheritance?1) Two or more genes interact to

produce the variation in the trait apparent in the population of the organism.

2) Two or more alleles interact to produce the variation in the trait apparent in the population of the organism.

3) Mendel’s principles do not apply to continuously varying traits.

4) Traits with continuous variability are not influenced by heredity.

ANSWERThe traits studied by Mendel were mostly discrete

traits with two phenotypes. Many traits such as human height vary continuously. Which of the following describe the inheritance of continuously varying traits using Mendel’s rules of inheritance?1) Two or more genes interact to

produce the variation in the trait apparent in the population of the organism.