The Work of Gregor MendelThe Work of Gregor Mendel

QuestionQuestion

Have you ever wondered why Have you ever wondered why everyone in your family is tall, but everyone in your family is tall, but you are short? you are short?

Have you ever wondered why Have you ever wondered why brothers and sisters can look brothers and sisters can look different, or look so much alike?different, or look so much alike?

MendelMendel Called the “Father of Genetics”Called the “Father of Genetics” Studied patterns of heredity of pea Studied patterns of heredity of pea

plants.plants. Genetics – the study of heredityGenetics – the study of heredity

Three Conclusions to His Three Conclusions to His ResearchResearch

Principle of Dominance and RecessivenessPrinciple of Dominance and RecessivenessOne allele in a pair may mask the effect of the otherOne allele in a pair may mask the effect of the other

Principle of SegregationPrinciple of SegregationThe two alleles for a characteristic separate during the The two alleles for a characteristic separate during the

formation of eggs and spermformation of eggs and sperm

Principle of Independent AssortmentPrinciple of Independent AssortmentThe alleles for different characteristics are distributed to The alleles for different characteristics are distributed to

reproductive cells independently.reproductive cells independently.

Mendelian geneticsMendelian genetics

Character Character (heritable feature, i.e., fur color)(heritable feature, i.e., fur color)

Trait Trait (variant (variant for a character, i.e., brown)for a character, i.e., brown)

True-bred True-bred (all offspring of same variety) (all offspring of same variety)

Hybridization Hybridization

(crossing of 2 different true-(crossing of 2 different true-breds)breds)

P generationP generation (parents) (parents) FF11 generation generation (first filial (first filial

generation)generation)

Genetic vocabulary…….Genetic vocabulary…….

Punnett square:Punnett square: Gene:Gene: point on a point on a

chromosome that controls the chromosome that controls the traittrait

AlleleAllele: an alternate form of a : an alternate form of a gene gene A or aA or a

Homozygous:Homozygous: identical identical alleles for a characteralleles for a character

Heterozygous:Heterozygous: different different alleles for a genealleles for a gene

Phenotype:Phenotype: physical traits physical traits Genotype:Genotype: genetic makeup genetic makeup Testcross:Testcross: breeding of a breeding of a

recessive homozygote X recessive homozygote X dominate phenotype (but dominate phenotype (but unknown genotype)unknown genotype)

How can the Chances of an Offspring’s How can the Chances of an Offspring’s Traits be Determined?Traits be Determined?

BY USING A BY USING A PUNNETT SQUAREPUNNETT SQUARE Get out your Punnett Squares Get out your Punnett Squares

ProblemsProblems

BbBb X X BbBbPhenotypic RatioPhenotypic Ratio

3:13:1Genotypic RatioGenotypic Ratio

1:2:11:2:1

B b

B

b

B B B b

b B b b

What about two traits?What about two traits? Complete the second half of your Complete the second half of your

Punnett Square ProblemsPunnett Square Problems

Dihybrid CrossDihybrid Cross

BbTtBbTt x x BbTtBbTt

BTBT BtBt bTbT btbt

BTBT BBBBTTTT BBTtBBTt BbTTBbTT BbTtBbTt

BtBt BBTtBBTt BBttBBtt BbTtBbTt BbttBbtt

bTbT BbTTBbTT BbTtBbTt bbbbTTTT bbTtbbTt

btbt BbTtBbTt BbttBbtt bbTtbbTt bbttbbtt

Exceptions to Mendel’s Exceptions to Mendel’s RuleRule

Incomplete DominanceIncomplete Dominance

The phenotype of The phenotype of the heterozygote the heterozygote is intermediate is intermediate between those of between those of the two the two homozygotes.homozygotes.

Ex) Snap Dragon Ex) Snap Dragon ColorColor

Red, Pink, WhiteRed, Pink, White

Co-dominanceCo-dominance

Phenotype of both Phenotype of both homozygotes are homozygotes are produced in produced in heterozygotes heterozygotes individuals.individuals.

Both alleles are Both alleles are expressed equally.expressed equally.

Ex)Roan Cattle Ex)Roan Cattle White-feathered White-feathered birds are both birds are both homozygotes for homozygotes for both B and W both B and W allelesalleles

Multiple AllelesMultiple Alleles

Ex )Blood typeEx )Blood type Blood type A and B are Blood type A and B are co-co-

dominantdominant, while O is , while O is recessiverecessive.. Forms possible blood types of A, B, Forms possible blood types of A, B,

AB, and O.AB, and O.

Blood Also Shows Blood Also Shows CodominanceCodominance

Sex-DeterminationSex-Determination

Chromosome pairs 1-22 are Chromosome pairs 1-22 are autosomesautosomes

Chromosome pair 23 are Chromosome pair 23 are sex sex chromosomeschromosomes

They determine the sex of an They determine the sex of an individualindividual

XX = femaleXX = female XY = maleXY = male

Sex-Linked InheritanceSex-Linked Inheritance

Traits that are only found on the X Traits that are only found on the X chromosomechromosome

Colorblindness and Hemophilia are Colorblindness and Hemophilia are examples of sex-linked traits. examples of sex-linked traits.

These genes are recessive and found These genes are recessive and found only on the X chromosome.only on the X chromosome.

Polygenic InheritancePolygenic Inheritance Inheritance pattern of a Inheritance pattern of a

trait that is controlled trait that is controlled by by two or more two or more genes.genes.

Gene may be on the Gene may be on the same chromosome or same chromosome or on different on different chromosomes.chromosomes.

Ex) Skin color and Ex) Skin color and HeightHeight