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?

G REGOR M ENDELBiography monk in Austria in mid-1800s studied statistics TeacherGardener

M ENDEL ’ S GARDEN PEAS heredity: transmission of traits from parents to offspring traits: 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

M ENDEL ’ S T RAITS TRAIT VARIANTS

M ENDEL ’ S METHODS mated known traits together methodically cross-pollination: involves flowers of two separate plants self-pollination: involves flowers of only one plant peas 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” plants Cross-pollinated P generation to create F 1 generation Self-pollinated F 1 generations to create F 2 generation Mendel was able to record the results of every cross M ENDEL ’ S E XPERIMENTS

M ENDEL ’ S R ESULTS AND C ONCLUSIONS Recessive and Dominant Traits All members of F 1 generations had similar traits, but F 2 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.

M ENDEL ’ S R ESULTS AND C ONCLUSIONS Law of Dominance when present, a dominant allele will be expressed

D OMINANT AND RECESSIVE TRAITS

Phenotypes when dealing with different genotypes

M ENDEL ’ S R ESULTS AND C ONCLUSIONS Law of segregation a 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).

M ENDEL ’ S R ESULTS AND C ONCLUSIONS Law of independent assortment factors 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

 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

G ENOTYPE AND P HENOTYPE Terms you need to know: GenotypePhenotypeHomozygousHeterozygous

P ROBABILITY : LIKELIHOOD THAT A SPECIFIC EVENT WILL OCCUR

B. P ROBABILITY OF A RECESSIVE TRAIT IN F2 GENERATION : 1/4

Mendel’s Monohybrid Cross – P to F 1 A Punnett square, something we’ll cover in a moment.

Homozygousxhomozygous PP p p Pp pp PP

Staying the Course – Mendel Continued Crosses to the F 2 (the grandchildren) What was learned? The green trait was not lost or altered, even though it disappeared in the F 1. One trait is dominant to the other in its expression. The reappearance of the recessive trait in ¼ of the F 2, suggests genes come in pairs that separate in the formation of sex cells.

PPPp pp Heterozygous x heterozygous Pp x Pp Pp p pP P

Homozygous x heterozygous Test crosses PP x Pp or pp x Pp Pp pp Pppp p p pP

Test Cross results for unknown hybrid Pp pp Pppp p p pP

PP p p Pp pp PP Test Cross results for unknown pure bred

Dihybrid: cross between individuals that involves comparing two traits P REDICTING RESULTS OF DIHYBRID CROSSES

RRYY rryy RY ry RrYy ry RrYy ry RrYy ry RrYy Homozygous x homozygous to produce dihybrids Predict the gametes Predict the F 1 generation

M AKING THE G AMETES RRYY Each gamete has exactly one allele for each trait RY

RrYy RRrr FOIL (R+r)(Y+y) All possibilities are equally likely Y yY y

independent assortment in action independent assortment in action

Predict the F1 generation RrYy RYRyrYry RY RRYYRRYyRrYYRrYy Ry RRYyRRyyRrYyRryy rY RrYYRrYyrrYYrrYy ry RrYyRryyrrYyrryy Heterozygous x heterozygous to produce dihybrids Predict the gametes Predict the offspring

With Complete Dominance, Different Genotypes Can Produce the Same Phenotype

Mendel Truly Stayed the Course, Creating an F 3 (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.

R ULES FOR W RITING G ENOTYPES 1. ALWAYS K EEP ALLELES FOR EACH TRAIT TOGETHER 2. 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

E XAMPLES OF P LEIOTROPY 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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SAsaSBsbSCscSAsaSBsbSCsc SAsaSBsbSCscSAsaSBsbSCsc Skin Color is polygenic

Fraction of population Skin color 1 –– –– 64 6 –– –– 64

S UMMARY Pleiotrop y GeneTrait Polygenism Trait 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 (I A, I B, 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 I A I B ) 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

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)? or Nature 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.

C ONCEPT C HECK The 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 F 1 gametes and the “p” allele to the other half? Independent assortment. Mitosis.Meiosis. Chromosome crossover.

A NSWER The 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 F 1 gametes and the “p” allele to the other half? 3)Meiosis.

C ONCEPT C HECK 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”.

A NSWER Mendel’s principle of independent assortment holds true only for traits with genes that 4)are both “1” and “2”.

C ONCEPT C HECK The 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.

A NSWER The 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.