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
Pea Color Trait
Generation
P G x Y
F
1 x
All Yellow
F
2
¼ of progeny green

What Does It Mean, Mendel???
• The disappearance of the green trait in F
1 is recessive means it
• The appearance of only the yellow trait in F
1 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
Pea Color Trait & Height Trait
Generation
P
G x
Y
F
1 x
All Yellow &
Tall

Mendel’s Dihybrid Cross
F
2
9/16 yellow & tall
3/16 green & tall
3/16 yellow & short
1/16 green & short

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 & Genes
Homologous Chromosomes
Sister
Chromatids
Maternal
Homolog
Paternal
Homolog
Loci
Non-Sister
Chromatids
Alleles

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: I A I A , I A O
• Phenotype: ABO type B
• Genotype: I B I B , I B O
• Phenotype: ABO type O
• Genotype: OO

Co-Dominance
• Phenotype: ABO type A
• Genotype: I A I B
• 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
Normal
• 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

DNA

Complementary Base
Pairing