Genetics

Gregor Mendel

Genetic Concepts

• Phenotype– Physical characteristics of organism– Result of gene expression– Biochemical properties of proteins determine

physical characteristics of organism

• Genotype– Set of alleles present in genome of organism– Alleles are versions of genes

Segregating Traits in Sweet Pea

Genetic Crosses

• Monohybrid – cross between two variants of a single trait

– ABO type A x ABO type B

• Dihybrid – cross between two variants of two traits

– ABO type A, Rh+ x ABO type B, Rh-

Mendel’s Monohybrid Crosses

G Y

Pea Color Trait

x

All Yellow

P

Generation

F1

x

F2¼ of progeny

green

What Does It Mean, Mendel???• The disappearance of the green trait in F1 means it

is recessive• The appearance of only the yellow trait in F1 means

it is dominant• A recessive trait is seen if only recessive alleles of

that gene are present• The dominant trait is seen if 1 or 2 alleles of that

gene are present• THERE MUST BE TWO ALLELES OF EACH

GENE• THE ALLELES SEPARATE DURING

REPRODUCTION

Mendel’s Dyhybrid Crosses

Y

Pea Color Trait & Height Trait

x

All Yellow & Tall

P

Generation

F1

G

x

Mendel’s Dihybrid Cross

9/16 yellow & tall3/16 green & tall

3/16 yellow & short 1/16 green & short

F2

What Does It Mean, Mendel???

• Yellow and Tall are dominant traits; green & short are recessive

• EITHER ALLELE CONTROLLING COLOR CAN ASSORT WITH EITHER ALLELE CONTROLLING HEIGHT

Mendel’s Principles

• Law of Segregation– There are two alleles (distinct copies) of each gene

in the genome of an organism– The two alleles are separated during meiosis and put

into separate gametes

• Law of Independent Assortment– Any allele of one gene may assort with any allele of

any other gene– All possible allelic combinations are possible if

enough gametes are produced

Chromosomes & GenesHomologous Chromosomes

Sister Chromatids

Non-Sister Chromatids

Loci

Alleles

Maternal Homolog

Paternal Homolog

Meiosis I

Crossing Over (Not with John Edward)

Y

Z

y

z

Meiosis II

Inheritance Patterns

• Complete Dominance

• Co-dominance

• Intermediate Dominance

Complete Dominance/Recessiveness

• Phenotype: ABO type A• Genotype: IAIA, IAO

• Phenotype: ABO type B• Genotype: IBIB, IBO

• Phenotype: ABO type O• Genotype: OO

Co-Dominance

• Phenotype: ABO type A

• Genotype: IAIB

• Both A & B alleles encode functional enzymes

• Both enzymes work at same time

• Both modifications are made simultaneously

• Phenotype is combined effect of both A & B enzymes

Intermediate Dominance

• A dosage effect

• B better than B’

• BB > BB’ > B’B’

• Red, pink, white flowers

• RR > RW > WW

Linkage

• Genes on same chromosome are separated by crossing over during meiosis I

• Genes located near each other, on the same chromosome, are separated infrequently

• Often they are inherited together

• This is termed “being linked”

Chromosomal Sex Determination

• In most organisms, males & females have a unique pair of chromosomes which determine sex

• XY mechanism– Males – XY; females - XX

• ZW mechanism– Males – ZZ; females - ZW

• X0 mechanism– Males – X_ ; females - XX

Sex Linkage

• Genes on the X or Y chromosomes are sex-linked

• A phenotype caused by a gene on the X chromosome will appear in males more frequently

Normal

Normal

DNA

Complementary Base Pairing