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
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-
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 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
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