Genetics Since MendelGenetics Since Mendel

Chapter 5, Section 2Chapter 5, Section 2

Genetics Since MendelGenetics Since Mendel

What have What have we found we found out since out since Mendel?Mendel?

Since Mendel’s Since Mendel’s experiments, scientists experiments, scientists have found that genetic have found that genetic inheritance can be much inheritance can be much more complicated than more complicated than the simple, dominant-the simple, dominant-recessive inheritance recessive inheritance that Mendel saw in pea that Mendel saw in pea plants.plants.

Incomplete DominanceIncomplete Dominance

Incomplete Incomplete DominanceDominance

Occurs when offspring of Occurs when offspring of two homozygous parents two homozygous parents show an intermediate show an intermediate phenotype.phenotype. The heterozygous form is The heterozygous form is

in between the dominant in between the dominant and recessive and recessive homozygous forms.homozygous forms.

Incomplete DominanceIncomplete Dominance

Incomplete Incomplete DominanceDominance

Example:Example: Snapdragons:Snapdragons:

Crossing a purebred white Crossing a purebred white snapdragon with a snapdragon with a purebred red snapdragon purebred red snapdragon produces a pink produces a pink snapdragon.snapdragon.

Multiple AllelesMultiple Alleles

Multiple Multiple AllelesAlleles

Trait is controlled by more Trait is controlled by more than two alleles.than two alleles.

Produce more than 3 Produce more than 3 phenotypes for the trait.phenotypes for the trait.

Multiple AllelesMultiple Alleles

Multiple Multiple AllelesAlleles

Example:Example: Blood Type:Blood Type:

Alleles: IAlleles: IAA I IBB and i and i Phenotypes: A, B, AB, OPhenotypes: A, B, AB, O

Type O is recessive to types Type O is recessive to types A and B.A and B.

Types A and B are both Types A and B are both expressed in the phenotype expressed in the phenotype when paired together.when paired together.

Polygenic InheritancePolygenic Inheritance

Polygenic Polygenic InheritanceInheritance

Occurs when a group of Occurs when a group of gene pairs act together to gene pairs act together to produce a trait.produce a trait. Traits are controlled by Traits are controlled by

alleles from more than one alleles from more than one gene.gene.

A wide variety of A wide variety of phenotypes are phenotypes are produced.produced.

Examples:Examples: Hair, eye, and skin colorHair, eye, and skin color

Impact of the EnvironmentImpact of the Environment

Environmental Environmental influence on influence on genetics:genetics:

Environmental Environmental influences can be influences can be internal or external.internal or external. Examples:Examples:

A boy who has genetics A boy who has genetics to be six feet tall is to be six feet tall is malnourished as a child malnourished as a child and is only five foot eight and is only five foot eight inches tall.inches tall.

Male birds exhibit Male birds exhibit brighter feathers than brighter feathers than female birds.female birds.

Human Genes and MutationsHuman Genes and Mutations

What is a What is a mutation mutation again?again?

Error in the DNA Error in the DNA sequence.sequence.

Can be harmful, helpful, or Can be harmful, helpful, or null.null.

Can be caused by Can be caused by mistakes in replication or mistakes in replication or by external agents called by external agents called mutagens.mutagens.

Human Genes and MutationsHuman Genes and Mutations

Chromosome Chromosome DisordersDisorders

An organism receives An organism receives too many or too few too many or too few chromosomes as a chromosomes as a result of mistakes in result of mistakes in meiosis.meiosis.

Usually fatalUsually fatal Example:Example:

Down’s syndromeDown’s syndrome 3 copies of 3 copies of

chromosome 21.chromosome 21.

Recessive Genetic DisordersRecessive Genetic Disorders

Recessive Recessive Genetic Genetic DisorderDisorder

Disorder only appears if Disorder only appears if an individual has both an individual has both recessive alleles.recessive alleles.

Parents are carriers Parents are carriers (heterozygous) and do (heterozygous) and do not show symptoms of not show symptoms of the disorder.the disorder.

Recessive Genetic DisordersRecessive Genetic Disorders

Recessive Recessive Genetic Genetic DisorderDisorder

Example:Example: Cystic fibrosisCystic fibrosis

Production of thick instead of Production of thick instead of thin mucous in the lungs and thin mucous in the lungs and intestinal tract.intestinal tract.

Most common genetic Most common genetic disorder leading to death in disorder leading to death in Caucasian Americans.Caucasian Americans.

Determination of GenderDetermination of Gender

Boy or Girl?Boy or Girl? Sperm cells have either Sperm cells have either an X or a Y an X or a Y chromosome.chromosome.

Egg cells have only an Egg cells have only an X chromosome.X chromosome.

Males – XYMales – XY Females - XXFemales - XX

Sex-Linked DisordersSex-Linked Disorders

Sex-linked Sex-linked DisordersDisorders

Alleles controlling the Alleles controlling the genetic disorder are genetic disorder are linked to either the X or linked to either the X or the Y chromosome.the Y chromosome.

Sex-Linked DisordersSex-Linked Disorders

Sex-linked Sex-linked DisordersDisorders

Example: Example: Color blindnessColor blindness

Recessive allele linked to Recessive allele linked to the X-chromosome.the X-chromosome.

All males with the All males with the recessive allele are recessive allele are colorblind.colorblind.

Females must have two Females must have two recessive alleles to be recessive alleles to be colorblind.colorblind.

PedigreesPedigrees

PedigreesPedigrees Family tree for a certain Family tree for a certain trait.trait.

Used to determine Used to determine inheritance patterns and inheritance patterns and probabilities and in probabilities and in breeding plants and breeding plants and animals.animals.

PedigreesPedigrees

How to How to read a read a pedigree:pedigree:

Males – squaresMales – squares

Females – circlesFemales – circles Have the trait – coloredHave the trait – colored Does not have the trait – Does not have the trait –

emptyempty

Is a carrier Is a carrier (heterozygous) – half-(heterozygous) – half-coloredcolored